Self-assembled rolled-up devices: towards on-chip … Bibliographic record Smith, Elliot J.: Self-assembled rolled-up devices: towards on-chip sensor technologies Dissertation Chemnitz

Self-assembled rolled-up devices: towards on-chip sensor technologies

von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz

genehmigte Dissertation zur Erlangung des akademischen Grades

doktor rerum naturalium

(Dr. rer. nat.)

vorgelegt von

M.Sc. Elliot John Smith

geboren am 13. Juni 1984

in St. Louis, MO, U.S.A.

eingereicht am 1. July 2011

Gutachter:

Prof. Dr. Oliver G. Schmidt

Prof. Dr. Rudolf Bratschitsch

Tag der Verteidigung: 29. August 2011

i

Bibliographic record

Smith, Elliot J.: Self-assembled rolled-up devices: towards on-chip sensor technologies Dissertation Chemnitz University of Technology, Faculty of Natural Science (2011) Keywords: Rolled-up, On-chip integration, Lab-in-a-tube, Hyperlens, Metamaterial optical fiber, Biosensor, Optical ring resonator, Micro-helix coil, Radial-magnetization, Corkscrew-magnetization, Hollow-bar-magnetization, Polymer delamination.

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Abstract

By implementing the rolled-up microfabrication method based on strain engineering, several

systems are investigated within the contents of this thesis. The structural morphing of planar geometries

into three-dimensional structures opens up many doors for the creation of unique material

configurations and devices. An exploration into several novel microsystems, encompassing various

scientific subjects, is made and methods for on-chip integration of these devices are presented.

The roll-up of a metal and oxide allows for a cylindrical hollow-core structure with a cladding layer

composed of a multilayer stack, plasmonic metamaterial. This structure can be used as a platform for a

number of optical metamaterial devices. By guiding light radially through this structure, a theoretical

investigation into the system makeup of a rolled-up hyperlens, is given. Using the same design, but

rather propagating light parallel to the cylinder, a novel device known as a metamaterial optical fiber is

defined. This fiber allows light to be guided classically and plasmonically within a single device. These

fibers are developed experimentally and are integrated into preexisting on-chip structures and

characterized.

A system known as lab-in-a-tube is introduced. The idea of lab-in-a-tube combines various rolled-up

components into a single all-encompassing biosensor that can be used to detect and monitor single bio-

organisms. The first device specifically tailored to this system is developed, flexible split-wall

microtube resonator sensors. A method for the capturing of embryonic mouse cells into on-chip optical

resonators is introduced. The sensor can optically detect, via photoluminescence, living cells confined

within the resonator through the compression and expansion of a nanogap built within its walls.

The rolled-up fabrication method is not limited to the well-investigated systems based on the roll-up

from semiconductor material or from a photoresist layer. A new approach, relying on the delamination

of polymers, is presented. This offers never-before-realized microscale structures and configurations.

This includes novel magnetic configurations and flexible fluidic sensors which can be designed for on-

chip and roving detector applications.

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Dedication

This thesis is dedicated to my Family.

To my Dad, who showed me the wonders of physics and optics since I was little.

To my Mom, who has always been there to support my dreams.

To my Brother, who has always looked out for me.

To my Girlfriend, who has made my time here truly wonderful.

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Table of Contents Bibliographic record i

Abstract ii

Dedication iii

Table of Contents iv

List of Figures and Tables vi

List of Abbreviations viii

1. General Introduction 1

1.1 Strain engineering ….….….…………………………….….…...…….... 1

1.2 Metamaterials …………….…………………………………………..… 4

1.3 Lab-in-a-tube …………………….…………………………………..…. 7

1.4 Polymer delamination ……………………………………………...…… 9

1.5 References ……………………………….……………..….……...……. 11

2. Rolled-Up Metamaterials 15

2.1 Introduction …………………………………………….……………….. 15

2.2 Rolled-up hyperlens ……………………………………..…………….... 17

2.2.1 Introduction …………………………………………….……... 17

2.2.2 Hyperlensing over entire visible spectrum …..….……….….… 19

2.2.3 Impedance matched systems ………………………………….. 22

2.2.4 Conclusion and outlook ………………………..….…………... 24

2.3 Metamaterial optical fibers ………………………………...…….……… 24

2.3.1 Introduction.…………………………………………..……….. 24

2.3.2 Theory …….……………………………………..…..…….….. 25

2.3.2.1 Material systems……………………………….……. 27

2.3.2.2 Combined classical and plasmonic guidance ………. 28

2.3.2.3 Sensing potential …………………………………… 30

2.3.2.4 Conclusions ...………………………………………. 31

2.3.3 Experiment ……….…………………………..………………. 32

2.3.3.1 On-chip integration of rolled-up systems ……….….. 32

2.3.3.2 Methods ……………………….….……………….… 34

2.3.3.3 Material investigation and characterization……..….. 35

2.3.3.4 Metamaterial optical fiber development …………..... 38

2.3.3.5 Metamaterial optical fiber characterization ………… 40

2.3.3.6 Conclusions and outlook ………….….…………….. 47

v

2.4 References ………………….….….……………………………………. 48

3. Towards Lab-in-a-Tube 52

3.1 Introduction ……………………………………………………………… 52

3.1.1 Rolled-up optical ring resonators ………………….………….. 52

3.2 Cell capture …………………………………………………………...…. 55

3.2.1 Microsyringe …………………………………………….......... 55

3.2.2 Capturing of 3T3 NIH embryonic mouse cells into microtubes . 56

3.3 Flexible split-wall microtube resonator sensors ………………………… 58

3.3.1 Microtube fabrication …………………………………………. 58

3.3.2 NIH 3T3 fibroblast mouse cells ………………………………. 58

3.3.3 Cell detection …………………………………………………. 59

3.3.4 FDTD analysis ………………………………………………… 62

3.3.5 Reproducibility ……………………....………………….….…. 65

3.4 Conclusions and outlook .…………………………………..…………… 66

3.5 References ……………….…………………………………..………….. 67

4. Rolled-Up Polymers by Delamination 70

4.1 Introduction ………………………………………………….….……… 70

4.2 Realizable geometrical structures …………………………………..….. 70

4.3 Magnetized micro-helix coil structures …………………………….…... 73

4.3.1 Introduction …………………………………………………… 73

4.3.2 Orientations of magnetization in coiled structures ..………….. 75

4.3.2.1 Experimental probing of magnetic configurations …. 78

4.3.2.2 Theoretical investigation of dynamics....………….… 83

4.3.3 Conclusions and outlook ……………………………...………. 88

4.4 Chemical sensing ……………………….……..………………….……... 90

4.4.1 Introduction …………………………………………………… 90

4.4.2 Media sensing via mechanical coil actuation ………………... 90

4.4.3 Conclusions and outlook ..…….….……………………….….. 94

4.5 Conclusions ……………………………………………………………... 95

4.6 References ………………………………………………………………. 96

5. Conclusions 99

Acknowledgements 102

Curriculum Vitae 104

Selbständigkeitserklärung 106

vi

List of Figures and Tables Introduction

Figure 1.1: Photoresist roll-up technique 3 Figure 1.2: Critical point drying 3 Figure 1.3: Manipulating light with metamaterials 5 Figure 1.4: Effective permittivity of a multilayer stack 7 Figure 1.5: Lab-in-a-tube concept 8 Figure 1.6: Coil-up and roll-up via polymer delamination 10

Rolled-Up Hyperlens

Figure 2.1: Defining coordinate systems 16 Figure 2.2: Layered plasmonic metamaterial vs. classical dielectric 17 Figure 2.3: Geometry of hyperlens 19 Figure 2.4: Rolled-up hyperlens system 20 Figure 2.5: Working range of a hyperlens 21 Figure 2.6: Immersion hyperlens 23 Metamaterial Fiber Optics Theory

Figure 2.7: Fiber ray diagrams 26 Figure 2.8: Effective permittivity of a 3:1 filling ratio of Ag/TiO2 27 Figure 2.9: Tuning between classical and plasmonic guidance 29 Figure 2.10: Metamaterial optical fiber sensing 30 Figure 2.11: Rolled-up metamaterial optical fiber simulated 31 Metamaterial Fiber Optics Experiment

Figure 2.12: Integration of rolled-up devices 33 Figure 2.13: Integration of an array of fibers 33 Figure 2.14: Ellipsometry measurements of TiOx 35 Figure 2.15: Ellipsometry and AFM measurements of Au 36 Figure 2.16: Ellipsometry measurements of Ag 37 Figure 2.17: Effective permittivity of TiOx/Au MOFs 38 Figure 2.18: Fabricated metamaterial optical fibers 39 Figure 2.19: Optical setup for metamaterial optical fiber characterization 41 Figure 2.20: Core vs. cladding output profile 42 Figure 2.21: Profiles of a TiOx (30 %)/Au (70 %) MOF at different wavelengths 43 Figure 2.22: Effective permittivity for a TiOx (30 %)/Au (70 %) metamaterial 44 Figure 2.23: Regions of guidance outlined for a TiOx (30 %)/Au (70 %) MOF 45 Figure 2.24: Regions of guidance outlined for a TiOx (20 %)/Au (80 %) MOF 46 Figure 2.25: Close-up of transition from Region 4 to Region 5 46 Figure 2.26: Output profile of rolled-up TiOx fibers 47 Towards Lab-in-a-Tube

Figure 3.1: Structures which support WGMs 53 Figure 3.2: Integration of rolled-up microtubes 54

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Figure 3.3: Microsyringe setup 56 Figure 3.4: Cell capture sequence 57 Figure 3.5: Cell detection with F-SWµRS 59 Figure 3.6: Polarization measurements 60 Figure 3.7: Schematic of sensing action 61 Figure 3.8: Fitting of data using FDTD 63 Figure 3.9: FDTD simulations comparing a tube with and without a nanogap 64 Figure 3.10: SEM of microtube reveals nanogap 64 Figure 3.11: Reliably reproducible F-SWµRS 65 Figure 3.12: Consecutive detection of cells using single F-SWµRS 66

Rolled-Up Magnetic Polymers

Figure 4.1: Realizable structures from rolled-up polymer method 71 Figure 4.2: SU8 polymer thicknesses 72 Figure 4.3: Magnetic hysteresis loop 74 Figure 4.4: Visualization of magnetic moment for in-plane and out-of-plane easy axis 75 Figure 4.5: Vibrating sample magnetometry measurements 76 Figure 4.6: Obtainable magnetic orientations of a micro-helix coil 77 Figure 4.7: Fit of the applied external magnetic field as a function of time 79 Figure 4.8: Coil tracking of trajectory vs. film magnetization orientation 80 Figure 4.9: A visualization of the uncompensated magnetic moment arising in radial-magnetized coils 81 Figure 4.10: Response of radial-magnetized coils to an external alternating magnetic field 81 Figure 4.11: Experimentally obtained dynamics of a coil vs. the rotation of an external magnetic field 82 Figure 4.12: Theoretical modeling of coil dynamics 85 Figure 4.13: Transition from rotative to OLR motion 85 Figure 4.14: Nuances of competing torques 86 Figure 4.15: Close up of competing torques 87 Figure 4.16: Phase portraits of coil dynamics 88 Figure 4.17: Cargo pickup and delivery 89 Rolled-Up Polymer Sensors

Figure 4.18: Chemical sensing via polymer swelling and shrinking 91 Figure 4.19: Contact angle 92 Figure 4.20: Dependence of the radius of curvature of a coil vs. immersion chemical 93 Figure 4.21: Mechanism of coiling and uncoiling 93 Figure 4.22: On-chip and roving sensors under development 95 Table 4.1: Contact angle of different materials 92 Table 4.2: Average coil radius of curvature vs. the immersion chemical 94

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List of Abbreviations Angstrom Å Atomic layer deposition ALD Aluminum oxide (sapphire) Al2O3

Angle of rotation of bar magnet θmag Angle offset between coil ends uncomp

coilΨ

Angular frequency of magnet ωmag Angular position of the coil Ψcoil Applied magnetic field B(t) Attenuated total reflectance ATR Atomic force microscopy AFM Average index of refraction navg Average diameter Davg Azimuthal mode number M# Bilayer thickness d Carbon dioxide CO2 Cell diameter Dcell Central angle of magnetic moment α Cladding index of refraction ncladding Cobolt Co Coercivity Hc Coil length Cl Contact angle θcon Copper Cu Core index of refraction ncore Critical angle θC Critical point dryer CPD Distance between hiccups ς Distributed Bragg reflector DBR Dulbecco’s modified eagle’s medium/ham’s F-12 nutrient DMEM/F-12 Drag coefficient γ Effective index of refraction neff Effective permittivity εeff Effective viscosity of medium ηv Electric field E-field Electromagnetic EM Electron beam E-Beam Ethylenediaminetetraacetic acid EDTA Embryonic fibroblast mouse cell NIH 3T3 Exchange parameter A External coil radius Rext Extracellular matrix ECM Filling ratio of dielectric fd

Filling ratio of material a fa Filling ratio of material b fb Filling ratio of metal fm Finite-difference time-domain FDTD Finite element method FEM Flexible split-wall microtube resonator sensor F-SWµRS Focused ion beam FIB

ix

Form factor FF(t) Free space wavelength λo

Free space wavenumber ko

Full width at half maximum FWHM Gold Au Gold palladium alloy AuPd Hafnium oxide HfO2 Helium cadmium HeCd Hydrodynamic torque (damping torque) TH Imaginary part of x Im{x} Imaginary part of index of refraction κ Imaginary part of permittivity ε2 Index of refraction n Index of refraction of medium nmedium Infrared IR Initial phase of the rotating magnetic field φ Inner diameter Din Inner radius r in Internal coil radius Rint Isopropanol IPA Lens magnification Lm Magnetic field H-field Magnetic field intensity H Magnetic moment M Magnetic moment of coil M (Ψcoil) Magnetic torque TM(Ψcoil,t) Magnetic saturation Ms Material length scale Mls Mass of coil m Metamaterial optical fiber MOF Micrometer µm Microliter µL Millibar mbar Milliliter mL Millitesla mT Milliwatts mW Molecular beam epitaxy MBE Moment of inertia I N-Methyl-2-pyrrolidone NMP Nanometer nm Negative index of refraction material NIM Number of windings N Numerical aperture NA Oscillatory-like-rotative OLR Outer diameter Dout Outer radius rout Overall average rotational angular velocity of the coil τ Oxygen O2 Parallel component of permittivity ε‖ Perpendicular component of permittivity ε⊥ Phosphate buffered saline PBS

x

Photoluminescence PL Platinum Pt Projection separation Ps Quality factor Q-factor Radial component of permittivity εr Radial component of wavevector kr Real part of x Re{x} Real part of index of refraction η Real part of permittivity ε1 Relative permeability µ Relative permittivity ε Relative permittivity of material a εa Relative permittivity of material b εb Relative permittivity of dielectric εd Relative permittivity of metal εm Revolutions per minute rpm Rolling length Rl Root mean square surface roughness Srms Scanning electron microscope SEM Shape factor of coil χ Silicon Si Silicon dioxide SiO2 Silicon monoxide SiO Silver Ag Size of quantum dot q Spatial object separation Os Strain of material 1 β1

Strain gradient ∆β Stress S Superlinear scaling factor ξ Surface-enhanced Raman spectroscopy SERS Surface plasmon SP Tangential component of permittivity εθ Tangential component of wavevector kθ Thickness of magnetic strip T Thickness of material a da

Thickness of material b db Three-dimensional 3D Time t Titanium dioxide TiO2 Titanium oxide TiOx Total torque T(Ψcoil,t) Transitional region of angular frequency transition

magω

Transverse electric TE Transverse magnetic TM Two-dimensional 2D Units U Vibrating sample magnetometry VSM Volume of coil V Wavelength λ

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Wavevector k Width of magnetic strip W Whispering gallery mode WGM x-component of applied magnetic field Bx x-component of magnetic moment Mx y-component of applied magnetic field By

y-component of magnetic moment My Young’s Modulus E

Chapter 1 General Introduction

1

Chapter 1: General Introduction

In this thesis a number of different micro-sized devices that can be created and developed for

on-chip sensing applications, will be introduced and discussed. Micro-fabrication techniques can be

used to create unique geometries of materials, which are fundamentally new or can lead to new

functionality. The ability to go beyond the restriction of two-dimensional (2D) planar devices

towards more complex three-dimensional (3D) architectures gives rise to many applications

including metamaterials,[1] photonic crystals,[2] smart cargo containers,[3] and micro-jet engines,[4]

just to name a very few. As the size of devices continues to diminish, researchers are challenged to

simplify the fabrication techniques while at the same time, increasing their functionality.[5] The

devices discussed within this work all move along this principle and towards functional on-chip

integration, employing rolled-up technology.[6] With this technology, a number of devices can be

created including metamaterials, like hyperlens’[7] and metamaterial fiber optics[8] discussed in

Chapter 2, lab-in-a-tube systems[9,10] discussed in Chapter 3 and functional rolled-up polymers[11,12]

introduced in Chapter 4.

1.1 Strain engineering

All of the devices and techniques, which will be discussed in this thesis, are based on a

technique using strain engineering for creating 3D architectures.[6,13] Stress, defined as an internal

force within an object,[14] is a common element present in many nanoscale objects. As described in

the book Thin Film Materials:

“Virtually any thin film bonded to a substrate or any individual lamina within a multilayer material supports some state of residual stress over a size scale on the order of its thickness. The presence of residual stress implies that, if the film would be relieved of the constraint of the substrate or an individual lamina would be relieved of the constraint of its neighboring layers, it would change its in-plane dimensions and/or would become curved.” [15]

Using this principle and assuming a sample consisting of a substrate covered by a sacrificial layer

with a thin nanomembrane deposited on top; if this sacrificial layer were removed, the

nanomembrane would be released, and it would bend.[6,16] If a nanomembrane consisting of a

bilayer of two materials with different biaxial strains (β1 and β2) is considered, this bending could

result in one of two structures:[17] It could wrinkle if the strain gradient (∆β = β2 – β1) in the film is


2

relatively small (∆β < 0.5%), or it could roll-up if the strain gradient is large enough (∆β > 0.5%).

The relationship between the strain (the change in length of a film divided by the original film

length) and the stress (S) in a film is given by the Young’s modulus (E), a material dependent value,

where S = Eβ.[14] Films which roll-up will be focused on here. Stress is an intrinsic property of

nanomembranes and it can be tuned by changing many factors like the thickness of the

nanomembrane, the deposition temperature, and deposition rate. A nanomembrane can be

composed of more than one material; often used is a bilayer. The most important aspect about

rolled-up strain engineering is that it allows for a planar deposition of all required components, then

upon release of these layers (i.e. by removing the sacrificial layer), three-dimensional, integrated,

functional devices can be created.

The strain gradient over a bilayer is tunable in different ways. For instance, with epitaxial grown

bilayers, the strain in the nanomembrane[18] can be calculated using the lattice mismatch of the two

materials. The resulting microtubes are well scalable knowing the crystalline lattice constant and by

changing the bilayer thickness.[19] However, given that epitaxy is an extremely expensive

technology, and that not all devices require single crystalline films, it’s of importance to introduce

other methods for creating rolled-up structures. In this case, a different sacrificial layer would need

to be suggested. There are a number of different alternative sacrificial layers which have been

investigated for this purpose: Photoresist,[20,21] Ge,[22] or polymer delamination.[11,12] All of these

alternatives eliminate the need for an expensive molecular beam epitaxy machine (MBE) as well as

more harmful dissolvents/etchants like hydrofluoric acid which is required for processing

semiconductor materials. Instead, each can be easily patterned by photolithographic techniques, as

well as dissolved in organic solvents. This thesis will focus on structures made using the photoresist

and polymer delamination method.

First, patterns are defined on Si or glass by a photolithography process. The materials are then

deposited onto the sample using any of a number of deposition tools. The techniques used for the

contents of this thesis include electron beam (E-Beam) deposition, sputter deposition, thermal

evaporation and atomic layer deposition (ALD). By being able to use these different types of

deposition techniques, the number of possible material combinations is very large. The angle at

which the samples are deposited can be adjusted from 0° to 90°, defined by the angle of the

deposition source to that of the sample. This process is known as angle deposition. The photoresist

technique employs this angle deposition method, Fig. 1.1, which allows for an “anchor” and

“window,” resulting in a preferential rolling direction.[20] The “anchor” defines the final resting

place of the rolled-up structure, whereas the “window” allows access for the dissolvent to contact


3

Figure 1.1 | Photoresist roll-up technique. A nanomembrane bilayer is deposited onto a sacrificial photoresist layer at an angle. The angle deposition creates a “window,” (created from the shadow the photoresist creates) so the dissolvent has access to the sacrificial layer, and an “anchor” which defines the final resting place of the microtube. When the sacrificial layer is released, the strained nanomembrane will relax, causing it to bend and roll-up. The samples can then be dried in a critical point dryer. An additional step can be made to coat the microtubes by ALD.

the photoresist. The “window” is created from the shadow the photoresist structure creates because

of the angle deposition of the materials. Dissolvents, which were used, include acetone, methanol,

isopropanol (IPA), and N-Methyl-2-pyrrolidone (NMP). After the tubes are rolled-up, the sample

can be dried using a critical point dryer (CPD). Given the thickness of the nanomembranes, on the

order of 10 – 50 nm, if the samples are air dried, due to the surface tension of the dissolvent, the

microtubes will be destroyed/flattened when the samples are brought from the liquid phase

(acetone) to the gas phase (air). However, a CPD avoids this by displacing acetone with liquid

carbon dioxide (CO2).[23] The displacement of acetone with liquid CO2 raises the pressure of the

drying chamber to around 50 bar, pt. 1 in Fig. 1.2. The CO2 is heated up into a supercritical phase

where the CO2 transitions into a quasi-liquid-gas phase. As the chamber is heated, the pressure

rises, pt. 2 in Fig. 1.2. The pressure is then released from the chamber, allowing for the substance to

be brought down from the supercritical phase, around the critical point (in the phase diagram) into

the gas phase,[23] pt. 3 in Fig. 1.2. This avoids any phase boundaries which can result in the collapse

of the microtubes. Once the samples are dry, an additional ALD step is optionally performed for

structural integrity. For these bilayer structures the strain gradient cannot be defined easily like

lattice mismatch in single crystalline materials. The strain gradient in the nanomembrane still arises

Figure 1.2 | Critical point drying. The sample is first placed in acetone which is then displaced by liquid CO2, pt. 1. The chamber is then heated, leading to a rise in pressure and the liquid morphing into a quasi liquid-gas phase, the supercritical region, pt. 2. The pressure is then released from the chamber, allowing the media to morph into the gas phase, pt. 3. This avoids any phase boundary, which would collapse a microtube.


4

from the different materials, however a number of factors can largely influence it; including the

interfaces of the unlike materials, growth temperature, film composition (which can be influenced

by deposition rate), and swelling of the sacrificial polymer layer, to name a few. Unlike rolled-up

semiconductors, whether or not a nanomembrane will roll-up upon release cannot be assumed a

priori. Due to this fact, each material combination must be individually investigated.

1.2 Metamaterials

The first of three parts of this thesis focuses on plasmonic metamaterials which can be designed

from rolled-up structures, so here a background on this subject is given. If one defines the material

length scale (Mls) as the distance between materials with different optical properties (index of

refraction, n) then certain classes of optics can be defined. In this way, classical optics deals with

light on a material length scale larger than the wavelength (λ) of light being investigated. As the

length scale is decreased to a size on the order of the wavelength, i.e. Mls ≈ λ, a class of optics

known as photonics is reached. Photonics includes structures like photonic crystals which

manipulate photons in much the same way that semiconductor crystals manipulate electrons.[24]

Both exhibit a forbidden region, known as a photonic band gap for photonic crystals[25] and an

electronic band gap for semiconductors.[26] A one-dimensional example of a photonic crystal is

known as a distributed Bragg reflector (DBR), which is composed of a periodic variation of

alternating layers of oxides (generally in the range 0.1λ < Mls < 0.9λ).[27] This device reflects a

deterministic region of light extremely efficiently, while allowing other wavelengths to pass,

lossless through the reflector. This reflected region is known as a photonic stop band. A common

use for this device is as mirrors of laser cavities or for enhancing the output of light emitting

diodes.[28] Devices extending this principle to 2D[29] and 3D[2] have also been highly investigated. If

the material length scale is brought down yet further (Mls << λ), a class of materials known as

metamaterials emerges. This class of materials includes plasmonics[30] which can break the

diffraction limit,[31,32] negative index of refraction materials (NIMs)[33] that bend light the “wrong”

way,[34,35] and cloaking devices which make an object appear invisible.[36,37]

Metamaterials are man-made materials, exhibiting optical properties not found in nature. These

materials can manipulate light in peculiar ways. Their optical properties derive from their sub-

wavelength material structure rather than their chemical makeup.[36] There are a number of light

phenomena which have been theorized and experimentally realized regarding such materials. One

such phenomenon is making light bend in the “wrong” direction, so called left-handed materials,[38]

Fig. 1.3(a). This effect is observed if the material has a negative index of refraction.[35,39,40] Given


5

that the index of refraction is defined as n = εµ , a Russian physicist Veselago showed that if a

material’s permittivity (ε) [the response of a material to an applied electric field] and permeability

(µ) [the response of a material to an applied magnetic field] were simultaneously negative, then the

index of refraction would be negative as well.[33] Obtaining a negative permittivity in the optical

range is relatively easy, conductors fall into this category of materials. However a negative

permeability is much harder to achieve; considering classically, µ is always assumed to be equal to

one at optical frequencies,[41] i.e. optically transparent bulk materials are intrinsically non-magnetic.

Although, there is a way of achieving a negative permeability by creating metallic (ε < 0) split ring

resonators which are embedded in a dielectric material. When a magnetic flux (originating from a

light source) passes through the split rings, rotating currents will be induced. This causes an

additional magnetic flux. In other words, the material will have a magnetic response to an applied

magnetic field, meaning the permeability of the material will no longer equal one. The rings have a

resonate frequency which is dependent on their dimensions and the gap size. If the rings are excited

at frequencies higher than resonance, then the real part of the permeability will become negative.[35]

In order to create a bulk metamaterial, multiple layers of these embedded split rings need to be

stacked and aligned on top of one another. Given this and the fact that the size of the split in the gap

is on the order of the wavelength, even with state-of-the-art nanofabrication techniques, it is

extremely difficult to fabricate the structures that respond at optical frequencies.[42,43]

Figure 1.3 | Manipulating light with metamaterials. Metamaterials are designed to manipulate light in peculiar ways. (a) One example is a negative index of refraction material which bends light in an opposite direction compared to classical refraction. (b) Another example is a cloaking device. Here a cylindrical object (white center) with a cloaking layer (surrounding layer) is shown. Top, A classical dielectric layer would not cloak the object, instead light would be scattered due to the object, and it would show up as a shadow in the exiting light. Bottom, if the layer is designed to cloak, then the phase of the wave is matched to what the phase of light outside of the layer is. This allows a “wrapping” of the light around the object and no shadow will be created, effectively hiding the object.

Another interesting device, which can be made from metamaterials, is referred to as a cloaking

device, as it cloaks any object held within the structure by wrapping the electromagnetic (EM)

waves smoothly around the object. Cloaking devices rely on a method employing transformation


6

optics which essentially allows for EM fields to be directed and shaped based on the material

makeup of the metamaterial.[36,37] An object which is not cloaked is detectable because it introduces

a shadow when inside an EM field, top Fig. 1.3(b). However, if the object is cloaked, and the EM

waves are wrapped around the object, then the object would leave no shadow, and in turn not be

seen, bottom Fig. 1.3(b). Assuming the cloak is a cylinder, in order to do this, the permittivity and

permeability of the cloak would need to vary as a function of the radius. Simply, the values for ε

and µ near the outer surface of the cloak would need to be very similar to the surrounding media,

say air, so that none, to very little Fresnel scattering off the surface would be observed. As light

travels from this outer surface to the interior surface, ε and µ would increase significantly, repelling

the EM waves from the center object, leading instead to a flow around it. These devices can also be

made assuming a non-magnetic material (µ = 1).[37]

Plasmonic metamaterials are considerably easier to fabric than NIMs (requiring nanopatterning)

and cloaking devices (requiring nanopatterning and particular object geometry). This is because

rather than needing to control a materials permeability and permittivity, only the permittivity needs

to be manipulated. This can be done through a layer combination which alternates metal (as the

conductor) and oxide (as the insulator). Plasmonics lead to an enhancement of the local electric

field because of the sub-wavelength confinement of light.[30] Due to this, sub-wavelength structures

are able to transfer and propagate light, allowing for the sizes of components to be decreased, as

well as opening opportunities in non-linear phenomenon because of this local confinement.[30] The

manipulation of light which plasmonics allows for can lead to increased data transfer as well as

reveal sub-wavelength information about surrounding objects. One example of this sensing is found

in surface-enhanced Raman spectroscopy (SERS).[44] Plasmonics refers to an optical field pertaining

to surface plasmons, which are waves that propagate along the surface of a conductor,[45] and can

arise on any interface between two materials of which one has positive permittivity and the other

has negative permittivity.[44] Surface plasmon polaritons are quasiparticles which form from the

coupling of photons with free electrons in the conductor. They are basically trapped waves of light

due to their interaction with the free electrons.[30] The electrons oscillate in resonance with the light.

