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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


• 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


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


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


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


• 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


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


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 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


• 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


• 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


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


Thank you

manipulating polariton condensates on a chip - ioffe … polariton condensates on a chip. ... for...

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