ELEMENTS OF NANOPHOTONICS: PHOTONIC CRYSTALS, PHOTONIC WIRES, METAMATERIALS...

OptoelectronicsResearch

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UNIVERSITY OFGLASGOW

Nano-Photo COMMAD 06 1

ELEMENTS OF NANOPHOTONICS: PHOTONIC CRYSTALS, PHOTONIC WIRES, METAMATERIALS

AND MORERichard M. De La Rue, Ali Z. Khokhar, Basudev Lahiri, Marc Sorel,

Faiz Rahman, Marco Gnan, Harold M.H. Chong, Pierre Pottier, Ahmad Md Zain, Charles N. Ironside, C.Walker, J. Wale

Ogungbenro, Nigel P. Johnson and Scott McMeekin*Optoelectronics Research Group, Department of Electronics and Electrical Engineering, University of Glasgow, RankineBuilding, Oakfield Avenue, Glasgow, G12 8LT, Scotland, UK.* Department of Electrical and Electronic Engineering, School of Science Engineering and Design, Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 0BA, Scotland, U.K.


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Introduction•Photonics is now well-embarked into the nano-metre regime •New ways of engineering,

•self-organising nano structures,• metamaterials (structures with new electromagnetic properties),• new types of gratings, waveguides •laser resonators

•In this presentation, will be diverse rather than comprehensive and reflects the work at the University of Glasgow.


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OutlineFabrication techniques for nanophotonicsNanophotonic structures, Opals, GaN structures, metamaterialsNanophotonic Devices, Compact Mach-Zehnderinterferometers, Photonic Band-gap Quantum cascade lasersSummary


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Fabrication ProcessesFabrication Processes

DirectDirect--Write Electron Beam LithographyWrite Electron Beam Lithography

substratesio2

SiO2 deposition Resist coating/bake E-beam exposure

Dry etching: 2-stage process usingsilica as pattern transfer layer

Resist removal/ cleavePattern development


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Cross-sectional scanning electron-micrograph of 2D photonic microstructure in AlGaAs/GaAs waveguide.

Deeply etched 2D array of holes

Pattern transfer to heterostructure waveguide by RIE (e.g. using SiCl4)

C.J.M. Smith et al, "Use of polymethylmethacrylate as an initital pattern transfer layer in fluorine and chlorine-based reactive ion etching", J. Vac. Sci. Tech., B17, pp.113-117, Jan/Feb (1999).


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Engineering “self-organising” Nano-photonic materials


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Studies of nearStudies of near--perfect, templateperfect, template--driven (100)driven (100)--oriented opal: 3D PhC structureoriented opal: 3D PhC structure

Large area crystal: ~ 1 mmLarge area crystal: ~ 1 mm22 --> or more > or more (possibly).(possibly).

Plan viewPlan view Tilted View of cleaved edgeTilted View of cleaved edgeC. Jin, M.A. McLachlan, D.W. McComb, R.M. De La Rue and N.P. Johnson, 'Template-assisted growth of nominally cubic (100)-oriented three-dimensional crack-free photonic crystals, Nano Letters, 5 (12), pp. 2646-2650, (2005).


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Inverted opal structureInverted opal structureSpheres provide a template for inverted structure formed by in-filling of interstices, followed by selective sphere removal. (Differential etching or combustion.)

300 nm

Polycrystalline TiOPolycrystalline TiO22(Anatase phase) inverted (Anatase phase) inverted structure formed by structure formed by calcination of latex calcination of latex sphere opal structure insphere opal structure in--filled with titanium filled with titanium ethoxide solution.ethoxide solution.

Black arrows show microBlack arrows show micro--voids formed voids formed during calcination.during calcination.


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GaN selective area growth (SAG) using MOVPE GaN selective area growth (SAG) using MOVPE ––towards real luminescent PhCs in the blue? towards real luminescent PhCs in the blue?

