Nurmikko , Arto

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Nurmikko , Arto

Nanophotonics – so what, and for what?
Arto V. Nurmikko*
Brown University
my own lesson in life: new technologies are unpredictable vs. long term impact
ZnSe green-blue
cw QW diode laser
Jeon et al (1993)
Nanoscale Energy Conversion Workshop – Sept 2006, Nice
Major Photonics Application/Technologies
Focus on “active devices” (vs. passive ‘optical wires’):
IC Lithography
Etc.
Displays
Optical telecom
Optical storage
Photovoltaics
0.2
•
•
•
•
0.6
1
λ(µm)
mostly single crystal epitaxy
e.g. highest diode laser efficiency >70% (VCSEL)
e.g. multijunction tandem PV cell ~40% (Spectrolab), and poly-Si
e.g. white light inorganic and organic LEDs
Nanophotonic Devices – Is smaller better ?
• smaller is better only if it is a lot better (performance, cost, application)
• otherwise (and additionally) need and explore novel application spaces
‘few photon’ (single photon)
coherent sources
Decreasing source size
FP/DFB lasers/VCSELs/RCLEDs
~λ
Photonic Crystal LEDs
and diode lasers
<λ
1) Require creative fabrication strategies for:
(i) Nanomaterial/composite assembly
(ii) Electrical access/junctions
(iii) nano-macroscale bridge for process
flow (compatibility issues)
2) Look for enhanced light-matter coupling
Nanophotonic regime
at least 2-dimensions of
(individual) elements are <<λ
Top-Down or/and Bottom-Up Fabrication
1 nm
10 nm
100nm
High resolution
lithography (ebeam)
Direct nanomaterials
synthesis
epitaxy
vs.
colloidal
QDs
1000nm
1x1 µm2
GaN QDs
Challenge: assembly, contacts
InGaN QWs
Challenge: size limit/expensive
Possible Elements of a New Toolkit
e.g. for a “few photon” or single photon source
1) Enhanced Light Matter Interaction
- semiconductor microcavities vs. atom microcavity physics
- near field (dipole-dipole) collective interaction
2) Efficient internal energy transfer on ‘nanoscale’ (Forster = dipole/dipole)
e.g. from pump or for multiple-element chromophores
3) For coherent sources (including single photon emitter), need strong
local feedback on sub-λ scale
Nanocomposite active optical material
Possible Elements of a New Toolkit
“Piecewise Material Examples”:
QD/J-aggregate
J-aggregate/
microcavity
InGaN nanopost
Arrays
Strong LightMatter
Coupling
Efficient
Internal
E-transfer
“Local
Feedback”
(resonatorless)
Electrical
Injection
Plasmonic particles
in gain medium
InGaN/organic
junction
• Interfaces and interactions: excitation vs. charge transfer
• Inorganic, organic, and noble metal nanomaterials
Cartoon Approach to Design
What is this ?
Nano-optical
antenna
Contact layer
Nano AND macroscale contacts
Photoelectronic
conversion; charge
and excitation transport
Nano composite layer
Contact layer
Flexible substrate
Need a spatially organized, optically high density,
electronically “flexible”, and low loss electrically
accessible nanomedium (for emitters and possibly PV)
1) Basic Semiconductor Microcavity Physics
ca. 1994-2005
e.g. organic
semiconductor
microcavity
(~4 monolayers)
e.g. ZnCdSe QW microcavity
Strong Light-Matter Coupling Regime in Semiconductors:
Single Exciton (atom) regime and QED
Reality Check:
Light-matter coupling strength:
ΩR = µEvac/ħ = (πe2f)1/2 / (4πεmoVm)1/2
Cavity modal volume:
n2εo|Evac|2 Vm = hν/2
Strong coupling criteria:
g2 > (γc- γx)2 /16
Possible to achieve ħΩR > kT near temperature for a nanostructured
Semiconductor-based structure for single-photon regime
Device Example : Single Photon Emitter
(a) Single organic molecule
in single mode 3D microcavity
(b) Single InAs quantum dot
in 3D microcavity (2004)
“random”
e.g. J-aggregate
Lcoh ~ 100 nm (RT)
Reichtmaier et al (2004)
Yoshie et al (2004)
Arakawa et al
• a special “zero-threshold” laser
• single exciton “molecule” (“two-level atom) within 3D confined optical field:
analog to single-atom-in-microcavity (!)
