Plasmon Assisted Nanotrapping

E. P. Furlani, A. Baev and P. N. Prasad

The Institute for Lasers, Photonics and Biophotonics University at Buffalo, SUNY

Overview

Introduction

Applications

Experimental Results

Modeling Nanotrapping Systems

Summary

Optical Trapping – Laser Tweezers

D. G. Grier Nature 424 2003

Powerful tool for remote manipulation of microscopic biomaterial.

Strongly focused laser beam creates

gradient optical force that traps particles.

Not ideal for nanoscale trapping (diffraction limitation, heating).

Not well suited for integration with Lab-

on-Chip systems (opto- fluidics).

Plasmonic-based Optical Nano-trapping

Locally enhanced field near illuminated metallic nanostructures creates gradient

optical force that traps nanoparticles.

Well suited for trapping sub-wavelength metallic or dielectric particles.

Potential for integration with Lab-on-Chip

systems (opto-fluidics).

Gold Nanocones

DielectricNanoparticle

Einc(t)

p

+ -

+ - + -

+ -

Surface Plasmon Resonance (SPR)and Localized SPR (LSPR) in Metallic Nanostructures

Plasmon: Quantized charge density wave in free electron gas.

LSPR: Resonant scattering modes in

sub-wavelength metallic nanoparticles

SPR: Surface plasmons confined to metal/dielectric interface.

1/2

sin( )m dsp d

m d

k kc

Wave vectors

E(t)

- - -

+ + + - - -

+ + +

- - - + + + - - - + + + - - -

m

d

E

H.

Strong Local Field

Motivation for LSPR Nanotrapping

Higher Resolution: optical nano-manipulation of sub-

wavelength particles (d << ) (overcome diffraction limit).

Reduced Power: optical intensity an order of magnitude

lower then conventional optical tweezers

Multiplexed Nano-trapping: multiplexed parallel

manipulation of particles using arrays of metallic nanopaticles

Microsystem Integration: integrated optical particle

manipulation/separation for BioMEMS, Lab-on-a-Chip systems.

E(t)

Local Field Enhancement

Metallic Nanoparticles

Optical Absorption - Scattering

Local Field Enhancement Re ( ) 2 ( )mp d

Absorption frequency/bandwidth depend on particle size, shape, composition and surrounding media etc.

P(t) = E(t)- - -

+ + + - - -

+ + +

30

( )4

( ) 2mp d

d pmp d

R

mp

d

Analytical Dielectric Function for Au Nanostructures Experimental and analytical

dielectric values vs.

Analytical Dielectric Function for Au used in Analysis*

2

2

1,2

1

1 1

1 1 1 1 1 1

n n

Au

pp

i in

n n

n n n n

i

A e e

i i

*P. G. Etchegoin et al. J. Chem. Phys. 125, 164705 (2006)

Einc

+ -

+ - + -

+ -

Optical Trapping of Sub-Wavelength Neutral Particles

Dielectric Nanoparticle

Metallic Nanostructures

Force on Dielectric Nanoparticle caused by Local Field Gradient produced by Illuminated Metallic Nanoparticles

J. Aizpurua et al., PRL 90 2003T. Atay et al., Nano Letters 4 2004

Nano-cone Array

Nano-Pillar Array

Nano-Ring Array

Fabricated Metallic Nanostructures

Experimental Results

Optical trapping of nanoparticles using tapered metallic nanopillars

Collaboration with A. N. Grigorenko et. al, Nanometric optical tweezers based on nanostructured substrates, U. Manchester UK

1 m

120 nm

90 nm

Optical Trapping of Microbubbleson Nanostructured Substrate

A.R. Sidorov et al. Optics Communications 278 (2007)

120 nm

90 nm

Enhanced Optical Trapping Au Nanoparticle Array

Array of Au Nanostructures

Trapped Dielectric SphereMoving Dielectric Sphere

X. Maio and L. Y. Lin, Opt. Letters. 32 2007, also unpublished work 2008

Size of Trapped Particle D = 6.8 m D = 1 m D = 0.8 m

Optical Trapping Intensity (W/m2) with Au NP Array 0.71 3.4 3.8

Optical Trapping Intensity (W/m2) with Glass Slide 7.1 6.0 7.6

Plasmonic Trapping of Cells

Single Yeast Cell Trapped in Square (other cells moving at constant speed)

Trapped Cell Moving Cells

X. Maio and L. Y. Lin, IEEE J. Sel. Topics Quant. Elec. 13 2007

Optical intensity required for stable trapping of single yeast cell is 78.8 W/m2

Array of Au Nanostructures

Modeling Optical Nanotrapping

Dielectric Model for Metallic Nanoparticles.