AmorphousSiO2 mask layer

Sapphiresubstrate

GaN seed layer

GaN micropyramids,used to template InGaNquantum wells (brown)


Sapphiresubstrate

GaN seed layer


Sapphiresubstrate

GaN seed layer

Sapphiresubstrate

GaN seed layer

Sapphiresubstrate

GaN seed layer

GaN micropyramids,used to template InGaNquantum wells (brown)

Schematic cross-section showing theformation of GaN micropyramids. Growthsteps that produce planar InGaN QWs onthe pyramid facets induce the formation

of InGaN quantum dots at the peaks.Typical setpoint temperatures are 1100˚C

for GaN growth, and 800˚C for InGaN growth.

5um


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GaN selective area growth (SAG) using MOVPE GaN selective area growth (SAG) using MOVPE –– towards real towards real luminescent PhCs in the blue? luminescent PhCs in the blue?

See also: D. Coquillat et al, PhysicaStatus Solidi (A) Applied Research,202 (4), pp. 652-655, March 2005. Measurements of PhC band-structure in the mid-IR.

HexagonalHexagonal--base 'GaN' pyramids grown through small apertures in silica maskbase 'GaN' pyramids grown through small apertures in silica mask layer layer --with with InGaNInGaN MQW layers included: I.M. Watson et al, BACG Meeting, SheffieldMQW layers included: I.M. Watson et al, BACG Meeting, Sheffield U.K., U.K., 2005.2005.


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Metamaterials


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MetamaterialsAn object that gains its (electromagnetic) material properties from its structure rather than inheriting them directly from the materials it is composed of.

Usually when the resulting material has properties not found in naturally-formed substances.

First metamaterials used split ring resonators at microwave frequencies D.Smith et al

Metamaterials with negative N have numerous startling properties:•Snell's law (N1sinθ1 = N2sinθ2) still applies, but rays refract on the same side of the normal on entering the material. •The Doppler shift is reversed (that is, a light source moving toward an observer appears to reduce its frequency) •Cherenkov radiation points the other way •The group velocity antiparallel to phase is velocity (as opposed to parallel for normal isotropic materials) •Higher frequencies have longer, not shorter, wavelengths in such a material

http://en.wikipedia.org/wiki/Snell%27s_law

http://en.wikipedia.org/wiki/Doppler_shift

http://en.wikipedia.org/wiki/Cherenkov_radiation

http://en.wikipedia.org/wiki/Group_velocity

http://en.wikipedia.org/wiki/Antiparallel

http://en.wikipedia.org/wiki/Phase_velocity



http://en.wikipedia.org/wiki/Wavelengths


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Metamaterials for optical frequencies Metamaterials for optical frequencies -- in gold (Au) on in gold (Au) on silicon (Si) silicon (Si) -- by EBL patterning and liftby EBL patterning and lift--off process.off process.

l

w

2/1)/( dwlLC πλ 2≈

d

a

Thickness t


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Reflectance Measurements and simulations Reflectance Measurements and simulations ssmall split (open) mall split (open) rings. rings. Dimensions: Dimensions: l=l= 217, 217, a=a=297297, w=, w=66, 66, dd=33 nm, =33 nm, t=t=2525 nmnm

Measurement Measurement

100

120

140

160

180

1 2 3 4 5 6 7 8wavelength (um)

100

120

140

160

180

1 2 3 4 5 6 7 8wavelength (um)

0.4

0.65

0.9

0.6 1.6 2.6 3.6 4.6 5.6 6.6wavelength (um)

0.4

0.6

0.8

1

0.6 1.6 2.6 3.6 4.6 5.6 6.6wavelength (um)

SimulationSimulation

Open RingsTE TM

N.P. Johnson et al: 'Increasing optical metaN.P. Johnson et al: 'Increasing optical meta--materials functionality', COO, SPIE meeting, Warszawa, Aug/Sept,materials functionality', COO, SPIE meeting, Warszawa, Aug/Sept, 2005.2005.


