Current

POLARITON LIGHT EMITTING DEVICES: RELAXATION DYNAMICS

Simos TsintzosDept of Materials Sci. & TechMicroelectronics GroupUniversity of Crete / IESL

Polariton emitter

Active region

Bottom Mirror

Top Mirror

Polariton Physics at Crete

Spectroscopy & FabricationProf. PG Savvidis

Prof. NT Pelekanos

Simos Tsintzos

Tingge Gao

Panos Tsotsis

Funding: Greek Research Council, ΕΠΕΑΕΚ, EU FP7

Prof. J. J. Baumberg G. Christmann

MBE Growth

Prof. Z. Hatzopoulos III-V

University of Cambridge

Collaborations

FORTH-IESL

University of Crete

Dr. G. KostantinidisDr. G. Deligeorgis

• Demonstration of a polariton LED device operating up to room temperature

• New schemes of electrical injection assisted by LO phonon enhanced relaxation

• Electro/Photo-luminescence imaging of polariton

dispersions to track relaxation dynamics

• Conclusions

Outline

FORTH Microelectronics Research Group Univ. of Crete

First demonstration of strong coupling in MC

Stimulated scattering of polaritons Parametric

amplification

Polariton lasingPolariton

condensationPolariton

SuperfluidityPolariton LEDs

J.R. Tischler et al, PRL 95, 036401 (2005) (organic)A. Khalifa et al., Appl. Phys. Lett. 92, 061107 (2008) T = 10 KD. Bajoni et al., Phys. Rev. B 77, 113303 (2008) T = 100 KS. I. Tsintzos et al., Nature 453, 372 (2008), APL (2009) T = 315 K

Polariton Physics

Polariton Devices

Polariton LaserDiodes ?

Ultrahigh speedSwitches ?

Time

• Spectacular physics related to bosoniccharacter of polaritons

• Mature understanding

Polariton LED

• Small progress

1992

• ApproachElectrical injection of polaritons in strongly

coupled GaAs semiconductor microcavity

Fabricate p-i-n diode microcavitiesfor electrical injection

Measure polariton electroluminescenceand dispersion relations

• Technical challenges/issues

High resistivity of the DBR mirrors

Doping related losses in DBR mirrorsand polariton robustness

Injection issues: e.g. inhomogeneous pumping of QWs

Electrical Injection in Microcavity

FORTH Microelectronics Research Group Univ. of Crete

high temperature

doping profile

Polariton LED

Microcavity Design

FORTH Microelectronics Research Group Univ. of Crete

• Designed to operate at high temperatures• Multiple QWs to enhance Rabi splitting

Polariton Electroluminescence

S. Tsintzos et al., Nature 453, 372 (2008)

Emission collected normal to the device

FORTH Microelectronics Research Group Univ. of Crete

260250240230220210200190Temperature (K)

1.355

1.350

1.345

1.340

1.335

1.330E

nerg

y (e

V)

Upper Polariton Lower Polariton exciton cavity

Temperature (K)

En

erg

y (e

V)

• Clear anticrossing observed

• Direct emission from exciton polariton states

Exciton ~ -0.38meV / KCavity ~ -0.102meV /K

Temperature tuning•Rabi splitting of 4.4meV at 219 K

Room temperature Polariton LED

•Polariton LED with 8 QWsto increase Rabi splitting

•Lateral injection scheme to improve injection

ΔΤ=5K

I=0.8mA

Rabi splitting of ~4meV at T=288K

FORTH Microelectronics Research Group Univ. of Crete

S. Tsintzos et al, APL 94,071109 (2009)

Large Rabi splitting in GaAs QW MCs at (T=300K)

FORTH Microelectronics Research Group Univ. of Crete

Θ

GaAs QWs

DBRAlAs

Al0.15Ga0.85As

DBRAlAs

Al0.15Ga0.85As

• Clear anticrossing

• Rabi splitting of 6.5mev observed

zero detuning

8

6

)8(

)6(

V

VV(8), the only adjustable parameter

Fitting of Rabi Splitting versus T and N

22 )(4 CXV

FORTH Microelectronics Research Group Univ. of Crete

(for zero detuning)

CX ,

Exciton, cavity modelinewidths

X with temperature

N=8GaAs

InGaAs

# QWs

Collapse of Strong Coupling Regime at High Densities

T=235K

2I~

Relaxation bottleneck

FORTH Microelectronics Research Group Univ. of Crete

• Injection density at 22mA ~ 1010 pol/cm2

need new injection schemes that bypass bottleneck

Microcavity structure exploiting LO phonon enhanced relaxation

FORTH Microelectronics Research Group Univ. of Crete

holes

DBR

polariton

LO-phononElectrons DBR

Using GaAs/AlGaAs QWs

Electroluminescence of LO phonon-designed MCs

FORTH Microelectronics Research Group Univ. of Crete

•Red shift in EL caused by heating• Due to series resistance of the DBRs

1.49 1.50 1.51 1.52101

102

103

104

105

106

EL

Inte

nsi

ty

Energy (eV)

5 8.7 13.1 21 25.4 29.7 36 45 52 60.5 73 83.7 91.4 100 120 140 160 180 200 231 250 280 303 351 401 501 610 658 709 760 827 920 1033

Single shot imaging of the polariton dispersions

θ

FORTH Microelectronics Research Group Univ. of Crete

• Look at polariton population along the lower branch.

•Applicable to both PL and EL measurements on small mesas

objective

sample 4K

pin-hole

θ

θ

CCDθ

λ

λ

confocal

E

EL

Power dependence Images

Nonlinearity at very low power

Weak CouplingStrong Coupling

Nonlinearity possibly due to screening of diode built-in field

FORTH Microelectronics Research Group Univ. of Crete

Enhanced Polariton Relaxation @ High InjectionEnhanced Polariton Relaxation @ High Injection

0 5 10 15 20 25 30

Angle (degrees)

0.1mW

0.4mW

Inte

grat

ed In

tens

ity (

a.u)

0.9mW

2mW

3mWT=70K

FORTH Microelectronics Research Group Univ. of Crete

Enhanced Polariton Relaxation @ High TemperatureEnhanced Polariton Relaxation @ High Temperature

T=140K

P=0.4mW

T=60K

P=0.4mW

• GaAs polariton LED device operating up to RT

• New approach to electrical injection exploiting LO phonon enhanced relaxation

• Imaging shows enhanced relaxation at higher Temperatures and Powers and collapse of bottleneck region.

Thank you

Summary

FORTH Microelectronics Research Group Univ. of Crete