Pavlos G. SavvidisUniversity of Crete, FORTH-IESL
ISSO 12St. Petersburg 14.7.12
Manipulating Polariton Condensates on a Chip
Acknowledgements
PG SavvidisSimos TsintzosPeter Eldridge Tingge GaoPanos Tsotsis
IESL-FORTHGrowth & Device Fabrication
Department of Materials Science and Tech. University of CreteSpectroscopy
Dr. G. KostantinidisDr. G. DeligeorgisProf. Z. Hatzopoulos
J. BaumbergG. ChristmannG. TosiP. CristofoliniC. Coulson
T. LiewNanyang Tech. University Singapore
N. Berloff
Collaborations
- Polariton lasing in high finesse microcavities- threshold temperature dependence (strong vs weak)
- Electrical and optical manipulation of polariton condensates on a chip
- polariton condensate transistor- interactions between independent condensates- electrical control of polariton condensate
- Dipolaritons: dipole oriented polaritons- control of quantum tunneling with light
Outline
Strong Coupling Regime in Semiconductor Microcavity
• Strongly modified dispersion relations new properties
• small polariton mass mpol ≈ 10-4me
FORTH Microelectronics Research Group Univ. of Crete
2//
22 kLn
cEcc
photon
exciton
IIex MkEkE
20
2//
2
Strong Coupling Regime
Bosonic character of cavity polaritons could be used to create an exciton-polariton condensate that would emit coherent laser-like light.
Bose‐Condensation and Concept of Polariton Lasing
Polaritons accumulate in the lowest energy state by bosonic final statestimulation.
The coherence of the condensate builds up from an incoherent equilibriumreservoir and the BEC phase transition takes place.
Non resonant excitation
Polariton condensate Extremely light
effective mass 5 4
010 10 m
Imamoglu et al., PRA 53, 4250 (1996)
reservoir
The condensate emits spontaneously coherent light without necesityfor population inversion
300K p-BEC
stimulated scatteringstrong coupling
p-OPO 10K p-BEC
vortices
spin intns
300K pLED1D
2000 20061992
PolaritonicsHopfield
2010
p-lasertheory
superfluid
traps
From a device perspective:- Near speed of light lateral transport- Light effective mass- Condensate regime readily available on a chip even at RT
New directions: electrically driven polariton devices
Room temperature Polariton LED
S. Tsintzos et al., Nature 453, 372 (2008)
Emission collected normal to the device
FORTH Microelectronics Research Group Univ. of Crete
• Clear anticrossing observed
• Direct emission from exciton polariton states
Transport driven device
•Rabi splitting of 4.4meV at 219 K
Collapse of Strong Coupling Regime at High Densities
T=235K
2I~
Relaxation bottleneck
• Injection density at 22mA ~ 1010 pol/cm2
Relaxation on lower branch governed by polariton‐polaritoninteractions (dipole‐dipole)
Dipolariton approach: weakly‐coupled double quantum wells
direct control of polariton dipole
Electrically pumped polariton lasers
new challenges:
‐ strong coupling in high finesse doped microcavities structures
‐ injection bypassing relaxation bottleneck
‐ control of polariton dispersions and scatterings
dipole-dipole
qkkkkqkqk
PPqkkA
aeffPP ppppVH B
,','',',2
1 ˆˆˆˆ2
P. Cristofolini et al., Science 336, 704 (2012) G. Christmann APL 98, 081111 (2011)
Temperature Dependence of Lasing Threshold in high finesse GaAs microcavities
GaAs QWs
32 period DBRAlAs
Al0.15Ga0.85As
35 period DBRAlAs
Al0.15Ga0.85As
High finesse GaAs microcavity
FORTH Microelectronics Research Group Univ. of Crete
Modeled Q factor ~ 20000
Experimental Q factor ~ 16000
FORTH MicroelectronicsResearchGroup Univ.ofCrete
Non-resonant optical excitation
• Reflectivity dips relatively small
• Rabi splitting of 9.2meV at 50K
objective
2θ CCDθ
λ
Sample
Realspaceimage
PLθ
lens lens
