Electrically Driven Polariton Devices Pavlos Savvidis Dept of Materials Sci. & Tech University of Crete / FORTH Polariton LED Rome, March 18, 211
Outline Polariton LED device operating up to room temperature Polariton lasing under CW nonresonant optical pumping in high finesse MC - effect of temperature Electrical control of stimulated parametric scattering Evidence of indirect / oriented polaritons in ADQW microcavities
First demonstration of strong coupling in MC 1992 Polariton Physics Plethora of new phenomena associated with bosonic character of polaritons Mature understanding Polariton Devices Time 28 Polariton LEDs Polariton Laser Diodes Polariton Transistors Polariton based entangled ph. sources Ultrahigh speed Switches A. Khalifa et al., Appl. Phys. Lett. 92, 6117 (28) T = 1 K D. Bajoni et al., Phys. Rev. B 77, 11333 (28) T = 1 K S. I. Tsintzos et al., Nature 453, 372 (28), APL (29) T = 315 K R. Johne et al., Phys. Rev. B 81, 125327 (21) A. Amo, et al., Nature Photonics, 4, 361 (21) H. Liew et al., Phys. Rev. B 82, 3332 (21). Polariton based devices
New Physics & Applications Strong-coupling provides a new insight into a number of very interesting fundamental physical processes Polaritons are Bosons - stimulated scattering - Bose condensation Polariton vs Photon Laser Deng, et al. Natl. Acad. Sci. 15318 (23) - ultralow threshold polariton lasers all optical switches and amplifiers - however any practical polariton device requires electrical injection
Polariton LED Design Considerations p-i-n diode microcavity Technical challenges/issues High resistivity of p-type DBR mirrors Doping related losses in DBR mirrors and polariton robustness high temperature doping profile Injection issues: e.g. inhomogeneous pumping of QWs
Polariton Electroluminescence Energy (ev) Energy (ev) Emission collected normal to the device Clear anticrossing observed Direct emission from exciton polariton states 1.355 1.35 1.345 1.34 1.335 1.33 Upper Polariton Lower Polariton exciton cavity 19 2 21 22 23 Temperature (K) Temperature (K) 24 25 26 Temperature tuning Exciton ~ -.38meV / K Cavity ~ -.12meV /K Rabi splitting of 4.4meV at 219 K S. Tsintzos et al., Nature 453, 372 (28)
Large Rabi splitting in GaAs QW MCs at (T=3K) zero detuning Θ DBR AlAs Al.15 Ga.85 As GaAs QWs DBR AlAs Al.15 Ga.85 As Clear anticrossing Rabi splitting of 6.5mev observed S. Tsintzos et al., APL (29)
Collapse of Strong Coupling Regime at High Densities Relaxation bottleneck ~I 2 need new injection schemes that bypass bottleneck T=235K Injection density at 22mA ~ 1 1 pol/cm 2 angle
Ways to overcome Bottleneck in Relaxation Stimulated parametric polariton scattering -Fast relaxation mechanism into polariton ground state - Difficult to implement for electrical injection LO phonon enhanced relaxation (injection 36meV above the polariton energy at k=) M. Maragkou et al., APL (21) High Q MicroCavities Increased polariton enough time to ralax Lifetime on the lower polariton branch E. Wertz et al., APL 95, 5118 (29)
High finesse GaAs microcavity CW pump 32 period DBR AlAs Al.15 Ga.85 As GaAs QWs 23K exciton 35 period DBR AlAs Al.15 Ga.85 As Experimental Q factor ~ 8 Modeled Q factor ~ 1
Experimental Setup Long Pass filter 78nm objective Beam Splitter lens Real space image lens λ 2θ CCD θ PL θ Sample 5μm Pinhole Excitation Beam Collection of light from a very small part of excited area Excitation spot ~4μm Collection spot ~5μm