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Effect of silicon substrate

0.3

0.350.4

0.450.5

0.550.6

2 4 6 8 10 12 14

Wavelength (µm)

Ref

lect

ance

(AU

) Si

SiO2

FDTD simulations of identical gold SRRs on silicon and silica substrates l=564, a =750, w =1 30, g=120 nm, t =30 nm


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Split Ring Resonators SRRsSRRs on Silicon made from different metals

Silver –prone to diffusionand granularity

Gold highest fidelityso far but not welcomein Si foundry

Aluminiumgood compatible with Si foundries


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hυ>Eg

e-h pairs

Light resonantwith SRRs

Silicon substrate

Proposed device – object is to open and close the rings by non-resonant light

Recent Use of LTGaAs W J Padilla Phys. Rev. Lett. 96, 107401 (2006)

The penalty of using silicon substrates in terms of wavelength can be more than compensated by the increased functionality that becomes available.

Non resonantLight aboveBandgap of Silicon


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Modeling the shift in plasma frequency

Reduced plasma frequency (from dilution) for isolated metal rods is given by

a

Need different model for SRRs

For arrays of SRRs define F which is the areaof exposed silicon in a unit cell ie total area of unit cell minus area covered by the metal SRR a

As the metal area of an individual SRR increases, for a fixed unit cell area, F decreases and the plasma frequency decreases. (But alsoeffective mass of electrons increases with magnetic field)

1.J B Pendry, A J Holden, D J Robbins and W J Stewart, pp4785- 4809 J. Phys.: Condens. Matter 10 (1998).

( ))/ln.2.

22

22

Faac

pπω =


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Gap capacitance

Inter-elementcapacitance

a

l

t

w

gd

TTLC CL

1=ω

Where LT is the total inductance of a rectangular wire and is given by the sum of the sum of the ring self inductance and the mutual inductance

For electrically induced magnetic resonance LC

LRing = 0.2µo −w2

sinh−11+w2

2 + l −w2

⎛ ⎝ ⎜

⎞ ⎠ ⎟ sinh−1

l −w2

w2

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

− l −w2

⎛ ⎝ ⎜

⎞ ⎠ ⎟

2

+w2

⎛ ⎝ ⎜

⎞ ⎠ ⎟

2⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

H .....(1)

LMutual = 0.2.l ln ld

+ 1+l2

d2

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟ − 1+

d2

l2 +dl

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟ µH..............(2)

1.Zahn M, “Electromagnetic Field Theory” pp343, John Wiley & Sons, 1979.2.Grover F. W. “Inductance Calculations, Working Formulas and Tables”, pp 35 Dover Pub. 1946.


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Previously only parallel plate capacitance ofthe gap in the SRR element taken intoaccount but must also include the inter-elementcapacitance using the standard expression.

SeparationAreaC orεε

=

Substrate dielectric taken in to account in coplanar capacitance calculated by conformal mapping techniques to take account of the field in the substrate.

For two metal strips of width p separate by distance q [1]where k’ is given by:

CPT CCC +=

Gap capacitance

Inter-elementcapacitance

a

l

t

w

g

d

Ccp =εr +1( )εo

21π

ln 21+ k '

1− k '

⎡

⎣

⎢ ⎢

⎤

⎦

⎥ ⎥

F /m k ' = 1−p

p + 2q⎛

⎝ ⎜

⎞

⎠ ⎟

2⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

1.Wheeler H. A. IEEE Trans Microwave Theory Tech. MTT-13, 1965, pp172-185.