FORTH MicroelectronicsResearchGroup Univ.ofCrete
BeamSplitter
ExcitationBeam
50μm Pinhole
Collection of light from a very small part of excited area
Long Pass filter 780nm
Excitationspot~40μm
Collectionspot~5μm
PLimagingSetup
• Lowest Threshold at 25K ~ 6.5mW strong couplingat 70K ~ 13mW weak coupling
• Nonresonant optical pumping above stopband
GaAs Polariton Laser 25K vs 70K
• Lasing threshold only doubles between polariton laser at 25K and photon laser at 70K
RabiSplittingvs Density
FORTH MicroelectronicsResearchGroup Univ.ofCrete
f : exciton oscillator strength: carrier density: saturation density
0
2
00 0
1
4
cav eff
cav
cRVn LR
e fn m c S
(PRB,M.Ilegems)
22 )(4 CXV
Exciton lifetime τ increases with temperature
FORTH MicroelectronicsResearchGroup Univ.ofCrete
• For same pumping rate carrier density increases dramatically with increasing T
(PRB M.Gurioli,V. Savona)
(steady state)dN ng n gdt
Crossover from Strong to Weak coupling Lasing
pump
N NN
TE
k//
P. Tsotsis et al., New Journal of Physics 14, 023060 (2012)Thermalization of
the reservoir to higher k// states
Polariton Condensate Transistor Switch
Polariton Condensate Transistor Switch
Motivation: Although photonic circuits have been proposed, a viable optical analogue to an electronic transistor has yet to be identified as switching and operating powers of these devices are typically high
Common perception: In the future, charged carriers have to be replaced by information carriers that do not suffer from scattering, capacitance and resistivity effects
Approach: Polaritons being hybrid photonic and electronic states offer natural bridge between these two systems
Excitonic component allows them to interact strongly giving rise to the nonlinear functionality enjoyed by electronsPhotonic component restricts their dephasing allowing them to carry information with minimal data loss and high speed
Macroscopic quantum properties of polariton condensates make them ideal candidates for use in quantum information devices and all optical circuits
Gao et al.,PRB 85, 235102 (2012) D.Sanvitto et al. Nature Photon. 5, 610 (2011) D.Ballarini et al. arXiv:1201.4071 (2012) E.Wertz et al. Nature Phys 6, 860 (2010)
Generating Polariton Condensate Flow
20μm
• Polariton condensate forming at the ridge end
• Local pump induced blueshift and lateral confinement forces polariton flow along the ridge
Only top DBRis etched
pump expanding condensate
Δ
,
0µm
5µm 10µm 15µm• blue shift at pump
• polaritons expand along the ridge
103
104
105
106
Inte
grat
ed in
tens
ity (a
rb. u
nits
)
12 4 6 8
102 4 6 8
1002
Power (mW)
Ballistic Condensate Ejection
G. Christmann et al., Phys. Rev. B 85, 235303 (2012)
Fourier plane
Polariton Condensate Built-up
• Ballistic transport of polaritons• Polaritons flow and relax in the direction of negative detuning• Condensate forming at the ridge end
FORTH MicroelectronicsResearchGroup Univ.ofCrete
CCD slit
objective
• spatially separated and angle resolved emission
Incr
easi
ng s
ourc
e in
tens
ity
Source
Gate
Polaritoncondensate
Detuning
E
GaAs microcavity
Polariton Condensate Transistor Switch
• Polariton propagation is controlled using a second weaker beam that gates the polariton flux by modifying the energy landscape
Gating Polariton Condensate Flow
• Gate beam power 20 times weaker than source
• Second condensate appears between source and gate at higher gate powers
• At higher powers gate re-pumps the condensate at the ridge end
Gating Polariton Condensate Flow
Theoretical modeling by Tim Liew
• gating efficiency up to 90% is demonstrated
Electrical and optical control of polariton condensates
Is electric gating of transistor feasible ?