Plasmonic properties can be tailored and manipulated by the makeup of a material. It is important to

note that plasmonics does not just pertain to metamaterials and vice versa, but here a focus is made

on where the two subjects overlap.

When designing plasmonic metamaterials, a requirement is that the metal-oxide bilayer

thickness (d) must be much smaller than the incident wavelength (d << λ), i.e. sub-wavelength.

With this, a large stack of bilayers can be considered as a bulk material because the light does not


7

Figure 1.4 | Effective permittivity of a multilayer stack. (a) A multilayer stack of bilayers composed of two different materials can be considered to have an overall anisotropic effective permittivity, (b).

“see” the individual layers, rather only the overall combined effect of them. When given a

multilayer stack, Fig. 1.4(a), comprised of two materials with different ε, a bulk effective

permittivity (εeff), Fig. 1.4(b), can be calculated for the material, revealing anisotropy. In other

words the material will exhibit a different permittivity parallel to the stacks (ε‖) than perpendicular

to the stacks (ε⊥). This phenomenon is known to occur in nature in some crystals exhibiting uniaxial

birefringence, where different polarizations of light will be refracted at different angles. This is due

to the light traveling at different velocities through a media depending on the propagation direction

and light polarization;[46] some examples include calcite and rutile. The perpendicular and parallel

components of the effective bulk material can be calculated by,[47]

abba

abeff

ff εεεεε+

=⊥ , (1)

bbaaeff ff εεε +=|| , (2)

where εa and εb are the permittivities of two different materials and fa and fb are their respective

filling ratios of the materials making up the bilayer, i.e. fa + fb = 1. This means that fa = da/(da + db)

and fb = db/(da + db), where da and db are the thicknesses of the individual layers making up the

bilayer. By making these stacked layers with a metal and dielectric, the plasma frequency inherent

to the metal can be shifted and tuned.[48] The plasma frequency is the resonant frequency of the free

electrons in the material. With these tunable structures, some interesting devices can be investigated

which will be discussed further in Chapter 2.

1.3 Lab-in-a-tube

Using strain engineering a variety of compact components have been fabricated and investigated

experimentally and theoretically. Because of the self-assembly process that allows for planar

components to be transformed into functional 3D structures, many different 3D devices have been

developed including electrical resistors[49] and energy storage devices;[22,50] optical ring


8

resonators;[51-53] 3D scaffolding for yeast,[54] animal fibroblast[10] and neuron cells;[55,56]

magnetically controlled fluidic sensors;[57] microjet engines;[4,20] and has opened the possibility for

designing future devices like tubular metamaterials.[7,8] The 3D scaffolding allows for “real life”

conditions given that in nature, cells are not confined to 2D as is typically investigated in the

laboratory. Using these small 3D structures also allows for the separation of cells, so that individual

cells may be investigated rather than looking at the group interaction of many.

The concept of lab-in-a-tube, proposed by Mei and Schmidt,[9] is an integration of many of the

individually-investigated rolled-up systems into a single ultra-compact bio-organism sensing

device, Fig. 1.5.[58] This concept allows researchers to go a step further than lab-on-a-chip[59] (that

works towards bringing the functionality of a laboratory down onto a single chip that can fit in the

palm of a hand), to a device which would be one of a thousand like devices on a single chip. This

sensing device would be for detecting changes and aspects of single cells, bacteria, and molecules.

A final version might have integrated electronics for monitoring resistive changes within the cells

Figure 1.5 | Lab-in-a-tube concept. An adaptation of the original proposal by Mei and Schmidt.[9] A lab-in-a-tube combines various rolled-up ultra-compact components into a single, highly-integrated bio-organism sensing device. Such a system would have optical, electrical and magnetic detectors and controllers, integrated within each device.

with each device supplied by its own rolled-up power source.[22] Since the system wouldn’t want to

rely on the random chance of trapping bio-organisms, a pump based on electroosmotic flow[60]

could be also available for capturing cells and other organisms. In the same device, integrated optics

would allow for the detection of structural changes a cell undergoes while within the structure,[10] as

well as optical observation of sub-wavelength cellular processes within the cell using a hyperlens[7]

could be possible.

Given the small size of this system, arrays of thousands of such devices would allow for a

highly parallel analysis of individual bio-organisms. The materials so far used for microtube cell


9

culture have all been found to be bio-compatible. Nevertheless the cells are able to recognize that

they are in contact with a foreign body and in this regard, tend to not flourish. Thus, the cells are not

comfortable with their surroundings when inside inorganic microtubes (the conditions existing for

previous cell culture investigations[54,55]). This can lead to changes in how a cell acts and

proliferates. Therefore, it is important to find a method to mimic the extracellular matrix (ECM)

which would deceive the cells into believing they are surrounded by other cells. This can be done

using self-assembled monolayers to coat the inorganic surface of the microtubes and the

functionalization of such tubes has been reported[61] offering a promising future in cell culture

experiments. The first work towards making this system a reality relies on the mechanical detection

of cells using optical ring resonators and will be discussed in further detail in Chapter 3.

1.4 Polymer delamination

As was mentioned in Section 1.1, two types of strain engineering are used in this work: a

photoresist method[20] which has been relatively well investigated and used for many different

structures;[21] and a second approach that was recently developed[11,12] relying on the delamination

of strained polymers,[62] Fig. 1.6. The concept and final obtainable geometries of this new approach

are intrinsically different from the photoresist and Ge sacrificial layer techniques. In both of those,

the rolling layer is on the order of or smaller than 100 nm. Also when the nanomembrane rolls, it

comes back into contact with itself after a single rotation, which is crucial for many applications;

particularly, mechanical contact of electrodes for electronics.[22] However, with the polymer

technique, since the polymer rolls with the nanomembrane, the nanomembrane of each following

roll is separated from the previous one by the thickness of the polymer. This nanomembrane

separation is a tunable property in such structures (500 nm – 1 µm).

Using the polymer technique, till now, there are two main structures which can be fabricated:

Coils and Swiss rolled spirals (and variants of these). The name Swiss roll[63] gets its name from the

geometry of the structure it describes, being that of a multiple thick-layer rolled-up geometry, which

looks similar to a Swiss roll or jelly roll dessert. For both, an inorganic layer is deposited onto a

photoresist patterned SU8 polymer, Fig. 1.6(a). SU8 is an epoxy which is a negative resist that was

developed by IBM.[64] The second step is to place the sample into NMP where the SU8 delaminates

and curls up. If the pattern consists of a narrow strip (< 5 µm in width), the curl up results in a

coiling, creating a helix coil in the end, Fig. 1.6(b). If instead, the patterns are square or rectangular

(tens of microns in width), instead of coiling, roll-up of the membrane occurs. The resulting

geometry is a truly Swiss rolled structure, Fig. 1.6(c). Using scanning electron microscopy (SEM)


10

Figure 1.6 | Coil-up and roll-up via polymer delamination. (a) An inorganic layer is deposited on top of a polymer which has been patterned via photolithography. This creates a strain gradient so that when the sample is placed in NMP it will delaminate from the substrate and curl up. (b) If the patterns are relatively thin strips (a few microns in width), then the result of delamination is a coil-up, resulting in a helix coil. (c) If the patterns are square or rectangular shaped (on the order of tens of microns), the result will be a roll-up ending in a Swiss rolled tube. (d) SEM images (made by Dr. S. Baunack) taken from a double-rolled structure, which was cut by FIB, reveals the Swiss roll geometry.

and cutting into the cross section of such a structure with a focused ion beam (FIB), the Swiss rolled

geometry is revealed, Fig. 1.6(d).

This approach makes it possible to fabricate novel structures, which are otherwise unobtainable,

some of which will be discussed in Chapter 4. The roll-up is not intrinsic to just certain types of

nanomembranes, therefore different inorganic layers can be deposited onto the SU8 and then self-

assembled into one of these structures. For example, if a magnetic layer is used, new magnetic

geometries can be fabricated.[11] Geometrical magnetic configurations on the microscale can be

created to mimic helical magnetic moments, which only occur in nature on the atomic level.[65] If

instead a gold (Au) layer were to be used, interesting plasmonic effects could be investigated using

the resonator-like structure. With such a structure it may be possible to create magnetic dipoles,

using non-magnetic layers, due to its Swiss roll design, much like split-ring resonators.[34,63] The

characteristic that distinguishes this Swiss roll structure, Fig. 1.6(d), from the similarly-looking

schematic in the right side of Fig. 1.1, is that the “active” nanomembrane layer is separated by the


11

thick polymer layer in these structures; whereas when using the photoresist technique, these layers

are in contact. Given that SU8 is a very biocompatible material and transparent, such structures

could offer another good architecture for cell culture experiments.

1.5 References

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3. T. G. Leong, A. M. Zarafshar and D. H. Gracias, Three-dimensional fabrication at small size scales, Small, 6, 792-806 (2010).

4. A. A. Solovev, Y. F. Mei, E. Bermúdez Ureña, G. S. Huang and O. G. Schmidt, Catalytic microtubular jet engine self-propelled by accumulated gas bubbles, Small, 5, 1688-1692 (2009).

5. R. P. Feynman, There’s plenty of room at the bottom, J. Micro. Electromech. Sys., 1, 60-66 (1992).

6. O. G. Schmidt and K. Eberl, Thin solid films roll up into nanotubes, Nature, 410, 168 (2001).

7. E. J. Smith, Z. Liu, Y. F. Mei and O. G. Schmidt, System investigation of a rolled-up metamaterial optical hyperlens structure, Appl. Phys. Lett., 95, 083104 (2009). [Erratum: Appl. Phys. Lett., 96, 019902 (2010)].

8. E. J. Smith, Z. Liu, Y. F. Mei and O. G. Schmidt, Combined surface plasmon and classical waveguiding through metamaterial fiber design, Nano Lett., 10, 1-5 (2010).

9. Y. F. Mei and O. G. Schmidt, Volkswagon project proposal, Grant No. I/8072 (2008).

10. E. J. Smith, et al., Lab-in-a-tube: detection of individual mouse cells for analysis in flexible split-wall microtube resonator sensors, Nano Lett., doi: 10.1021/nl1036148 (2010).

11. E. J. Smith, D. Makarov, S. Sanchez, V. M. Fomin and O. G. Schmidt, Magnetized micro-helix coil structures, Phys. Rev. Lett., 107, 097204 (2011).

12. E. J. Smith, D. Makarov and O. G. Schmidt, Polymer delamination: towards unique three-dimensional microstructures, Soft Matter, doi: 10.1039/CiSM06416A (2011).

13. V. Y. Prinz, et al., Free-standing and overgrown InGaAs/GaAs nanotubes, nanohelices and their arrays, Physica E, 6, 828-831 (2000).

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12

15. L. B. Freund and S. Suresh, Ed. Thin Film Materials, pg. 60, [Cambirdge University Press, Cambridge, 2003].

16. G. G. Stoney, Tension of metallic films deposited by electrolysis, Proc. R. Soc. Lond., A82, 172-175 (1909).

17. P. Cendula, S. Kiravittaya, Y. F. Mei, Ch. Deneke and O. G. Schmidt, Bending and wrinkling as competing relaxation pathways for strained free-hanging films, Phys. Rev. B, 79, 085429 (2009).

18. R. Songmuang, Ch. Deneke and O. G. Schmidt, Rolled-up micro- and nanotubes from single-material thin films, Appl. Phys. Lett., 89, 223109 (2006).

19. Ch. Deneke, C. Müller, N. Y. Jin-Philipp and O. G. Schmidt, Diameter scalability of rolled-up In(Ga)As/GaAs nanotubes, Semicond. Sci. Tech., 17, 1278-1281 (2002).

20. Y. F. Mei, et al., Versatile approach for integrative and functionalized tubes by strain engineering of nanomembranes on polymers, Adv. Mater., 20, 4085-4090 (2008).

21. Y. F. Mei, A. A. Solovev, S. Sanchez and O. G. Schmidt, Rolled-up nanotech on polymers: from basic perception to self-propelled catalytic microengines, Chem. Soc. Rev., 40, 2109-2119 (2011).

22. C. C. Bof Bufon, et al., Self-assembled ultra-compact energy storage elements based on hybrid nanomembranes, Nano Lett., 10, 2506-2510 (2010).

23. M. E. Smith and E. H. Finke, Critical point drying of soft biological material for the scanning electron microscope, Invest. Ophthalmol., 11, 127-132 (1972).

24. E. Yablonovitch, Inhibition spontaneous emission in solid-state physics and electronics, Phys. Rev. Lett., 58, 2059-2062 (1987).

25. P. L. Gourley, Microstructured semiconductor lasers for high-speed information processing, Nature, 371, 571-577 (1994).

26. C. Kittel, Ed. Introduction to solid state physics, 8th edition, pg. 163-167, [John Wiley & Sons, Inc., Hoboken, NJ, 2005]

27. C. J. R. Sheppard, Approximate calculation of the reflection coefficient from a stratified medium, Pure Appl. Opt., 4, 665-669 (1995).

28. E. F. Schubert, Y. H. Wang, A. Y. Cho, L. W. Tu and G. J. Zydzik, Resonant cavity light-emitting diode, Appl. Phys. Lett., 60, 921-923 (1992).

29. T. F. Krauss, R. M. De la Rue and S. Brand, Two-dimensional photonic-bandgap structures operating at near-infrared wavelengths, Nature, 383, 699-702 (1996).

30. W. L. Barnes, A. Dereux and T. W. Ebbesen, Surface plasmon subwavelength optics, Nature, 424, 824-830 (2003).

31. J. B. Pendry, Negative refraction makes a perfect lens, Phys. Rev. Lett., 85, 3966-3969 (2000).


13

32. B. Wood, J. B. Pendry and D. P. Tsai, Directed subwavelength imaging using a layered metal-dielectric system, Phys. Rev. B, 74, 115116 (2006).

33. V. G. Veselago, The electrodynamics of substances with simultaneously negative values of ε and µ, Sov. Phys. Usp., 10, 509-514 (1968).

34. D. R. Smith, J. B. Pendry and M. C. K. Wiltshire, Metamaterials and negative refractive index, Science, 305, 788-792 (2004).

35. R. A. Shelby, D. R. Smith and S. Schultz, Experimental verification of a negative index of refraction, Science, 292, 77-79 (2001).

36. J. B. Pendry, D. Schurig and D. R. Smith, Controlling electromagnetic fields, Science, 312, 1780-1782 (2006).

37. W. Cai, U. K. Chettiar, A. V. Kildishev and V. M. Shalaev, Optical cloaking with metamaterials, Nature Photon., 1, 224-227 (2007).

38. O. Hess, Farewell to flatland, Nature, 455, 299-300 (2008).

39. V. M. Shalaev, Optical negative-index metamaterials, Nature Photon., 1, 41-48 (2007).

40. J. Valentine, et al., Three-dimensional optical metamaterial with a negative refractive index, Nature, 455, 376-379 (2008).

41. E. Hecht, Ed. Optics, 4th Ed., Pg. 66, [Pearson Education Inc., San Francisco, CA, 2002].

42. C. M. Soukoulis, S. Linden and M. Wegener, Negative refractive index at optical wavelengths, Science, 315, 47-49 (2007).

43. N. Liu, et al., Three-dimensional photonic metamaterial at optical frequencies, Nature Mater., 7, 31-37 (2008).

44. J. Homola, S. S. Yee and G. Gauglitz, Surface plasmon resonance sensors: review, Sensors Actuat. B, 54, 3-15 (1999).

45. R. H. Ritchie, Plasma losses by fast electrons in thin films, Phys. Rev., 106, 874-881 (1957).

46. S. K. Shevell, Ed. The science of color, 2nd Ed., Pg. 297, [Elsevier, Oxford, 2003].

47. S. M. Rytov, Electrodynamic properties of a finely stratified medium, Sov. Phys. JETP, 29, 605-616 (1955) [Sov. Phys. JETP, 2, 466-475 (1956)].

48. S. Schwaiger, et al., Rolled-up three-dimensional metamaterials with a tunable plasma frequency in the visible regime, Phys. Rev. Lett., 102, 163903 (2009).

49. F. Cavallo, R. Songmuang and O. G. Schmidt, Fabrication and electrical characterization of Si-based rolled-up microtubes, Appl. Phys. Lett., 93, 143114 (2008).

50. H. X. Ji, Y. F. Mei and O. G. Schmidt, Swiss roll nanomembranes with controlled proton diffusion as redox micro-supercapacitors, Chem. Commun., 46, 3881-3883 (2010).


14

51. R. Songmuang, A. Rastelli, S. Mendach, Ch. Deneke and O. G. Schmidt, From rolled-up Si microtubes to SiOx/Si optical ring resonators, Microelec. Eng., 84, 1427-1430 (2007).

52. G. S. Huang, et al., Optical properties of rolled-up tubular microcavities from shaped nanomembranes, Appl. Phys. Lett., 94, 141901 (2009).

53. T. Kipp, H. Welsch, C. Strelow, C. Heyn and D. Heitmann, Optical modes in semiconductor microtube ring resonators, Phys. Rev. Lett., 96, 077403 (2006).

54. G. S. Huang, Y. F. Mei, D. J. Thurmer, E. Coric and O. G. Schmidt, Rolled-up transparent microtubes as two-dimensionally confined culture scaffolds of individual yeast cells, Lab Chip, 9, 263-268 (2009).

55. S. Schulze, et al., Morphological differentiation of neurons on microtopographic substrates fabricated by rolled-up nanotechnology, Adv. Eng. Mater., 12, B558-B564 (2010).

56. M. Yu, et al., Semiconductor nanomembrane tubes: three-dimensional confinement for controlled neurite outgrowth, ACS Nano, 5, 2447-2457 (2011).

57. E. Bermúdez Ureña, et al., Fabrication of ferromagnetic rolled-up microtubes for magnetic sensors on fluids, J. Phys. D Appl. Phys., 42, 055001 (2009).

58. E. J. Smith, Y. F. Mei and O. G. Schmidt, Optical components for lab-in-a-tube systems, Proc. SPIE 8031, 80310R (2011), doi:10.1117/12.885246.

59. A. Manz, N. Graber and H. M. Widmer, Miniaturized total chemical analysis systems: a novel concept for chemical sensing, Sens. Act. B1, 244-248 (1990).

60. H. Sørensen, S. Sørensen, C. Bjergegaard and S. Michaelsen, Ed. Chromatography and capillary electrophoresis in food analysis, pg. 181, [The Royal Society of Chemistry, Cambridge, 1999].

61. S. Sanchez, A. A. Solovev, Y. F. Mei and O. G. Schmidt, Dynamics of biocatalytic microengines mediated by variable friction control, J. Am. Chem. Soc., 132, 13144-13145 (2010).

62. V. Luchnikov, O. Sydorenko and M. Stamm, Adv. Mater., 17, 1177-1182 (2005).

63. J. B. Pendry, A. J. Holden, D. J. Robbins and W. J. Stewart, Magnetism from conductors and enhanced nonlinear phenomena, IEEE Trans. Microwave Theory Tech., 47, 2075-2084 (1999).

64. H. Lorenz, et al., SU-8: a low cost negative resist for MEMS, J. Micromech. Microeng., 7, 121-124 (1997).

65. M. Uchida, Y. Onose, Y. Matsui and Y. Tokura, Real-space observation of helical spin order, Science, 311, 359-361 (2006).

Chapter 2 Rolled-Up Metamaterials

15

Chapter 2: Rolled-Up Metamaterials

2.1 Introduction

This thesis gives an investigation into a number of different rolled-up devices which can be put

towards on-chip sensing applications. It was mentioned in Section 1.2 that plasmonics offers good

sensing capabilities because of their sub-wavelength size attributes. Given that plasmonic

metamaterials can be realized by creating a multilayer stack composed of the bilayer combination of

a metal and an oxide, rolled-up technology[1] utilizing the photoresist technique,[2] provides a good

platform for creating these metamaterials. A single deposition step of each material, followed by the

self-assembly nanomembrane release, will result in a tubular structure which has a hollow interior

and a metamaterial cladding (comprised of a metal-oxide multilayer stack). Again, using Eqs. 1 and

2 and now accounting for the material properties of the individual oxide (dielectric) and metal

(conductor) layer, what is left is:[3]

dmmd

dmeff

ff εεεεε

+=⊥ , (3)

mmddeff ff εεε +=|| , (4)

where εd and εm are the permittivities of the dielectric and metal respectively and fd and fm are their

respective filling ratios. For ease of understanding, at times the systems will be analyzed using

Cartesian coordinates (like Eqs. 3 and 4) and other times, because of the cylindrical geometries of

the system, polar coordinates will be used instead. When considering the system in polar

coordinates, the effective permittivity is:

dmmd

dmeffeffr ff εε

εεεε+

== ⊥ , (5)

mmddeffeff ff εεεεθ +== || , (6)

where εr and εθ are the radial and tangential permittivities associated with a cylindrical structure.

This makes intuitive sense given that perpendicular ( ⊥ ) to a multilayer stack, would be the same as


16

radial (r ) to a cylinder, and looking at the permittivity of a multilayer stack parallel (||) to the

planar geometry is the same as if looking at the permittivity tangentially (θ ) around a cylinder, Fig.

2.1. A matrix can be written to represent the overall effective permittivity describing the geometry

of the system of a flat multilayer stack, Fig. 2.1(a):

[ ]

=⊥

||

||

00

00

00

εε

εε eff

. (7)

A matrix can also describe the cylinder, resulting from a bilayer rolled-up. This can be written for

either: Cartesian coordinates, Eq. 8, by taking into account the coordinate conversion for the

permittivity matrix going from a planar stacked structure to that of a rolled one, Fig. 2.1(b):

[ ]

+−−+

= ⊥⊥

⊥⊥

||

2||

2||

||2

||2

00

0)(cos)(sin)cos()sin()(

0)cos()sin()()(sin)(cos

εθεθεθθεεθθεεθεθε

ε eff

; (8)

or polar coordinates, Eq. 9,

[ ]

=

θ

θ

εε

εε

00

00

00r

eff

. (9)

Figure 2.1 | Defining coordinate systems. (a) The coordinate system for a planar multilayer stack is defined. (b) When a bilayer is rolled-up, it forms a cylindrical geometry, which can be defined in Cartesian or polar coordinates.

This chapter will focus on two different metamaterial devices which can be developed using

rolled-up technology. The first will be on a theoretical system investigation of a rolled-up

hyperlens.[4] An examination of the materials needed to achieve such a device and the working

conditions over the entire visible spectrum will be made. The second part of the chapter will


17

introduce a new metamaterial device known as a metamaterial optical fiber (MOF)[5] and will be

divided into two parts: First, a theoretical introduction of the system, emphasizing its importance

and novelty; and second, the first experimental progress which has been made to realize this device

and its integration with other on-chip components.

2.2 Rolled-up hyperlens

2.2.1 Introduction

As was introduced in Section 1.2, plasmonic metamaterials allow for the transmission of sub-

wavelength information. One example of this is known as the hyperlens,[6-12] which can break the

diffraction limit by revealing sub-wavelength details about an object. The hyperlens requires only

the cross section of a 3D cylindrical structure. In other words, a cross section of a metamaterial

cylinder can be used as a hyperlens. This permits a simplification of Eq. 9 to only account for the

radial and tangential components of the system, so

[ ]

=

θεε

ε0

0reff

. (10)

Light traveling through a hyperlens has a preferential radial direction, out through the walls of the

cylindrical configuration[6] as shown in Fig. 2.1(b). What happens, is surface plasmons (SP) couple

to the evanescent field (near field) of an object. This coupling of SPs through alternating layers of

oxide and metal, leads to a conversion from an evanescent wave into a propagating wave,[6] which

Figure 2.2 | Layered plasmonic metamaterial vs. classical dielectric. (a) An evanescent wave in contact with a layered structure, alternating metal and oxide, can excite surface plasmons which will couple from one layer to the next. The result is a conversion from an evanescent wave in the near field to a propagating wave in the far field. (b) In a classical dielectric the evanescent wave decays exponentially away from the source, and is undetectable in the far field. (c) A schematic showing the orientation of the E-field and H-Field with respect to TE and TM polarized light.


18

can then be detected using classical optics.[11] This effect is shown in Fig. 2.2, comparing the

coupling effect of the evanescent wave through a metamaterial which results in a propagating wave,

to that of an evanescent wave in a classical dielectric material.[6,8] This is possible if transverse-

magnetic (TM) polarized waves are considered, bottom of Fig 2.2(c). The dispersion relation (i.e.

how light moves within an object) for transverse-electric (TE) [top of Fig. 2.2(c)] polarization relies

on a manipulation of the permeability,[13] which is assumed equal to one in these materials.

Therefore imaging with TE modes doesn’t improve the resolution below the diffraction limit. If a

circular geometry is considered, the result of this propagation can be seen in Fig. 2.3.

The TM dispersion relation of light traveling in a cylinder is given by,[6]

r

ro

kkk

εεθ

θ

222 +=

, (11)

where ko is the free space wavenumber (ko = 2π/λo where λo is the free space wavelength), and kr

and kθ are the radial and tangential wavevectors, respectively. Given that the light propagates

outward, radial through the cladding of the metamaterial, the ratio kr/ko is proportional to the

propagating wavelength and kθ/ko is representative of the transmittable size resolution. As a quick

note, the imaginary part of the permittivity is proportional to loss in a material and a wave is

evanescent if the wavevector k is imaginary, whereas the wave will propagate if k is real. If light

propagates through a classical, isotropic material [for instance through air (εr = εθ = 1)], then as kθ/ko

becomes larger than 1 (sub-wavelength object), then kr/ko becomes imaginary, therefore the wave is

evanescent. The isofrequency dispersion curve for such a material is circular.[10] Only for smaller

kθ/ko, objects larger than the free space wavelength λo of light, will kr/ko be real and the wave will

propagate. This intuitively gives the diffraction limit of the system. However, if an anisotropic

metamaterial is used instead, εr ≠ εθ, this rule can be broken.

If Re{εr} < 0 and Re{εθ} > 0, then the isofrequency dispersion relation becomes hyperbolic, Fig.

2.4(c), and therefore for any value of kθ/ko, large or small, kr/ko will be real. This means that sub-

wavelength resolution, i.e. (kθ/ko > 1), can be achieved and light from the object will propagate out

through the material. This hyperbolic dispersion is what led to the name “hyperlens.” The ground

work has been laid out theoretically[6-10] and a 2D half cylindrical hyperlens demonstrated the

concept.[11] More recently a 3D version was also created.[12] Another important point to mention

here is that since light propagates radially through such a structure,[6] so the amount of lens

magnification (Lm) of two objects separated spatially by a sub-wavelength distance, is the ratio of

the inner radius, rin, to that of the outer radius, rout, i.e. Lm = rout/r in.[7,8,10] This is the same as the


19

Figure 2.3 | Geometry of a hyperlens. Light propagates radially through a hyperlens. The geometry of the lens is defined by the inner (r in) and outer radii (rout), the sub-wavelength object separation (Os), and the spatial separation of the objects’ projections (Ps) at the outer wall of the lens.

difference between the spatial separation of the projections of the objects (Ps), at the outer wall of

the lens, to that of the distance of their actual separation (Os) at the inner wall, Fig. 2.3. However, it

will be shown in the following sections that if the losses and scattering are limited and reduced, the

projections of the objects can be transmitted a larger distance from the outer wall. This allows for a

higher separation resolution. In other words, the imaging plane of the classical optics can be moved

away from the outer surface of the lens.

