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Nanoscience II: Semiconductor nanostructures

11.11.2010

Markku SopanenMICRONOVA

Department of Micro- and NanosciencesAalto University School of Science andTechnology

Acknowledgments: Prof. Harri Lipsanen, Dr. Mikael Mulot, Dr. Marco Mattila, Dr. Teppo Hakkarainen

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1 Semiconductor nanostructures2 Quantum dots3 Photonic crystals

Outline

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1 Semiconductor nanostructures

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What is a ”semiconductor nanostructure”?

Classification by properties Electronic tailoring (quantum dots, wires) Optical tailoring (photonic crystals)

Classification by nanostructure dimensionality 1D (quantum wells, superlattices, Bragg mirrors) 2D (quantum wires, nanowaveguides, planar photonic crystal) 3D (quantum dot, nanoparticle, photonic crystal)

[Charge carrier system dimensionality is the opposite way.]

Semiconductors do not usually play a crucial role in metamaterials .

Obviously a structure containing at least one semiconductor material and having at least one dimension in nanometer scale. However, usually one-dimensional structures are not considered as ”nano”.

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Covalent bonds in semiconductorsElectronic structure of Si: 1s22s22p63s23p2

4 valence electrons, 4 electrons missing to fill the outer shell

Some semiconductors have more ionic bonds (II-VI, etc.).Electrons involved in the bonds are trapped in the bonds, and are not available for conduction.

Pure semiconductor is a poor conductor

But free carriers can be easily created by doping.

Ga

Ga

Ga Ga

Ga

Ga

As

As

AsAs

As

As

Electronic structure of Ga: 1s22s22p63s2p63d104s24p1

3 valence electrons, 5 electrons missingElectronic structure of As: 1s22s22p63s2p63d104s24p3

5 valence electrons, 3 electrons missing

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Phosphorus impurity atom (extra valence electron ) in silicon lattice: the extra valence atom is weakly bond: an energy Ec - Ed << Eg is required to create a free electron. This type is called donor defect/impurity => n-type semiconductor

Doping

Ev

Ec

Eg

Filled valence band

Ed

Ev

Ec

Eg

Filled valence band

Ea

Boron impurity atom: acceptor defect/impurity => p-type semiconductor

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Diamond structureDiamond structure = FCC lattice + 2 identical

atoms in the primitive cell: (0,0,0) and (a/4, a/4, a/4)

– Examples: Si, Ge and diamond

Crystal viewer (diamond and Zinc blende structure):http://jas2.eng.buffalo.edu/applets/education/solid/unitCell/home.html

Zinc-blende lattice = FCC lattice + 2 different atoms in the primitive cell

– Examples: GaAs, InP, GaP, GaSb, InSb, ZnS, ZnSe, …

(GaN, SiC and ZnO are difficult to manufacture in zinc-blende structure)

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Semiconductor band structure

Electronic structure of Si: 1s22s22p63s23p2

N Si atoms: 2N electrons in 3s orbital, 2N electrons in 3p orbitals

3s

3p

Empty upper bands

Filled lower bands

Conduction band

• Energy states of Si atoms expand into the energy bands of Si crystal• The lower bands are filled and higher bands are empty• The highest totally filled band is the valence band• The lowest empty band is the conduction band

2N electronsValence band

2N electrons

4N electronsEne

rgy

N Si atoms in crystal formN isolated Si atoms

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GaAs band structure (E-k diagram)

Eg

Valence band

Conduction band

X-valleyL-valley

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Direct and indirect bandgap

Direct band gap: The conduction band is formed only by overlap of s-orbitals

Indirect band gap: The conduction band is a mix of p- and s-orbitals

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Quantum well

yz

xInP

InP

• Quantum well: a thin semiconductor layer (Lz<20nm) embedded between two semiconductors with larger bandgaps.

• Electrons and holes trapped in the well are free to move in the x-yplane, but are strongly confined in the z-direction = 2D electron gas.

InAs0.65P0.35 (5nm)

Lx

Lz

Ly

Cross-sectional TEM picture of a GaInNAs QW grown on GaAs.

