Claude Weisbuch

Claude Weisbuch Materials Department, University of California, Santa Barbara, USA Laboratoire PMC, Ecole Polytechnique, Palaiseau, France Strong coupling 1 : Fermi Golden rule not valid: tri-­‐dimensional systems-­‐ excitonic polariton, resonant Brillouin sca<ering, resonant polariton fluorescence, RIRS, RRS R G Ulbrich, N Van Hieu, A Nakamura end of 70ies Low D1 quantum wells: free exciton luminescence (2D/3D), interface disorder, R Dingle , A C Gossard, W Wiegmann, PM Petroff, WT Tsang end of 70ies QW laser J Nagle, S Hersee, JP Duchemin, M Razeghi ‘ 80ies Quantum dots, bo<leneck, … end of ‘ 80ies H Benisty, E Bockenhoff Strong coupling 2 cavity polaritons Y Arakawa, M Nishioka, R Houdre, R Stanley, U Oesterle, M Ilegems ‘ 90ies controlling flow of light : weak coupling -­‐ Fermi’s Golden rule-­‐modify photonic density of states Microcavity LEDs, photonic crystals for telecoms or LEDs H Benisty, D Labilloy, M RaMer, E Schwoob, A David, S Denbaars, S Nakamura, J Speck, E Hu, E MaPoli, E Rangel ‘ 00ies plan 1 Strong coupling I : the environment in late 80’ies 2 Strong coupling II : how was it discovered? 3 microcavity light emi<ers a) strong coupling ? b) weak coupling design rules c) microcavity LEDs the limits no photon recycling 4 visible light emi<ers today Summary 1 Strong coupling I : the environment •  QWs/QWWs/QDs –  e-­‐h –  Excitons in 1991: 3D vs 2D •  Purcell •  Microcavi\es : CQED with atoms Strongly-coupled 3D excitons and photons, excitonic polaritons
are the quasi particles of the system
Exciton ω.k
photon ω.k
photons
excitons
Phys. Rev. B 7, 4568 (1973)
In the polariton picture, light scattering
becomes a first order process, due to Resonant Brillouin scattering
the scattering of an excitonic polariton. By bulk excitonic polaritons
Outside photons transform into
polaritonswith unit efficiency at crystal
interface
Efficiency of Brillouin scattering ≈ 10-12
in opaque materials
Brillouin scattering is the
Need of multipass FP spectrometer to reject
scattering of an excitonic polariton photoluminecence
by an acoustic phonon.
Efficiency of resonant Brillouin scattering ≈ 10-3
Surprise: in quantum wells, free excitons dominate luminescence!
Rashba theory
of bound excitons
GaAS QW
Bulk GaAS
Explanation: Agranovitch& Dubovskii, 1966
3D exciton-polariton
luminescence is a forbidden
process (needs break of
symetry): 3D excitons
transform into 3D photons and
vice-versa, back and forth.
3D luminescence is due to the
outcoupling of a polariton at
the air interface.
QW luminescence is the
coupling of 2D electronic
system with 3D photons = no
polariton effect as reversibility
is possible, k is not conserved.
Exciton luminescence is
allowed, and fast!
More on low D excitons and 3D photons:
Temperature dependance of lifetime of QW and Q wire excitons:
Fraction of population within light cone (depends on DOS)
~  T for QWs,
~ T 1/2 for QWRs
Gershoni & Katz, Phys.Rev. ‘ 94
Trying to understand luminescence: Linewidth increases with increasing quantum confinement Measuring all samples grown in Bell Labs
1974-1980
No data in this region! Linewidth is due to single atomic layer fluctua\ons! Macroscopic manisfestation
Of microscopic disorder
Solid state communs 1981 Resonant Raleigh sca<ering of an inhomogeneously broadened transi\on Hegarty et al. Phys.Rev. Lett. 49, 930, 1982
absorption
PL
Resonant
Raleigh
scattering
Mobility
edge
Fabry-Perot with internal source
Microcavity emission:
atoms in a Fabry-Pérot resonator
Idea from ~1960 (Kastler, Schawlow&Townes), Applied Optics, 1, p.17 (January 1962)
cf single mirror !
rings
output
mirror
mc= Int L
λ2
For a point source :
( Fabry-Perot interf.) × (2-beam interf)
(metal)
2
2
E = Eo ×
T1
1− r1r2 e
2iφ 2
× 1 + r2 e
2iφ ' 2
2
= Eo ×
Exaltation or Inhibition
due to the modal structure of the
whole cavity
T1
1− r1r2e
2iφ 2
× 2 ζ (z, θ )
Factor from 0 to 4 depending on
the source location with respect to
the mode antinodes at the
considered angles
Controlled Atomic Spontaneous Emission from
Er 3+ in a Transparent Si/Si02 Microcavity
Vredenberg, et al.