A missing study in earlier hyperlens explorations was an investigation using values of the

optical properties of real materials and looking over the entire visible range. Past works looked at

and investigated a single wavelength and material. In the next section, an investigation of the

material makeup and surrounding medium of an optical rolled-up hyperlens over the entire visible

range will be given.[4] A study over the working spectral range of a hyperlens with different

material combinations is examined. An introduction to the concept of an immersion hyperlens,

which can suppress Fresnel diffraction at the outer surface and increase the output intensity, is given

as well. This type of technique would be ideal and could be implemented for cellular culture

investigations. Other sub-wavelength observation tools, like a standard SEM, require a vacuum

which only permits imaging the cells after they are dead, rather than in situ.

2.2.2 Hyperlensing over the entire visible spectrum

A program based on the finite element method (FEM) called COMSOL Multiphysics was used

to perform simulations on these metamaterial structures in order to map out, understand, and

investigate this system. In short, FEM is a numerical method for finding approximate solutions to

complex equations like solutions to boundary value problems involving Maxwell’s equations.


20

Instead of solving the system as a whole, which is complicated, the method relies on the solving of

finite pieces of the system, individually and to nearest approximations. The finer these finite pieces

are, referred to as the mesh, the more accurate the solution is. With such a method, the areas of most

interest can be given a finer mesh than not-so-interesting areas of the model, alleviating the

computer time and memory needed to solve a problem. This is a standard method for investigating

how light propagates through metamaterial systems.[10,11] The method allows for a crosscheck of the

effective permittivity of a bulk material given by Eqs. 5 and 6. This effective permittivity can be

compared directly to a system which is comprised of individual layers from the roll-up of a bilayer,

Figs. 2.4(a) and (b). One can see that both solving for the layered system Fig. 2.4(a) and the

effective bulk system Fig. 2.4(b) results in a very similar behavior. Because of this, from here on

the hyperlens will be considered as an effective bulk material, which is computationally simpler.

Here, realistic material parameters[14,15] for a hyperlens made from a titanium dioxide (TiO2) and

silver (Ag) multilayer stack are used to show this effect.

Figure 2.4 | Rolled-up hyperlens system. A diagram showing both a hyperlens using a realizable rolled-up TiO2/Ag structure (a) and one using the effective permittivity theory (b). Objects being imaged are λ/30, separated by a distance of λ/2. (c) Using material combinations which result in anisotropy with characteristics of Re{εr} >> Re{εθ}, elliptical dispersion, or Re{εr} < 0 and Re{εθ} > 0, hyperbolic dispersion, allow for the transmission of sub-wavelength information. (d) The elliptical isofrequency dispersion relation curves for a TiO2/Ag 5:1 ratio structure are plotted, showing a lens which is capable of transmitting sub-wavelength information over the entire visible spectrum.

Titanium dioxide is implemented because of its especially high permittivity. This gives the

possibility for the plasma frequency of the metamaterial to be tuned throughout the visible range.

This effectively shows that the hyperlens can work over the entire visible spectrum. Earlier it was

mentioned that a hyperbolic dispersion breaks the diffraction limit. However, this dispersion

relation is not the only one which leads to this effect. If instead, Re{εr} >> Re {εθ} is considered,


21

Eq. 10 results in a highly elliptical dispersion. Figure 2.4(c) shows that this dispersion relation also

allows for the transmission, in the radial direction, of sub-wavelength information. The difference

between the hyperbolic relation and the elliptical relation is that the hyperbolic is unbounded (no

limit to the transmittable spatial information), while the highly elliptical is not. This means that

there is still a limit to the spatial resolution. If a single TiO2/Ag rolled-up bilayer is created with a

5:1 ratio, an elliptical dispersion is present over the entire visible range, i.e. the diffraction limit is

broken, Fig. 2.4(d). The flatter the dispersion relation is, the better the resolution of differently sized

objects.[10] Although the dispersion is bounded over this whole spectral range, the size of an object

roughly 3 times smaller than the free space wavelength would be resolvable.

There are two important characteristics to consider in an optical imaging system: resolution and

transmission. The working range of a hyperlens is considered [for hyperlenses made from two

different material combinations] so that a clearer picture of what materials and particular

wavelengths can be used for hyperlensing. The working range is defined as when the dispersion

relation is either hyperbolic or highly elliptical. This will give researchers a better idea of which

materials they consider/focus-on for optimal sub-diffraction limited imaging. By looking at the

dispersion relation over the visible spectrum and considering various material combinations, a plot

can be made of where sub-diffraction limited imaging is possible, shaded regions of Fig. 2.5(a). The

investigated material combinations consist of TiO2 and Ag as well as aluminum oxide (Al2O3) and

Figure 2.5 | Working range of a hyperlens. The anisotropic range of a hyperlens with different ratios of oxide to metal is presented (Al 2O3/Ag plot overlaid on TiO2/Ag plot). (a) The tunable hyperbolic dispersion range (solid color with bars) is defined to be where Re{εr} < 0 and Re{εθ} > 0, given the criteria for the hyperlens effect[6-8] and elliptical dispersion range (solid color) which is defined as Re{εr} >> Re{εθ}. The dotted lines are the wavelengths required for impedance matching the given metamaterial lens with air, whereas the dashed are those required for matching with water resulting in a high transmission into the far-field. (b) The dispersion relation for different material makeups is shown at different wavelengths in the visible spectrum. This illustrates the fact that the isofrequency dispersion relation, whether hyperbolic or elliptical, is relatively flat which results in high spatial resolution.


22

Ag, with varying filling ratios of each. The top boundaries of the shaded regions in Figure 2.5(a) are

the plasmon frequencies of the metamaterial.[4,16] The dotted and dashed lines are where the

condition for impedance matching of the lens is met for air and water respectively and will be

discussed in the next section. The dispersion relation for a few selected material combinations at

different wavelengths over the visible spectrum are given in Fig. 2.5(b). This illustrates that flatness

of the isofrequency dispersion relation is achievable for many different combinations and

wavelengths, in turn improving the resolution of the lens.

2.2.3 Impedance matched systems

When working with metamaterials, it is important to keep in mind that the metal adds loss to the

system because the imaginary part of its permittivity is quite large.[15] In order to increase the

transmission of the lens, losses must be lowered or avoided. One way to do this in a hyperlens

would be to increase the filling ratio of the oxide. However, at some point the isofrequency

dispersion relation becomes elliptical, as shown in Fig. 2.5(a), which in turn sets an upper bound to

the smallest resolvable object. The other way of lowering losses in the system is by impedance

matching the lens to the surrounding medium.[4,9] Alternatively, an impedance mismatch leads to

Fresnel reflections[17] at the interface of the medium and lens, which results in a lower transmission.

An impedance match can be achieved by working at a wavelength where the material meets the

requirement, mediumn=θε , where nmedium is the index of refraction of the medium the hyperlens is

immersed in. The wavelengths and material filling ratios at which this matching occurs for TiO2/Ag

and Al2O3/Ag for air and water have been calculated and are plotted as the dotted and dashed line,

respectively in Fig. 2.5(a). This is calculated for air because of interest in imaging objects in

laboratory settings and for water because cell culture medium has the same index of refraction.[18]

With this, the study of sub-wavelength cellular activity in situ could then be undertaken. If

impedance matching is made, effectively an immersion hyperlens would be created, similar to its

counterpart in classical optics.

The importance of impedance matching is illustrated in Figure 2.6. It is shown that a hyperlens

which is impedance matched for water, performs poorer in air [Fig. 2.6(a)]. However, when the lens

is working in water, the resolution in the far field is greatly improved, Fig. 2.6(b). For this particular

configuration, two objects, say quantum dots of the size q = λo/30 (11 nm), emitting light at a free

space wavelength λo = 342 nm, separated by a distance Os = λo/2 (171 nm), are magnified by a

hyperlens with r in = λo and rout = 3λo [Fig. 2.6(d)]. If the image plane is defined at the outer surface

of the lens, which is normally considered,[7,8,10] then the lens magnification would be Lm = 3×.

However, given the low loss of this hyperlens configuration, the objects’ projections go well


23

Figure 2.6 | Immersion hyperlens. Impedance matching the lens to the surrounding medium becomes important for higher transmission and can give way to higher resolution than unmatched systems. This condition is

met when mediumn=θε , and results

in suppressed Fresnel reflection at the outer interface of the hyperlens for a higher output and better resolution. The normalized magnetic field distribution for air (a) and water (b) are shown. (c) A cross section of the magnetic field profile taken at 3.5 µm [dashed line in (a) and (b)] from the outer surface is shown to have higher resolution in water than in air. For this particular simulation, the effective permittivity is used for a 2:1 ratio of Al 2O3/Ag at λo = 342 nm leading to a

θε = 1.32 and an index of refraction

for water used was 1.33. (d) The geometry used is as follows; an inner radius r in = λo, outer radius rout = 3λo, a separation of dots Os = λo/2 and a dot size q = λo/30.

beyond the outer surface, meaning the imaging plane can be moved further from the lens. This

allows for even greater magnification, here Lm ≈ 14× [Fig. 2.6(c)], when the imaging plane is

moved 3.5 µm from the outer surface of the lens. A cross section of the magnetic field profile in the

far field, Fig. 2.6(c), also shows how much better the resolution is when the lens is properly

impedance matched.

Having a higher index of refraction medium in the core of the lens will lead to higher resolution

between the objects being imaged. Impedance matching of the external medium leads to higher

transmission of light since less scattering will occur. This in turn allows the imaging plane to be

brought further from the outer surface, lending to a higher magnification of the objects.

The ultimate goal of the hyperlens is to create a device which can be used practically. Until

now, there has been no hyperlens developed to look into the sub-wavelength unknown. Rather,

proof-of-concept demonstrations were made for imaging known/predefined sub-wavelength objects,

i.e. shapes were cut into chromium which was on the inner surface of the lens.[11,12] The chromium

reflects illuminated light, so only light going through the sub-wavelength slots can be transmitted

through the lens. These conditions (implementing an inner chromium layer) were used for a number

of simulations performed by other groups to avoid scattering. In the simulations presented here,


24

“real” objects were assumed, meaning “quantum dots” emitting in all directions were chosen as the

light sources. Assuming these real objects, the hyperlens was shown to perform well.

2.2.4 Conclusion and outlook

In this section a theoretical approach to analyze the performance of a hyperlens based on rolled-

up technology was introduced. The principles on how a hyperlens can be used to break the

diffraction limit were laid out. An exploration of a hyperlens, assuming different material

combinations, was made. It was found that a rolled-up hyperlens could be developed for imaging

over the entire visible spectrum, given a particular bilayer combination. The concept of creating an

immersion hyperlens by impedance matching the hyperlens to the surrounding media was also

investigated. This would improve the resolution and transmission of such a lens.

A real life usage of a hyperlens could be for cell culture where colloidal quantum dots are

implanted in the cells, or used to label certain parts of the cell, and then imaged with a hyperlens.

Therefore, the requirements for impedance matching to water (having the same index of refraction

as a cell culture medium) were considered as well. This compact optical device would fit well with

lab-in-a-tube systems which will be further discussed in Chapter 3.

2.3 Metamaterial optical fibers

2.3.1 Introduction

As was introduced in Section 1.2, many metamaterial applications and devices have been

theoretically conceived[6,19-23] and many experimental realizations have followed.[11,12,16,24-27] The

subject continues to grow because it is of much interest to many researchers today. Given the youth

of such a topic, many opportunities exist for developing novel systems which implement

metamaterials. Another area which stays in the realm of plasmonics is plasmonic waveguides,[28]

which can transmit sub-wavelength information leading to faster data transfer rates. These devices

can also be very small, which is necessary as devices are shrunk down and also for optoelectronic

sensor integration.[29,30]

Here, a novel metamaterial integration for fiber optics is given, allowing for the dual ability to

guide light plasmonically, as well as classically within a single device. This can be designed using

the same structure needed to create a cylindrical hyperlens. However, instead of guiding light

perpendicular to the tube, light is guided down its length. This device is known as a metamaterial

optical fiber.[5] Depending on the wavelength of light used and the material combination, the device

would be able to guide light plasmonically, due to surface plasmons in the cladding, or classically


25

within the core. Given that plasmons can be confined to materials smaller than the incident

wavelength, a sub-wavelength cladding is also allowed. This admits the possibility of designing this

device for on-chip integration. The first sub-section 2.3.2 will introduce the principle theory of such

a fiber, followed by the work towards the first realization and integration of these fibers (sub-

section 2.3.3).

2.3.2 Theory

Classical guidance of light through the core of a fiber is a subject of great importance and

includes many subcategories of fibers for sensing as well as communication.[31] One of the most

investigated solid core fibers is based on a classical silica glass core/cladding fiber which has

allowed for extremely low loss light transmission and is an industry standard when it comes to long

distance telecommunication, Fig. 2.7(a). However, solid core fibers have some disadvantages which

include high insertion loss, end reflections and large beam divergence.[32] A way around these issues

is to design hollow (air) core fibers. However, this is a challenge because light is guided best in a

core which has a higher index of refraction than the cladding surrounding it. Given that nair ≈ 1, it is

difficult to fabricate efficient hollow core fibers. Some methods have been investigated to create

such fibers including: Metallic cladding fibers (cladding acts like a classical mirror);[32] Bragg fibers

(cladding acts like a Bragg mirror),[33-35] Fig. 2.7(b); and attenuated total reflectance (ATR) fibers,

meaning ncladding < 1, found naturally in sapphire at 10.6 µm (classical guidance).[36] Bragg fibers

rely on a cladding with either alternating layers of dielectrics in order to form a 1D Bragg

mirror,[33,34] or on a 2D photonic crystal structure.[35]

The device that we have proposed[5] is a metamaterial-based, novel type of fiber which can

guide light classically by means of leaky or ATR guidance through the core or plasmonically via SP

guidance in the cladding. These different forms of light propagation are outlined in Figure 2.7(c) by

ray diagrams in order to draw a picture of the different forms of guidance. This type of fiber allows

for more versatility because of the tunable dispersive properties exhibited by the metamaterial

compared to fibers based on naturally occurring bulk materials. Depending on the filling ratio of the

metal to oxide and the free space wavelength, different mode profiles can propagate through this

structure, Fig. 2.7(c). A sub-wavelength metamaterial cladding can be used to be guide light either

classically through its core, Fig. 2.7(c1), or plasmonically through its cladding, Fig. 2.7(c2). If a

liquid or gas fills this core, then a coupling of SP cladding modes with the classical core modes

takes place which could lead to the sensing of sub-wavelength information within the liquid or gas,

Fig. 2.7(c3). A fiber consisting of only a metamaterial core could also be designed, which would

allow the total size of the fiber to be sub-wavelength, Fig. 2.7(c4).


26

With considerable attention put towards the material design, a deliberate tuning of the effective

permittivities can be made for a metamaterial making up the core or cladding of a fiber. This design

would allow for the metamaterial structure to be sub-wavelength in size. That means that the

cladding [Figs. 2.7(c1-3)] or the core [Fig. 2.7(c4)], could be smaller than the free space

wavelength, yet still propagate light via surface plasmons. Compared to classical waveguides that

are restricted to being larger than the wavelength and usually are around 10 and 125 µm in size for

Figure 2.7 | Fiber ray diagrams. (a) A ray diagram for a conventional, solid core, silicon-oxide-based dielectric waveguide. (b) A ray diagram representing the mechanics in which a Bragg photonic bang gap waveguide works through Bragg reflection of light, leading to propagation through the core. (c1-3) A ray diagram for a fiber using a metamaterial as cladding. Given the dispersive characteristics of the metamaterial, one can tune between core propagation (c1) or surface plasmon propagation in the metamaterial cladding (c2). (c3) Combined coupling of surface plasmon modes with classical core modes that occurs when a liquid or gas is introduced into the core. (c4) Propagation of plasmons when the metamaterial is used as the core. Depending on the wavelength, and material makeup, light can be guided with this special fiber utilizing component geometries smaller than the incident wavelength, and can propagate light using attenuated total reflectance, leaky, or plasmonic guidance.

the core and cladding, respectively,[37] for single mode fibers and even larger for multimode. This

size is much too large to be considered for on-chip guidance and integration. Another setback with

classical single mode fibers is their low numerical aperture,[37] NA = no[2(no–n1)/no]1/2, of NA ≈

0.15, Fig. 2.7(a). As mentioned before, photonic band gap fibers are promising because of their

ability to incorporate a hollow core. However, like classical fibers, their overall structure is quite

large with a core ≈ 15 µm and a cladding around 100 µm[35] [Fig. 2.7(b)]. There was work done

which reported a NA ≈ 0.28 implementing a solid core.[38] The fiber optic device which we have

proposed for metamaterial integration not only allows for many types of guidance including Leaky,


27

ATR, and SP, but also has the advantage of shrinking the overall device; including a cladding with

a thickness of the order 40 – 500 nm, a core which is sub-wavelength and a NA approaching one.

2.3.2.1 Material systems

A discussion over the material design and requirement for creating these metamaterial optical

fibers (MOFs) is made here. As mentioned in the introduction, a geometry and structure that is

exactly the same as was analyzed for a rolled-up hyperlens, can be used as a MOF. The difference

being that the cladding for an MOF can be much thinner, even as thin as a single 40 nm bilayer,

compared to the hyperlens which must be relatively thick. Given that the same structure can be

used, the same equations defining the effective permittivity apply. Although the effective

permittivity is better for a higher number of bilayers,[6] this approximation still motivates the

concept if thin layers are used. Assuming a metamaterial comprised of a multilayer stack of

Figure 2.8 | Effective permittivity of a 3:1 filling ratio of Ag/TiO2. The effective permittivity is calculated for a 3 to 1 ratio of Ag/TiO2 for the entire visible range using Eqs. 3 and 4. (a) The real part of the permittivity is calculated, revealing the plasmon frequency around 375 nm in the perpendicular component. (b) The imaginary part, proportional to loss, is given as well, revealing a high loss near the plasma frequency which dissipates at further away wavelengths. Inset, an illustration of a multilayer stack.

TiO2/Ag, and inserting realistic parameters for Ag and TiO2[14,15] (with a 3:1 filling) into Eqs. 3 and

4, typical plots can be made showing how dispersive such a material is, Fig. 2.8. This metamaterial

integration into a fiber optic device provides many degrees of freedom and many desirable

permittivity combinations and can be achieved by adjusting the wavelength or changing the filling

ratio of the material. This fiber design is 3-fold anisotropic (meaning the permittivity is different in

the tangential, radial and z direction), described by Eqs. 8 and 9. For our interests, we again assume

µclad = 1 and we look at the effects on guidance which arise from varying the permittivity of the

material at different wavelengths.


28

As mentioned before, the plasma frequency can be shifted over the entire visible spectrum by a

change in the metal:oxide ratio.[16] It was pointed out that the type of guidance can be shifted by

changing the wavelength of light which is being guided. Given that how light is guided, is directly

related to the permittivity of the material in which it is propagating, one could imagine that such a

material (Fig. 2.8) would offer very interesting guiding properties.

2.3.2.2 Combined classical and plasmonic guidance

In order to get a feel for the types of guidance which can be achieved with such a fiber, the FEM

program COMSOL Multiphysics was employed. In the simulation, boundary valued problems

involving Maxwell’s equations were solved for the magnetic field component using the modal

eigenvalue solver and searching for possible hybrid modes which are supported. Hybrid modes are

modes which have both an electric and a magnetic field component in the direction of wave

propagation. A probing of different permittivity combinations which could arise in such a material

was made and material losses were neglected for simplicity. This exploration was undergone

assuming real experimental optical values for Al2O3 and Ag.[14,15] Again, like for the hyperlens

analysis, a 2D cross section of the fiber was simulated given that 3D simulations are too large and

complicated to perform. The regions which were investigated were determined by the relationship

which can occur between Re{ε⊥} and Re{ε‖}, resulting in different material anisotropy. These

include: [Re{ε⊥} < 0; Re{ε‖} > 0], [Re{ε⊥} > 0; Re{ε‖} < 0], [Re{ε⊥} > 0; Re{ε‖} > 0], [0 <

(Re{ε⊥}, Re{ε‖}) < 1] and [(Re{ε⊥}, Re{ε‖}) = 0].

The geometry which was investigated, consisted of a hollow waveguide with a core radius of 2

µm surrounded by a 500-nm-thick metamaterial cladding layer, Fig. 2.9(a). A few examples of

typical mode profiles present in each of the permittivity regions are shown in Fig. 2.9(c). Region III

is a region in which the fiber takes on permittivities between 0 and 1 allowing for classical ATR

guidance within the core of the fiber. The fiber continues to guide classically as the permittivities

approach 0 from both the negative and positive side, Region II. The negative and anisotropic

features of the other regions allow for a bulk plasmonic guidance within the cladding. This is seen

in Regions I, IV and V [Fig. 2.9(c)], where high order modes can be supported. It is important to

point out that propagation length (proportional to loss arising in SP waveguides) is determined by

loss from the absorbing metal (coming from Im{εm}). The attenuation of an MOF can be addressed

by looking at the effective permittivity derived from Eq. 8. Given that the light propagates down the

fiber in the z direction, Fig. 2.1, the parallel component (z component in Eq. 8) of the effective

permittivity is what mostly influences the propagation. Because of this, Im{ε‖} needs to be

considered and would be the main reason for attenuation in the system. As seen from the spectral


29

range in Fig. 2.8, the imaginary part of the parallel component of permittivity is approximately a

constant which is very low (for the combination shown Im{ε‖} < 0.1 for most of the spectrum). This

suggests that the fiber will have longer propagation lengths than standard 2D SP waveguides which

consist of a single metallic strip, sandwiched between oxides.[28] The higher the oxide filling ratio of

the cladding layer, the lower this value will become.

Figure 2.9 | Tuning between classical and plasmonic guidance. A layout showing the different wavelength dependent regions of interest for investigation of different material compositions is presented. (a) The geometry of our design where we use a hollow waveguide in air with an inner radius of 2 µm and a cladding thickness of 500 nm. (b) Shows the different regions of effective permittivities in which we can achieve with our anisotropic metamaterial (the region in which both perpendicular and parallel permittivities are negative is for an effective anisotropic metal and will not be discussed here). (c) Gives an example from each of the regions shown in (b). Region I is when Re{ε‖} is positive and Re{ε⊥} is negative, which can propagate surface plasmons. Region IV (Re{ε‖} > 0; Re{ε⊥} > 0) and region V (Re{ε‖} < 0; Re{ε⊥} > 0) also allow for the propagation of surface plasmons. Region II shows hollow ATR waveguidance as the perpendicular effective permittivity approaches zero from the positive and negative sides. Region III is an area of ATR propagation, where the both effective permittivities are between zero and one.

If a side view of the metamaterial optical fiber, cut length-wise, is taken, like in Fig. 2.7(c), an

assessment of the achievable numerical aperture can be made. The NA is calculated by the angle

which is necessary to give total internal reflection in order to guide light. Light coming in at steeper

angles will be lost and not be guided. For total internal reflection to occur, the incident angle of

light must be greater than the critical angle of the material interface, where θc = sin-1(nclad/ncore). In

Section 1.2, the index of refraction was defined as n = εµ , with µclad = 1 mentioned in Section

2.3.2.1. If an air core is assumed, ncore = 1, we are left with θc = sin-1[(εeff)1/2]. If first, Regions II and

III are considered, where εeff is less than one, as εeff � 0 then θc � 0, therefore the numerical

aperture, NA = cos(θc), approaches one. In Region I, where ε⊥ is negative, the result would be total

internal reflection for the nonevanescent waves, given the metal-like properties, also leading to a


30

NA approaching unity. This means that such a fiber design would be extremely efficient at

collecting light for guidance even if the angle, in which the light is injected, is very large.

2.3.2.3 Sensing potential

Investigations have been undergone by a number of groups to explore the use of surface

plasmons as sensing mechanisms in order to detect small details of a medium, material, gas, etc,

because of the ability of SPs to reveal sub-wavelength information.[29] One example already

mentioned in the introduction was using SERS[39] as sub-wavelength probes which can even detect

single molecules. If a liquid or gas (n > 1) is introduced into the metamaterial optical fiber, a

coupling of the plasmonic modes in the cladding with the classical modes in the core, takes place

[Fig. 2.10(a)]. This suggests that the MOFs could be used as a gas or liquid detector. This coupling

could be a good way to detect and transmit sub-wavelength information about the medium

contained inside of the fiber, eventually leading to more sensitive sensors in the realm of

plasmonics. Going back to the idea of a fiber composed of a metamaterial core [Fig. 2.7(c4)], a

simulation representing a typical sub-wavelength mode profile (here λo = 342 nm and the diameter

of the fiber is 200 nm) is given. Although, as of yet, there is not a proposed method of how to

produce such a fiber, this demonstrates the ability for guiding light with a sub-wavelength fiber

design, which would allow for even smaller on-chip opto-electronic components.

In the same capacity as was mentioned in Section 2.2.2, it is important to show that the effective

permittivity simulations result in similar mode profiles which are present if a layered structure is

Figure 2.10 | Metamaterial optical fiber sensing. (a) Using the same geometry from Figure 2.9 and values from Region I, an introduction of a liquid or gas into the core [shown here is methanol, n = 1.3288 (index of refraction from Ref. {40})], leads to an effect of coupled plasmonic and classical modes. (b) a metamaterial core smaller than the incident wavelength (here the core has a diameter of 200 nm and the incident wavelength is 342 nm) can act as a conventional multimode fiber. The fiber can be smaller than the incoming wave and still support the propagating mode.


31

Figure 2.11 | Rolled-up metamaterial optical fiber simulated. (a) The geometry of the cross section for a hollow core rolled-up bilayer consisting of Al2O3 and Ag in air. The rolled-up bilayer is comparable to an effective anisotropic material presented earlier. (b) Bulk plasmonic mode propagation. (c) Classical hollow core waveguidance due to a cladding with an effective permittivity between 0 and 1.

assumed. This is also important given that the goal is create these fibers using rolled-up technology.

If simulations are made using the individual stacked layers, rather than the effective permittivity, it

was shown that these modes can exist in such a layered structure, Fig. 2.11 (here Al2O3/Ag was

considered with an inner diameter of 2 µm and a cladding of 500 nm). These being comparable to

the modes found when considering a bulk effective metamaterial.

2.3.2.4 Conclusions

This section introduced a theoretical approach for the integration of metamaterials with fiber

optics.[5] This optical fiber can be created by taking a planar metal/oxide bilayer and rolling it up to

form a multilayer stack cladding and is known as a metamaterial optical fiber. The cladding can be

represented as an effective bulk metamaterial which can be simulated and light propagation

properties can be investigated. Such a device was found to guide light both classically and

plasmonically by changing the propagating wavelength or bilayer material combination. The

metamaterial cladding is highly dispersive, this along with the anisotropy of the material are the key

components to this system. Such a fiber, if experimentally developed, could further the field of

plasmonic sensors and detectors as well. A method of how to produce such fibers by rolled-up

technology was suggested. Other work has been done which reported on using rolled-up layers as

optical fibers for the infrared (IR),[41] as well as devices were proposed for X-ray waveguiding,[42]

showing that indeed rolled-up structures can be used in such an optical fiber capacity. The next

section will present the work which has been performed for realizing such MOF structures, as well

as their integration into preexisting on-chip components.


32

2.3.3 Experiment

This section will present the experimental work on rolled-up metamaterial optical fibers. First, a

technique for integrating rolled-up structures into preexisting on-chip architectures will be

introduced. After this, material characterization of the films necessary for creating these

metamaterials will be performed. Then methods for creating rolled-up metamaterials will be given

followed by characterization of the first MOF devices.

2.3.3.1 On-chip integration of rolled-up systems

Rolled-up technology has mainly focused on individual structures,[2,16,43] or integration with

functional components necessary for the devices to work, like electrodes.[44-46] Functionality has

also been added to rolled-up devices as a post process, like creating micro-fluidic channels.[47]

However, to date, no work has presented the integration of these rolled-up devices into preexisting

on-chip structures. This is of importance, for instance, if an opto-electronic integration is desired;

like the coupling of a MOF to an already existing Si-detector on a substrate. Here the method and

demonstration of this integration of rolled-up devices into preexisting structures will be

presented.[48] This is possible using the angle-deposition technique,[2] which creates an anchor,

defining the microtube’s final resting place. The purpose of this integration is to have one or two

ridge waveguides that can be used to inject and/or collect light into and from a MOF which is

positioned at their ends.