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2

222

2 zee Lm

E ∗=

π

∗

+++=

e

yxeC m

kkEEE

2)( 222

EC

EV

Eg,1 Eg,2

Lz

z

E

Lz

EC

In the infinite well approximation, the energy levels are given by:

e2 (ℓ=2)e1 (ℓ=1)

e3 (ℓ=3)

e4 (ℓ=4)

Electron energy:

Energy levels for electrons

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Energy levels for holes

Lz

2

222

2 zhhhh Lm

E ∗=

π

EC

EV

Eg,1 Eg,2

EV

Lzz

E

hh2 (ℓ=2)lh2 (ℓ=2)

lh1 (ℓ=1)hh1 (ℓ=1)

In the infinite well approximation, the energy levels are given by:

2

222

2 zlhlh Lm

E ∗=

π

∗

+++=

hh

yxhhV m

kkEEE

2)( 222

Heavy hole energy:

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E

Density of states (electrons): 2D vs. 3D

D(E)

Ee1 Ee2 Ee3EC

3D2D

( )*

2( ) el

l

mD E H E E dEπ

= −∑

( )1, when E E0, when E < E

ll

l

H E E≥

− =

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Superlattices

Intersubband emission

Superlattice consists of two (or more) different materials in alternating layers. The periodicity induces subbands within the conduction band and the valence band.

For electronic effects layer thicknesses are 1-10 nm and for optical effects 10-100 nm.

Superlattice structure

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Microelectronics and -photonics

Microcavity LED

There are already nm-scale layers in present devices.

E.g., the QW’s are 2-3 nm thick in white LEDs.

Transistor + pin-photodiode

Integrated optics

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Quantum wire

yz

x

• Quantum wire: 1D electronic system (confinement in 2D)

• Electrons and holes trapped in the wires are free to move only along the y-direction

InP

InP

InAs0.65P0.35

Ly

LzLx (110) cross-section TEM picture of

stacked InAs QWires in InAlAs matrix lattice matched to InP.

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E

Density of states: 3D, 2D and 1D

D(E)

E1e E2e E3eEC

3D2D1D

Note: At the absorption edge, the density of states is 0 in the bulk (3D) case. However, it is very large in quantum wires (1D).

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Fabrication of quantum wires

Top-down methods: wires, e.g., defined by lithography and consequent etching

Bottom-up methods: wires, e.g., grown by VLS (vapor-liquid-solid) method using metal particles as seeds

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Example: InP nanowires on InP by MOVPE

VLS growth of InP using In droplets

SEM image of InP nanowires on InP

TEM image of InP nanowires: the metal droplet can be seen at the end of the wire

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Applications of quantum wires

- Nanowire transistors, logic elements, electronic waveguides

- Optical waveguides,

optical emitters

- Sensors utilizing functionalized surface

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Density of states in QDsDensity of states in • 3-dimensional (bulk), • 2-dimensional (well), • 1-dimensional (wire) and • 0-dimensional (dot) semiconductors

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2 Quantum dots

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QD classification

Classification by structure

Particles

Composites

Single crystals

Classification by confinement potential Strongly confinedWeakly confined

Classification by fabrication Homogeneous nucleation Heterogeneous nucleation Kinetically confined synthesis Physical techniques (lithography, nanoimprinting, etc.)

Quantum dots (QDs): nanosize structures of crystalline nature, confined in three dimensionsClassification of quantum dots by various criteria:

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QD nanoparticles

QD band gap is effectively shifted in proportion to 1/R2. The size causes different colors in optical absorption and emission.

Fluorescence (emission) of CdTe quantum dots in solution. Color variation is due to diameter from 2 nm (green) to 5 nm (red).

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Core-shell QD- core-shell structure has a core QD surrounded by a thin shell of another material

- surface consists of a large fraction of the atoms in the quantum dot

=> surface structure important factor for the properties, e.g. biotin activated quantum dots (Evident Technologies)

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Examples of the fabrication methods of quantum dots

8 nm

CdSe

Smält kiseldioxid

Mask

AlGaAsGaAs

AlGaAs

-e

EtsningKvantpunkt

GaAs

InAs2 Självorganiserad tillväxt InAs

Homogeneous nucleation:nanoclusters in glass

Physical technique: patterning of heterostructures- e-beam lithography- maskless FIB lithography

Heterogeneous nucleation: self-assembled growth

SiO2 (insulator)=> optical color filters

GaAs QDetching

~20 nm

no artificial patterning!