Predict EM Field (Full-Wave Time-Harmonic Analysis)

Compute Time-averaged Optical Force Fopt on Dielectric Nanoparticles (Dipolar Force)

Identify Regions of Trapping

Use Fopt to Predict Particle Motion.

Optical Force on a Dielectric Nanoparticle

( )pillarsP E

*, 0 0

1Re ( )

2i

opt i p j jj

F E E

0 inc scatE E E

0 0

23

0

4

21

3

dp

p

kik

R

30 2

p dp

p d

R

Time-averaged Optical Dipolar Force Fopt is a function of

several variables: , p, mp (), d, and the geometry,

composition and coupling of metallic nanostructures.

d

Einc(t)

p

+ -

+ - + -

+ -mp()mp()

p

Trapping and Scattering Force Components

Re[ ] Im[ ]p p pi

3grad pF R

*, 0 0

1Re

2i

opt i p j jj

F E E

2

0

1Re[ ]

4grad pF E

6scat pF R

2

0

1Im[ ]

2scat pF E k

3pR

6pR

Trapping Potential Vtrap: Vgrad trapF

(Electric Field Energy Density)V trap eW

2 m

k

Symmetry Boundary Conditions: PEC, PMC

Full-Wave Time-Harmonic Analysis(Array of Nanopillars: Glass Substrate covered with H2O)

Computational Domain

3.4 m

2 m 2 m

PML

PML

y x

k

PEC

PMCGlass

H2O p

Symmetry Boundary Conditions: PEC, PMC

Computational Model

Computational Domain

Surface current Jx BC chosen to produce plane wave: Ex = 2106 V/m. FEA Model: 43,904 cubic vector elements with 838,485 degrees of freedom.

3.4 m

2 m 2 m

PML

PML

y x

k

PEC

PMCGlass

H2O

Jx

Incident Intensity5.3 mW/m2

CPU Platform Dual Processor (3 GHz)

Quad Core Windows XP 64 Bit

32 GB Ram Time: 15 min per given

p

Axial Optical Force

vs. Field Polarization

p

Einc

k

TE

TM

Fz along this line

Glass

H2O

k

TrapTrap

k

Trap

TM polarization

Optical Force Analysis

Force Vectors in x-y Plane

Glass

H2O

TrappingPotential

-<We> J/m3

Glass

H2O

k

Trap

Rp = 50 nm

= 1000 nm

TE Analysis

TM Analysis -<We> J/m3TE Analysis

Einc

k

TE

TM

p

Trapping Force Analysis

kk

Force vs. Particle Size Force vs. Cone Separation

No Trapping for Large ParticlesScattering Force Dominates

Einc

k

TE

TM

d

p

TE Polarization = 635 nm

Rp = 50 nm

Axial Optical Force

vs. Field Polarization

p

Einc

k

TE

TM

Fz along this line

Glass

H2O

k

TrapTrap

k

Trap

Induced Electromagnetic Modes

Top View – Induced Ez Side View

E(t)

-z

x

Induced Ez

+ +

+

+ +

+

Top View

---

---

+ -

- + - +

+ -

= 635 nm

= 1000 nm

p

Einc

k

TE

TM

Fz along this line

Glass

H2O

2D Array of PillarsTE Trapping vs.

k

Rp = 100 nm

TE Analysis

k

Rp = 100 nm

Trap

Glass/Air

Glass/H2O

TE Analysis

Trap

p

Einc

k

TE

TM

Fz along this line

Glass

H2O

2D Array of RingsTE Trapping vs.

600 nm

200 nm

300 nm

k

Air Only

Trap

Rp = 100 nm

Rp = 100 nm

k

Glass/H2O

Trap

Optical trapping of neutral sub-wavelength particles can be achieved using local field enhancement near illuminated metallic nanostuctures.

Nano-trapping can be achieved with plane wave illumination.

Trapping force depends on particle size, , polarization and background permittivity.

Integration in Lab-on-Chip applications: Opto-fluidics

Summary and Conclusions