The process begins with the construction of ridge waveguides made from SU8 (a photoresist

epoxy), defined through a photolithographic step, Fig. 2.12(a1). These waveguides are 5 × 5 µm

(width × height) and are 1 mm in length. A second mask is then used to define a rectangular

sacrificial layer, here AZ MIR 701, from which the fiber will roll-up. This is aligned in a way so

that the pattern is positioned within the gap between the ridge waveguides, Fig. 2.12(a2). A bi-

/trilayer is then deposited at an inclined angle (in the same fashion as Fig. 1.1), so that a window is

defined, opposite of the ridge waveguides, Fig. 2.12(a3). After this, the sample is then placed within

a dissolvent (i.e. NMP) and the nanomembrane rolls up. The fiber has a final resting position

directly in between the two ridge waveguides, Fig. 2.12(a4). The result of this process can be seen

in Fig. 2.12(b) where an optical image is made after the roll-up process revealing a good alignment

of the fiber to the ridge waveguides. A close up of the junction shows that the gap between the two

devices is quite small. This can be adjusted by using a more accurate mask. Images taken with SEM

show the same fiber where one can see the 80-µm pattern that the nanomembrane rolled-up from.


33

Figure 2.12 | Integration of rolled-up devices. (a) An illustration outlining the engineering process for integrating rolled-up devices into preexisting structures. (a1) First the substrate is patterned with SU8 ridge waveguides having a predefined gap in between. (a2) Second, a photoresist sacrificial layer, here AZ MIR701, is aligned within the gap and developed. (a3) A bi-/trilayer is then deposited on top at an inclined angle. (a4) Finally, the sacrificial layer is dissolved with NMP and the nanomembrane rolls up, having a final resting place within the predefined gap. (b) An optical image of a fiber which has been integrated. Inset, a close-up of the fiber-waveguide junction with a relatively small gap. (c) An SEM image (made by D. Thurmer) of the same fiber shows clearly the pattern which the fiber rolled-up from.

Such a process can be expanded from the integration of single devices, to any number of arrays of

devices using the same number of deposition steps, as shown in Fig. 2.13. Here, the samples are

still in the rolling medium.

Figure 2.13 | Integration of an array of fibers. An array of rolled-up devices can be integrated into an array of preexisting structures. Inset, a closer view of 5 structures which have been rolled-up and positioned at the end of ridge waveguides.


34

2.3.3.2 Methods

A detailed description on the methods for this work will be made here. It took some time to

perfect the engineering required for the fiber integration discussed in Section 2.3.3.1; the difficulty

being finding the right exposure times needed for each lithography step. In the end, the following

necessary steps are required. For cleanroom processing of the samples, first a monolayer of Ti-

Prime [@ 3,500 revolutions per minute (rpm) for 20 s] is applied in order for the SU8 to properly

adhere to the substrate. SU8-10 is then spin-coated on the substrate at 5,000 rpm for 30 s (results in

a 5 µm thick layer). A soft-bake [60°C � 90°C (10 min) � 60°C] is made before a

photolithographic exposure of 15 sec to define the waveguides. A post-bake follows, [60°C � 90°C

(5 min) � 60°C] in order to fully crosslink the SU8. The samples are then developed using 100 %

MR Dev 600 for 45 – 60 s. A second post-bake [60°C � 120°C (10 min) � 60°C] is then made in

order to “ash” the SU8. If this step is not performed, then the SU8 delaminates once placed in NMP,

a phenomenon employed and discussed in Chapter 4. The waveguides are now finished. The next

step is to create the sacrificial layer from which the nanomembranes will be released. For this, a

layer of AZ MIR 701 is spin-coated onto the substrate [@ 5000 rpm for 35 s (results in a 700 nm

thick sacrificial layer)]. A soft-bake [80°C � 60°C] is made before a photolithographic exposure of

50 seconds. This long exposure time is required so that no residual photoresist on top of the SU8

waveguides is leftover. The alignment of the mask for the sacrificial layer is made using a 20×

objective lens. This alignment is necessary for accurately determining the final resting position of

the fibers after roll-up. The sample is then developed in 100 % MIF 756 for 35 s after a post-bake

[80°C � 60°C].

The nanomembrane was then deposited using either E-Beam or thermal evaporation. For the

characterized fibers the E-Beam was used. The samples were deposited at a 60° angle to that of the

source (0° being the angle at which the sample lies flat, facing the deposition source). A layer of

titanium oxide (TiOx) [0.1 – 0.8 Angstroms per second (Å/s), 2 × 10-4 mbar O2 background] came

first, followed by the Au or Ag [0.6 Å/s, 7 × 10-6 mbar], then capped with a protective 0.5 – 2 nm

layer of TiOx [0.1 Å/s, 2 × 10-4 mbar O2 background]. The removal of the sacrificial layer is done by

placing the samples in NMP. They are then transferred to acetone in order to dry using the CPD.

One setback from collecting a large statistical data set for the MOFs comes from the fabrication.

With the current fabrication method, 90 – 100 % of an array of patterns will roll-up in NMP, Fig.

2.13. When they are transferred to acetone (for the drying phase), they can become larger in

diameter (this may be due to the wettability of the materials within these given media,[49] i.e. how

much the materials resist being in contact with the medium). Some of the tubes will also become

unaligned with the SU8 ridge guides at this point. After the critical point drying step (which can be


35

quite abrasive to the samples when displacing the acetone with CO2), about 5 % of these types of

tubes survive with a good structure intact (optically). About 1 % of these are still aligned with the

injection waveguides. Work has been done to alleviate this problem like making a smoother critical

drying step. However, the issue of tubes changing in radius in different media is, in general,

unavoidable.

2.3.3.3 Material investigation and characterization

The materials which were investigated for MOF development include gold (Au) and Ag for

their plasmonic attributes, as well as TiO2 as an oxide because of its high permittivity.[14] A

discussion over the material properties will be given here because of the extreme importance of

knowing these properties when trying to confirm the theoretical predictions set out in the last

section. Note the index of refraction, like the permittivity, can be split into real and imaginary parts,

n = η + iκ. The conversion to permittivity (for µ = 1), is given by Re{ε} = ε1 = η2 – κ2 and Im{ε} =

ε2 = 2ηκ.[15] Ellipsometry and atomic force microscopy measurements (AFM) were performed. The

first material which was investigated was TiO2. The TiO2 was deposited using an E-Beam with

TiO2 material as a source, a variety of oxygen (O2) background pressures (1 – 2 × 10-4 mbar) were

used. The ellipsometry measurements were compared to literature,[14] and are plotted in Fig. 2.14.

Figure 2.14 | Ellipsometry measurements of TiOx. Ellipsometry measurements for different E-Beam deposition conditions reveal a lower index of refraction than reported in literature. However, the material is not lossy.

It is clear that the permittivity values are lower than those reported in literature. This is probably

due to the fact that normally TiO2 is grown at 300°C or higher, whereas the deposition temperature

of our samples is limited to 80°C because of the photoresist used. The TiO2 is most likely an

amorphous layer and possibly has oxygen vacancies lending to the lower permittivity. Regardless,

the layer will be referred to from this point on as TiOx. The average index of refraction was found to

be n = 1.8 + 0i, whereas literature puts the values between (2.5 – 2.8) + 0i.[14]


36

Next, layers of Au were studied. Different thicknesses were investigated; all Au films were

deposited at the same rate, 0.6 Å/s. Ellipsometry measurements were performed on samples with

three particular thicknesses, Fig. 2.15. The thinnest of the layers, 12 nm, does not converge to the

values reported in literature.[15] In fact, when η and κ are converted to permittivity, it shows that Au

exhibits, although lossy, dielectric properties (positive ε) over most of the visible range. In 1972,

Johnson and Christy (Ref. 15) discussed this phenomenon. This is of importance to discuss here

because the layer thicknesses used in the next section for creating MOFs rely on Au thicknesses

Figure 2.15 | Ellipsometry and AFM measurements of Au. (a) The real part of the index of refraction of Au for a number of different thicknesses compared to literature. (b) The imaginary part. (c) AFM measurements (performed by B. Eichler) reveal the Au layers to be smooth and continuous with 0.8 nm < Srms < 1.37 nm.

between 2 and 23 nm. The question is: Which permittivity values should be considered when

calculating the effective permittivity? In literature, when discussing the effective permittivity it was

assumed that the thickness of the layer stack must be much smaller than the incident wavelength (d

<< λo). This is so that the EM waves effectively see a bulk material rather than the finely stratified

layers.[3] The lower limit of the individual material thicknesses is not discussed. Intuitively, the

lower limit must come at some point in which the layer is too thin to have a collective behavior

within the material. Johnson and Christy speculated that the divergence is caused by the layer not

being homogenous or continuous (for layers < 25 nm).[15] However our measurements contradict

this speculation. AFM measurements made on a 12 nm film shows a root mean square surface

roughness (Srms) between 0.8 nm < Srms < 1.37 nm, Figure 2.15(c1-3), which is smaller than the total


37

layer thickness. Given this smoothness, the layers must be continuous. Furthermore, if discontinuity

would be the case, then the loss (κ) is expected to be higher than literature value.[15] However, it is

shown in Fig. 2.15(b), that κ converges to literature values. Given this fact, an assumption is made,

that although an individual ultrathin layer is divergent with regards to the bulk material permittivity,

that when there are many stacked layers of the metal and oxide, the material will still act as dictated

by the effective permittivity theory, Eqs. 3 and 4. This would imply that there are enough free

electrons within the multilayer stack to support plasmonic guidance. The profile data at the output

of the MOFs, presented in Section 2.3.3.5, is compared with the effective permittivity using these

experimental values and is further supporting of the assumption made here. The same divergence

from literature is seen with E-Beam deposited Ag layers as well, Fig. 2.16. Again, for thicker layers

the data converges to literature values.

Figure 2.16 | Ellipsometry measurements of Ag. Experimental Ellipsometry measurements for Ag reveal the real (a) and imaginary (b) parts of the index of refraction for various film thicknesses.

The white light laser source which was used for the MOF characterization does not emit light

with a very high intensity at wavelengths below 440 nm. Since the plasma frequency for Ag-based

metamaterials lies in the blue and ultra violet regions of the spectrum, they were not investigated

here, however they were developed. Instead, a focus on the characterization of Au-based MOFs was

made. If the data from Figs. 2.15 and 2.14 are inserted into Eqs. 3 and 4, a more accurate effective

(given the use of TiOx) permittivity can be calculated for the materials used in fabrication. The

result of this calculation can be seen in Fig. 2.17. This gives a basis in which to characterize these

devices.


38

Figure 2.17 | Effective permittivity of TiOx/Au MOFs. Using Eqs. 3 and 4 and the experimentally obtained permittivity values of our films, the effective bulk permittivity can be calculated. Here, Au filling ratios of 20, 40, 60 and 80 % were calculated. Including the real (a) and imaginary (c) parallel component and the real (b) and imaginary (d) perpendicular component of the effective permittivity.

2.3.3.4 Metamaterial optical fiber development

There are a number of different materials investigated for creating metamaterial optical fibers.

Using these materials, rolled-up tubes are made from TiOx/Ag [Fig. 2.18(a), trilayer of TiOx (10

nm)/Au (2.5 nm)/TiOx (2 nm)], TiOx/Au, (Silicon dioxide) SiO2/Au and SiO2/Ag [Fig. 2.18(b),

trilayer of SiO2 (7 nm) /Ag (3 nm)/SiO2 (2 nm)]. Attempts were made with Al2O3/Ag, but the

samples did not roll. However, it is possible that temperature deposition would allow for such tubes

to be formed. So-called temperature deposition is a method in which materials are deposited at

different temperatures in a single deposition step. This adds additional stress to the nanomembrane

due to expansion or contraction of the sacrificial photoresist layer caused by the difference in

temperature of the sample during the deposition. There are a number of different approaches

attempted for developing metamaterial optical fibers. One of these is the standard bilayer deposition

(also under the same principle: trilayer deposition) deposited at a constant temperature. The fibers

characterized in Section 2.3.3.5 are made with this method. The other investigated technique is a

temperature deposition approach which adds additional stress in the materials for roll-up. This can

increase layer compactness [Fig. 2.18(c) SiO2 (9 nm)/Ag (3 nm)] and can even be used to create


39

Figure 2.18 | Fabricated metamaterial optical fibers. Rolled-up fibers formed from a nanomembrane trilayer of (a) TiOx/Ag and (b) SiO2/Ag using the standard deposition technique at constant temperature. (c) Depositing two layers at different temperatures also results in a compact cladding layer. Here an SEM image (made by D. Thurmer) of a fiber whose cross section was revealed by a FIB cut, shows a compact multilayer stack (lighter contrast is SiO2, darker contrast is Ag). (d) A plotting of the temperature vs. time for a pure Au nanomembrane which is deposited at a constant rate. The strain induced from the temperature deposition creates enough strain to allow for single nanomembrane to roll-up.

fibers from single materials. For instance, single materials like Au (a potentially interesting

plasmonic structure), which do not have enough strain to roll if the deposition is made under

constant temperature, Fig. 2.18(d). The important aspect to consider here is that the temperature

deposition is done as a single deposition step with only changing the substrate temperature during

deposition. The temperature deposition is carried out using a sample holder which has a water-

cooled Peltier cooler attached. The temperature is adjusted by hand and read out from a temperature

probe placed directly next to the samples.

Originally Ag was used as the plasmonic metal. This was before Ag was ruled out as a viable

candidate for an in-depth investigation because of the spectral limit of the light source which was

used. It is also a possibility that Ag will oxidize, classically known as tarnish, which requires a

method to insure this process would be minimized. Because of this, instead of focusing on bilayer


40

deposition, trilayers were considered in the form of oxide/metal/oxide in order to insure that Ag

would be protected from oxidizing in air when the samples were taken out of vacuum. It was also

considered a possibility that the oxide would bond back on itself much better than with the metal

when the nanomembrane was rolled-up. The plasmonic guidance is based on a bulk guidance of

surface plasmons, and surface plasmons require a good metal/oxide interface to propagate. Due to

this, the third reason to use a trilayer is that it guarantees an oxide/metal/oxide interface, meaning

that even if the roll-up is not fully compact due to gaps, the interface will remain intact. All samples

were designed using a 15 – 20 nm thick bi-/trilayer; thicker nanomembranes tended to not roll-up,

but rather remained planar. In the other material system investigated Au is used as the plasmonic

metal. The MOFs characterized in the next section consist of the following material makeup: The

TiOx (30 %)/Au (70 %) sample consisted of TiOx (4 nm)/Au (10.5 nm)/TiOx (0.5 nm); The TiOx (20

%)/Au (80 %) sample consisted of TiOx (2.5 nm)/Au (12 nm)/TiOx (0.5 nm). As mentioned earlier,

using Au as the plasmonic metal allows for the plasma frequency of the metamaterial to be shifted

throughout the entire visible range.

2.3.3.5 Metamaterial optical fiber characterization

Here, a discussion of the setup used for the characterization of metamaterial optical fibers is

given. In order to do this, a way of getting the light into the fibers is needed. The original method

attempted was to try and focus light down to the opening of a fiber, using an objective lens, and

then have a second objective lens for collecting the output of the fiber. This is experimentally

difficult. The issue is that the input and output are too close to one another. Given that there is no

method to insure that insertion loss will not occur, a lot of light is scattered at the input. With this,

and the fact that the fibers are 100 – 400 µm in length, the outputs of the fibers do not have a strong

enough output signal to be picked out of the background “noise” created from the scattered light at

the input. This is why the method for on-chip integration, presented in Section 2.3.3.1, has been

developed. This allows for the insertion point to be a large distance from the output of the fiber, in

essence lowering the amount of noise at the output. By using this integrated arrangement, an optical

setup can be developed for characterization, Fig. 2.19. The light source is a white light laser, called

a supercontinuum source, which emits light from 400 nm to 2 µm. The IR is then filtered out using

a mirror filtering system. The light is either sent directly to a 50× objective, or first through a

monochromator in order to filter out individual wavelengths [2 nm full width at half maximum

(FWHM)], then directed to the 50× objective lens. This filtering of individual wavelengths is

important for the investigation, given how dispersive the MOFs are. The objective lens is used to

focus the light down to the input of one end of an SU8 ridge waveguide. The light is then guided


41

Figure 2.19 | Optical setup for metamaterial optical fiber characterization. A supercontinuum light source (white light laser), emitting from 0.4 – 2 µm, is used as a light source. The IR is then filtered out with a filtering mirror module. The beam is sent either directly to a focusing objective lens or first filtered to single wavelengths with a monochromator, then directed to the focusing objective lens. The light is focused down onto one end of an SU8 ridge waveguide using a 50× objective lens. Light travels down the waveguide and couples to a metamaterial optical fiber positioned at the other end. A second 50× objective lens collects the output of the light where it can then either be sent to a fiber spectrometer or expanded and imaged onto a beam profiler. (a) A bright field microscopy image reveals the ridge waveguide and MOF positioned at its end. (b) When the microscope observation light is turned off, the image shows the input of the light from the ridge waveguide (left side), the coupling position, and the scattering light from the output of the fiber (right side).

down the waveguide to the other end where it is injected into a MOF positioned at its end. The

aligned ridge waveguide and MOF can be seen in Fig. 2.19(a). When the observation lamp of the

microscope is turned off, the light in the ridge guide is visible as well as the injection point left side

Fig. 2.19(b) and the fiber output in the right side of Fig. 2.19(b). The output of the fiber is then

collected by a second 50× objective lens. The fiber can then be analyzed with a fiber spectrometer or

a beam profiler. For the purpose of this work, the data was analyzed using the beam profiler.

It is important to mention that surface plasmons are highly confined to the surfaces in which

they propagate. An argument could be made that any plasmonic waveguiding effect would only be

detectable if the near field of the fibers is probed. However, light guided plasmonically can be

scattered by objects, like defects or gratings,[50] which allow them to be detected in the far field.

When the fibers are created from the roll-up process, they are required to tear-away at the edges of

the pattern in which the nanomembranes originate. This tearing leads to slightly coarse ends, which

create the necessary condition, i.e. end defects, for the plasmonic waves to be scattered away from

the fiber.

The measurement method used is the beam profiler. The reason for this is that the theoretical

work focused on the mode profile of metamaterial optical fibers. With this method the sub-


42

wavelength transverse modes which show up in theory will not be detectable in experiment because

the measurement is limited by the diffraction limit of light. Regardless of this, there should be a

clear shift of the intensity of light in the profile. That means the output profile of light emitted from

the fiber should reveal if the light was guided more plasmonically or classically by the position of

Figure 2.20 | Core vs. cladding output profile. (a) The output of the MOFs is investigated and reveals a preference of being concentrated to the core, left, or shifting concentration to the cladding, right, as the visible spectrum is scanned over. The white line is the measured FWHM of the profile. (b) Cross sections taken from the dotted line in (a) reveal that indeed the profile spreads out towards the cladding of the fiber.

the profile with respect to the spatial orientation of the core and cladding. For wavelengths at which

light is guided mostly classically the beam profile should have a FWHM in the center of the fiber

output. However, for wavelengths in which the light was mostly guided plasmonically, the profile

should be more spread out since the light will have been mostly confined to the cladding. This

effect was observed, Fig. 2.20(a), so a more in-depth analysis was undertaken. Using an achromatic

Keplerian telescope to expand the output of the beam before it was imaged on the beam profiler, the

geometric cross section of the fiber (taken from the fiber diameter measured optically) can be

mapped on the screen, Black circle in Fig. 2.20(a). This fiber consists of TiOx/Au with a 70 % Au

filling. It was found that indeed, depending on the wavelength, the output profile of the fiber was

concentrated at the center of the fiber [670 nm of Fig. 2.20(a)], or at the outer edges of the fiber

[710 nm of Fig. 2.20(a)]. The white line in all of the profile data is the measured FWHM of the

profile. The analysis of the cross section [dotted black line in Fig. 2.20(a)] suggests that light with

λo = 670 nm is concentrated in the core and shifts to being concentrated in the cladding of the fiber

for the wavelength of 710 nm.


43

Figure 2.21 | Profiles of a TiOx (30 %)/Au (70 %) MOF at different wavelengths. The trilayer consists of TiOx (4 nm)/Au (10.5 nm)/TiOx (0.5 nm). Distinct changes in the output profiles can be seen as the input wavelength is swept across the visible spectrum.


44

An investigation of how the output profile of a TiOx (30 %)/Au (70 %) MOF changed over the

visible spectrum is performed. The profile, taken in input-wavelength steps of 10 nm, is found to

take on a number of different changes over the spectrum. The results are displayed in Fig. 2.21.

At this point it is important to consider the effective material permittivities of the fiber which

were calculated in Section 2.3.3.3. A comparison, of how these permittivities change according to

wavelength, to how the output of the fiber changes, must be made. Using the permittivity values

from Figs. 2.14 and 2.15 and calculating for a filling ratio of TiOx/Au of 30 % to 70 %, what comes

out is represented in Fig. 2.22. In the figure, distinct regions are labeled. The boundaries of these

regions represent when a change from positive to negative of either Re{ε⊥} or Re{ε‖} occurs. These

regions should have different guiding characteristics given that the anisotropy is different for each

Figure 2.22 | Effective permittivity for a TiOx

(30 %)/Au (70 %) metamaterial. The trilayer consists of TiOx (4 nm)/Au (10.5 nm)/TiOx (0.5 nm). The calculated real (a) and imaginary (b) parts of the effective permittivity of a metamaterial cladding made up of a material combination of TiOx (30 %)/Au (70 %) is given. Distinct regions, in which the anisopotry of the material undergoes a change (i.e. from positive to negative), can be labeled for comparing to the output profiles of the experiment.

one. In Fig. 2.22, there are five distinctive regions which exist (for this material combination).

These are as follows: Region 1 where Re{ε⊥} > 0 and Re{ε‖} > 0, where [εeff ] is slightly

anisotropic; Region 2 where Re{ε⊥} > 0 and Re{ε‖} < 0 and [εeff ] is slightly anisotropic; Region 3

where {ε⊥} < 0 and Re{ε‖} < 0; and Region 4 and Region 5 where Re{ε⊥} > 0 and Re{ε‖} < 0,

where [εeff ] is highly anisotropic. The difference between Region 4 and Region 5 is that in Region

4, Im{ε⊥} >> 0. If the output profiles of the fiber, which were shown in Fig. 2.21, are separated by

regions compared to those defined in Fig. 2.22, a comparison between the calculated theoretical


45

permittivity and experimental output profile can be made. A separation of these regions is given in

Fig. 2.23.

It is noticeable that indeed the profile seems to take on different characteristics when the

permittivity shifts from one region to another as the wavelength is varied. It appears that Region 1

(Fig. 2.23) is defined by core guidance. This is a region where 0 < Re{ε‖} < 1, which was calculated

in the theoretical section to be a region taking on ATR guidance and could be associated with

Region III in Fig. 2.9. In Region 2 (Fig. 2.23) the light seems to be mainly confined to the core, but

is appears that the FWHM lies at the edge of the cladding and there could be some light here which

is guided plasmonically. This region corresponds to Region V in Fig. 2.9, and should support

plasmonic guidance. Region 3 (Fig. 2.23) is where both of the components of permittivity are

negative, a region not discussed in the theory, and the profile takes on a definite change from the

previous region. The FWHM lies much further out from the core than seen earlier, however the

signal is quite weak in this region, possibly due to losses expected in the metal. This would be

expected if most of the light is confined to the cladding given that both Im{ε⊥} and Im{ε‖} are quite

large in this region. In Region 4 (Fig. 2.23) the output profile of the fiber shifts again, more towards

Figure 2.23 | Regions of guidance outlined for a TiOx (30 %)/Au (70 %) MOF. The trilayer consists of TiOx (4 nm)/Au (10.5 nm)/TiOx (0.5 nm). The different anisotropic regions which occur in the effective permittivity, left, are mapped onto the output profile of a fiber, right. A clear change is noticeable as the wavelength is varied across the numerous regions.

the core of the fiber. According to the expectation of theory, again Region V of Fig. 2.9, this region

should support plasmonic guidance. However, if the magnitude of Im{ε⊥} is noted, it is clear that

the cladding is very lossy in this region, causing more of the light to be confined to the core. In

Region 5 (Fig. 2.23), the losses are not as high, and the light appears to be confined in both the core

and the cladding.

If the profile regions are indeed defined by the effective permittivity of the cladding, then when

a different MOF is developed (made up from a different filling ratio of metal to oxide) the profile


46

regions should likewise shift with the new calculated effective permittivity. Investigations were

carried out to confirm this assumption. The left side of Figure 2.24 gives the calculated effective

Figure 2.24 | Regions of guidance outlined for a TiOx (20 %)/Au (80 %) MOF. The trilayer consists of TiOx (2.5 nm)/Au (12 nm)/TiOx (0.5 nm). As the filling ratio of the cladding material is changed, and in turn the effective permittivity, the regions of guidance will also shift, left. This change should show as a shift in the regions observed in the output profile, right, of a metamaterial as well. This shift is indeed observed experimentally.

permittivity for a metamaterial cladding made up of TiOx (20 %)/Au (80 %). The right side of

Figure 2.24 shows the recorded output profile of the fiber. Indeed, there are similarities to the

profile versus the region the permittivity lies within. Region 1 no longer falls within this spectral

range but the other regions are comparable to the MOF which contained a 70 % Au filling [referring

to Fig. 2.23, the trilayer consisting of TiOx (4 nm)/Au (10.5 nm)/TiOx (0.5 nm)].

Figure 2.25 | Close-up of transition from Region 4 to Region 5. A close-up comparison of two MOFs with different filling percentages of Au (70 % and 80 %) is shown. Both the 70 % (a) and 80 % (c) result in similar output profiles, but shifted by 30 nm. The effective permittivities reveal that the Regions in the sample with 70 % Au (b) data are blue-shifted from the samples with 80 % Au (d) data by 30 nm.


47

The results are further suggestive if a close-up of a region from both concentrations is

considered. Taking, for instance, a close-up of the transition from Region 4 to Region 5 from both

MOFs, the same profiles occur for both samples, Figs. 2.25(a) and (c). The difference being a 30

nm shift at which the profiles exist. This can be directly traced back to the effective permittivity

calculation, Figs. 2.25(b) and (d). Relatively the same material anisotropy occurs in both spectra.

However, for the sample with an 80 % Au filling [Fig. 2.25(d)], the same anisotropy is 30 nm red

shifted from where it arises in the permittivity of the sample with a 70 % Au filling, Fig. 2.25(b).

This is a reasonable region to compare the two permittivities of the materials because in the other

regions there are much larger differences in the anisotropy of each. The comparison is not exact, but

differences between the two could be due to differences in experimental error. One detail which is

not understood is the source of the antisymmetric lobes which appear in Region 5 of Figure 2.23

and 2.24. This could be caused from interaction with the substrate or due to the fiber being elliptical

rather than circular, as presumed.

It is also of interest to compare the output of these fibers to the mode profile which comes from

an isotropic rolled-up TiOx fiber, Fig. 2.26. However, these results are not conclusive at this point

because all of the oxide fibers that were measured will not guide light below 730 nm. This might be

because the NA for such a fiber is small and only light entering at 0° can pass from one end to the

Figure 2.26 | Output profile of rolled-up TiOx fibers. In order to compare with tunable metamaterial fibers, pure oxide fibers were also investigated. It is revealed that light with shorter wavelengths than 730 nm cannot be guided down a fiber (400 µm in length, diameter ≈ 5 µm). The black circle is the calculated tube diameter. The white outline is the FWHM of the output intensity.

other. It is important to keep in mind that guiding light with such a thin cladding (≈ 70 nm) with a

high index of refraction material has not really been considered before, or found to have been

reported in literature.

2.3.3.6 Conclusions and outlook

In this section, the fabrication of rolled-up metamaterial optical fibers and their properties were

investigated in-depth. Metamaterial optical fibers composed of a number of different materials were

fabricated using an angled deposition technique as well as a new temperature deposition technique.

