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  1. POLARITON LIGHT EMITTING DEVICES: RELAXATION DYNAMICS Dept of Materials Sci. & Tech Microelectronics Group University of Crete / IESL Simos Tsintzos Polariton emitter Top Mirror Active region Current Bottom Mirror

  2. Polariton Physics at Crete Spectroscopy & Fabrication Prof. PG Savvidis Prof. NT Pelekanos Simos Tsintzos Tingge Gao Panos Tsotsis MBE Growth Prof. Z. Hatzopoulos III-V University of Crete Collaborations University of Cambridge FORTH-IESL Prof. J. J. Baumberg G. Christmann Dr. G. Kostantinidis Dr. G. Deligeorgis Funding: Greek Research Council, ΕΠΕΑΕΚ, EU FP7

  3. Outline FORTHMicroelectronics Research GroupUniv. of Crete • 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

  4. First demonstration of strong coupling in MC 1992 Polariton Physics Time Stimulated scattering of polaritons Parametric amplification • Small progress Polariton Devices Polariton lasing Polariton condensation Polariton Superfluidity Polariton LEDs • Spectacular physics related to bosonic • character of polaritons • Mature understanding Ultrahigh speed Switches ? Polariton Laser Diodes ? J.R. Tischler et al, PRL 95, 036401 (2005) (organic) A. Khalifa et al., Appl. Phys. Lett. 92, 061107 (2008) T = 10 K D. Bajoni et al., Phys. Rev. B 77, 113303 (2008) T = 100 K S. I. Tsintzos et al., Nature 453, 372 (2008), APL (2009) T = 315 K Polariton LED

  5. Electrical Injection in Microcavity FORTHMicroelectronics Research GroupUniv. of Crete • Approach • Electrical injection of polaritons in strongly • coupled GaAs semiconductor microcavity • Fabricate p-i-n diode microcavities • for electrical injection • Measure polariton electroluminescence • and dispersion relations • • Technical challenges/issues • High resistivity of the DBR mirrors • Doping related losses in DBR mirrors • and polariton robustness • Injection issues: e.g. inhomogeneous pumping of QWs Polariton LED high temperature doping profile

  6. FORTHMicroelectronics Research GroupUniv. of Crete Microcavity Design • Designed to operate at high temperatures • Multiple QWs to enhance Rabi splitting

  7. Polariton Electroluminescence FORTHMicroelectronics Research GroupUniv. of Crete • Clear anticrossing observed • Direct emission from exciton polariton states Emission collected normal to the device Energy (eV) Temperature (K) Exciton ~ -0.38meV / K Cavity ~ -0.102meV /K Temperature tuning • Rabi splitting of 4.4meV at 219 K S. Tsintzos et al., Nature 453, 372 (2008)

  8. Room temperature Polariton LED FORTHMicroelectronics Research GroupUniv. of Crete • Polariton LED with 8 QWs • to increase Rabi splitting • Lateral injection scheme • to improve injection I=0.8mA ΔΤ=5K S. Tsintzos et al, APL 94,071109 (2009) Rabi splitting of ~4meV at T=288K

  9. Large Rabi splitting in GaAs QW MCs at (T=300K) FORTHMicroelectronics Research GroupUniv. of Crete zero detuning DBR AlAs Al0.15Ga0.85As Θ GaAs QWs DBR AlAs Al0.15Ga0.85As • Clear anticrossing • Rabi splitting of 6.5mev observed

  10. Fitting of Rabi Splitting versus T and N FORTHMicroelectronics Research GroupUniv. of Crete # QWs N=8 GaAs (for zero detuning) InGaAs Exciton, cavity mode linewidths with temperature V(8), the only adjustable parameter

  11. Collapse of Strong Coupling Regime at High Densities FORTHMicroelectronics Research GroupUniv. of Crete Relaxation bottleneck ~ need new injection schemes that bypass bottleneck T=235K • Injection density at 22mA ~ 1010 pol/cm2

  12. Microcavity structure exploiting LO phonon enhanced relaxation FORTHMicroelectronics Research GroupUniv. of Crete LO-phonon Electrons DBR DBR polariton holes Using GaAs/AlGaAs QWs

  13. Electroluminescence of LO phonon-designed MCs FORTHMicroelectronics Research GroupUniv. of Crete • Red shift in EL caused by heating • Due to series resistance of the DBRs

  14. Single shot imaging of the polariton dispersions pin-hole λ CCD θ θ EL confocal objective E θ λ sample 4K FORTHMicroelectronics Research GroupUniv. of Crete θ • Look at polariton population along the lower branch. • Applicable to both PL and EL measurements on small mesas

  15. Power dependence Images Strong Coupling Weak Coupling Nonlinearity possibly due to screening of diode built-in field Nonlinearity at very low power

  16. FORTHMicroelectronics Research GroupUniv. of Crete Enhanced Polariton Relaxation @ High Injection

  17. T=60K P=0.4mW T=140K P=0.4mW FORTHMicroelectronics Research GroupUniv. of Crete Enhanced Polariton Relaxation @ High Temperature

  18. FORTHMicroelectronics Research GroupUniv. of Crete Summary • 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