=> degradation of optical properties due to processing steps

=> defect-free

structure

large surface/volume ratio

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Colloidal growth (kinetically controlled synthesis)

- monodisperse nanocrystals (diameter variation <5%) needed

- chemical synthesis (fig.): reagents are rapidly injected into hot solvent, colloids are formed in the supersaturated solution

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Group II-VI semiconductor nanocrystals

- group II-VI semiconductors ME, where M = Zn, Cd, Hg and E = S, Se, Te are the most common nanocrystals due to their ease of chemical synthesis (CdSe, ZnS...)

- more complex coated nanocrystals, such as CdSe/ZnS core-shell structure important (Evident Technologies)

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Group III-V semiconductors

- group III-V semiconductor nanocrystals such as InP and InAs can be produced similarly as the II-VI structures

- not very useful in applications

Epitaxial growth:

Fabrication of nanocrystals on surface by epitaxy (layer growth)

- growth from vapor phase (CVD), molecular beam epitaxy (MBE), laser ablation etc.

- good control of growth conditions required (amount of material, choice of materials, temperature)

- typically mismatch of lattice constants between deposited thin layer and substrate causes nucleation into nanoscale islands (quantum dots)

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Modern epitaxial techniques

- good control of layer thickness d(∆d < 1Å) and composition needed

MBE (molecular beam epitaxy)- ultra-high vacuum- like vacuum evaporation- often solid sources- several systems in Tampere, one in Micronova

MOVPE or MOCVD (metalorganic vapor phase epitaxy)- sources: vapors or gases- two systems in

Optoelectronics Lab., Micronova

P

As Al InGa

In

As

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Growth modes in epitaxy

Frank-van der Merwe (2-d) Volmer-Weber (3-d)

Stranski-Krastanow (2-d + 3-d)

Transition to 3-d growth after ultrathin strained wetting layer

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Coherent Stranski-Krastanow growth mode

Ge islands on Si not dislocatedEaglesham, Cerullo, Phys. Rev. Lett. 64, 1943–1946 (1990)

Stress is not released by dislocation formation. Strain energy is accumulated both in the island and in the substrate.

TEM image of Ge island on Si

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Self-assembled growth of III-V QDs

- InAs has 8% larger lattice constant than GaAs- after deposition of >1.7 monolayers of InAs,

small islands (~10 nm wide) are formed (energetically favorable) on a very thin 2D layer (wetting layer)

- islands are defect-free and act as quantum dots with a high density (~1010 cm-2)

E.g., InAs island formation on GaAs surface

Stranski-Krastanow growth mode AFM

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Example: self-assembled InP islands on GaAs

- from vapor phase or molecular beam at 500 - 650°C

AFM images of InP nanocrystals on GaAs surface. InP layer thickness is 3 monolayers (~0.9 nm). Density of 20 nm high nanocrystals is about 109 cm-2.

GaAs

InP

InP

ultrathin strained layer,~3 ML InP on GaAs

Tg=635°C

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Shape engineering of quantum dots

TEM cross section of InAs nanocrystal on GaAs surface.

AFM image of InAs(P) quantum rings fabricated at our laboratory.

Annealing of InAs dots in P atmoshere results in shape change.

- nanocrystals can be capped (e.g. with GaAs) to form buried quantum dots

- the shape can be altered either by the capping process or by annealing

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Stacked quantum dots

TEM cross section of stacked InAs quantum dots.

Multilayer stacks of quantum dots can also be grown- the quantum dots have laterally statistical distribution in position- vertical coupling due to strain fields causes vertical ordering- size and shape of dots can be tuned by GaAs

barrier layer thickness

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Pyramidal InAs QDs

40x40 nm2 cross-section STM current image of cleaved InAs quantum dot and the wetting layer.