Both methods resulted in MOFs. A method for integrating rolled-up devices into preexisting on-


48

chip structures was also put forth and demonstrated. The calculated effective permittivity allowed

for a basis in which to understand the characteristics of these metamaterial optical fibers. The

experimental data from the fiber characterization, and the way in which the mode profile changes

with respect to the calculated effective permittivity, shows a definite tuning of the cladding

material’s effective permittivity. It may be that plasmonic guidance has yet to be proven for these

fibers; however it is clear that there is a tuning of the fiber output by adjusting the filling ratio of

metal to gold or the propagating wavelength. A clear shift in the regions of guidance is shown for

differing concentrations of Au. The material characteristics of these fibers are unique. The fact that

they are able to guide light throughout the entire visible range, with a calculated cladding thickness

of 70 nm (calculated from the nanomembrane thickness and distance of roll-up) is a promising

characteristic: More so, because an isotropic oxide fiber with the same cladding thickness was

unable to efficiently guide light throughout the visible spectrum.

Another interesting aspect is an exploration of the possible non-linear effects which MOFs

could exhibit, given that plasmonics are known to result in interesting non-linear effects because of

the large localized electric fields.[28,51] In order to make this investigation, the SU8 will have to be

replaced, because the power threshold before SU8 burns is quite low. A possible replacement would

be some sort of flowable oxide which can be spin-coated on a wafer and processed in much the

same way.

2.4 References


2. Y. F. Mei, et al., Versatile approach for integrative and functionalized tubes by strain engineering of nanomembranes on polymers, Adv. Mater., 20, 4085-4090 (2008).

3. S. M. Rytov, Electrodynamic properties of a finely stratified medium, Sov. Phys. JETP, 29, 605-616 (1955) [Sov. Phys. JETP, 2, 466-475 (1956)].

4. E. J. Smith, Z. Liu, Y. F. Mei and O. G. Schmidt, System investigation of a rolled-up metamaterial optical hyperlens structure, Appl. Phys. Lett., 95, 083104 (2009). [Erratum: Appl. Phys. Lett., 96, 019902 (2010)].

5. E. J. Smith, Z. Liu, Y. F. Mei and O. G. Schmidt, Combined surface plasmon and classical waveguiding through metamaterial fiber design, Nano Lett., 10, 1-5 (2010).

6. B. Wood, J. B. Pendry and D. P. Tsai, Directed subwavelength imaging using a layered metal-dielectric system, Phys. Rev. B, 74, 115116 (2006).

7. A. Salandrino and N. Engheta, Far-field subdiffraction optical microscopy using metamaterial crystals: theory and simulations, Phys. Rev. B, 74 075103 (2006).


49

8. Z. Jacob, L. V. Alekseyev and E. E. Narimanov, Optical hyperlens: far-field imaging beyond the diffraction limit, Opt. Express, 14, 8247-8256 (2006).

9. A. V. Kildishev and E. E. Narimanov, Impedance-matched hyperlens, Opt. Lett., 32, 3432-3434 (2007)

10. H. Lee, Z. Liu, Y. Xiong, C. Sun and X. Zhang, Development of optical hyperlens for imaging below the diffraction limit, Opt. Express, 15, 15886-15891 (2007).

11. Z. Liu, H. Lee, Y. Xiong, C. Sun and X. Zhang, Far-field optical hyperlens magnifying sub-diffraction-limited objects, Science, 315, 1686 (2007).

12. J. Rho, et al., Spherical hyperlens for two-dimensional sub-diffraction imaging at visible frequencies, Nature Comm., 1, 144-149 (2010).

13. W. Zhang, H. Chen and H. O. Moser, Subwavelength imaging in a cylindrical hyperlens based on S-string resonators, Appl. Phys. Lett., 98, 073501 (2011).

14. E. D. Palik, Ed. Handbook of Optical Constants of Solids, pg. 676, 770 and 799-800, [Academic, San Diego, CA, 1998].

15. P. B. Johnson and R. W. Christy, Optical constants of the noble metals, Phys. Rev. B, 6, 4370-4379 (1972).

16. S. Schwaiger, et al., Rolled-up three-dimensional metamaterials with a tunable plasma frequency in the visible regime, Phys. Rev. Lett., 102, 163903 (2009).

17. E. Hecht, Ed. Optics, 4th Ed., Pg. 119-124, [Pearson Education Inc., San Francisco, CA, 2002].

18. D. Borja, et al., Optical power of the isolated human crystalline lens, Invest. Opthalmol. Visual Sci., 49, 2541-2548 (2008).

19. V. G. Veselago, The electrodynamics of substances with simultaneously negative values of ε and µ, Sov. Phys. Usp., 10, 509-514 (1968).

20. D. R. Smith, J. B. Pendry and M. C. K. Wiltshire, Metamaterials and negative refractive index, Science, 305, 788-792 (2004).

21. J. B. Pendry, D. Schurig and D. R. Smith, Controlling electromagnetic fields, Science, 312, 1780-1782 (2006).


23. W. Cai, U. K. Chettiar, A. V. Kildishev and V. M. Shalaev, Optical cloaking with metamaterials, Nature Photon., 1, 224-227 (2007).

24. R. A. Shelby, D. R. Smith and S. Schultz, Experimental verification of a negative index of refraction, Science, 292, 77-79 (2001).


50

25. J. Valentine, et al., Three-dimensional optical metamaterial with a negative refractive index, Nature, 455, 376-379 (2008).

26. C. M. Soukoulis, S. Linden and M. Wegener, Negative refractive index at optical wavelengths, Science, 315, 47-49 (2007).

27. N. Liu, et al., Three-dimensional photonic metamaterial at optical frequencies, Nature Mater., 7, 31-37 (2008).

28. W. L. Barnes, A. Dereux and T. W. Ebbesen, Surface plasmon subwavelength optics, Nature, 424, 824-830 (2003).

29. J. Zeng and D. Liang, Application of fiber optic surface plasmon resonance sensor for measuring liquid refractive index, J. Intell. Mater. Syst. Struct., 17, 787-791 (2006).

30. A. K. Sharma, R. Jha and B. D. Gupta, Fiber-optic sensors based on surface plasmon resonance: a comprehensive review, IEEE Sens. J., 7, 1118-1129 (2007).

31. C. Yeh and F. I. Shimabukuro, Ed. The essence of dielectric waveguides, [Springer, New York, NY 2008].

32. J. A. Harrington, A review of IR transmitting, hollow waveguides, Fiber Integr. Opt., 19, 211-217 (2000).

33. P. Yeh, A. Yariv and E. Marom, Theory of Bragg fiber, J. Opt. Soc. Am., 68, 1196-1201 (1978).

34. M. Ibanesca, et al., Analysis of mode structure in hollow dielectric waveguide fibers, Phys. Rev. E, 67, 046608 (2003).

35. R. F. Cregan, et al., Single-mode photonic bang gap guidance of light in air, Science, 285, 1537-1539 (1999).

36. T. Hidaka, T. Morikawa and J. Shimada, Hollow-core oxide-glass cladding optical fibers for middle-infrared region, J. Appl. Phys., 52, 4467-4471 (1981).

37. J. S. Sanghera and I. D. Aggarwal, Ed. Infrared Fiber Optics, [CRC Press, Boca Raton, FL 1998].

38. A. Argyros, T. A. Birks, S. G. Leon-Saval, C. M. B. Cordeiro and P. St. Russell, Guidance properties of low-contrast photonic bandgap fibers, Opt. Express, 13, 2503-2511 (2005).

39. J. Homola, S. S. Yee and G. Gauglitz, Surface plasmon resonance sensors: review, Sensors Actuat. B, 54, 3-15 (1999).

40. D. R. Lide, Ed. CRC handbook of chemistry and physics 2003-2004, [CRC Press, Boca Raton, FL 2003].

41. S. Mendach, et al., Light emission and wave guiding of quantum dots in a tube, Appl. Phys. Lett., 88, 111120 (2006).


51

42. Ch. Deneke and O. G. Schmidt, Structural characterization and potential x-ray waveguiding of a small rolled-up nanotube with a large number of windings, Appl. Phys. Lett., 89, 123121 (2006).


44. F. Cavallo, R. Songmuang and O. G. Schmidt, Fabrication and electrical characterization of Si-based rolled-up microtubes, Appl. Phys. Lett., 93, 143114 (2008).

45. C. C. Bof Bufon, et al., Self-assembled ultra-compact energy storage elements based on hybrid nanomembranes, Nano Lett., 10, 2506-2510 (2010).

46. D. J. Thurmer, C. C. Bof Bufon, Ch. Deneke and O. G. Schmidt, Nanomembrane-based mesoscopic superconducting hybrid junctions, Nano Lett., 10, 3704-3709 (2010).

47. D. J. Thurmer, Ch. Deneke, Y. F. Mei and O. G. Schmidt, Process integration of microtubes for fluidic applications, Appl. Phys. Lett., 89, 223507 (2006).

48. E. J. Smith, R. Engelhard, S. Kiravittaya and O. G. Schmidt, Demonstration of on-chip integration of metamaterial optical fibers, unpublished, (2010).

49. E. G. Shafrin and W. A. Zisman, Constitutive relations in the wetting of low energy surfaces and the theory of the retraction method of preparing monolayers, J. Phys. Chem., 64, 519-524 (1960).

50. R. H. Ritchie, E. T. Arakawa, J. J. Cowan and R. N. Hamm, Surface-plasmon resonance effect in grating diffraction, Phys. Rev. Lett., 21, 1530-1533 (1968).

51. J. Merlein, et al., Nanomechanical control of an optical antenna, Nature Photon., 2, 230-233 (2008).

Chapter 3 Towards Lab-in-a-Tube

52

Chapter 3: Towards Lab-in-a-Tube

3.1 Introduction

This chapter will focus on a sensor system which has been investigated for on-chip applications.

The goal is to develop an optofluidic biosensor for integration into lab-in-a-tube devices.[1,2] The

overall concept of lab-in-a-tube was already introduced in Section 1.3 and in this chapter an

introduction to the first sensor, specifically developed for this system, will be given.

The ability to sense and study single cellular activity and behaviors is important in the fields of

cell biology and biophysics.[3,4] How these cells react when confined to rigid structures and cell-

material interactions is also of great interest. This is because in situ reactions of a cell, along with a

cell’s environmental interactions, hold clues for the understanding of some physiological and

biochemical responses. These in situ observations include studies of cell proliferation, cancer

growth,[5,6] cellular motility,[7] and wound healing,[8] to name a few. Means in which to develop

inexpensive and noninvasive biosensing methods for in situ detection of the dynamic changes of

soft biological systems is of great interest to numerous scientific fields including biology,

biophysics and micro-/nanotechnology. The concept of lab-in-a-tube would meet these

requirements, and would allow for the in situ measurement and observation of large numbers of

individual cells over a large multiarray system.

3.1.1 Rolled-up optical ring resonators

The compact component, which will be focused on in this chapter, relies on light confinement in

optical ring resonators, enabled by employing rolled-up technology.[9-15] Optical ring resonators

have recently been developed as label-free on-chip devices, utilizable for different applications

because of the ability to precisely measure shifts in whispering gallery modes (WGMs). Label-free

refers to the ability of identifying a bio-organism without the use of special binding proteins. The

shifts in a resonator’s WGMs are caused by the resonator’s changing environment.[13,16-21] Some

detectable changes in the environment include the refractive index of the surrounding medium,[13]

attachment of single viruses or molecules,[17,19] nanoparticles on the surface of the resonators,[20,21]

or the sensing mechanism discussed here, which detects structural changes in the resonator itself

due to cellular interactions.[15]


53

The principle under which these optical resonators work is through the constructive and

destructive interference of light as it resonates around the tube wall of a resonator. The peaks in a

spectrum, which come from the constructive interference of light, are known as WGMs.[22] These

look like standing optical waves in a circular structure. The name WGM originates from the

acoustic waves which travel around St. Paul’s Cathedral’s dome in London;[22] allowing one to

whisper on one side of the dome and be heard on the other. This phenomenon for light was first

investigated by Richtmyer in 1939 who proposed that light could resonate around a dielectric which

was bent back on itself. This structure was referred to as a dielectric resonator.[23] It was considered

that if a dielectric waveguide was taken and formed into a cylinder, the light waves confined to this

dielectric structure, would resonate around indefinitely. This assumed that the wave traveling

around had its original phase after one rotation[23] (leading to constructive interference). Such waves

cannot resonate indefinitely and Richtmyer proved that light traveling in such a resonator must

indeed radiate out. This radiated light can be collected and analyzed, the basis of modern resonator

detectors. The principle formula representing the WGMs in such a resonator is given by:

o

avgeff DnM

λπ

=# (11)

where, M# is the azimuthal mode number (a measure of the number of antinodes in the standing

Figure 3.1 | Structures which support WGMs. A number of different structures have been developed to support optical WGMs including (a) microspheres, (b) microcylinders, (c) planar microrings and (d) toroids developed on micropillars. (e) Definition of TM and TE polarization in a cylindrical ring resonator.


54

wave), neff is the effective index of refraction (dependent on the index of refraction making up the

resonator and the surrounding media), Davg is the average diameter [Davg = (Dout + Din)/2 where Dout

and Din are the outer and inner diameter of the resonator, respectively].

A number of structures can support these WGMs as it was outlined by Vollmer and Arnold, Fig.

3.1.[17] Roll-up technology can provide cylindrical geometries similar to the one shown in Fig.

3.1(b). Since light resonates around these objects, it is also possible to use them as filters. If a

resonator is in contact with a fiber, the resonator can pull out particular wavelengths from the fiber,

effectively filtering them out, similar to a band-stop/notch filter in electronics. This is done by a

number of research groups by contacting a tapered fiber to the resonator,[17] as illustrated in Figs.

3.1(a), (b) and (d). However, another way exists, for instance the fiber could be designed on-chip

with a resonator fabricated in 2D next to the fiber, Fig. 3.1(c).

Our group, in collaboration with IHP in Berlin (Dr. L. Zimmermann), is currently investigating

using the concept of rolled-up integration, presented in Section 2.3.3.1, for the integration of rolled-

up microtubes on top of on-chip waveguides. In other words, it is of interest to fabricate optical

drop filters in the third dimension, on top of preexisting, on-chip waveguides. Promising results

have come out of this venture, and there is ongoing work for improving and characterizing these

structures. Figure 3.2 shows the first work for realizing this integration technique.[24] A patterned

sacrificial layer is developed onto a substrate which contains on-chip waveguides, Fig. 3.2(a). The

Figure 3.2 | Integration of rolled-up microtubes. (a) A sacrificial layer is developed onto a substrate containing on-chip waveguides. A bilayer is deposited onto this sample. (b) After the sacrificial layer is removed, the bilayer rolls-up to create microtubes resting on top of the waveguides (c). Inset of (c) reveals a contrast-enhanced image showing the contact of the microtube to the waveguide (SEM image made by D. Thurmer).


55

bilayers for the roll-up are then deposited. The sacrificial layer is removed, resulting in rolled-up

microtubes [Fig. 3.2(b)] which rest on top of the waveguides. This is confirmed by SEM images,

Fig. 3.2(c). After these ring resonators are in place, another step, integrating the rolled-up

microtubes as a microfluidic channel could then be made.[25] This would allow a fluid to pass

through only the center of the resonator, and in turn, the change in resonance due to this new

medium would be picked up and sensed by the on-chip waveguide.

The outer diameter of resonators which are obtained from the roll-up process can be measured

with optical microscopy. Using this, and knowing the rolling length (Rl) [length of sacrificial layer],

and knowing the inner diameter (measured by SEM or approximated knowing the bilayer

thickness), the number of windings can be obtained:

inout

l

DD

RN

+=

π2

. (12)

This value is needed when the structures are analyzed and simulated later. Many works have

focused on the optical properties of these rolled-up resonators with respect to their light

confinement[9,12,14,26] and, as mentioned earlier, their environmental sensing abilities.[13] It was noted

in Section 1.3, that these rolled-up structures also offer a 3D scaffolding which has shown promise

in yeast cell[27] and neuron cell culture,[28,29] as it provides a more realistic environment for the cells

to flourish. This is an improvement over conventional laboratory culturing experiments, which are

based on 2D culturing. Despite these advances in cell culture, no developed optofluidic detection

scheme for the cells had been put forward. This is where our work comes in; these rolled-up devices

are implemented as individual animal cell sensors.[15] A presentation of a method used to efficiently

capture embryonic fibroblast mouse cells (NIH 3T3), and sequentially, detect them within on-chip

microresonators will be given in the following sections of this chapter.

3.2 Cell capture

3.2.1 Microsyringe

An important tool which was not used or thought up for earlier cellular culture, with regard to

microtube-studies,[27,28] was a technique for capturing cells within these structures.[15] Rather, the

capturing of cells within such structures was left completely to chance. This required hundreds of

microtubes in order to increase the probability that cells would enter the microtubes on their own

accord, just so that there would be a few for observation. This problem is addressed, and a

technique for cell manipulation and capturing is put forth, known as a microsyringe, Fig. 3.3. The


56

microsyringe consists of a tapered capillary in conjunction with a microfluidics pump. The capillary

is secured to an XYZ micromanipulator at a 10° angle (with respect to the sample). Before

pumping, the tapered end of the capillary is positioned at one open end of a microtube using the

micromanipulator.

Figure 3.3 | Microsyringe setup. A diagram for illustrating the microsyringe setup used for the on-chip manipulation of NIH 3T3 fibroblast mouse cells. The large end of a tapered capillary is connected to a microfluidic pump. The tapered end is positioned to one end of a microtube using an XYZ micromanipulator. The microsyringe “sucks-up” the cell culture medium through the microtube, in doing so, capturing cells within the structures.

3.2.2 Capturing of 3T3 NIH embryonic mouse cells into microtubes

A manipulation of the fibroblasts is performed on-chip with this microsyringe. The

microsyringe is used to “suck-up” cells which are suspended within a cell culture medium,

Dulbecco’s Modified Eagle’s Medium/Ham’s F-12 Nutrient (DMEM/F-12). After the microsyringe

tip is in place at one end of a microtube, the pump is switched on, pumping medium through the

microtube at a rate of 0.01 – 0.1 microliters per second (µL/s) until a cell is pumped to the opposite

opening of the tube, Fig. 3.4(a). The pump rate is then increased to 0.5 – 1 µL/s, which creates

enough force to slowly suck even oversized cells into the microtubes, Figs. 3.4(b) and (c). After a

relatively short time (seconds to tens of seconds), the cell is sucked completely within the

microtube, Fig. 3.4(d), at which point the microsyringe is turned off and moved away from the

microtube.

An important characteristic is that capillary forces originating from the microsyringe tip

(tapered capillary) lead to a small, intrinsic flow through the microsyringe. A delay of flow also

occurs (anywhere from 5 – 30 s depending on the previous rate) after the pump is switched off.

Therefore, it is important to smoothly and quickly remove the capillary tip from the open end of the

resonator after a cell is fully inside. A failure to do so will result in the cell being pumped through

the entire microtube and in turn, lost.

The tapered capillary is formed by pulling a 1 mm (in diameter) glass capillary with a gravity-

assisted system. The length of the capillary is secured within the middle of a small tungsten coil


57

Figure 3.4 | Cell capture sequence. Imaging sequence of the effects of the microsyringe pumping method. (a) The tapered capillary (here ≈ 18 µm) is positioned at one opening of a microtube (here Dout ≈ 10 µm). (b) The flow of medium (typically 0.01 – 0.1 µL/s) through the microtube is in the direction of the capillary, left. (c) Even oversized NIH 3T3 fibroblast mouse cells (here ≈ 15 µm) can be sucked within the microtubes due to the pulling force of the microsyringe. (d) The result is a cell which is fully captured within the confinement of the microtube and ready for analysis. (e) An array of eight microtubes which have cells (labeled in green) captured within six of them, showing the reproducible precision of this method.

which is then heated. The capillary is slowly pulled (by adjustable weights) and eventually the

bottom half breaks away smoothly from the top half. This tip is adjustable between 0.5 – 80 µm

depending on the temperature and weight used. However, an optimized technique is implemented

for the purpose of the microsyringe system, in order to create tips ranging from 6 – 18 µm in

diameter. The optimal size being slightly smaller than the diameter of the microtube which medium

is being sucked through. The other end of the capillary is attached to a tube connected to the fluidics

pump by gluing the tube into the capillary with super glue. This allows for an easy exchange of the

tip if it is damaged.

Not only can this pumping method capture a single cell within a single microtube, it can also

consistently capture individual cells within arrays of microtubes, Fig. 3.4(e). This gives a good

platform for a multiple analysis of individual cells within a small observation area. This pumping

method can be used to manipulate cells within the microtubes for further analysis. For instance,

mechanical sensing which will be discussed later in this chapter, also future devices like rolled-up

hyperlenses which were discussed in the last chapter, as well as other biological analysis including

cell proliferation rates and behavior when confined within such structures. To date, cells of various

sizes have been captured. If the cell diameter (Dcell) is compared to the microtube in which they

were captured, the range is from Dcell = 0.7Dout to Dcell = 2.3Dout. The microsyringe process is a

straightforward method which is precise and reproducible; offering the solution of cell capturing

which was lacking in previous studies.[27,28] Related videos of cell capture can be found in the

online supplementary information of Ref. [15].


58

3.3 Flexible split-wall microtube resonator sensors

3.3.1 Microtube fabrication

The resonators used for cell detection are known as flexible split-wall microtube resonator

sensors (F-SWµRS), deriving their name from the sensing mechanism within the resonators which

will be clarified later in this section. The microtube resonators, which are utilized, are designed to

be comparable to, or larger than, the diameter of the 3T3 NIH cells (Dcell). The preparation is made

using angle deposition on a sacrificial photoresist layer, ARP 3510. The sacrificial layers are

circular in shape and have a pattern diameter of 50 µm. Quartz substrates are used. The microtubes

are formed from a nanomembrane consisting of a SiO/SiO2 bilayer. The silicon monoxide (SiO) is

7.5 nm thick (grown at 4 Å/s) and the SiO2 is 42 nm thick (grown at 0.5 Å/s with a 5 × 10-5 mbar O2

background pressure). Both are deposited using E-Beam deposition. The sacrificial layer is removed

by placing the samples in acetone. The bilayers can then relax leading to a roll-up. The microtubes

are on the order of 6 – 10 µm in diameter with a total number of windings ≈ 1.2 – 1.7, calculated

from Eq. 12. Following roll-up the samples are dried using a CPD. After drying, an additional ALD

step is performed to coat the microtubes with a 73 nm layer of hafnium oxide (HfO2). This step is

crucial in order to add stability to the microtubes, given that the cells can destroy less stable

structures. It also increases neff because of its higher index of refraction which is important for

increasing the optical confinement when the resonators are immersed in a liquid.

3.3.2 NIH 3T3 fibroblast mouse cells

As mentioned earlier, this work focuses on the detection of NIH 3T3 fibroblast mouse cells. The

choice of using this cell line (NIH 3T3) for this work is for a number of reasons. These include: 1)

Such cells play an important role within the body when it comes to healing wounds[8] by

maintaining and fabricating the extracellular matrix (ECM). 2) They are cells, which are broadly

present in the bodies of animals, providing the structural framework required for connective tissues.

3) This particular type of cells notice and respond to structural and mechanical changes within their

microenvironment. For instance, they are able to distinguish the difference between stiff and elastic

structures.[30] 4) These cells are “static” cells and their actions are strongly dependent on their

physiomechanical environment.[31] It is these features which make fibroblast cells interesting to

study and analyze with rolled-up structures and sensors.

The cells were supplied by our collaborators S. Schulze and Dr. S. Sanchez, but in brief they

were prepared in the following way: The NIH 3T3 fibroblast mouse cells were cultured in

DMEM/F-12 with 10 % fetal bovine serum, supplemented with 100 units per milliliter (U/mL) of


59

penicillin-G and 100 mg/mL of streptomycin. The cells were separated weekly at a 1:4 splitting

ratio using 0.25 % trypsin per 1 millimole ethylenediaminetetraacetic acid (EDTA). The fibroblasts

were seeded at a low density between 1 × 104 to 4 × 104 cells/mL and preserved at a temperature of

37°C under a 5 % CO2 atmosphere. Directly before using the cells for an experiment they were

washed with a phosphate buffered saline (PBS), treated with trypsin, and re-suspended in the

DMEM/F-12 medium. The cells were then centrifuged for 4 min at 4400 rpm, washed again with

PBS and placed back in the culture medium (~ 107 cells/mL).

A sample, which had the microresonators integrated on-chip, was placed within ~ 5 mL of

DMEM/F-12. A drop of the cell solution (2 – 50 µL) was then dropped into the solution. After this

point, the microsyringe pumping technique took place.

3.3.3 Cell detection

Optical ring resonators were introduced in the beginning of this chapter and it was said that light

can resonate around these structures. It was mentioned that the light source can come from a tapered

fiber [Figs. 3.1(a),(b) and (d)] or an on-chip waveguide [Fig. 3.1(c) and Fig. 3.2].[24,32] However, the

Figure 3.5 | Cell detection with F-SWµRS. The PL measurements of a microresonator comparing the spectrum before and after the capture of a cell. A vertical shift of each spectra was made for clarity. Inset, an optical image (using a 50× objective) indicating the points on the resonator which were measured: Pt. 1 opposite of cell, Pt. 2 taken on side containing cell. A sharpening in the peaks of the WGMs appears in the spectra at both points. A blue-shift is observed at Pt. 1. TE modes also begin to appear with the cell’s presence [as defined in Fig. 3.1(e)].


60

particular rolled-up resonators discussed from here on, rely on room temperature

photoluminescence (PL). Instead of depending on an outside broadband source of light from a fiber,

the light source is embedded within the resonator itself. When this source is illuminated with a short

wavelength laser, the embedded light source has a broadband fluorescence over the visible

spectrum. The laser source is a narrowband helium-cadmium (HeCd) laser which emits light at 442

nm and the power used is between 0.15 and 3 milliwatts (mW). The embedded light sources are

randomly distributed silicon (Si) nanoclusters which are formed within the SiO layer during

growth.[33]

Whispering gallery mode measurements are performed on a number of samples in air and in

DMEM/F-12, with and without a NIH 3T3 cell captured inside the resonators. Photoluminescence

spectra before and after a cell’s capture for a typical resonator are given in Fig. 3.5. The inset of

Fig. 3.5 shows an optical image of the resonator which is measured, indicating the locations where

the measurements are taken. The cell here lays half outside of the microtube. A number of features

Figure 3.6 | Polarization measurements. Polarization dependent PL measurements performed in air, made for understanding the origin of the secondary peaks in Fig. 3.5. This is a confirmation that the secondary peaks are indeed TE modes. Note the arbitrariness of the angle, only relative angles are of interest. Between 90° – 130° and 270° – 315° the polarizer only allows TM polarization to pass. When the polarizer is rotated from 0° – 45° and 180° – 225°, the TM modes are substantially blocked, revealing the TE polarized modes.

are present in the spectra: 1) At point 1 (Pt. 1, the side containing no cell), there is a blue-shift once

a cell is captured and the modes become narrower. 2) At point 2 (Pt. 2, the side containing the cell),


61

the modes also become narrower, but no shift is present once a cell is captured. The narrowing of

the peaks means that the quality factor (Q-factor) of the resonator is increased. The Q-factor is a

measure of how well light resonates within a structure [defined as Q = λ(at center of mode

peak)/∆λ(at FWHM)].