• fJ-aggr ~ 10 – 100 fQD Æ room temp operation in strong coupling regime
• QED and ‘special’ photon statistics for quantum information processing
Prelude: J-aggregate Organic in a Microcavity
• Organic semiconductors and organic/inorganic hybrids/nanocomposites
• Microcavity effects to enhance light-matter interaction: Exciton-Polariton
0.25
Absorbance (αL)
J-Aggregate
J-band 694nm
Monomer
0.20 Cyanine dye/PVA-J aggregate
0.15
monomer
(solution)
FWHM
= 20nm
0.10
“extended Frenkel”
e.g. J-aggregate (vs.monomer):
• giant exciton oscillator strength
• fast relaxation time
• imbed in inorganic microcavity
• recently: layer-by-layer deposition
α > 106 cm-1 (Bradley et al 2006)
0.05
0.00
400 450 500 550 600 650 700 750 800
Wavelength (nm)
Room Temperature absorbance
Extraordinarily dense
Potential optical
Gain medium
An Organic Exciton-Polariton Microcavity
optical pumping: e.g. Lidzay et al
• λ/2 microcavity
• Normal mode (Rabi) splitting
~ 200 meV >> kT
J. Tischler at al, PRL (2005)
Exciton-Polariton Organic
Microcavity LED
• implementation with metallic reflectors
• emission from lower polariton band
• possibilities for a polariton laser ?
J. Tischler PRL (2005)
(A)
(B)
Energy (eV)
3 2.7 2.4 2.1
3 2.7 2.4 2.1
1.8
70
UB
LB
50
40
30
20
10
500
600
700
Wavelength (nm)
1.8
1.0
PL
0.9
0.8
0.9
0.8
Abs
0.7
0.6
UB
0.5
LB
0.4
0.3
0.7
0.6
0.5
0.4
LB
UB
0.3
0.2
0.2
0.1
0.1
0.0
400
Energy (eV)
3 2.7 2.4 2.1
EL Intensity (a.u.)
80
Absorbance, PL Intensity (a.u.)
90
(%) Reflectivity
1.8
1.0
100
60
(C)
Energy (eV)
0.0
400
500
600
700
Wavelength (nm)
400
500
600
700
Wavelength (nm)
Current efforts: J-aggregates as superhigh gain medium
• employ layer-by-layer synthesis
• measure coherence area by fsec 4-wave mixing spe’cy
• aim at a 2D “crystal” of 100 nm coherence area: giant dipole
for a single photon emitter
2) Examples of Interactions and Interfaces
Organic/inorganic semiconductors and metal nanoparticles:
Energy transfer:
Charge transfer:
• InGaN nanopost arrays
• InGaN/organic heterojunction
• J-aggregate-QD transfer
• Plasmon focusing
B850nm
B800nm
e.g. variable
D-A length
Eg. Rhodopsoremnas acidophilia:
• a truly multichromophore system: beyond Förster theory
• very high local chromophore density
(a) Multichromophore, High Density Nanoparticle
“Artificial” Composite Material Systems ?
InGaN nanoposts/ODs
i) Simple Forster (inelastic photon tunneling):
organic
medium
KR ~ (n-4)(Ro/R)6
ii) Multichromophore enhancements:
P
Photons
P
~10-50 nm
• multiple length scales over which D-A centers interact
• degenerate multiexciton systems (‘vanishing Stoke shifts’)
• quantum mechanical coherence and collective effects
Silbey, PRL 2004,
Dicke: superradiance
InGaN Nanorod Mesoscopic Active (Optical Gain) Media
Yiping He (2004-2005)
e.g. 10 InGaN QWs
~ 40-60nm pillar diameter
~ <50 nm edged-to-edge separation
360
380
400
420
440
Wavelength (nm)
• high resolution ebeam litho, etching
• high spontaneous emission efficiency:
low surface state recombination
• stimulated emission at very low threshold
• physics: photon localization vs. dipoledipole interaction (nanoscale resonators)
AlGaN
200nm
InGaN MQW
Active medium
sapphire
substrate
GaN buffer
layer ~2µm
Enhanced Photon-Exciton Coupling in High
Density Nanorod Arrays
(1) Evidence for enhanced photon-electron interaction on ~1 um scale:
•
•
Photon scattering (localization/effective mean free path)
Near-field electrodynamics (dipole-dipole interaction: multichromophore)
(2) Nonideality factors from surface roughness and fabrication
imperfections: a form of inhomogeneous broadening
very strong coupling/short photon mean free path in a high fosc medium
Prior work in “random lasers”:
a) Molecular dyes in “ground glass”
(e.g. Lawandy et al, 1995)
b) Random ZnO nanocystallites in
dielectric host (Cao et al, 2000)
Photon diffusion length < 100 nm
(b) Energy Transfer from Colloidal QDs to J-aggregate
Colloidal QD as the “pump”
QD in silica spheres,
Organic ‘cladding’
Absorbance Spectrum
0.6
TTBC J-agg
0.5
Abs
CdSe/ZnS QD emission: 565nm
J-aggregate emission: 580nm
0.7
0.4
0.3
QD & TTBC
0.2
Optical pumping:
0.1
0.0
400
• QD emission was significantly
quenched
• The presence of QDs may
interfere with formation of Jaggregates
450
100
500
550
600
650
700
Wavelength (nm)
PL Spectrum
Ex@380nm
QD565
div by 5
80
Intensity (a.u.)