Cross-sectional scanning tunneling microscopy (STM) of cleaved InAs quantum dots shows structural and compositional information with atomic resolution (fig.)

- the typical structure for capped dots is a truncated pyramide (below)

5 nm high and 15 nm wide InAs quantum dot

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Optical properties of self-assembled quantum dots

Modeling of the self-assembled quantum dot potential using a hemispherical cap of InAs on top of an InAs wetting layer embedded in a GaAs substrate and cap layer.

- density of state of quantum dots resemble that of atoms: sharp energy levels

- modeling of the quantum dot can be approximately done by using a simple structure (fig.)

Schematic of the energy levels in an InAs/GaAs self-assembled quantum dot having 5 electron and hole shells (s, p, d, f, g) with a degeneracy (2,4,6,8,10 particles / energy). The shells here are partially filled (state-filling process).

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PL spectra- ideal quantum dot system would

give narrow lines in optical spectra

- in real systems the size and shape fluctuation of the quantum dots broadens the spectra (fig. below)

- typical photoluminescence (PL) spectra of >>103 dots consists of Gaussian peaks (note state-filling)

State-filling of the quantum dot shells with increasing excitation intensity in low temperature photoluminescence (PL) spectroscopy. The inset shows a Gaussian fit used to deconvolute the contributions from the various shells.

PLpumpλ

ground state

excited states

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Stressor quantum dot structure- strain field of a self-assembled island causes local decrease of bandgap of a quantum well just below the island. The quantum dot has nearly parabolic potential for electrons and holes.

- almost perfect crystal structure

=> narrow intense PL peaks

1.20 1.25 1.30 1.35

Energy (eV)

QW

QD

CB

VB

BAND DIAGRAM

PL SPECTRUM

self-assembled island

quantum well

high excitation

low excitation( )substratea a>

AFM (1 x 1 µm)

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QD applications

Semiconductor quantum structures are already commonly used in optoelectronic applications such as telecom lasers, CD & DVD read-write heads, light emitting diodes (LEDs) etc.

QD structures are expected to improve performance, e.g, in near-infrared QD lasers (900 - 1300 nm), QD vertical cavity surface-emitting lasers (VCSEL), QD photodetectors. They might also enable new devices in, e.g., quantum computing.

QD VCSEL

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Photonic crystals

page 44Sea mouse

Natural photonic crystals

a = 510nm

a < 100nm

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Natural opals

2 μm

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Photonic crystal classification

Photonic Crystals (PhCs)

1D PhCsBragg, 1887

2D PhCs 3D PhCsYablonovitch et al., 1991

PhC fibersRussel et al., 1995

Planar PhCsKrauss et al., 1996

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• Studied by Lord Rayleigh in 1887

• Quarter wave layers: ,

Bragg grating mirror (1D PhC)

LL n

d4

0λ=

nLnH

dL dH

λ0

HH n

d4

0λ=

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Bragg grating mirror example: SiN/SiO2 mirror

When the incidence angle decreases, the reflection band becomes narrower and eventually vanishes

∆λ

λ0

Wavelength (nm)

Rel

fect

ivity

« stop band »

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From Bragg mirrors to photonic crystals

• Photonic crystal: generalization of the Bragg mirror concept to 2D and 3D periodic structures

• A 3D photonic crystal can have a full bandgap: it then reflects light for any incident angle.

• Full bandgap requires a large refractive index contrast in the structure.

Joannopoulos et al., MIT

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120°

120°

35°35°35°

120°

The ”Yablonovite”

• Manufactured by the Yablonovitch group at MIT in 1991• First 3D PhC with a full photonic bandgap in microwave range• Consists of a periodic pattern of holes drilled into plexiglas. Each

hole is drilled three times in three different directions• The obtained 3D pattern reproduces the diamond structure

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Artificial opals

Vos et al. Nature 430 654

• Opal can be manufactured by sedimentation of SiO2 spheres of controlled size (Left picture).

• Only inverted opals with refractive index above 2.2 exhibit a full photonic bandgap (Right picture).