The dominant modes in the spectra are TM [Fig. 3.1(e)], whereas TE modes do not resonate as

well due to the thinness of the resonator wall.[14,34] This is reconfirmed by polarization

measurements, Fig. 3.6. These measurements[9,14] are performed on the resonators to determine the

origins of the peaks shown in Fig. 3.5. As mentioned, the TE modes are much lower in intensity

than the TM modes. Because of this, it is possible to completely suppress these modes with the

polarizer, seen here in Fig. 3.6 at approx. 270° and 90°. However, the TM modes are much more

intense so the polarizer does not fully suppress these modes. Regardless, this effectively shows that

indeed the stronger peaks observed in Fig. 3.5 are TM, as defined by Bolaños, et al.[14]

A physical picture of the resonator’s sensing mechanism is outlined in Fig. 3.7. Before a cell is

captured inside a microtube, there exists an intrinsic nanogap within the wall of the resonator. The

origins of this nanogap are from an imperfect roll-up process. This imperfect roll-up is a

consequence of creating resonators with a diameter as large as a cell. This nanogap provides a split

in the wall of the resonator. The presence of the cell has two major outcomes: 1) When a cell is

sucked into the resonator, it exerts in outward force on the walls from inside of the microtube. This

force compresses the nanogap, effectively closing it. This more-compact wall results in an increased

Q-factor, a point which will be reaffirmed with a modeling of the sensor in the next section. This is

because less light is scattered out from resonator, in turn, providing better confinement. This is an

Figure 3.7 | Schematic of sensing action. An explanation of the PL spectra from Fig. 3.5. The microresonators have intrinsic nanogaps within their walls. When a cell is sucked into the resonator (a) the windings of the rolled-up bilayer become more compact, compressing the nanogap throughout the length of the microtube [(b) and (c)], increasing the light confinement. (b) At Pt. 1, the tube winds tighter, leading to a blue shift in the spectra because of the smaller diameter.


62

effect observed at all points and in all sensors which are measured. 2) At Pt. 1, the tube becomes

wound tighter. This leads to a decrease in its diameter which causes a blue-shift in the spectrum.

This effect is not found at Pt. 2 because the resonator cannot become smaller at this point because

of the cell, and cannot become larger because of the outside ALD coating around the microtube. It

is for these reasons that these microresonators are given the name “flexible split-wall microtube

resonator sensors.”

3.3.4 FDTD analysis

In order to confirm the validity of the sensing mechanism represented in Figure 3.7, numerical

simulations were performed using the finite-difference time-domain (FDTD) method. This is a

numerical method for solving Maxwell’s equations in the time domain. This approach is commonly

used to compute numerical solutions for EM wave problems. This method works on the relationship

between the electric field (E-field) and magnetic field (H-field) in the sense that any change in the

E-field temporally, depends on the H-field spatially, and vice-versa. Each field component is solved

for in time steps. These time steps are related to spatial steps smaller than the wavelength; the

requirement being that the EM field does not significantly change over this temporal/spatial step.[35]

A number of commercially available FDTD solvers exist. The fitting results presented here were

performed by Dr. S. Kiravittaya based on the values extracted from the experiment which were

provided to him. Only the results will be discussed, rather than the details behind the calculations

performed, as they are pertinent to the verification of the sensing mechanism.

For solving the problem, details of the resonator must be known and taken into account. For the

resonator in question from Fig. 3.5, Davg = 9 µm (measured with an optical microscope) and the

optical properties of the materials, taken from literature,[13,16,36] making up the resonator are as

follows: nSiO = 1.55, nSiO2 = 1.46, and nHfO2 = 1.95. The average index of refraction is taken from

the filling ratios of the materials making up the resonator, taken from Section 3.3.1; navg = y(nSiO) +

x(nSiO2) + [1 – (x + y)](nHfO2) and neff is numerically estimated from the navg and index of refraction

of the surrounding medium, nmedium. This is found to be navg (neff) = 1.83 (1.53). Using these values,

and looking in the spectral range from Fig. 3.5, particularly λo = 659 nm, we find from Eq. 11, M# =

66. This gives the starting point necessary for solving the problem with the FDTD method.

First the microtube diameter and the size of the nanogap are determined in the simulation

starting with the optical mode spacing (≈ 9 nm) of the spectrum. By varying this nanogap width,

and assuming loss within the SiO layer (i.e. κ ≠ 0), the FDTD simulations are able to support the

qualitative description given in Fig. 3.7. A fitting of the mode peaks along with their FWHM is


63

Figure 3.8 | Fitting of data using FDTD. (a) By varying the size of the nanogap it is found that the Q-factor increases the smaller the gap, and the spectra has a blue-shift tendency. (b) and (c), PL spectra (note background subtracted) over a narrow wavelength range are given from experiment and fitted by FDTD simulation (triangle/crossbar).

made for the individual spectrum. The parameters found to lead to the best fit of the experimental

data is assuming the SiO/SiO2 bilayer as a single layer with a complex index of refraction. An inner

tube diameter of 9.03 µm is used, comparable to the 9 µm measured with a microscope. The tube

consists of N = 1.22, a SiO/SiO2 (n = 1.46 + 0.015i) layer 50 nm thick and an additional 75 nm of

HfO2 (n = 1.95) inside and outside of the tube. The DMEM/F-12 is assumed to have the same index

of refraction as water (n = 1.333).[37]

Using the above parameters for the ring resonator, an initial nanogap size of 190 nm is found to

exist in the wall of the sensor. Once the cell is sucked into the microtube, the nanogap is

compressed to 0 nm, Fig. 3.8(a). The peak position of the WGM and their FWHM found by

simulation, are represented within the experimental spectra as the triangles and crossbars,

respectively in Figs. 3.8(b) and (c). The higher compactness of the walls does indeed lead to an

increase in the Q-factor.

In order to present another visual to the simulations which were performed, electric field

profiles for the case of a 190 nm nanogap, Fig. 3.9(a), and a 0 nm nanogap, Fig. 3.9(b), are given.

Here, the azimuthal mode number is 69. The intensity of the profiles indicates the higher light


64

confinement within the resonator with a compressed nanogap. The tube diameter is also slightly

decreased due to a tightening in the windings of the rolled-up layer.

Figure 3.9 | FDTD simulations comparing a tube with and without a nanogap. Profiles of the simulations (performed by Dr. S. Kiravittaya) are presented here. (a) A loosely wound microtube leads to high scattering and low light confinement. (b) When a cell is sucked inside, the nanogap compresses, resulting in less scattering and higher confinement of the light resonating.

Figure 3.10 | SEM of microtube reveals nanogap. The presence of a nanogap within the walls of these sensors is revealed through SEM images (made by Dr. S. Baunack) of a typical sensor which has been cut by a FIB. The top right inset illustrates the geometry of the FIB cut with respect to the microtube. The bottom left inset shows a close up of a nanogap where distinct layers are discernable.


65

To further validate the assumption of a nanogap residing within the wall of these sensors, a FIB

cut was performed on a typical sensor, allowing for the details of its inner wall to be probed. An

image of the cross section of this sensor is made with a SEM in Figure 3.10. The upper right inset

depicts the manner in which the FIB cut was executed, blue line. The nanogap is clearly visible,

marked by the arrows in the image. If a further close-up is taken of the cross section, lower left inset

Fig. 3.10, the individual layers of the structure are exposed. A color coding was made for better

visualization. A gold palladium alloy (AuPd) and carbon layer were sputtered so that the samples

would be more conductive, and would not charge due to the electrons in the SEM. The fact that the

nanogap contains no HfO2 is noteworthy, a point that was taken into account when simulating the

material.

3.3.5 Reproducibility

A sensor like the one described in this chapter, only has a worth in the big picture of detectors,

if it is reliably reproducible. If so, then there would be a place for it in the family of lab-in-a-tube

devices. There would have to be a relatively high yield of sensors which performed the way they

should. So, as an additional investigation, multiple F-SWµRSs were measured in order to

demonstrate this reproducibility. Microresonators were chosen at random, an initial PL spectrum

was taken, a cell was then sucked into the sensor, and a second PL measurement was made. Point 1

was measured in all of these sensors; each revealed a detection of the captured cell, Fig. 3.11. It

Figure 3.11 | Reliably reproducible F-SWµRS. Multiple sensors were used to detect different cells. The background is removed from these spectra. PL taken at Pt. 1 shows the before and after results of the resonate modes in the sensor.

does not matter if the original spectra hardly revealed any modes, the spectra after the cell capture

showed an increased Q-factor. The background has been subtracted from the spectra shown here.

The intensity is in arbitrary units, however the before and after spectra are plotted on the same scale

relative to one another. The blue-shift mentioned earlier is also observed in some of the plots.


66

It was also a question as to whether or not a single F-SWµRS could be used to detect

consecutively captured cells. This exploration was also made and it was found that a single sensor

can indeed be used to detect multiple cells, Fig. 3.12. The plot shows the Q-factor, taken at 704 nm,

of the measured PL spectrum of a particular sensor. Again, first a base PL spectrum was taken (Q ~

50), a cell was then captured and the PL measurement was repeated (Q increased almost one order

of magnitude). After this, the microsyringe was used to suck out the cell. The resonator was again

measured. The Q-factor did in fact decrease because the nanogap reopened, confirming the validity

for using “flexible” in the name of the sensor. However, there was a “memory” leftover in the

signal, meaning that the Q-factor did not return to its base value (Q decreases by 2). A second cell

was then sucked into the resonator and the PL was again repeated (Q increases by 1.5). This shows

that despite the memory, the detector does sense the second cell. This cell was also evacuated with

Figure 3.12 | Consecutive detection of cells using single F-SWµRS. A single detector can be used to capture cells, one after another. When a cell is captured, the nanogap compresses until it is closed. When the cell is evacuated the flexible nanogap reopens slightly. A memory of the previously captured cell is still present. A second cell then compresses the nanogap upon entering. The nanogap returns to its memory position when the cell is evacuated.

the microsyringe. The Q dropped back down to its memory value. This demonstrates the ability of

using an F-SWµRS to detect cells caught one after another. This memory could be considered an

unwanted effect in these sensors. Rather on the contrary, sensor memory would be a valuable tool

for recognizing which sensors had cells inside of them even after an experiment is finished and the

samples have been washed.

3.4 Conclusions and outlook

In conclusion, within this chapter a scheme was presented for using flexible split-wall microtube

resonator sensors as cell detection devices for optofluidic cell-sensing applications. An introduction

to optical ring resonators, which can be designed with rolled-up technology, was given. The new

optical resonator discussed here brings the concept of lab-in-a-tube systems closer to a realizable

rolled-up detection device. A method for precision capturing of cells within these microtube

resonators, known as a microsyringe, was also put forth. This technique was a missing tool for


67

earlier cellular/microtube investigations. Cells of various sizes have been successfully captured

within these structures.

The findings of the investigation of these optical ring resonators show that mechanical

interactions of the cells with the resonator can be detected optically. The sensing mechanism was

due to nanogaps which are present in such a structure. This hypothesis was verified through FDTD

simulations as well as experimentally obtained SEM images of sensors which were cut by FIB. The

F-SWµRS responded to the presence of cells through WGM peak sharpening and shifting. The

reproducibility for such a sensor was investigated as well. It was found that all sensors, arbitrarily

chosen, responded in the same manner. Also it was tested and shown that a single sensor could be

used more than once for detection, thus detecting consecutively captured cells. These results of

reproducibility and consecutive cell detection give promise for the efficient production of these

sensors for lab-in-a-tube systems. It was also found that these resonators have a “memory” of past

cells which can be used for finding out which structures had cellular interactions after the

experiment is finished and the samples have been washed.

Preliminary results have also shown that if a number of PL measurements are taken periodically

after the capture of a cell, the Q-factor also begins to decrease. This can be attributed to the fact that

the cells begin to spread out within the confines of the microtube, a detectable observation with this

rolled-up resonator. This hints at the ability to study a cell which is undergoing apoptosis, because a

cell will go into rigor upon death. This is a phenomenon which should be detectable with this

rolled-up microresonator.

The next steps could include in situ PL investigation during pumping. Further studies of

capturing fibroblast cells should also be undertaken given that nanoscale environmental changes can

have strong influence on a cell’s morthology, proliferation, adhesion and gene regulation.[38]

3.5 References

1. Y. F. Mei and O. G. Schmidt, Volkswagon project proposal, Grant No. I/8072 (2008).

2. E. J. Smith, Y. F. Mei and O. G. Schmidt, Optical components for lab-in-a-tube systems, Proc. SPIE 8031, 80310R (2011), doi:10.1117/12.885246.

3. C. Grashoff, et al., Measuring mechanical tension across vinculin reveals regulation of focal adhesion dynamics, Nature, 466, 263-266 (2010).

4. G. D. M. Jeffries, et al., Using polarization-shaped optical vortex traps for single-cell nanosurgery, Nano Lett., 7, 415-420 (2007).

5. N. A. Bhowmick, E. G. Neilson and H. L. Moses, Stromal fibroblasts in cancer initiation and progression, Nature, 432, 332-337 (2004).


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6. S. E. Cross, Y. S. Jin, J. Rao and J. K. Gimzewski, Nanomechanical analysis of cells from cancer patients, Nature Nanotech., 2, 780-783 (2007).

7. R. J. Pelham and Y. L. Wang, Cell locomotion and focal adhesions are regulated by substrate flexibility, Proc. Natl. Acad. Sci. U. S. A., 94, 13661-13665 (1997).

8. G. C. Gurtner, S. Werner, Y. Barrandon and M. T. Longaker, Wound repair and regeneration, Nature, 453, 314-321 (2008).


10. R. Songmuang, A. Rastelli, S. Mendach, Ch. Deneke and O. G. Schmidt, From rolled-up Si microtubes to SiOx/Si optical ring resonators, Microelec. Eng., 84, 1427-1430 (2007).

11. A. Bernardi, et al., On-chip Si/SiOx microtube refractometer, Appl. Phys. Lett., 93, 094106 (2008).

12. G. S. Huang, et al., Optical properties of rolled-up tubular microcavities from shaped nanomembranes, Appl. Phys. Lett., 94, 141901 (2009).

13. G. S. Huang, et al., Rolled-up optical microcavities with subwavelength wall thicknesses for enhanced liquid sensing applications, ACS Nano, 4, 3123-3130 (2010).

14. V. A. Bolaños Quiñones, et al., Optical resonance tuning and polarization of thin-walled tubular microcavities, Opt. Lett., 34, 2345-2347 (2009).

15. E. J. Smith, et al., Lab-in-a-tube: detection of individual mouse cells for analysis in flexible split-wall microtube resonator sensors, Nano Lett., doi:10.1021/nl1036148 (2010).

16. I. M. White, H. Oveys, X. Fan, T. Smith and J. Zhang, Integrated multiplexed biosensors based on liquid core optical ring resonators and antiresonant reflecting optical waveguides, Appl. Phys. Lett., 89, 191106 (2006).

17. F. Vollmer and S. Arnold, Whispering-gallery-mode biosensing: label-free detection down to single molecules, Nature Meth., 5, 591-596 (2008).

18. H. Zhu, I. M. White, J. D. Suter, P. S. Dale and X. Fan, Analysis of biomolecule with optofluidic ring resonator sensors, Opt. Express, 15, 9139-9146 (2007).

19. F. Vollmer, S. Arnold and D. Keng, Single virus detection from the reactive shift of a whispering-gallery mode, Proc. Natl. Acad. Sci. U. S. A., 105, 20701-20704 (2008).

20. J. Zhu, et al., On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator, Nature Photon., 4, 46-49 (2010).

21. T. J. Kippenberg, Particle sizing by mode splitting, Nature Photon., 4, 9-10 (2010).

22. V. B. Braginsky, M. L. Gorodetsky and V. S. Ilchenko, Quality-factor and nonlinear properties of optical whispering-gallery modes, Phys. Lett. A, 137, 393-397 (1989).


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23. R. D. Richtmyer, Dielectric resonators, J. Appl. Phys., 10, 391-398 (1939).

24. E. J. Smith, S. Kiravittaya, and O. G. Schmidt in collaboration with L. Zimmerman at IHP Berlin who supplied waveguide samples, not published, (2010).

25. D. J. Thurmer, Ch. Deneke, Y. F. Mei and O. G. Schmidt, Process integration of microtubes for fluidic applications, Appl. Phys. Lett., 89, 223507 (2006).

26. K. Dietrich, et al., Optical modes excited by evanescent-wave-coupled PbS nanocrystals in semiconductor microtube bottle resonators, Nano Lett., 10, 627-631 (2010).

27. G. S. Huang, Y. F. Mei, D. J. Thurmer, E. Coric and O. G. Schmidt, Rolled-up transparent microtubes as two-dimensionally confined culture scaffolds of individual yeast cells, Lab Chip, 9, 263-268 (2009).

28. S. Schulze, et al., Morphological differentiation of neurons on microtopographic substrates fabricated by rolled-up nanotechnology, Adv. Eng. Mater., 12, B558-B564 (2010).

29. M. Yu, et al., Semiconductor nanomembrane tubes: three-dimensional confinement for controlled neurite outgrowth, ACS Nano, 5, 2447-2457 (2011).

30. D. E. Discher, P. Janmey and Y. L. Wang, Tissue cells feel and respond to the stiffness of their substrate, Science, 310, 1139-1143 (2005).

31. H. M. Langevin, et al., Fibroblast spreading induced by connective tissue stretch involves intracellular redistribution of α- and β-actin, Histochem. Cell Biol., 125, 487-495 (2006).

32. F. Vollmer, Optical microresonators: label-free detection down to single viral pathogens, SPIE Newsroom, doi: 10.1117/2.1201002.002619 (2010).

33. L. X. Yi, J. Heitmann, R. Scholz and M. Zacharias, Si rings, Si cluster, and Si nanocrystals-different states of ultrathin SiOx layers, Appl. Phys. Lett., 81, 4248-4250 (2002).

34. M. Hosoda and T. Shigaki, Degeneracy breaking of optical resonance modes in rolled-up spiral microtubes, Appl. Phys. Lett., 90, 0181107 (2007).

35. K. S. Yee, Numerical solution of initial boundary value problems involving maxwell’s equations in isotropic media, IEEE Trans. Ant. Prop., 14, 302-307 (1966).

36. E. D. Palik, Ed. Handbook of Optical Constants of Solids, pg. 676, 770 and 799-800, [Academic, San Diego, CA, 1998].

37. D. Borja, et al., Optical power of the isolated human crystalline lens, Invest. Opthalmol. Visual Sci., 49, 2541-2548 (2008).

38. D. A. Fletcher and R. Dyche, Cell mechanics and the cytoskeleton, Nature, 463, 485-492 (2010).

Chapter 4 Rolled-Up Polymers by Delamination

70

Chapter 4: Rolled-Up Polymers by Delamination

4.1 Introduction

This chapter will focus on a new approach for creating and developing self-assembled

microstructures. It will be shown that the structures which can be developed by employing this

fabrication method are unique to this technique, and are otherwise unobtainable.[1,2] Given the

infancy of this process, the scope of devices and sensors which have been developed and

investigated up until this point, is not huge. However, the outlook for the creation of new and

attractive devices is promising. This method uses a polymer similar to the photoresist technique

mentioned earlier, used for the experiments of the previous chapters. Patterns are defined in the

same way using photolithography; the different implemented patterns will be discussed shortly.

Any type of material can then be deposited on top of this photoresist polymer layer; even a single

layer of Au, which does not roll with the standard technique, can be used. The difference with this

method compared to the photoresist approach, is that the polymer applied here is not dissolvable.

Instead, upon entering an organic solvent, NMP, the layer delaminates[3] from the substrate. The

nanomembrane deposited on top, leads to a large strain gradient in the layer, and with the

delamination the layer is forced to curl up. Other solvents (including acetone, isopropanol and

methanol) have also been investigated, but the delamination was observed only in NMP. This is

because the polymer absorbs the NMP, causing it to swell, increasing the strain in the layer. A few

structures and devices which have been developed up until now will be discussed in the next

sections of this chapter. These include novel magnetic helical coils and thick Swiss roll structures,

as well as structures revealing chemically sensitive mechanics which may be used as chemical

detectors and actuators.

4.2 Realizable geometrical structures

The general fabrication of these structures was already laid out in Section 1.4 and will not be

repeated in this chapter; the general description being given in Fig. 1.6. Here a number of different

geometrical structures, which are obtainable using the delamination technique, are shown. The

polymer which has the desired characteristics of delaminating but not dissolving in the organic

solvent is known as SU8.[4] The SU8 patterns are made in much the same way as the ridge


71

waveguides were made in Chapter 2 except the ashing step is left out. It is the ashing step which

keeps the SU8 from delaminating. The structures formed from this self-assembled technique are

outlined in Fig. 4.1, and include helix coils [Fig. 4.1(a)], helix spirals [Fig. 4.1(b)], double Swiss

rolled spirals [Fig. 4.1(c)], and Swiss rolled spirals [Fig. 4.1(d)]. The fabrication of helix coils are

possible if long narrow striped patterns of SU8 are defined on the substrate. In Figure 4.1(a), the 2D

pattern from which this helix coil was formed was approximately 5 µm × 5 µm × 1 mm. During

delamination these patterns roll-up in a corkscrew shape. This action is referred to as coil-up. For

coiled-up geometries, an SU8 layer thickness between 2 µm – 6 µm was shown to work well. The

Figure 4.1 | Realizable structures from rolled-up polymer method. (a) Optical images taken of helix coil structures. (b) Dark field image of a helix spiral formed from off-axis rolling of rectangular pattern. (c) Bright field image of a double Swiss rolled structure, formed from a rectangular pattern. (c) Bright field image of a Swiss roll spiral geometry formed from a square pattern. (e) SEM image (made by Dr. S. Baunack) of a Spiral cut by FIB, revealing the 700 nm thickness of the layer. Inset, a close up of the cross section showing the compact walls as well as the large separation of the nanomembranes due to the polymer thickness. (f) A similar structure as in (e) but with a thicker 900 nm polymer thickness.


72

chirality (left-handed or right-handed) of the coils is found to be determined by fluctuations during

the delamination process, rather than intrinsic strain,[5] resulting in a equal number of each. From 3

samples, 49 coils were investigated and of those it was found that 24 exhibited right-handed

chirality and 25 exhibited left-handed.

The other geometries [Figs. 4.1(b)-(f)] can be formed from roll-up, rather than coil-up, using an

SU8 thickness between 500 nm and 1 µm. Thicker layers did not roll with the size of patterns

investigated and led to cracking in the nanomembrane layer which was deposited onto the SU8.

This roll-up is achievable if the SU8 pattern is rectangular or square. The shape which emerges is

dependent on which axis the roll-up occurs. Which edge rolls-up is highly dependent on the

topography of the polymer layer.[6] For rectangular patterns: 1) If the roll-up starts on the long edge,

then long single [if roll-up happens from one side] or double [if roll-up happens from both sides,

Fig. 4.1(c)] Swiss rolls are formed. 2) If the roll-up starts from the short edge, then either many-

winding Swiss rolls are created, or more often, an off-axis rolling takes place, resulting in helix

spirals, Fig. 4.1(b). For square patterns: 1) If the roll-up starts from a corner (i.e. off-axis), the

resulting shape is much like a croissant which is rolled-up straight (not shown). 2) However, most

square patterns result in a short Swiss roll spiral, Fig. 4.1(d). Whether or not a structure will form a

rolled spiral or coiled structure is dependent on the relationship between the structure’s width,

length and thickness before delamination.[7]

The SU8 thickness is the major degree of freedom in this method. SU8 comes in a variety of

viscosities, each resulting in a range of thicknesses depending on the substrate spin-on speed. The

two types of SU8 which were used for this investigation are SU8-10, whose thickness is variable

from 4 – 11 µm (green squares in Fig. 4.2), and SU8-2, whose thickness is variable between 8 µm

down to 500 nm (red circles in Fig. 4.2). The lines in Fig. 4.2 are for illustrative purposes and do

not represent a fit to the data. These samples were measured using a Dektak profilometer which can

Figure 4.2 | SU8 polymer thicknesses. The thickness of the SU8 polymer layer can be tuned by either using a different viscosity of the photoresists, SU8-10 or SU8-2, or by adjusting the spin-on speed.


73

give a 2D cross section of a patterned sample. Figure 4.1(e) shows an image taken with SEM of the

cross section of a Swiss roll cut by FIB, which has a 700 nm thick polymer layer. Figure 4.1(f)

shows a similar structure, but the polymer layer is 900 nm thick. This highly tunable barrier

thickness (0.5 – 1 µm) is another unique aspect of this microfabrication approach.

These structures allow for the creation of novel hybrid structures at the microscale which have

not been realized before. One of these in particular is in the realm of magnetics which will be

discussed in further detail in the next section.

4.3 Magnetized micro-helix coil structures

4.3.1 Introduction

Nature has come up with many fascinating magnetic structures. Magnetic structures are defined

by the magnetic spin orientations in a material. There are a number of typical orderings in magnetic

materials including ferromagnetic, where the individual spins all align themselves in the same

direction, and antiferromagnetic, where neighboring spins align themselves antiparallel to one

another.[8] However, there are more complex structures that exist, which include numerous helical

or toroidal magnetic configurations.[9,10] These complex magnetic orientations lead to interesting

electro-magnetic properties and new effects which can be observed.[11,12] Therefore, these complex

structures are of great interest to better understand solid state magnetism. One example is

helimagnetic materials[13] where the topological Hall effect was observed.[14,15] Essentially a

helimagnetic material is given its name due to a tilt in the angle of discrete adjacent spins, which

can form skyrmions and helical spin structures.[16] These have been explored theoretically.[17,18]

However, given the unlikely properties a material must exhibit, only a few experimental works

have been reported.[19,20] Another difficulty is that these spin behaviors exist only on the atomic

level.

There are other methods for obtaining complex magnetic configurations, rather than finding a

material existing in nature. That is by creating microscale structures which give a continuous

distribution of the magnetic moment. One example where this has been done was for creating a

magnetic vortex from a thin ferromagnetic layer which was shaped in the form of a disk.[21,22]

Going along this mentality, we concentrated on developing microscale architectures exhibiting

similar magnetic configurations to those existing on the atomic scale in nature, mainly

helimagnetic-like magnetization configurations. The focus led to a single structure which could

posses as many as three distinct and unique magnetic configurations.[1]


74

Knowing a film’s magnetic characteristics is a crucial basis for the structures which will be

investigated in the following sections. Therefore, it is important to give a short introduction into the

basics of magnetism which describes the physics of the magnetic moment orientation within a

ferromagnetic film in the presence of an applied magnetic field.[23] Without an applied magnetic

field, the magnetic moment is aligned along the so-called easy axis of the magnet which is mainly

determined by the symmetry of the crystal structure and the film dimensions. If a strong enough

external magnetic field is applied, the magnetic moment of the film will align itself to this field.

After the field is removed the alignment of the magnetic moment will relax back to the direction

determined by the easy axis. The magnetization of a film is introduced as the magnetic moment per

unit volume.[23]

In these materials, the relationship between the magnetic moment (M) and the magnetic field

strength (H) is in the form of a hysteresis loop, Fig. 4.3. A hysteresis loop is measured by applying

a large magnetic field in one direction then lowering it to zero and raising it to a large value in the

opposite direction, all while measuring the magnetic moment. In such a measurement, there is a

point where the relationship plateaus, where an increase in the magnetic field no longer has an

effect on the magnetic moment. This is called magnetic saturation (Ms) and occurs when all the

individual magnetic dipoles are aligned in the same direction. If the field is applied in the opposite

direction with a large enough strength, the moment will flip orientation within the film. The field

required to bring the magnetic moment to zero is known as the coercivity of the film (Hc).

Assuming a sharp hysteresis loop, if an applied magnetic field is H < Hc, then the magnetic moment

of the film is largely unaffected. However, if H > Hc, then the orientation of the magnetic moment

of the film will be flipped, aligning itself with the external field.

The easy axis of a magnetic film is defined as the coordinate where the relationship between H

and M results in a hysteresis loop [meaning there is a preferred orientation direction of the

magnetization (i.e. up or down)] rather than a linear relation [where there is no preferred magnetic

Figure 4.3 | Magnetic hysteresis loop. In a ferromagnetic film, the relationship between the orientation of the magnetic dipoles within the film, to that of an externally applied magnetic field, results in a hysteresis loop. When the external field is strong enough, the film will magnetically saturate and all the dipoles will point in the direction of the external field. For applied magnetic fields lower than the coercivity of the film (Hc), the orientation of the dipoles will be largely unaffected. The magnetic field required to bring the magnetic moment to zero is Hc.


75

orientation direction]. The two types of magnetic films which will be utilized for these structures

consist of: 1) Films with an in-plane easy axis, meaning the magnetization of the films lies within

the plane of the film, Fig. 4.4(a); 2) Films with an out-of-plane easy axis, meaning the

magnetization points either into or out of the flat film, Fig. 4.4(b).