Phys. Rev . Lett.71, 517
(1993)
Physics and Device Applications of Optical Microcavities
Yokoyama Science 256, 66 (1992)
Spontaneous Emission Experiments
with Optical Microcavities
The Thresholdless
Laser
Altering Spontaneous Emission
Bulk System dimensionality Optical phenomenon 3D exciton 3D photon conservation de k3D et E(K) Strong coupling Coupled exciton –photon state= Exciton polariton Extrinsic radiative process 2D Quantum 2D exciton Weak coupling wells in 3D 3D photon Intrinsic radiative process: free optical non-­‐conservation de exciton luminescence dominates environment k⊥ Quantum wells in microcavity 2D exciton 2D photon conservation de k// Strong coupling Cavity polaritons Extrinsic radiative process Summary 1 Strong coupling I : the environment 2 Strong coupling II : how was it discovered ? & early days Excitons in 2D (QWs) interacting with 2D
photons (planar cavity)
Energy and wavevector conserved: 1exciton interacts with 1 photon
Cavity polaritons
Erice, 1993, Eli 2 & Eli 1)
Dipole or field
1/ ΩRabi
time
cw, arakawa et al,
PRL ‘ 92
U. Oesterle
EPFL
Room-temperature cavity
polaritons in a
semiconductor microcavity
Houdre et a., Phys.Rev. B 49, 16 761
(1994)
Inhomogeneous Broadening
FP
mode"
Exciton"
CavityPolariton"
+
If Rabi splitting is larger than
inhomogeneous broadening
All exciton states are coupled together
through cavity photon exchange
Splitting is hardly perturbed
γc /E0 = 3.10−5 , γ/E0 = 1.10−4 , σ/E0 = 1.10−3
coupling strength are 50 (1), 5 (2) and 1 (3)
Houdré et al.
Phys. Rev. A 53 (1996) 2711
While QW width is >1 meV, polariton linewidth can be as low as 50 µeV
QW fluctuations are averaged over polariton wavefunction, i.e. photon
Mode size, ≈ tens of µm, instead of exciton size, ≈ ten nm.
Phonon
broadening
Phonon scattering in LPB
Is diminished due to
small DOS
photon-like
Measurement of Cavity-Polariton
dispersion Curve from AngleResolved Photoluminescence
Experiments
R. Houdre,
Phys. Rev. Lett.
73, 2043 (1994)
Cavity-polariton photoluminescence in
semiconductor microcavities:
Experimental evidence
R. Stanley et al., Phys. Rev. B 53, 10995 (1996)
photolum
absorption
Summary 1 Strong coupling I : the environment 2 Strong coupling II : how was it discovered? 3 microcavity light emiCers a) strong coupling ? b) weak coupling design rules c) microcavity LEDs the limits no photon recycling Experimental:
Norris, CW, Arakawa
1994
Mode counting & cavity order
modes ~ Dirac peaks , antinode factor taken constant for simplicity
ko
extracted to air
kz
ko
guided
modes
θc
kz
mc
Airy
peaks
guided
modes
k//
k//
(b) : micro-cavity of order mc ≤ 2n2
~ Same amount of emission in each mode
1 #
mc
mc
Airy
peaks
θc
(a) : “macro”-cavity of high order mc
=> Extracted fraction =
SINGLE
extracted
mode
#in the ideal limit (R →1)
ζextracted mode
ζi
∑
all modes
2
→
(< 1 !)
mc
Schubert et al., Science 265, 943 (1993)
Impact of Planar Microcavity Effects on Light Extrac\on Part I: Basic Concepts and Analy\cal Trends Benisty et al. IEEE Journal Of Quantum
Electronics, 34, 1612 (1998)
GaN microcavity emi<er modeling Extracted intensity as function of cavity length and QW position in cavity.
Quantum well must be 100nm from top and 150 from
bottom with 20nm accuracy(λ/8n)
GaAs microcavity LED Ø 400µm
28% QE
top contact
oxydation
trench
intracavity
contact
external QE
0.3
0.2
0.1
0
AlOx DBR
in air
single-facet
substrate-based
0.1
current density (A.cm
1
10
-2
)
M. Rattier, JSTQE 2002
HYBRID
ORGANIC
INORGANIC QWs
IN CAVITY
• principle : resonant energy transfer by resonant microcavity photons between electrically-created excitons and radiative excitons
Organic
layer
III-V
Frenkel exciton
light out
photon
Wannier-Mott
exciton
QW
• solution : use coupled cavities
electrical injection
Agranovitch, Benisty, Weisbuch
Solid state commun;102
,
631 (1997)
organic QW (pentacene, J-aggregates, terrylene,...)
GaAs / GaAlAs QW
GaAs substrate
Summary 1 Strong coupling I : the environment 2 Strong coupling II : how was it discovered? 3 microcavity light emi<ers a) strong coupling ? b) weak coupling design rules c) microcavity LEDs the limits no photon recycling 4 visible light emiCers today Light Emitting Diodes LEDs
Another important metrics: luminous efficacy of
light source for lighting: lumens per watt
Luminous efficacy of radiation
Spectral efficacy of source
Source LER/max LER=411
lm/W
History of LED Efficiency
Dupuis and Krames, IEEE J. Lightwave Tech. 26, 1154 (2009).
