Present status of polaritonic
nonlinearities in planar III-nitride
microcavities
Raphaël Butté
Institute of Condensed Matter Physics
ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE,
SWITZERLAND
1
Motivations
Fundamental:
Study of bosonic condensation phenomena up to room temperature
Lasing threshold
Wide band gap semiconductors  large effective masses
GaN
DHS
QW
III‐As compounds
QD
 high threshold current density for lasing, best GaN edge‐emitting QW LDs (Nichia/Samsung) ~ 1‐2 kA/cm2
What could be the next step?
 GaN polariton LDs SST 26, 014030 (2011)
Cf. talk Marlene Glauser
N. N. Ledentsov, SST 26, 014001 (2011)
Applied:
Achieve room temperature electrical injection of cavity polaritons characterized
with an ultra‐low effective mass
 Low threshold “lasers” without population inversion
A. Imamoglu et al., PRA 53, 4250 (1996)
ESF polaritonics, Rome, March 22 2012
2
Outline
• Polariton condensation in planar microcavities: role of the detuning 
and temperature
• Impact of biexcitons on the relaxation mechanisms of polaritons
• A few hints on renormalization effects in III‐nitride microcavities
• Conclusion and perspectives
ESF polaritonics, Rome, March 22 2012
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Polariton nonlinearities in planar microcavities
GaAs
CdTe
VRS ~ 15 meV, NQW = 12
TC ~ 40 K
VRS ~ 26 meV, NQW = 16
TC ~ 50 K
• Relaxation bottleneck to overcome • Key role of cavity photon lifetime to achieve spontaneous condensation
• Large QW number to reduce the exciton density per well
1
Kinetics
2
Kinetics
Thermod.
Thermod.
• Temperature‐dependent optimum  but limited T range accessible
ESF polaritonics, Rome, March 22 2012
1E. Wertz
2J.
et al., APL 95, 051108 (2009);
Kasprzak et al., PRL 101, 146404 (2008)
4
Planar III-N microcavity for polariton studies
MQW spectrum (half cavity)
Thin GaN/AlGaN
MQWs with a high
exciton saturation
density:
~ 1012 cm-2/QW
3.75
C
UPB
Large QW number to
increase Rabi splitting
Energy (eV)
3.7
3.65 high Q
MC with
factor at local
3.6
scale
XX
LPB
VRS = 56 meV
3.55
0
10
20
30
40
50
60
70
Angle (degrees)
ESF polaritonics, Rome, March 22 2012
PRB 77, 085310 (2008)
5
Experimental setup
1‐ Fourier PL setup: Ar+ (244 nm, cw) or Nd:YAG (266 nm, quasi‐cw)
Access to the polariton dispersion curve E(k//)
real space
CCD
BS cube
CCD
spectrometer
BS plate
Fourier imaging
UV objective
NA = 0.55
He cryostat
X-Y stage
2‐ Time‐resolved PL setup: Ti:sapphire (280 nm, 2 ps) + monochrom. + streak camera
Temporal evolution of the PL at k// = 0
ESF polaritonics, Rome, March 22 2012
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Polariton
condensation phase diagram
Polariton lasing threshold
(, T, Pthr) diagram1‐2
GaN
1‐2
3
Thermalized population in the vicinity of the ground state with Teff= 300  10 K  signature of nonequilibrium
polariton BEC at room temperature
T = 300 K
ESF polaritonics, Rome, March 22 2012
1PRB 80, 233301 (2009); 2PRB 81, 125305 (2010); 3PRL 104, 166402 (2010)
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Polariton
condensation phase diagram
Polariton lasing threshold
Large negative detuning and low temperature
‐ inefficient polariton relaxation
 increasing threshold (kinetic regime)
Less negative detuning and elevated temperatures
‐ enhanced scattering efficiency
 decreasing threshold (toward/or thermod. regime)
T = 4 K
T = 300 K
Competition between kinetic and thermodynamic condensation regimes: impact of phonon scattering term  shift of opt(T) toward more negative  values with increasing T(K)1‐2
Extra kink observed in the evolution of opt(T) once escape from parabolic region of the LPB is allowed1‐2
 thermal detrapping from polariton
ground state(specific feature due to matter‐like character of polaritons)
ESF polaritonics, Rome, March 22 2012
1PRB 80, 233301 (2009); 2PRB 81, 125305 (2010)
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Outline
• Polariton condensation in planar microcavities: role of the detuning 
and temperature
• Impact of biexcitons on the relaxation mechanisms of polaritons
• A few hints on renormalization effects in III‐nitride microcavities
• Conclusion and perspectives
ESF polaritonics, Rome, March 22 2012
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Fourier imaging spectroscopy: accurate
farfield emission pattern
VRS = 60 meV
Upper polariton
X
Two
unidentified
PL lines!
Lower
polariton
Relaxation
bottleneck
Cavity detuning  = -55 meV
ESF polaritonics, Rome, March 22 2012
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Dark excitons in real multiple QW microcavities
Transfer matrix simulations
Energy
UPB
C
NQW -1
dark
excitons
fosc ≠ 0
…
LPB
NQW disordered
quantum wells
LP
In presence of disorder, redistribution of the oscillator strength from the polaritons to the dark modes
R. Houdré et al., Phys. Rev. A 53, 2711 (1996)
UP
« Dark » excitons
Dark excitons can bind to form cavity biexcitons
G. C. La Rocca et al., J. Opt. Soc. Am. B 15, 652 (1998)
ESF polaritonics, Rome, March 22 2012
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X
Exciton and biexciton dynamics
TR‐PL decays taken at k// = 0
UP
X
X
XX
LP
PL Intensity (arb. units)
680 ps
PL transients
10 K - zero-angle
X
XX
X
0
250
2
500
750
1000
1250
1500
Time (ps)
At quasi‐thermal equilibrium
2X  XX 
I XX  I X2
Observation of “cavity biexcitons”
Do biexcitons play a role in the relaxation mechanisms of polaritons?
ESF polaritonics, Rome, March 22 2012
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Energy
Biexciton luminescence in the full cavity
X+X
XX
EXX,b
XX to UPB
XX to X
XX to LPB
Possible cavity biexciton radiative dissociation channels:
XX  photon + LPB
XX  photon + UPB
C
UPB
XX  photon + X
X
Xloc
LPB
k//
ESF polaritonics, Rome, March 22 2012
Uncoupled excitons get localized and recombine
LPs accumulate in the reservoir
13
LPB relaxation dynamics
PL transients
10 K - zero-angle
X
XX
LP
0
250
Intensity (arb. units)
Intensity (arb. units)
PL transients
10 K - zero-angle
ELP > EXX – ELO
500
750
1000
Time (ps)
XX‐LO
X
XX
LP
X‐LO
1250
0
1500
ELP = EXX - ELO
250
500
750
1000
AlGaN
k// = 0
PL transients
10 K - zero-angle
Intensity (arb. units)
XX
X
XX
X
‐40
LP
‐80
ELP < EXX - ELO
X
XX
LP
‐‐120
0
‐‐160
ESF polaritonics, Rome, March 22 2012
1500
Time (ps)
XX
X
LP
LP
1250
250
500
750
1000
1250
1500
Time (ps)
Energy (eV)
14
Enhanced relaxation efficiency
CdTe
F. Boeuf et al., Phys. Rev. B 62, R2279 (2000)
GaAs
M. Maragkou et al., Appl. Phys. Lett. 97, 111110 (2010)
GaN
P. Corfdir et al., submitted to Phys. Rev. B
GaAs, CdTe and GaN
Increased LP relaxation efficiency when EX – ELPB(k// = 0) = ħLO
GaN only Increased LP relaxation efficiency when EXX – ELPB(k// = 0) = ħLO
Extra polariton relaxation channel mediated by cavity biexcitons
ESF polaritonics, Rome, March 22 2012
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Outline
• Polariton condensation in planar microcavities: role of the detuning 
and temperature
• Impact of biexcitons on the relaxation mechanisms of polaritons
• A few hints on renormalization effects in III‐nitride microcavities
• Conclusion and perspectives
ESF polaritonics, Rome, March 22 2012
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Renormalization of polariton branches:
the standard picture
Two contributions
n
 EX n  E 
nsat
Coulomb interaction