Figure 4.4 | Visualization of magnetic moment for in-plane and out-of-plane easy axis. (a) In-plane easy axis means the magnet moment points in-plane to the film. Bottom, the relationship between the applied magnetic field to the magnetic moment is a hysteresis loop for a magnetic field applied in-plane, and linear for a magnetic field applied out-of-plane. (b) Out-of-plane easy axis means the magnetic moment points out-of-plane to the film. Bottom, the resulting relationship between the magnetic moment and applied magnetic field is the opposite as for the in-plane film.

4.3.2 Orientations of magnetization in coiled structures

It will be shown here that three different magnetic configurations were created using the coil-up

method.[1] These magnetic structures include radial-, corkscrew-, and hollow-bar-magnetized

micro-helix coils. These structures are helimagnetic-like in configuration,[9,10] but on a micron scale

rather than the atomic scale ones found in nature. The way these are created is by depositing an

“active” nanomembrane film on top of the previously mentioned strips of SU8. These active films

have either an in-plane magnetic easy axis, or an out-of-plane magnetic easy axis. The out-of-plane

magnetic easy axis film was created from the deposition of a multilayer stack of Cobalt (Co) and

Platinum (Pt) [Co (0.4 nm)/ Pt (0.6 nm)]5. This structure was used to create a radial-magnetized

coil structure. The film with an in-plane magnetic easy axis was a 20 nm thick Co layer. This in-

plane magnetic layer was used to create the corkscrew and hollow-bar-magnet configurations

described later.

In order to be certain that indeed the active layer had the easy axis which was expected from the

deposition, their magnetic properties were investigated by Dr. D. Makarov using vibrating sample

magnetometry (VSM). Magnetic hysteresis loops were taken for the Co layer (in-plane easy axis),


76

Fig. 4.5(b), and the Co/Pt multilayer stack (out-of-plane easy axis), Fig. 4.5(a). The measurements

were performed on samples which were deposited onto a planar Si substrate covered by

unpatterned SU8. Both film systems with in-plane and out-of-plane easy axis were investigated,

revealing a well-defined in-plane magnetic anisotropy for the Co layer and a well-defined out-of-

plane magnetic anisotropy for the Co/Pt multilayer stack. The coercive field (Hc) was measured to

be ≈ 22 mT for the out-of-plane, and ≈ 7 mT for the in-plane. Samples which were non-magnetic,

made from a multilayer stack of copper (Cu) and Pt [Cu (0.4 nm)/Pt (0.6)], were also prepared in

order to act as a reference/control. These layers were all prepared using sputter deposition. The

depositions were carried out at room temperature in a vacuum chamber with a base pressure of 1 ×

10-7 mbar. Sputtering was made with an Ar background pressure of 5 × 10-3 mbar at a rate of 0.5

Å/s. In order to improve the growth conditions for the active layers, a 2 nm Pt buffer layer was

sputtered prior to the active layer. An additional 2 nm Pt layer was then sputtered as a cap to protect

the active layer from oxidization.

Figure 4.5 | Vibrating sample magnetometry measurements. (a) VSM measurements taken with an in-plane and out-of-plane geometry of the applied magnetic field for the Co/Pt active layer, revealing a well-defined out-of-plane easy axis of magnetization. (b) The same measurements were made for the Co active layer which showed an in-plane easy axis of magnetization. Both films were studied on top a Si wafer covered by SU8.

The samples were magnetically saturated in the desired direction directly before coil-up, in

order to assure the initial magnetic moment configuration. Going back to the SU8 strip geometry, if

the magnetic moment is in-plane, and the sample is saturated perpendicular to the orientation of the

SU8 strips, then a coil-up of the structure will result in a geometry which has a hollow-bar-

magnetic orientation, Fig. 4.6(a). If alternatively, the sample is saturated parallel to the strips, then

coil-up will result in a magnetic moment in the shape of a corkscrew, Fig. 4.6(b). If instead the

Co/Pt multilayer stack, which has a magnetic moment pointing out-of-plane is considered, then a


77

coil-up of the strip will result in a radial-magnetization, Fig. 4.6(c). This magnetic moment

configuration is unobtainable by any other fabrication method. An optical image of a radial-

magnetized coil is depicted in Figure 4.6(dI), accompanied by an artist illustration of the structure,

Fig. 4.6(dII). The structures created from a 5 µm × 5 µm × 1mm SU8 strip typically result in a coil

whose dimensions are a 35 µm radius and 50 µm length. The simulations were performed by Dr. D.

Makarov using a commercially available FEM micromagnetic program MAGPAR.[24] A quick

discussion of these results will be given here because of their importance in allowing a full story to

be formed around these structures. The sizes of the coils in reality are quite large to be considered

directly using micromagnetics. However, the results motivate the intuitive magnetization

orientations which are formed upon coil-up. In order to model the coils so that information can be

obtained from the model, the simulations were performed using a nanomembrane that

experimentally has the same thickness, 5 nm. However, the width (15 nm) and length (1.9 µm)

needed to be a few orders of magnitude smaller than in experiment, so that the simulation area

would be small enough to be solvable. This is because, as mentioned earlier in Chapter 2, 3D

Figure 4.6 | Obtainable magnetic orientations of a micro-helix coil. Micromagnetic simulations carried out on planar and coiled-up magnetic strips. (a) Initial magnetization of the strip is in-plane, perpendicular-to-strip, left; hollow-bar-magnetized helix coils are created after coil-up, right. (b) Initial magnetization, parallel-to-strip, left; Corkscrew-magnetization forms after coil-up, right. (c) Initially out-of-plane magnetized strip, left; Radial-magnetized coils are created upon coil-up, right. (d) An optical image (I) and an artist rendition (II) of a hybrid helix coil composed of an SU8 outer organic layer and an inorganic magnetic “active” inner nanomembrane.


78

simulations are very computer intensive. Nevertheless, the presented simulations can be used as

illustrative visualizations.

The mesh for the simulated area was divided into tetragonal cells with a side length of 10 nm.

For the Co/Pt multilayers, resulting in an out-of-plane easy axis of magnetization, the following

values were assumed for simulation: A saturation magnetization Ms = 0.6 T; an anisotropy constant

KU = 3 × 105 J/m3, as determined from experiment.[25] For the Co layer which results in an in-plane

easy axis of magnetization, the following values were used: The saturation magnetization Ms = 1.7

T; and the anisotropy constant of KU = 4.5 × 105 J/m3. These values are for typical bulk Co.[26] The

exchange parameter (A) for both simulations was set constant at A = 1 × 10-11 J/m, which is a

typical value for a film in which exchange coupling is high.[27] The exchange parameter defines

how much coupling occurs between adjacent magnetic dipoles.

4.3.2.1 Experimental probing of magnetic configurations

Experimentally verifying the magnetic configurations in Fig. 4.6 is not a trivial task. The most

straightforward method of probing these different magnetic configurations is to follow the

dynamics of the coils in response to an external alternating magnetic field. In order to have the

magnetic orientations which were predicted in the right side of Figs. 4.6(a)-(c) it would mean that

the magnetic dipoles were unaffected by the coil-up, i.e. the magnetic dipole direction in which the

films were saturated while in the planar geometry, left side of Figs. 4.6(a)-(c). Indeed, in using this

alternating external magnetic field, the magnetic states are revealed through three distinct types of

dynamic motion, each unique to the particular magnetic orientation of the coil. The experimental

results of this probing are explained in this section. Taking into account the parametric magnetic

excitation from the external field, in the Section 4.3.2.2 a theoretical framework is also developed

which explains the non-linear behavior which is observed. It is important to point out that the

alternating applied magnetic field has no influence on the Cu/Pt reference samples mentioned

earlier.

The experimental setup consists of a Petri dish on top of an XYZ micromanipulator, seen as the

black object under the sample in the right side of Figure 4.7. This Petri dish is observed under a

microscope which is equipped with a high-speed camera. The external magnet consists of a bar

magnet which is 1 cm in radius and 12 cm in length. This magnet is clamped to a step motor which

is used for a controlled manipulation over the rotation (that is, end over end rotation, not planar

rotation) of the magnet. This motor is controlled by a computer program in which the desired

rotational speed can be entered. All of the dynamics which will be presented here were performed

in the delaminating medium, NMP. The dynamics of the coils were observed on glass and on the


79

magnetic substrates in which they coiled-up from. The rotational data presented later was extracted

from images recorded by a high speed camera, at 10 frames per second, under a 50× objective lens.

Figure 4.7 | Fit of the applied external magnetic field as a function of time. The magnetic field vs. time which was measured using a Hall-probe, red dots. A number of functions were investigated for fitting. The best fit is a result of an average of the

first five cosine functions, ∑=

+ +4

0

12 )(cos5 n

magn t ϕωρ , blue line. The right side shows the

alignment orientation in the experimental setup at two different positions.

The spacing of the bar magnet from the substrate is approximately 3 cm, which gives a

maximum field strength of ±10 mT. Due to this spacing, the magnetic field which is exerted on the

coils is rather complex and leads to a parametric excitation. This excitation is what determines the

specifics of the coil dynamics. Figure 4.7 shows the time evolution of the external magnetic field

measured with a Hall-probe. The red dots are the experimentally obtained measurements of the

field, whereas the lines are fits using different functions. The best fit is found to be an average of

the first five odd powers of a cosine function, ∑=

+ +4

0

12 )(cos5 n

magn t ϕωρ

, which is shown as the blue

line in Fig. 4.7. In this equation, ρ is an arbitrary constant for the fit, φ is the initial phase of the

rotating magnetic field and ωmag is the angular frequency of the rotating magnet. The source of

force acting on the coil can be attributed to the magnetic torque which originates from the bar

magnet. The magnetic field gradient has, if any, only a minor influence over the dynamics.

Supporting evidence of this is that the field gradient is insufficient for moving light iron oxide

(Fe3O4) microparticles which are highly susceptible to field gradients.[28] This was experimentally

confirmed.


80

As mentioned before, all three differently magnetized coils have unique dynamics associated

with them. The radial-magnetized coils are found to be directionally-deterministic (meaning that

they travel in a straight line), Fig. 4.8(a). The corkscrew-magnetized structures respond to the

applied torque with a forward dancing motion, Fig. 4.8(b). The hollow-bar-magnetized coils

perform end-over-end forward tumbling, which can be attributed to the magnetic moment being

similar to that of a bar magnet, Fig. 4.8(c). Videos of these different trajectories can be found in

Ref. [1]. The coil-up process is found to be independent of the active nanomembrane layer which is

deposited on top of the SU8. However, because of the less complicated dynamics associated with

the radial-magnetized coils, compared to the other two types of coil dynamics, these are focused on

for a more in-depth analysis.

Figure 4.8 | Coil tracking of trajectory vs. film magnetization orientation. (a) Radial-magnetized structures respond with straight and deterministic forward motion of the coils. (b) Corkscrew-magnetized coils respond to the external field through a forward dancing motion. (c) Coils which exhibit hollow-bar-magnetization respond in an end-over-end tumbling motion.

Given the symmetry of a radial-magnetized cylinder, it is expected to be fully magnetically

compensated. However, a structure with a coiled geometry has an uncompensated magnetic

moment because of the windings on either end of the coil, Fig. 4.9. The angle at which these coil

ends lie from one another is given by uncompcoilΨ . Figure 4.9 illustrates the case for the most

compensated case (uncompcoilΨ = 0°) to that of the least compensated case (uncomp

coilΨ = 180°) given the

offset of the coil ends to one another. This uncompensated moment tries to align with the


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Figure 4.9 | A visualization of the uncompensated magnetic moment arising in radial-magnetized coils. Given the coil structure, the magnetic moment isn’t fully compensated given the ends of the coil. This effect is outlined here, showing the various degrees of uncompensation, with respect to the angle between the two ends, which can arise. Given experimental imperfections, even if

uncompcoilΨ = 0, there would be uncompensated

magnetic moment.

Figure 4.10 | Response of radial-magnetized coils to an external alternating magnetic field. (a) and (c) Optical microscopy images showing the rotation of a coil over approximately 120° when the external bar magnet is rotated the same number of degrees. A tracking of either end of the coil can be made, red dotted line. The axis of the coil is orientated perpendicular to the field lines of the permanent magnet. (b) and (d) The corresponding schematic revealing the coil’s rotation angle, Ψcoil, after a rotation of the bar magnet over the angle, θmag. The uncompensated magnetic moment is highlighted in yellow. The inset in (b) shows the geometry which makes up the uncompensated magnetic moment.


82

orientation of the external magnetic field. Because the coercive field of the out-of-plane magnetized

structures (Hc ≈ 22 mT, Fig. 4.5) is quite a bit higher than the maximum externally applied

magnetic field (≤ 15 mT, Fig. 4.7), a switching of the magnetic moment orientation in the

multilayer stack is unlikely to transpire. It is due to this, that an alignment of the uncompensated

magnetic moment can only come about if the coil undergoes a physical rotation around z , Fig.

4.10(d). A tracking of the coil’s ends is possible and can be studied in comparison to the rotation of

the external magnet, Fig. 4.10. In other words, the angular position of the coil (Ψcoil) for each frame

of data taken can be compared to ωmag.

Given that these coils are untethered to the substrate on which they coil-up from, the substrate

over which the dynamics are investigated, can be interchanged. The rotative motion of these coils

can either result in stationary rotation or forward translational motion. This is controlled by the

friction between the coil and the substrate. The scope of this work focuses on coils which were

studied on a glass substrate. If a magnetic substrate is used, there is an additional attractive force of

the coil to the substrate. This force can be more influential than the external applied field. How

influential it is, depends on the thickness of the coils. For coils consisting of an SU8 layer thickness

of less than 4 – 5 µm, this force can be strong enough to fully prevent rotation. However, on glass,

the friction is considerably lower, and can be low enough that a coil’s dynamics is stationary

rotation. The controlled motion which is exhibited by these coils on different substrates is crucial to

achieve directed propulsion,[29,30] which is important to applications including fuel-free microcargo

Figure 4.11 | Experimentally obtained dynamics of a coil vs. the rotation of an external magnetic field. (a) Time evolution of the coil’s rotation angle, Ψcoil, for various rotational speeds of the bar magnet, ωmag. The slope, τ, of each curve reveals the average rotation speed of the coil for the corresponding ωmag. (b) A 3 s close-up view of two full rotations of the coil for values of

ωmag < transitionmagω shows that the rotation is rotative but not linear. (c) A close-up view for ωmag >

transitionmagω reveals an oscillatory-like-rotative (OLR) motion that the coil undergoes.


83

delivery. This can be compared to some other microtransporters requiring a fuel to operate, like

hydrogen peroxide,[31] which is toxic for many applications.

The external magnet was rotated at various angular velocities and the resulting angular position

of the coil was recorded and analyzed, Fig. 4.11. The slopes of the data (τ) were used to derive an

overall average rotational angular velocity of the coil. However, due to the nonlinear response of

the coils, τ does not encompass the real full angular velocity of a coil. Rather, as ωmag is increased, τ

begins to lag behind and eventually drops off above a critical frequency region, transitionmagω . This

overall behavior is characteristic of all coils which were investigated and found to be dependent on

a coil’s dimensions. Given the particular coil analyzed in Fig. 4.11, a critical frequency region was

found to be 250° s-1 < transitionmagω < 300° s-1. This is described as the region in which the dynamics of

the coil transition from rotational [with small oscillations superimposed on top, Fig. 4.11(b)] to

oscillatory-like-rotative (OLR) motion, Fig. 4.11(c). If a 3 s time lapse is taken for ωmag = 150° s-1

and 200° s-1 [Fig. 4.11(b)] it is shown that Ψcoil vs. time (t) has an average positive slope. However,

after the transition region, ωmag > transitionmagω , OLR motion is observed, characterized by large

oscillations superimposed over the rotative motion, Fig. 4.11(c).

4.3.2.2 Theoretical investigation of dynamics

In order to have a full understanding of the coil characteristics which were presented in Section

4.3.2.1, a theoretical model describing the dynamics has been laid out by Prof. V. M. Fomin. The

fitting parameters and particulars of the system were provided to Prof. Fomin in order to simulate

the coil dynamics. The dynamics of a coiled structure driven by a parametric magnetic excitation,

Fig. 4.7, and immersed in a liquid can be modeled by the equation of motion:

),(2

2

tTdt

dI coil

coil Ψ=Ψ, (13)

where I = m( 2extR – 2

intR )/2 is the moment of inertia for a coil with a mass of m having an internal and

external radii of Rext and Rint, respectively. The total torque[32] is given by,

T(Ψcoil,t) = TM(Ψcoil,t) + TH, (14)

and consists of a magnetic torque, TM(Ψcoil,t) and a hydrodynamic, or damping, torque (TH). The

hydrodynamic torque is determined by the drag coefficient (γ) and the rotational velocity of the

coil:


84

dt

dT coil

H

Ψ−= γ , (15)

where γ = χVηvξ. Determined by the shape factor of the coil (χ),[33] the total volume of the coil V = π

2extR Cl (where Cl is the length of the coil), and the viscosity of the medium, (ηv). The scaling factor

(ξ) accounts for a superlinear behavior of the damping torque which is a function of the rotational

velocity.[34] The magnetic torque is defined by the magnetic moment of the coil, M (Ψcoil), and the

applied magnetic field, B(t):

TM(Ψcoil,t) = [M (Ψcoil), B(t)]z = Mx(Ψcoil)By(t) – My(Ψcoil)Bx(t). (16)

The Cartesian components of the coil’s uncompensated magnetic moment are Mx = M cos(Ψcoil)

and My = M sin(Ψcoil), Fig. 4.10. The magnitude of the uncompensated magnetic moment (M) for a

magnetic strip with a width (W) and thickness (T) which is on a cylindrical surface with a radius R

= (Rext + Rint)/2 and a central angle α is M = 2MsRTW sin(α) [inset of Fig. 4.10(b)].

The magnitude of the applied field, found from data given in Fig. 4.7, has a form factor

∑=

+=4

0

2 )(cos5

)(n

magn ttFF ϕωρ

. Using this, a mathematical description of the Cartesian coordinates

of the applied magnetic field can be expressed: Bx = BoFF(t) sin(ωmagt + φ) and By = BoFF(t)

cos(ωmagt + φ). Taking these into account, the magnetic torque from Eq. 16 can be fully expressed:

TM(t) = MBoFF(t)sin(ωmagt + φ – Ψcoil). (17)

If Eq. 15 and Eq. 17 are inserted into Eq. 13, the full equation of motion can be laid out:

dt

dttFFMB

dt

dI coil

coilmagocoil Ψ−Ψ++=Ψ γϕω )sin()(2

2

. (18)

The presence of Ψcoil in the sinusoidal function of Eq. 18, implies nonlinear dynamics of the coil,

whereas the time-dependent form factor FF(t) represents a parametric excitation of the coil. It

should be noted that equations of motion with and without inertia were considered. The estimated

Reynold’s number of this system was found to be approximately 7 × 10-5. The motion of the coil

can be described neglecting inertia because of working at low Reynolds number, which is in

agreement with the classical work by Purcell.[35] Even though this non-inertia model allows for a

simulation of the coil’s dynamics, it is an approximation. The physical picture which is based on a

balance of the torques acting on the model, namely when T(Ψcoil,t) ≠ 0, is not accessible in this

simplified model. It is for this reason that the model represented by Eq. 18, which takes inertia into


85

Figure 4.12 | Theoretical modeling of coil dynamics. Theoretical modeling of the time evolution of a coil’s rotation angle. (a) The corresponding data to that of the experimental values shown in

Fig. 4.11(a). (b) The corresponding close up values for ωmag < transitionmagω compared to the

experimental values in Fig. 4.11(b). (c) The corresponding close up values for ωmag > transitionmagω

compared to the experimental values in Fig. 4.11(c).

account, is used for understanding the dynamics of these coils. With this model, the dynamics

which were experimentally obtained for Fig. 4.11, can be recreated theoretically in Fig. 4.12. The

values used to acquire the plots shown in Fig. 4.12 are as follows: m = 5.02 × 10-11 kg, Rint = 30

µm, Rext = 37 µm, Cl = 38.5 µm, η = 1.002 × 10-3 Pa·s, the superlinear factor, ξ, was 0.625 for ωmag

= 250° s-1. This was increased from 0.625 to 1.0 over the transition region, transitionmagω , and was 1.0

for ωmag = 300° s-1. The Ms = 0.6 T, T = 5.0 nm, W = 7.0 µm, sin(α) = 0.0226 and φ = π/2.

Using this model also allows for an investigation of the transition region, Fig. 4.13, which is

difficult to observe experimentally. When ωmag begins to slightly exceed 250° s-1, the motion

Figure 4.13 | Transition from rotative to OLR motion. The detailed dynamics a coil undergoes when transitioning from rotative to OLR motion is shown for the transition frequency region (250°

s-1 < transitionmagω < 300° s-1).


86

consists mainly of periods of rotation (with weak oscillations). These periods of rotation are

interrupted by hiccups, observed as back-and-forward motions of the coil. These are seen as dips in

the upward slop of Fig. 4.13. These hiccups occur at a higher and higher rate (i.e. the time between

hiccups, ς, decreases) as ωmag is increased, until the motion consists only of hiccups. This is the

point where the coil has fully entered the OLR regime.

In order to investigate these hiccups, an observation of the competing torques within the model was

made. It is shown in Fig. 4.14 that the magnetic and damping torques compensate themselves to a

high degree. Here, the magnetic and damping torques are multiplied by a factor of 100, whereas the

total torque must be multiplied by a factor of 8,000 in order to be resolved. This shows that the

classical approximation of assuming an inertia equal to zero is, to a high extent, accurate,[32,35] i.e.

T(Ψcoil,t) = 0. However, this compensation is not whole. There is a remaining uncompensated

torque, although very small, which lends to the nuances of the coil dynamics, namely these hiccups.

Figure 4.14 | Nuances of competing torques. (a) ωmag = 250° s-1; (b) ωmag = 252° s-1; (c) ωmag = 260° s-1; (d) ωmag = 300° s-1. The arrows show the points at which a hiccup occurs. The vertical bar shows a point in which the hiccups are interrupted by a slightly pure rotation once the coil is almost fully within the OLR region. Please note the different scaling factors of the torques.

Figure 4.14(a) reveals that the rotation, which is superimposed by weak oscillations, originates

from the periodic sequence of positive and negative pulses of the total torque. The hiccups occur

when there is a pair of negative pulses of this total torque which act on the coil consecutively,

arrows in Figs. 4.14(b) and (c). After the coil is in the transition from rotative to the OLR range,


87

Figure 4.15 | Close up of competing torques. (a) Trajectory of a coil responding to an external magnetic field with a rotational frequency of 252° s-1. (b) The magnetic TM (blue line) and damping TH (aqua line) torques are shown for this region. The resulting total torque due to an uncompensation of TM and TH (magenta curve) is shown at the same scaling factor and appears to be very close to zero. (c) A close up of T(Ψcoil,t) shows that the total torque is in fact non zero. The hiccups, marked by the red arrows, in the dynamics can be traced directly back to this uncompensated total torque due to inertia in the system.

here 300° s-1, pairs of positive and negative pulses of the total torque act in sequence, producing the

OLR motion. Only rarely are these interrupted by a compensation of a positive and negative pulse,

bar in Fig. 4.14(d), which causes a slightly pure rotation. Above this frequency, the coil is

completely within the OLR region. Figure 4.15 reemphasizes the fact that indeed the total torque is

small in comparison to the magnetic and damping torques, however it is not zero. This close-up

shows the torques at a hiccup region in the dynamics. The nonlinear dynamics of this system,

which is excited by a parametric external magnetic field, is shown by plotting the phase trajectories

in the phase plane.[36] Plotting the phase trajectories in the phase plane is done by plotting the

angular velocity, dt

d coilΨ, of the coils versus their angular position, Ψcoil, shown in Fig. 4.16. The

region which is considered as the rotative region (superimposed by weak oscillations) is

characterized by an oscillating phase portrait, black curve in Figs. 4.16(a)-(c). Every hiccup, or

back-and-forth motion of the coil, is seen as a loop in the phase space. Increasing the rotation

frequency of the external magnet, ωmag, leads to a decrease in the angular separation of these loops,

ς (or increase in occurrence of hiccups). This is typical for the transition between two regions of

motion of a coil’s dynamics. For ωmag = 270° s-1 and 280° s-1, a periodic pattern of an envelope of

the minima and maxima of these loops becomes apparent, Fig. 4.16(b). When ωmag = 300° s-1


88

Figure 4.16 | Phase portraits of coil dynamics. Phase portraits (dt

d coilΨ vs. Ψcoil) of the

coil dynamics for (a) ωmag = 252° s-1 and 254° s-1; (b) ωmag = 270° s-1 and 280° s-1; (c) ωmag = 300° s-1 and 350° s-1. In (a)–(c) a comparison is made with ωmag = 250° s-1. (d) The angular separation between loops, ς (i.e. angular distance between hiccups), versus the magnetic field frequency ωmag.

different loops begin to overlap each other, a characteristic of the OLR region, Fig. 4.16(c). This

sequence of overlapping loops which emerge for higher ωmag, resembles a phase portrait of a

parametrically driven oscillator. Figure 4.16(d) shows that the angular separation of the loops

decreases as ωmag increases, till a point where it levels off, representative of the OLR region.

4.3.3 Conclusions and outlook

Here, it has been shown that micro-helix coil structures can be fabricated exhibiting one of

three distinct magnetic configurations: radial-, corkscrew-, or hollow-bar-magnetization. These

different magnetic states were experimentally probed by exploring the coil dynamics in response to

an alternating external magnetic field. The dynamics were unique to each of the three magnetic

configurations. The motion of the radial magnetized coil was reconstructed using a theoretical

model.

The coiling method presented here allowed for a continuously distributed macroscopic

magnetic moment which bears resemblance to the discrete spin patterns arising on the atomic scale

in helimagnetic materials.[13,16] The developed technique offers the opportunity on the microscale,

to experimentally explore complex magnetic transitions which are characteristic of helimagnetic

structures. Additionally, these coil structures, as well as the magnetic spiral structures presented in


89

Figs. 4.1(e) and (f), show promise as a geometrical realization of the so-called magnetic toroidal

moment[9,10] on the microscale. Therefore, allowing for the experimental investigation of this novel

multiferroic material.

Another important aspect concerns the possibility of applying these coil structures as fuel-free

microrobotic devices which can be used to pickup and deliver microcargo within a system. Given

that the radial-magnetized micro-helix coils exhibit directionally-deterministic motion, there is

potential use of these structures in the realm of remote controllable microrobots and roving

sensors.[37] To this extent, we have demonstrated that these devices can pickup microcargo,

magnetic microspheres here in Fig. 4.17, and transport them across a sample. Again, here radial-

magnetized coils, approximately 60 µm in length with a 40 µm diameter, are used to transport cargo

consisting of a superparamagnetic particle (approx. 7 µm diameter).

Figure 4.17 | Cargo pickup and delivery. (a) A radial-magnetized coil approaching a Fe3O4 microparticle. (b) The coil is positioned upright on top of the particle for pickup. (c) The particle is transported away, attached to the coil.

Magneto-optical effects with these coils are also of great interest. Ongoing work is being done

to measure polarization changes in light which passes through the center of these coils; induced by

the different obtainable magnetic configurations. Within this scope, the coils may function like a

Faraday polarizer.[38] The Faraday effect manifests a change (rotation) in the polarization of light

that passes through an optically transparent magnetic material. The magnetic field in the material

causes the polarization of light to be rotated after passing through. Light going the other way

through the material will be rotated in the opposite direction. These devices are used in polarization

dependent optical isolators, behaving in the same manner as a diode does in an electrical circuit, by

allowing light to only pass through the device in one direction. If these coils do indeed exhibit this

effect, then they could be integrated into and used as an on-chip micro-optical isolator.