• TTBC J-agg emission was redshifted and greatly enhanced
QD565
60
QD & TTBC
40
20
TTBC J-agg
0
500
550
600
Wavelength (nm)
650
(Zhang 2006)
Forster Energy Transfer from QDs to J-aggregate
1.2
300
Absorbance Spectrum
1.0
250
Abs
0.8
Intensity (a.u.)
QD & TTBC
TTBC J-agg
0.6
0.4
0.2
QD655
200
150
TTBC J-agg
100
QD655
0
400
450
500
550
600
650
700
550
Wavelength (nm)
PLE Spectrum
Em@653nm
300
200
1-T
0.6
0.4
QD655
0.2
0
450
500
700
0.8
QD & TTBC
400
650
QD emission: 655nm
J-aggregate emission: 580nm
1.0
100
600
Wavelength (nm)
500
Intensity (a.u.)
QD & TTBC
50
0.0
400
PL Spectrum
Ex@530nm
550
600
Wavelength (nm)
• QD emission was greatly enhanced
when the excitation wavelength is in
the neighborhood of TTBC absorption.
• Monomers and dimers seems to
couple better to QDs than J-aggregate
• J-aggregate emission was still redshifted
(c) Metal Nanoparticles (resonant plasmons) to enhance
efficiency local field/feedback on sub-λ scale?
•
“Plasmonics”: giant optical antenna effects on sub-λ size scale
– means to couple, guide, and concentrate optical field
– also for providing interconnects to nanorods/nanocrystals
•
Metal nanoparticle-enhanced semiconductor quantum dot emission ?
– Plasmon Extinction = Absorption + Scattering
photons-in
QD
“cast a giant
Shadow”
Enhancement
Quenching
photons-out
M
“absorptive”
QD
photons-in
M
“scattering”
Concentration/Scattering of Light on Nanoscale (<<λ)
0nm
“cast a giant
shadow”
E
20nm
80nm
NSOM image
SEM image
(Atay et al,
Nanolett 2005)
100 nm
100 nm
after ultrashort pulse
laser irradiation (at ωp)
• tailoring the surface plasmon resonance by adjacent nanoparticle interaction
• optical field local concentration 10-100 fold at touching point(estimate)
• colloidal QDs added (so far) for resonant energy transfer/interaction
Nanoscale
Energy in
Conversion
– Sept 2006,
• immerse
optical Workshop
gain medium:
giantNice
scattering cross section (low loss) (Lawandy)
(c) Nanocomposite II-VI QD-Ag Structures
Colloidal II-VI and InGaN nanocrystals
Patterned area
(100 μm × 100 μm )
PMMA
200~400nm
50nm
QDs
30nm
Excitation
•
SEM image after developing
(pattern #4)
• Most QDs within SPP field
• Localized + propagating
SPP
PL
• Samples: Diameter – lattice
constant
#1: 100nm – 200nm
#2: 100nm – 260nm
#3: 140nm – 300nm
#4: 160nm – 300nm
J.H. Song, Nanolett (2005)
(d) Hexagonal Dense Array of Colloidal II-VI QDs
Before J-aggregate cladding: closely packed
Silica
sphere
Q. Zhang (2006)
Centered
CdSe/ZnS
QD
Spatial control of QD or nanorod placement
Colloidal particles
Single (coated) QDs in nanofabricated 100 nm “wells”
Conductive
back electrode
CdSe/ZnS QD/silica captured in well
• Self-assembling of colloidal QDs onto electron-beam lithography patterned
PMMA template (by capillary and other driving force).
• aim at single photon statistics (photon antibunching) under optical pumping
Summary: Can we really make something like this?
Nano AND macroscale contacts
Nano-optical
antenna
Contact layer
Photoelectronic
conversion; charge
and excitation transport
Nano composite layer
Contact layer
Flexible substrate
Acknowledgements:
V. Bulovic, J. Tischler, S. Bradley (MIT)
Jung Han (Yale)
T. Atay, Q. Zhang, Y. He, Y.-K. Song, R. Zia (Brown)

Nurmikko , Arto

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