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Comparison of models with FDTD (Finite

Difference Time Domain) simulations

02468

101214

0.7 0.9 1.1 1.3

Change in Lx and Lz

Wav

elen

gth

(µ

m)

0

2

4

6

8

10

12

0.7 0.9 1.1 1.3

C hang e in Lx

Wav

elen

gth

(µ

m)

LC resonance LC resonance

Plasma ResonancePlasma Resonance

Plasma frequency model FDTD simulationFDTD simulation LC model

N. P. Johnson, A. Z. Khokhar, H. M. H. Chong, R. M. De La Rue and S. McMeekin, Electronic Letters 42 p1117-1118 (2006)


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Bragg Grating Structures


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Micrograph of Micrograph of PhWPhW Bragg Bragg grating and transmission grating and transmission spectrumspectrum

1460 1480 1500 1520 1540 1560 15800.00.10.20.30.40.50.60.70.80.91.01.11.2

Tran

smis

sion

(a.u

.)

Wavelength (nm)

See also: See also: M.GnanM.Gnan et al, 'et al, 'ModellingModelling of photonic wire Bragg of photonic wire Bragg gratings, Opt. gratings, Opt. Quant.ElectronQuant.Electron., 38, pp. 133., 38, pp. 133--148, (2006).148, (2006).


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PhC Mirror Fabry-Perot filtersGuided-wave PhC microcavity for WDM add-drop filtering?

Good potential performance from PhC F-P filter with moderately high-Q, extended PhC mirror, microcavity in waveguide.

C. Ciminelli et al, ECOC 2004, Stockholm, (5th-9th Sept. 2004), Post-Deadline papers, pp.26-27, Th4.2.6.

PhC MirrorsCompact FPstructure

MicrocavityMicrocavity


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PhC Fabry-Perot filtersTransmission characteristics of an 8 µm long PhC F-P filter.

Free Spectral Range = 37 nm

Resonance λ[nm]

Estimated Q-factor

1486.65 1900

1525.78 1400

1563.17 1500


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Tuning of PhC F-P filters by thermo-optic effect

PhC Reflectors

NiCr

HeaterIntegration of thin film metal heater on to cavity region.

Nichrome thickness = 120 nm. Resistance ~ 1kΩ.

Silica buffer layer thickness is ~300 nm: to reduce optical losses.


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Tuning of PhC Tuning of PhC FF--P filters by P filters by thermothermo--optic effectoptic effect

0

5

10

15

20

25

30

35

1526.4 1526.5 1526.6 1526.7 1526.8 1526.9 1527 1527.1 1527.2

Wavelength / nm

Det

ecto

r sig

nal /

µV 0V

1.5V

0

5

10

15

20

25

30

35

1526.4 1526.5 1526.6 1526.7 1526.8 1526.9 1527 1527.1 1527.2

Wavelength / nm

Det

ecto

r sig

nal /

µV 0V

1.5V

Transmission characteristics of the PhC F-P filter.Individual spectral features produced by double F-P cavity behaviour associated with sample ends.Demonstration of blueblue--shiftshiftof the filter response when heater switched on at 1.5 V. Small shift of ~ 0.06 nm.Shift is in opposite Shift is in opposite direction to the expected direction to the expected one.one.

Heat going in wrong place(heating silica) High Q by multiple cavities Short inside a long cavityOpposite shift could be arranged for selfcompensation


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Mach-ZehnderInterferometers (MZIs)


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Integrated Mach-Zehnder Interferometers (MZI) Integrated MZIBulk optics MZI

DetectorDetector

Signal @detector

⎟⎠⎞

⎜⎝⎛= 2Path Optical- 1Path Optical22

λπACosSD

Path 1

Path 2DS

Path 1

Path 2

Small differences in path length used to switch the device


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Photonic wire in Silicon on Photonic wire in Silicon on Insulator (SOI)Insulator (SOI)

500nm

SiO2

Si

SiO2

PECVD Grown Layer

Guiding Core

Lower Cladding

150 nm

260 nm

1000 nm

SOINarrow waveguide: 500nm Trade-off: low propagation loss/Single even mode behaviourHighly confined fundamental mode @ λ=1.55µmPropagation Loss Figure: ~ 10 dB/cm


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MachMach--ZehnderZehnder simulationssimulations

SymmetricSymmetricMZIMZI

AsymmetricAsymmetricMZIMZI


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Fabrication Fabrication ImperfectionsImperfections

1. Making a sharp wedge is a tricky business…

2. Roughness introduced by the etching of the silica mask


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Properly etched samplesProperly etched samplesH.ZhangH.Zhang et al. IPRA meeting, Connecticut, USA, April 2006.et al. IPRA meeting, Connecticut, USA, April 2006.