90

4.4 Chemical sensing

4.4.1 Introduction

Due in part to their flexibility, polymers have been used as the main component in a number of

recently explored sensors and devices. This includes active actuators for Braille displays,[39]

pressure sensors for use as artificial skin[40] and cantilever devices for seismometers,[41] to mention

a few. During the studies which were mentioned earlier in this chapter, it was found that SU8 is

extremely sensitive to the chemical in which it is submerged. In line with this, a study has been

made on the swelling and shrinking of SU8 structures when immersed in different SU8 processing

chemicals.[42] Our original observation of the media dependence on the shape of these structures led

to the concept of exploring these SU8 coils, and similar architectures, as on-chip chemical sensing

detectors and possibly future actuator devices. This section will show the first obtained results

towards the characterization and development of working sensor devices in this capacity.[43]

The coiled structures react, to the chemical in which they are immersed, by either expanding or

contracting (coiling-up or uncoiling). A study can be made on the average radius of curvature the

SU8 structures exhibit. There are a number of physical contributions to consider, in order for

understanding the behavior of the system. The main factors which play a role include: 1) The

competition of stress due to particular nanomembrane structures on top of the SU8. 2) Influence the

nanomembranes and SU8 have with regards to wettability[44] to a given media. 3) The absorption of

media into the SU8 polymer layer, in turn resulting to swelling and shrinking of the structure.

4.4.2 Media sensing via mechanical coil actuation

The first physical phenomenon to consider is what and how big of a role the deposited

nanomembrane layer plays in the sensing mechanism of these structures. In other words, is the

coiling and uncoiling of the structures due to the material making up the nanomembrane on top of

the SU8? Firstly, the nanomembrane does contribute as a crucial component of the system. The

added stress from the nanomembranes has a large impact because standalone SU8 will delaminate

but it will not coil-up; rather only slight bending is observed. So there is indeed a dependence on

the nanomembrane layer that is deposited on top of the SU8 polymer. As known from rolled-up

strain engineering,[5,45] the nanomembrane affects the overall strain, as well will play a role as to

whether one face of the coil is to a greater or lesser degree, wettable.

Figure 4.18 shows the results of placing coiled structures within a number of different media.

Here it is shown that the immersion liquid leads to very large changes in the coil structure. For this

investigation, SU8 structures with three different nanomembranes (SiO, TiOx and Au) deposited on


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top, were considered. All samples started out in the delamination medium, NMP. The steps in

which the samples were transferred to different media were performed by first a transfer into a Petri

dish containing the new media, for washing of the last medium, then a transfer to a different Petri

dish containing the same medium as that of the previous washing Petri dish. This made the medium

in which the sample was immersed practically 100 % concentration. A different order in which the

samples were placed in different media was taken for each of the different samples. The reason for

this approach is so a number of different aspects of the sensing mechanism could be observed at

once. These include the effect that a particular nanomembrane has on the mechanics, as well as if

there is an effect from a particular medium. It is also important see if the order in which the

samples are placed in the different media affects the sensing, i.e. if previous media are a factor in

the sensing mechanism. This can be used to narrow down which aspect of the detectors contributes

the most to the observed coiling and uncoiling sensing mechanism.

Figure 4.18 | Chemical sensing via polymer swelling and shrinking. Optical images of coils originating in NMP with three different inner layers (a) SiO (b) Au and (c) TiOx placed in various media in different order. The coils react to the different media with an increase or decrease of their radius of curvature, thus sensing each medium in turn. The coils return close to their original size when placed back into NMP, despite being much larger and open in other media.

The changes appear to be largely independent of the top nanomembrane. This hypothesis is

arrived upon given that even though the intrinsic strain of TiOx is much higher than Au, both

appear to respond similarly in like media. This difference in strain is known from other roll-up

experiments where a single layer of TiOx, deposited in a standard way as detailed in Section

2.3.3.2, has enough strain to roll-up from a sacrificial layer, whereas for instance, Au would not

roll-up. Another factor is that the wetting properties, as seen by their contact angles (θcon)


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(measured by Dr. S. Sanchez on samples he was provided) of the materials are similar in various

media (Table 4.1). There is a large difference in the wettability for water and IPA, however the

coils act similar in these two liquids and IPA and NMP have similar wetting properties, but the

coils act completely

NMP H2O Acetone Isopropanol Amyl Acetate

TiOx θcon ≈ 15° θcon = 35° ± 3° θcon = N/A

(Wettable)

θcon = N/A

(Wettable)

θcon = N/A

(Wettable)

Au θcon = N/A

(Wettable)

θcon = 39° ± 4° θcon = N/A

(Wettable)

θcon = N/A

(Wettable)

θcon = N/A

(Wettable)

SiO θcon = N/A

(Wettable)

θcon = 31° ± 6° θcon = N/A

(Wettable)

θcon = N/A

(Wettable)

θcon = N/A

(Wettable)

SU8 θcon = N/A

(Wettable)

θcon = 70° ± 4° θcon = N/A

(Wettable)

θcon = 14° ± 2° θcon = N/A

(Wettable)

Table 4.1 | Contact angle of different media on different materials. Most materials are wettable to the various fluids except for water. For the values with N/A the angle was too small to accurately measure.

different in these two. The contact angle is a measure to how a droplet of a medium interacts with a

particular material surface it comes into contact with. The larger the contact angle, the more a

material resists being in contact with the given medium, or rather, the less wettable the material is,

Fig. 4.19. These results suggest the wettability of the given materials plays a trivial roll in the

sensing mechanism.

Figure 4.19 | Contact Angle. The contact angle is defined in the following manner and is a measure to how much a material resists being in contact with a medium.

The radius of coil curvature also appears to be mostly independent of the order in which the

immersion media were investigated. These considerations all seem to point to the SU8 itself being

the main contributor of the particular geometry which is observed in various media. This means

that most likely, the dominating contributor comes from a media diffusion into the SU8.[42] The

radius of curvature is plotted out for each of the different SU8 nanomembrane devices, Fig. 4.20.

The colored line represents the average radius of curvature of the devices in various media shown

in Fig. 4.18. There is a relatively large spread of curvatures for many of the media, however this


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spread is quite small in NMP and acetone, both of which are media resulting in a relatively small

radius of curvature and high compactness of windings, Fig. 4.18.

Figure 4.20 | Dependence of the radius of curvature of a coil vs. immersion chemical. A plot showing the dependence on the SU8 sensors’ radius of curvatures within numerous media and having one of three inner nanomembranes (a) SiO, (b) Au or (c) TiOx.

The mechanical mechanism which causes the coils to coil and uncoil is depicted in Fig. 4.21.

When the coils are in NMP, the SU8 absorbs the molecules making up the medium, causing it to

swell, Fig. 4.21(a). This swelling increases the strain in the polymer layer, causing the coils to coil-

up [Bottom of Fig. 4.21 (a)]. When the coils are placed in another media, the polymer desorbs

molecules, causing the polymer layer to shrink. This shrinkage lowers the strain in the polymer and

the coiled SU8 strips relax, causing them to uncoil [Fig. 4.21(b)].

Figure 4.21: Mechanism of coiling and uncoiling. (a) When the hybrid polymer strips are within NMP, the SU8 absorbes molecules from the media causing the layer to swell. Due to the swelling, the strain in the polymer layer is increased, leading to a coil-up. (b) When the strips are placed in another media, the polymer desorbs molecules and shrinks. This shrinking leads to a lower strain in the layer and the strips uncoil.

It is true that the devices with different nanomembranes have slightly different radius of

curvatures relative to one another, shown in Table 4.2 (the error bars are calculated by average

deviation from the mean). This potentially originates from the effect the strain and contact angle, of

a given material, have on the devices. These differences in the exact curvature (seen in Fig. 4.19

and Table 4.2) may also be attributable to not leaving enough waiting time in each medium before


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NMP Acetone Amyl Acetate Water Isopropanol SiO

nanomembrane 35 ± 2 µm 61 ± 3 µm 66 ± 22 µm 192 ± 44 µm 314 ± 100 µm

Au nanomembrane

30 ± 3 µm 41 ± 6 µm 41 ± 7 µm 80 ± 61 µm 87 ± 48 µm

TiOx nanomembrane

41 ± 4 µm 61 ± 5 µm 167 ± 108 µm 237 ± 99 µm 365 ± 203 µm

Table 4.2 | Average coil radius of curvature vs. the immersion chemical. The average radius of curvature (from smallest to largest) for various nanomembranes-on-SU8 sensors, when immersed in various liquids.

measurement. However, further investigations are needed to confirm this. The wait times were on

the order of five minutes, whereas it has been shown that full diffusion into SU8 can take up to one

hour.[42] Despite this, the initial diffusion (observed as expansion or contraction of the coils) is

almost instantaneous where wait time of not more than a couple of seconds to minutes was needed

to see the observed sensing phenomenon. This gives promise in the device’s rapid sensing

capability. These could also quite possible be used as temperature probes given that polymers are

highly susceptible to temperature variations.[46]

4.4.3 Conclusions and outlook

In conclusion, a fluidic sensor based on the SU8 polymer delamination technique, has been

developed and investigated. The sensing mechanism lies in the swelling and shrinking of the SU8

due to absorption and desorption of the various media into the polymer. The sensor reacts to

various fluids through the action of coiling or uncoiling. From this, the radius of curvature can be

measured and also allows for a calibration of such sensor systems since it is shown that the radius

of curvature returns to its original values, even after being in various other media.

The next step, and a work which is currently ongoing, is to have a further detailed study on

these sensors. It is also important to develop a full, on-chip integration for these devices. The way it

stands now, is the coils are loosely attached to the substrate; however after numerous transfers

between media, many coils are lost and washed away. A solution for this which is currently being

tested is a way in which to attach the “arms” of the coil to a central anchor. For instance,

developing an anchor of SU8 in a manner in which it will not delaminate, as described in Chapter

2, then making a second SU8 processing step for the arms, under the condition that the arms will

delaminate, Fig. 4.22(a). Another possibility is to create remote controllable roving sensor devices.

This would be possible if a magnetic layer, like described in the previous section, is used as the

nanomembrane on top of the SU8 structure. The entire structure could delaminate from the

substrate, and then the whole structure could be maneuvered around and sense different


95

concentrations of media in different locations. An artist’s depiction of this is represented in Fig.

4.22(b).

Figure 4.22 | On-chip and roving sensors under development. Work is underway for fabricating on-chip and roving sensors. (a) The on-chip sensors are comprised of an SU anchor (here the circular pattern with the number 9 in the middle) which shouldn’t delaminate, and the delaminating SU8 arms. (b) Another detector would be built to fully delaminate from the substrate and could be maneuvered within a media for detection. Here is an artist’s depiction of such a device.

4.5 Conclusions

Presented in the second to last chapter of this thesis, was a novel fabrication method for

obtaining unique hybrid structures. In so doing, by depositing magnetic nanomembranes onto SU8

strips, the creation of microscopic magnetic structures, having as many as three otherwise-

unobtainable magnetic configurations, was made possible. These different magnetic patterns were

investigated in the presence of an alternating magnetic field which revealed that each particular

magnetic configuration responded with unique dynamics. The dynamics of the radial-magnetized

structures were further investigated theoretically, which showed the peculiarities of the non-linear

system. With these new magnetic configurations, further investigations into the geometrical

realization of a toroidal moment, as well as microscopic optical isolators can now be made. It is

also expected that these Swiss roll coils can be applied for creating metamaterial structures since it

is known that Swiss rolled plasmonic spirals[47] can allow for the induction of artificial

magnetism.[48] This means that an array of these structures could be used to create a negative index

of refraction material.

This new technique, known as polymer delamination, has also opened the doors to creating new

flexible on-chip fluidic sensors. It was shown that on-chip detectors, responding to a fluid by the

coiling or uncoiling of sensor “arms,” can be developed. Given the number of devices and sensors


96

which have already been investigated or considered, along with the infancy of this technique, there

is a very good probability that this method will be of high impact to the roll-up community.

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Chapter 5 Conclusions

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Chapter 5: Conclusions

This doctoral thesis took an in-depth look at investigations which were performed on self-

assembled rolled-up sensing devices. An exploration was made for developing the novel micron-

sized apparatuses for integration into on-chip systems. Both theoretical and experimental studies

were performed. This work covered systems which relate to a variety of scientific fields including,

but not limited to, optics, magnetics, fluidics, non-linear dynamics and bio-physics.

The first chapter was used as an overall introduction to the systems and materials which were

investigated. The second chapter focused on metamaterial systems, realizable using a rolled-up

fabrication technique which is based on an angle deposition of materials onto a patterned sacrificial

layer of photoresist. A novel lens, known as a hyperlens, which can break the diffraction limit of

light, was introduced and it was shown how such a lens can be fabricated by rolling-up a

nanomembrane bilayer composed of metal and dielectric. Light emitted from two objects which

have a sub-wavelength spatial separation can be guided radially out through the wall of the tube,

which results in a projected magnification of the objects. The material requirements needed for

achieving hyperlensing over the entire visible range were made for the first time. The concept of

creating an immersion hyperlens was also investigated and a description of how such a technique

would be achievable, when working in a water-like medium or in air, was given. This technique

would allow for a higher achievable resolution and transmission with such a lens. The rolled-up

hyperlens also has a foreseeable future as a component designed for lab-in-a-tube systems for in-

situ sub-wavelength biological studies.

Next, a novel device which integrates classical fiber optics with plasmonic waveguidance,

known as a metamaterial optical fiber (MOF), was theoretically introduced. If the same structure

which was suggested for the hyperlens is used, but light is guided through the microtube, rather than

radially-out through the wall, this fiber can be formed. By creating a metamaterial cladding that

surrounds a hollow air core, tuning of the light guidance from classical within the core to plasmonic

within the cladding is possible with a modification of the material makeup or wavelength change of

the propagating light. Due to the interesting characteristics of this fiber, light can be guided even

with a sub-wavelength cladding thickness. This aspect allows for a shrink-down so that it can be

used and integrated into on-chip systems. Such devices can be created using rolled-up technology.

The experimental realization of metamaterial optical fibers was presented. A method was put

forth for the integration of these fibers and other rolled-up devices into preexisting on-chip


100

structures. In this way, ridge waveguides could be created and utilized for the injection and

collection of light from the rolled-up fibers. An in-depth characterization of the materials making up

the layers within these fibers was made. Using the known material properties, an effective

permittivity could be calculated in order to have a theoretical representation of the cladding. Due to

the dispersive metamaterial properties of the cladding, the wavelength dependence of the output

profile of MOFs (having various filling ratios of metal and oxide) was investigated. This revealed

distinct regions of guidance which were then correlated with the calculated effective permittivity.

This comparison showed that the wavelength regions of the output profiles can be directly related to

the anisotropy existing in the cladding. When multiple fibers with varying metal filling ratios were

compared, it was found that the regions were shifted in both the output profiles as well as the

effective permittivities. This suggested a true tunability when guiding light with these fibers.

Comparing these MOFs to isotropic oxide fibers of the same dimension, it was found that the

cladding thickness of oxide fibers was too thin to effectively guide light throughout the visible

range. This implies that the ability to guide light with a sub-wavelength cladding thickness lies

within the anisotropy of the cladding material.

The third chapter focused on the first ultra-compact component which was developed for lab-in-

a-tube systems. A method for the efficient capturing of embryonic mouse cells within rolled-up

tubular structures, known as a microsyringe, was laid out and demonstrated. This technique allows

for the precise capturing of cells into large arrays of microtubes, opening up the opportunity to

investigate many individual cells within a small viewing window. A component used to optically

detect mechanical-cellular interactions was introduced as well. This component, known as a flexible

split-wall microtube resonator, is an optical ring resonator that, for detection, relies on a flexible

nanogap residing within its wall. This nanogap either scatters light when expanded, or increases

light confinement within the resonator when closed. The presence of a cell influences whether this

nanogap is open or closed. When a cell is present and the nanogap is closed, the quality factor of the

whispering gallery modes in the photoluminescence spectra increases by an order of magnitude. It

was demonstrated that this detection method is reproducible and a single device can be used to

detect consecutively captured cells.

The fourth chapter presented a new approach for creating rolled-up devices and sensors. The

method relied on the delamination of a photoresist polymer so that it coiled-/rolled-up with the

nanomembrane which was deposited on top; rather than acting as a sacrificial layer and dissolving

away. Due to this effect, instead of creating rolled-up microtubes which come back into contact

with themselves after a single rotation, this new method allows for a partitioning between these

nanomembrane layers by a tunable barrier. This new approach, as well as the characteristic of the


101

built-in polymer partition, can be used to create components and architectures which were

previously unrealizable. The first architectures which were considered were made from a coil-up

process of a magnetic nanomembrane layer. Using either a layer with an in-plane easy axis of

magnetization or a layer with an out-of-plane easy axis of magnetization, the creation of unique

corkscrew-, radial- and hollow-bar-magnetized microstructures is possible. These structures are

unrealizable by any other known approach. The magnetic configurations were probed by analyzing

the distinct dynamic response of the coils when subject to an applied alternating magnetic field. The

dynamics were reconstructed with a theoretical model which revealed non-linear dynamics in the

system. The coil structures also demonstrated the ability to act as microrobots and could collect and

transport microcargo. The corkscrew-magnetized structures, as well as the spiral magnetized

structures, offer a promising solution for the geometrical realization of a toroidal moment; a

phenomenon known to only exist on the atomic scale in nature.

The second section of chapter four demonstrated that these polymer structures can be used as

on-chip and roving fluidic sensors as well. Given the ability of the SU8 polymer to swell and shrink

rapidly in a given media, it offers good promise to be implemented as a sensing device. The sensors

respond to a given immersion medium through a coil-up or uncoil of SU8 polymer arms. An

investigation on the nanomembrane and media dependence of this coil-up was made. It was

reasoned that the SU8 polymer swells or shrinks in a given medium. This is due to the change in

strain of the polymer layer because of the polymer expansion and contraction when absorbing and

desorbing the medium. This polymer delamination approach also gives promise in creating new

metamaterial structures which rely on a Swiss-roll configuration of a metal in order to induce non-

unity permeabilities in non-magnetic materials.

The work presented within this thesis offers a large variety of scientific advancements on which

new investigations may build, towards the further exploration of a number of devices and material

systems in the future. In so doing, the contents of this work leave profound scientific progress in the

field of on-chip rolled-up detectors and sensors.

102

Acknowledgements

Imagination is more important than knowledge… ~ Albert Einstein ~

I had discovered that learning something, no matter how complex, wasn’t hard when I had a reason to want to know it.

~ NASA Engineer Homer Hickam ~

First and foremost, I would like to acknowledge my Family. I want to especially thank my parents, Judy and R. J. Smith, for raising me and providing every opportunity they could to give me a bright future and the life I have enjoyed up until now. I am happy to see that my brother, Sean Smith, who has always taken a protective position beside me, has grown to be a truly great friend. I want to thank my girlfriend, Susann Bewernick, for reminding me, constantly, that there are more things in life than simply work. Du machst mich glücklich. The few lines I write here could not possibly encompass what I feel for these people and the importance they hold for me.

I would like to thank Professor Dr. Oliver G. Schmidt for giving me the opportunity to pursue my doctoral degree in this beautiful city and country. The support given towards my research has been invaluable and I have truly appreciated it.

I greatly appreciate Professor Dr. Rudolf Bratschitsch for taking the time in his busy schedule to be a referee for my doctoral work and would like to thank him for doing so.

I would also like to thank Professor Dr. Uriel Nauenberg for convincing me to pursue graduate school and advising me throughout my Bachelor’s and Master’s work.

I would like to thank Dominic Thurmer for suggesting I come pursue my PhD in Germany, as crazy as an idea that it had sounded at the time, it was one of the best decisions I have made in life. I also want to thank him for the time he took to get me adjusted to life here and as well thank him and Eva Gottmanns for their friendship over the years. Thanks go to Dr. Suwit Kiravittaya for help with providing FDTD simulations and many helpings with writing code as well as providing many helpful hints for my experiments and critical reading of the work provided within this thesis. Thanks also to Dr. Denys Makarov for his knowledge on micromagnetics and drive behind our recent manuscript as well as the cheerful and inspirational motivation in his work. I also want to thank him for critical reading of this thesis. Thank you Ronny Engelhard for endless discussions on deposition techniques and invaluable tips and help throughout my time working here. Thanks to Dr. Samuel Sanchez for lots of help with the lab-in-a-tube experiments and for being a driving force behind his members to work hard and create results. Thanks to Sabine Schulze for providing the animal cells. Thanks also go to Prof. Dr. Yongfeng Mei for providing some initial paths in which my work could take and offering many concepts. I would also like to thank Dr. Daniel Grimm for taking on the difficult task of keeping the cleanroom up and running as well as making valuable improvements within. I would also like to thank him for discussions with me on photolithographic processes. Thanks to Prof. Dr. Vladimir M. Fomin for helping make sense out of theoretical physics and doing it with a smile.

When it comes to the other numerous experiments which I performed while here, they could not be done alone, so I would like to especially thank those who helped along the way: Dr. Stefan Baunack and Dominic Thurmer for SEM images and FIB cuts; Barbara Eichler for AFM measurements and for cutting all my wafers; Conny Krien for providing sputtered samples and for the many hours spent teaching me the workings of the DCA sputtering machine; Prof. Dr. Zhaowei

Curriculum Vitae

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Liu for help with understanding metamaterial systems and help with the program COMSOL; Sandra Sieber for processing the masks I designed; and Ulrike Steere for handling much of the business side of things required for travel, etc.

Thank you also to the members of the rolled-up optics group for being there to bounce ideas off of. Thanks go out to all the friends I’ve had the pleasure of spending time with and discussing ideas: Keith Drake, Stefan Harazim, Dr. Cesar Bof Bufon, Esteban Bermúdez, Eugenio Zallo, Johannes Plumhof, Dr. Rinaldo Trotta, Dr. Christoph Deneke, Alex Solovev, Dr. Fabio Pezzoli, Santosh Kumar, Dr. Armando Rastelli, Dr. Thomas Dienel, Michael Melzer, Peter Cendula, Vladimir Bolaños Quiñones, Dr. Hengxing Ji, Martin Bauer, Dr. Larysa Baraban, Dr. Roman Rezaev, Dr. Paola Atkinson, Dr. Fei Ding, Daniil Karnaushenko, Dr. Ping Feng, Tobias Kosub, Dr. Hong Seok Lee, Sabine Schulze, Diana Iselt, Juan Diego Arias Espinoza, Jose David Cojal, Irma Sujanski, Darius Pohl and Darijan Kantor all of which were there at some point during my PhD work for bouncing ideas off of and for being there when it was necessary to simply go out and have a beer, forget about work and complain about our problems with one another.

Thanks to all of my friends in the U.S. and to anyone else who has helped me on my venture that I may have forgotten, but hopefully did not.

There is no harm in doubt and skepticism, for it is through these that new discoveries are made. ~ R. P. Feynman ~

Well I won’t back down, no I won’t back down You can stand me up at the gates of hell

But I won’t back down ~ T. Petty ~

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Curriculum Vitae Elliot John Smith

13th

June, 1984

St. Louis, MO, U.S.A.

United States of America Citizen

Education

04/2008-08/2011 Doctor rerum naturalium (Physics)

Chemnitz University of Technology, Chemnitz, Germany

Leibniz Institute for Solid State and Materials Research, Dresden, Germany

Advisor: Prof. Dr. O. G. Schmidt

“Self-assembled rolled-up devices: towards on-chip sensor technologies”

08/2006-05/2007 Masters of Science (Physics)

University of Colorado, Boulder, CO, U.S.A.

Advisor: Prof. Dr. U. Nauenberg

08/2002-05/2006 Bachelors of Arts (Physics/Minor in Mathematics)


08/1998-06/2002 High School Diploma

Boulder High School, Boulder, CO, U.S.A.

Work Experience

05/2007-04/2008 Professional research assistant

“Detector Studies for the International Linear Collider”


05/2006-05/2007 Graduate research assistant



01/2005-05/2006 Undergraduate research assistant



06/2003-06/2005 Solder technician

Fire Scout, Boulder, CO, U.S.A.

05/2001-04/2008 Volunteer wildland and structural firefighter

Boulder Mountain Fire Protection District, Boulder, CO, U.S.A.

Language

German: Good written and spoken

Spanish: Basic knowledge

Grants and Awards

05/2006 Physics tradesmen award, University of Colorado

10/2004-05/2006 Undergraduate research opportunity program grant, University of Colorado

Curriculum Vitae

105

Publications

1) Vladimir M. Fomin,* Elliot J. Smith, Denys Makarov, Samuel Sanchez and Oliver G. Schmidt

Dynamics of radial magnetized microhelix coils.

Submitted to Physical Review B (2011).

2) Elliot J. Smith,* Denys Makarov and Oliver G. Schmidt

Polymer delamination: towards unique three-dimensional microstructures.

Soft Matter, doi: 10.1039/C1SM06416A (2011).

3) Elliot J. Smith, Denys Makarov,* Samuel Sanchez, Vladimir M. Fomin and Oliver G. Schmidt*

Magnetized micro-helix coil structures.

Physical Review Letters, 107, 097204 (2011).

4) Alexander A. Solovev, Elliot J. Smith, Cesar Bof Bufon, Samuel Sanchez* and Oliver G. Schmidt

Light-controlled propulsion of catalytic micro-engines.

Angewandte Chemie, In Press (2011).

5) Elliot J. Smith, Yongfeng Mei and Oliver G. Schmidt*

Optical components for lab-in-a-tube systems.

Proc. SPIE 8031, 80310R (2011), doi:10.1117/12.885246.

6) Elliot J. Smith,* Sabine Schulze, Suwit Kiravittaya, Yongfeng Mei, Samuel Sanchez and Oliver G. Schmidt

Lab-in-a-Tube: Detection of individual mouse cells for analysis in flexible split-wall microtube resonator sensors.

Nano Letters, doi: 10.1021/nl1036148 (2010).

7) Ping Feng, Ingolf Mönch, Gaoshan Huang, Stefan Harazim, Elliot J. Smith, Yongfeng Mei* and Oliver G. Schmidt

Local-illuminated ultrathin silicon nanomembranes with photovoltaic effect and negative transconductance.

Advanced Materials, 22, 3667-3671 (2010).

8) Elliot J. Smith,* Zhaowei Liu, Yongfeng Mei* and Oliver G. Schmidt

Combined surface plasmon and classical waveguiding through metamaterial fiber design.

Nano Letters, 10, 1-5 (2010).

Research Highlight, Nature Photon., 3, 310 (2009).

9) Elliot J. Smith,* Zhaowei Liu, Yongfeng Mei* and Oliver G. Schmidt

System investigation of a rolled-up metamaterial optical hyperlens structure.

Applied Physics Letters, 95, 083104 (2009).

[Erratum: Applied Physics Letters, 96, 019902 (2010)].

Presentation at conferences

Invited talks:

1) “Ultra-compact optofluidic components for lab-in-a-tube applications”

22nd

Edgar Lüscher Seminar, Klosters, Switzerland.

February 12-18, (2011).

2) “Towards lab-in-a-tube: Individual cell manipulation and detection in a single on chip microtube optical ring

resonator”

NanoBioTech-Montreux 2010, Montreux, Switzerland.

November 15-17, (2010).

3) “Death of the phototube: Birth of the SiPM”

High Energy Physics Seminar, University of Colorado at Boulder, U.S.A.

December 27, (2006).

Curriculum Vitae

106

Contributed talks:

1) “Lab-in-a-tube: an optofluidic sensor for the detection of individual animal cells”

1st

European Optical Society Conference on Optofluidics, Munich, Germany.

May 23-25, (2011)

2) “Single cell sensing through interfacial contact within rolled-up microresonators”

IFW Dresden winter school: Interfaces, Oberwiesenthal, Germany.

January 16-19, (2011).

3) “Rolled-up metamaterial devices on silicon chips”

2009 AFOSR nano-photonics, silicon photonics and nanomembrane review, Cambridge, MA, U.S.A.

November 4-6, (2009).

4) “Combined surface plasmon and classical fiber optic waveguiding through metamaterial design”

IFW Dresden winter school: Nanomembranes, Oberwiesenthal, Germany.

January 12-14, (2009).

Posters:

1) “Rolled-up metamaterial optical fibers”

Tri-Service 6.1 metamaterial review, Virginia Beach, VA, U.S.A.

May 24-27, (2010).

Journal cover pages:

1) Nano Letters, vol. 11, issue 10 (2011).

2) Nano Letters, vol. 10, issue 1 (2010).

Selbständigkeitserklärung Hiermit erkläre ich an Eides statt, die vorliegende Arbeit selbständig und ohne unerlaubte Hilfsmittel

durchgeführt zu haben.

Elliot J. Smith

01/07/2011

Documents

Self-assembled rolled-up devices: towards on-chip … Bibliographic record Smith, Elliot J.: Self-assembled rolled-up devices: towards on-chip sensor technologies Dissertation Chemnitz