Material Institute of Madrid

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Band diagram

Wavevector k

Nor

mal

ized

freq

uenc

y a/λ

Wav

elen

gth λ

(μm

)

full bandgap, transmission forbidden in all directions

no transmission in Γ−L (111) direction → reflectance maximum

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Self-assembled opals• Made by self-assembly of SiO2, PMMA or polystyrene nanospheres.

• Structure must be inverted with Si to obtain a complete bandgap

• Typical sphere size for bandgap around 1.5µm: 900nm

• Possibily to sediment nanospheres onto Si patterned substrates.

Material Institute of Madrid

3D Photonic crystals

Difficult to insert defects in the lattice… 2 µm

VTT+Tyndall (Cork)

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Lithography defined structuresTime consuming and complex…

Sandia Nat. Lab

3D Photonic crystals

D. N. Sharp et al., Opt. Quant. Elec. 34, 3 (2002)

10µmM. Qi, H. Smith, MIT

…or difficult to add defects

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Planar 2D PhCs

2D array of holesVertical structureConfines light in the

vertical directionControls light propagation

in the plane

2D PhCs• Relatively simple structure • Have most of the properties of 3D PhCs• Existing technologies can be directly applied or developed further• Compatible with planar optoelectronics

n1

n1

n2 > n1

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The InP/GaInAsP/InP system

xy

z

GaInAsPInP

InP substrate

Provides light confinement in the vertical direction

Hx

Hy

EzTM

2 polarizations: Transverse Magnetic like (TM)

Hz ~ 0

TE

Ex

HzEy

Tranverse Electric like (TE)Ez ~ 0

GaInAsP (n=3.35)

-2

-1

0AirInP (n=3.17)

InP (n=3.17)z

(µm

)

Low index contrast system (∆n = 0.18)

Weak confinement in the core

Field profile

Active system

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The Silicon-on-Insulator (SOI) system

xy

z

SiO2

Si

Si substrate

Provides light confinement in the vertical direction

Hx

Hy

EzTM

2 polarizations: Transverse Magnetic like (TM)

Hz ~ 0

TE

Ex

HzEy

Tranverse Electric like (TE)Ez ~ 0

High index contrast system (∆n = 1.95)

Strong confinement in the core

Si (n=3.4)

Air

SiO2 (n=1.45)

1

0

Field profile

Passive system

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W1 waveguide

2D PhCs etched in InP membranes

600 nm InGaAs

300 nm InP

Facet view

Top view

InP membrane = high index contrast system (∆n = 2.17)improved light confinement compared to InP/GaInAsP/InP

Sample facet

M. Mulot, M. Swillo, M. Qiu, M. Strassner, M. Hede, S. Anand,J. Appl. Phys. 95, p.5928, 2004

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1 µm

PhC waveguides

W1 waveguide

W1

wav

egu

ide

• Line defects in PhCs can be used to guide light

• 1 line defect = W1 waveguide, 3-line defect = W3 waveguide

• PhC waveguides are essential building blocks of a PhC integrated circuit

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Single defect resonant

wavelength: λi

λ1, λ2, ...,λi

λi

λ1, λ2, ...,λi-1

Filter combining cavity and waveguide

GaInAsP membraneNoda et al., Nature 2000

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Point-defect cavity

• One hole removed = defect in the PhC lattice• Simulation by 2D Finite Difference Time Domain method

detector

Normalized frequency (a/λ)

Det

ecto

r sig

nal (

a.u.

)bandgap

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Point-defect cavity

At the resonance wavelength, light is trapped in the defect

The point-defect defect acts as a trap for photons. Lightcannot escape the structure due to the surrounding bandgap.

Normalized frequency (a/λ)

Det

ecto

r sig

nal (

a.u.

)

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Single-cell photonic crystal laser

Q = 2500 (measured)

Hong-Gyu Park et al., Science 305, p. 1444 (2004)

Ith = 260 μAMax power: a few nW

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Fabrication – the stacking method

© Crystal Fibre A/S

Photonic crystal fibers

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• Large mode area fibers

• Nonlinear fibers

• Polarization maintaining fibers

• High numerical aperture fibers

• Double cladding active fibers

• Air-guiding fibers

Photonic crystal fibers: Applications