Symmetric MZ Asymmetric MZ


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PhC Mach-Zehnder Interferometers

The image shown was used in specifying the lithographic pattern generation process. With a constant lattice period throughout the structure, the different block colors indicate small changes in the hole diameter that are designed to reduce back-reflection in the waveguide channels. Additional holes in the Y-junctions increase the bandwidth-transmission product


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Looking CloserLooking Closer

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Smallest MZI its length is approximately the diameter of an optical fibre


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Asymmetric MachAsymmetric Mach--Zehnder modulatorZehnder modulatorThermoThermo--optic transmission characteristicoptic transmission characteristic

0

1

2

3

4

5

6

0 20 40 60 80

Power / mW

Ligh

t Out

put /

A.U

.

SwitchingCurve Fit


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Photonic-Band gap structures for Quantum Cascade Lasers


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Why Photonic Band Gap and Quantum cascade laser

TM polarised therefore no vertically propagating modes - so 2 dimensional grating gives quasi- 3 dimensional confinementOnly electrons, no holes - so no surface recombination


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Photonic Band-Gap Quantum Cascade Laser

Photonic band-gap surrounding laser cavityBack reflector - reduced loss from back facetBetter confinement of lightReduce overall laser dimensions

Reduce threshold current and device heating

Laser cavity

Photonic Band-Gap (PBG) columns

PBG forms high reflectivity mirror

and waveguide

Cleaved facet

Laser output

Current injected into laser cavity and columns


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Photonic Band-Gap Design and Simulation

Laser wavelength ~ 10 mmLaser is TM polarisedFrom band-gap map

Columns rather than air holesChoose triangular crystal lattice

Photonic Band-GapIndex contrast = 2.4Period = 3 mmPillar diameter = 1.5 mm

2D Simulation using FullWAVE

ΓK

ΓM

High reflectivity stop band at the lasing wavelength

4 5 6 7 8 9 10 11 12 13 14 15 160

102030405060708090

100ΓK

ΓM

Hig

h re

flect

vity

sto

p-ba

ndfo

r qua

ntum

cas

cade

lase

r

Ref

lect

ivity

(%)

Wavelength (µm)


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Dry Etched PBG Pillars

SiCl4 RIEDeep, vertical etching

Ti/Au/NiCretch mask

Very durable

Demonstrates a process suitable for device fabrication


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Laser Results – First Generation Device

First attempt at devicePhotonic Band-Gap

Not fully etchedPoor mirror

Tested at Sheffield University

Acknowledgements to J. W. Cockburn and R. P. Green.

0 1 2 3 4 5 6 7 8

10.9 11.0 11.1 11.2 11.3 11.4

FTIR measurementT = 14 K

λ = 11.17 µm

Wavelength (µm)

1 kHz, 100 ns pulse

13 K30 K

50 K

77 K100 K120 K

Ligh

t (a.

u.)

Current (A)Current injection through metal etch mask

Epi –layer down mounting technique (using forming gas)


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Summary

•Nano-photonics has promised and delivered on new electromagnetic properties, applications and devices –it certainly has provided some startlingly good illustrations of interesting physics

• Issues of manufacture, losses and broadband operation need to be addressed before wide scale application on a commercial scale becomes feasible.

ELEMENTS OF NANOPHOTONICS: PHOTONIC CRYSTALS, PHOTONIC WIRES, METAMATERIALS...

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