Ti/Au
n+
i
Electrical control of polariton dispersions
V
DBR
DBR
QWs
Schottky diode
FORTH MicroelectronicsResearchGroup Univ.ofCrete
• Application of electric field to the QW tunes the exciton energy through QCSE
• Reduction in exciton oscillator strength & Rabi splitting have to be considered
Electrical control of polariton dispersions
FORTH Microelectronics Research Group Univ. of Crete
• Clear tuning of the lower polariton branch energy• Schottky diode allows local spatial field to be applied
25K 25K
side contacttop contact
Control of polariton dispersions in nonlinear regime
0V
30V
UP
XLP
• electric tuning of the lasing energy observed
below threshold
above threshold
Control of polariton dispersions in nonlinear regime
FORTH MicroelectronicsResearchGroup Univ.ofCrete
Interactions Between Condensates
Space
Ene
rgy
• Can we make two independent condensates interact on a chip?• What happens if we launch two condensates against each other
Buildup of Coherence and Phase Locking
Time resolved measurement & interferometry
Pulsed excitation, interference of one with
the other
+
CCD
Sample10K BS
BS
Pump laser
Streak
Polariton condensates in a parabolic optical trap
G. Tosi et al. Nature Physics 8, 190 (2012)
pump1 pump2
• equal spaced energies SHO wavefunctions• harmonic potential - quantum pendulum
tomography
Condensate theory
decay
, , Γrelaxation
reservoir pumppolariton potential
P1 P2
5μm
y
P1 P2
E
, Γ , ,decay
• complex Ginzburg-Landau equation (cGL)
• reservoir dynamics condensate feed
laser pump
diffusion
Nature Physics 8, 190 (2012)
Simulation results
5μm
Resembles oscillating dark-solitonsHow to measure?
Condensate dynamics
0
100
g(1)(%
)0
100
g(1)(%
)
• modelocking condensates
80
60
40
20
020100-10-20
Delay time (ps)
tr
∆t
Visi
bilit
y (%
)fringes
visibility
• self-interference every round trip time (exact match)• all the simple harmonic oscillator levels are phase coherent • implies spontaneous soliton oscillations, not static
nonlinear optics
, ,
cf: ultrafast lasers, supercontinuum generation
Nature Physics 8, 190–194 (2012)
0ps 6ps 13ps
0
100g
(1)(%)10µm
Tuneable oscillator
18161412108Time delay (ps)
90
80
70
60
50
Coherence decay (%
)
0.5
0.4
0.3
Ener
gy s
paci
ng,
E, (m
eV)
tr theory t = h / E coherence
Wavepacket frequency (THz)0.8 0.40.6 wavepacket revival is not perfect
decays over 40ps
due to coherent wavepacket - dispersion (SHO spacings)- decay- dephasing- diffusion
80
60
40
20
020100-10-20
Delay time (ps)
tr
∆t
Visi
bilit
y (%
) temporal widthset by number of SHO states ( =10)
Δ ≃ /
∗
2
Oscillations observed under pulsed excitation regime
2 spots separated by 25 microns
Streak camera measurement
Increasing pump power
Polariton Quantum Pendulum
Laser arriving
Multiple spot excitation
5 µm
Spot1
Spot2 Spot3
Vortex lattices• honeycomb lattice of up to 100 vortices and anti-vortices
K vector
7 mW14 mW25 mW
25 mW14 mW7 mW
Blueshift
Stretching the lattice
• Vortices formed by a linear superposition of 3 waves outflowing from each spot.