n 
g  n   g0 1  ' 
 nsat 
Phase‐space filling (saturation fosc)
b
X
Renormalized dispersion curve
1
ELPB / UPB  k / / , , n   EC  k / /   E X  k / /    E X  n  
2
2
1
2
4

E
k

E
k


E
n

g






 n


C
//
X
//
X
2
Nearly rigid blueshift of LPB and predicted linear shift of the LPB ground state with n
ESF polaritonics, Rome, March 22 2012
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Renormalization of polariton branches:
experimental facts
•
Nearly no change observed at large k//  saturation is the dominant renormalization effect
•
VRS extracted from coupled oscillator model with constant EX
and EC values
•
VRS decreased by 20% between 0.15 and 1 Pthr
ESF polaritonics, Rome, March 22 2012
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Renormalization of polariton branches:
impact of  and T
• Saturation effects seem to decrease with increasing 
values!
• Slow down of VRS decrease when crossing Pthr
•
Blueshift at k// = 0 showing a clear deviation from linearity for T < 200 K
•
b
Possible role of biexcitons ( ~ 22 meV) E XX
in a system dominated by saturation effects (GaAs model not applicable)
 several remaining open questions likely due to specificities of saturation effects in a aB2D
system with small !
ESF polaritonics, Rome, March 22 2012
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Conclusion and perspectives
•
Polariton condensation phase diagram from 4 to 340 K in GaN MQW
microcavity (access to kinetic and thermodynamic relaxation regimes)
•
Experimental signature of biexciton-mediated polariton relaxation
•
Anomalous renormalization behavior (sublinear blueshift of LPB), key
role of saturation effects + biexcitons?
•
Study the properties of polariton condensates over a wide range of
temperatures including renormalization, biexcitonic effects
•
Electrical injection of polaritons in III-N microcavities (Marlene Glauser)
•
System a priori suitable for (i) investigating ultrafast OPA and OPO
properties @ 300 K (solitons?), (ii) realizing coherent THz light emitters
(nonpolar microcavities)
ESF polaritonics, Rome, March 22 2012
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Acknowledgments
• PhDs: J. Levrat, G. Rossbach, M. Glauser, G. Christmann (Cambridge, UK)
• Post‐docs: Drs. M. Cobet, E. Feltin (Novagan) • Prof. N. Grandjean and Dr. J.‐F. Carlin
• Drs. G. Malpuech and D. D. Solnyshkov, Clermont University/CNRS (F)
• Prof. B. Deveaud‐Plédran, Drs. P. Corfdir (Cambridge, UK) and J.‐D. Ganière (EPFL)
ESF polaritonics, Rome, March 22 2012
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Thank you for your attention!
22
Enhanced relaxation efficiency
ESF polaritonics, Rome, March 22 2012
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