Present geometrical op\cs solu\ons Pa<erned substrate Roughened surface shaped substrate Thin film roughened surface Light extrac\on study for different devices a) Plain GaN chip b) Surface roughening c) Roughened GaN chip d) Pa<erned Sapphire substrate e) Flipchip III-nitride photonic-crystal light-emitting diodes "
with high extraction efficiency "
Jonathan J. Wierer, Jr1, Aurelien David1 * and Mischa M. Megens2 , Philips Lumileds"
NATURE PHOTONICS 2009"
The optical mode control results in a high-performance"
light-emitting diode with an estimated "
unencapsulated light extraction of 73 %, higher than
any unencapsulated III-nitride LED measured to date. "
Lumileds
Determining the extracMon efficiency: photonic crystal LEDs Need to determine EQE (external quantum efficiency) and IQE (internal quantum efficiency) ηextraction = EQE / IQE
8
10
Encapsulated
Unencapsulated
6
)
%
(e
EQE measurements (integraMng sphere):
IQE measurement: Room temperature EL method*:
ηIQE = 7.9%
η
q
e
)
W
m
4(
5
P
2
0
0
200
400
J (A/cm2)
600
0
800
*Getty et al. Appl. Phys. Lett. 94, 181102, (2009)
Extraction efficiency : Very high light extraction!
Due to a more efficient extraction than absorption mechanisms*
73% in air
however in a poor IQE device…
94% in epoxy dome
Matioli et al., Appl. Phys. Lett., 96, 031108 (2010) Appl. Phys. Express, 3, 032103 (2010)
Solid-­‐State Ligh\ng Research and Development: DOE Mul\ YearProgram Plan March 2011 LED Package and Luminaire Loss Channels & Efficiencies
Best: 70% in blue. 10mA /3V=20mW i.e. equiv. in IR 9mW, in 2π st =1.5 mW/st
15 times the 1993 Science MicroCavity LED!
Light efficiency in LED: why do we want more? If power injection is limited by heat extracted from chip,
improved efficiency allows to inject more current on given
chip surface, hence diminishing cost
Going to 80% efficiency from 60% increases the output by
166%!
Cost is decreased by 62%
50
LEDs Magazine June 2011
Japanese conversion to LED lights would slash energy usage
Replacing all the incandescent lights with LED lamps is more feasible, since
this would have an initial cost of 850 billion yen ($10.5 billion), but would save
27.3 billion kWh annually, worth the output of four nuclear reactors, according
to the institute of energy economics (Japan).
Summary 1 Strong coupling: the basic proper\es were evaluated in the early 90’ies and shown to be sound for quantum behaviour -­‐  Long coherence \mes & distances -­‐  Strong interac\ons between photon like par\cles -­‐  Many profound discovered, more later in the conference 2 Applica\ons: a big challenge! Supplementary slides Lighting
life cycle analysis:
fabrication
energy
requirements
Lighting:
energy
requirements
over life
@10 lm/W
@50 lm/W
@50 lm/W
Cost of energy saving: 2600kWh@10c= US $ 260!
Why LED lamp so expensive today?
2” substrate
≈ 20 cm2 @30 A/cm2
(droop conservative)
= 0.6 kA
= 1.8 kW electrical input @ 3V
0.72 optical kW @ 40% Wallplug Efficiency
140 klm @ 200 lm/W color efficiency
= 140 x 1klm light bulbs! (@ 80lm/W)
[≈ 4mmx4mm each]
Today: processed wafer cost $ 1000
Wafer cost per bulb: $7!(≈ ASP 4GB flash memory)
Diminishing costs requires diminished processing costs and
current density increase
A glimpse into the future: Production throughput
Suppose large factory: 100 machines each1 1000 runs/year2 1000 x 100 x 42x 20= 8 107 cm2/year Aixtron produc\on MOCVD machine 42 substrates 2” 1 substrate ≈ 20cm2 Pump power, full wafer surface made into LEDs ≈ 8 107 cm2x 1253 A/ cm2 x 3V≈ 3.1010 W ≈ 6 1010 W saved power (replaces lamps with x 3 efficiency, 150lm/W) 3hrs lamp use4/365 days => 6 1010 kWh saved 1 GW nuclear power plant: 1.2 106kW x 8000h produces ≈ 1010 kWh LED producMon each year saves energy of 6 new (or old) electricity plants! Value of saved energy: 6 1010 kWh = 6 109US $ @ 10c LEDs produced: 8 107 cm2 = 8 109 1mmx1mm LEDs5 Comments 1. 1200 MOCVDs were ordered in 2010,not all that large and with this throughput 2. 3 runs per day- should be possible with high throughput machines
3. Conservative number to avoid too much droop. Might become better
4. 3 hrs/day on average for every LED produced-maybea little too much
5. i.e. 8 billions per year-big factory- today’s LED production 50-60 billions ,
but it should largely increase for wide scale lighting use.