• Average distance between neighbouring vortices: 4 3 3⁄
• Outflow momentum dependent on power:
CGL simulations
Ferromagnetic like Antiferromagnetic like
Square lattice
Indirect polaritons: Dipolaritons
Dipolariton approach: weakly‐coupled double quantum wells
direct control of polariton dipole
Indirect polaritons: Dipolaritons
dipole-dipole
qkkkkqkqk
PPqkkA
aeffPP ppppVH B
,','',',2
1 ˆˆˆˆ2
P. Cristofolini et al., Science 336, 704 (2012) G. Christmann APL 98, 081111 (2011)
- reduction of lasing threshold- electrically-pumped polariton lasers and BECs
Biased samples: Polariton LED structure
+
_
100μm
“A GaAs light-emitting diode operating near room temperature”, Nature 453, 372-375 (2008)
Au bottom electrode
n-doped DBR
p-doped DBR
2 cavity with QWs
Au top electrode, recessed
etched mesa top, =350μm
InGaAs
GaAsAlAs
AuGaAs (001) substrate
Motivation
-1.20
-1.16
-1.12
-1.08Ener
gy (e
V)
40200Position (nm)
0.40
0.36
0.32
0.28
LP
LQW RQW
τΩ700fs τc
8ps
τe
τt τLO
τo20ps
-20
-15
-10
-5
0
Cur
rent
(A
)
1.00.80.60.4Bias (V)
x6
c)
b)
V
40
30
20
10
0
Peak
gai
n
1.00.80.60.4Bias (V)
100mV>90%
sharp dip in gain
extra photocurrent at same bias
Control of polariton parametric scattering due to LO phonon assisted tunnelling with a double QW microcavity
“Control of polariton scattering in resonant-tunneling double-quantum-well semiconductor microcavities”, Phys. Rev. B 82, 113308 (2010)
Dipolaritons
DX - IX 2
δ+
δ-
D
Ec
Ev
Ec
Ev
DX IX
“tunnelling off”
electron levels off resonance
electron levels at resonance
direct excitonsDX
“tunnelling on”
mixed excitonsDXIX,static dipole moment
DX
10-18
2.0
1.5
1.0
0.5
30x10325201510
Field (V/cm)
DXDX
IX IX
Dipo
le m
oment
1.0
0.8
0.6
0.4
0.2
0.0
Ove
rlap
50x10340302010
Field (eV)
2* he
QCSE
tunnelling
Oscillator strength is kept
Strong dipole moment
“Oriented polaritons in strongly-coupled asymmetric double quantum well microcavities”, Appl. Phys. Lett. 98, 081111 (2011)
Dipolaritons
strong coupling
UP
IX
C DX
LP
MP
HΩ/2 0
Ω/2 /20 /2
Combining tunnel coupling (J) and Rabi splitting (Ω)
Observation of dipolaritonsPhotoluminescence of the the system versus
increasing bias for detuned and resonant
cavity
tunnel-split excitons, uncoupled cavity
dipolaritons, strong coupling of J and
PL is lost because electron tunnel out
of the system
“Coupling Quantum Tunneling with Cavity Photons”
Science 336, 704 (2012)
Dipolaritons at resonance
peak extraction + MP – state: no DX!
HΩ/2 0
Ω/2 /20 /2
Barrier width dependence
no tunnel coupling normal polariton regime
Influence of the tunnelling barrier
thickness (4,7,20nm) on the bare tunnelling rate J
ADQW simulation from solving Schrödinger
equation
Excellent agreement with solution of the
Schrödinger equation for tunnel coupling
Observation through photocurrentExperiment: Tuning laser energy while monitoring
photocurrent
Strong coupling observed on photocurrent measurements
Hysteresis observed at resonance positions due to
charge build-up
Mapping of the full dispersion curvesExcellent sensitivity on otherwise
poorly optically active modes
- lasing threshold at 25K vs 70K (strong vs weak)
- Electrical control of polariton dispersionstuning of the condensate energy
- Electrical and optical manipulation of polariton condensates on a chip
polariton condensate transistorpolariton condensate pendulumInteractions between condensated in confining potentials
- Dipolaritons: Oriented polaritonsnew possibilities for enhancing nonlinear Interactions threshold reduction, control of parametric scattering
Summary
FORTH Microelectronics Research Group Univ. of Crete
Thank you