GaAs Polariton Laser at 23K P<P th P>P th Nonresonant optical pumping above stopband Threshold at 23K ~ 6.5mW
Photon laser at 77K P<P th P>P th 77K No pinhole 77K Transition to photon lasing in the weak coupling regime Lowest threshold at 77K ~ 13mW
Power Dependence Why such small difference in threshold? Lasing threshold only doubles between polariton laser at 23K and photon laser at 77K
Linewidth (mev) Rabi Splitting vs Temperature T T ex inh ac LO 1 exp kt 1 LO 2 T V T 2 4 ex c 6 5 f[x]=a + c T + b 1/(Exp[35/kT[T]] - 1) a=.64mev, b=12.3mev, c=1.128μev/k a=1.8mev, b=15.2mev, c=4.4μev/k (InGaAs/GaAs) 4 3 2 1 (b) 5 1 15 2 25 3 Temperature (K) Exciton linewidth broadens at high T Increasing exciton linewidth decreases Ω(Τ)
Rabi Splitting vs density 2 T V T V 1 e 4 n 2 cav 2 4 ex c R c R n L f m c S cav eff f : exciton oscillator strength : carrier density : saturation density (PRB, M. Ilegems)
Crossover from Strong to Weak coupling Lasing dnn Nn g N n g dt (steady state) pump Exciton lifetime τ increases with temperature (PRB M.Gurioli) For same pumping rate carrier density increases dramatically with increasing T E T k //
pump Polariton Parametric Amplification probe screen microcavity 2 4 Angle pump only probe only pump & probe Stimulated scattering of polaritons ultrafast amplification process Savvidis et. al. PRL 84, 1547 (2)
Electrical control of parametric amplification monomode fibers p-i-n diode Spectrometer Idler Sample Pump Signal Lower polariton grating V BS BS τ delay fs pulse Electrical control: reverse bias Quantum confined Stark effect
Reflectivity (arb. units) Stark tunable polariton modes Energy (ev) Probe reflectivity Cavity -2.4V 1.416 1.414 1.412 Exciton 1.41 1.48 1.46 1.44 1.42 LED 1.4-1 1 2 Bias (V) 1.441 2.5V T=7.5 K LP UP 1.48 1.412 Energy (ev) 1.416 Stark tuning of the excitons Rabi splitting 6 mev
Parametric Amplification vs Pump-Probe Delay Photodiode intensity Gain No pump with pump Normalized Probe reflectivity 3 3ps LP UP 25 pump 2 15 Time delay 1 5 1.46 1.49 1.412 Energy (ev) 1.415 ps Ultrafast gain dynamics (~3 ps)
Gain Bias Controlled Parametric Gain Gain Gain Current ( A) Current (μa) Energy (ev) Peak gain Energy (ev) Energy (ev) Peak gain Peak gain a) 5 4 3 2 1 1.44 LP(k p ) I pump 5-1.5V 4 3 2 1 2.5V 5 a) 4 3 2 1 b) 5 c) 1.416 1.414 a) 1.412 1.41 1.48 1.46 1.44 UP LP(k p ) pump on pump off c) c) -2 2 Bias (V) 2.5V 1.48 1.412 1.416 1.44 1.48-1 1.412 1 1.416 2 2.5V 1.44 1.48 1.412 Bias (V) 1.416 1 8 6 4 2 Pump partially screens electric filed LP -1 LP(k p ) 1 Bias (V) I pump I pump -1.5V -1.5V 2 b) b) Energy (ev) Gain loss at negative bias Energy (ev) Dispersionless gain peak Energy (ev) 1.416 1.416 1.414 1.414 1.412 1.412 UP UP 1.41 1.41 1.48 1.48 1.46 1.46 1.44-1 1.44-1 1 1 8 8 6 6 4 4 2 2-1 -1 LP LP 1 Bias (V) 1 Bias (V) 1 2 Bias (V) 1 Bias (V) Gain dip around.7 V G. Christmann et al. PRB 82 11338 (21) 2 2 2
Gain Bias Dependent Gain Suppression Peak gain Current ( A) 8 a) b) 4 3.4 V 2 6 1 4 c).4.6.8 Bias (V) 1. -5 2-1 -15 x6 1.47 1.48 1.49 1.41 1.411 Energy (ev) 1.1 V 1.412-2.4.6.8 Bias (V) 1. Sharp dip
Energy (ev) Mechanism.4.36.32.28-1.8-1.12-1.16-1.2 LP LQW 2ps τ t τ Ω 7fs τ o τ LO τ e τ c 8ps RQW transport competition: tunnelling separates e and h Rabi coupling: polaritons redistribute eh pairs between QWs E e (ev).6.5 ω LO LQW RQW.5 Bias (V) 1. LO phonon-induced tunnelling 1fs 2 Position (nm) 4 carrier escape 18ns, 25fs extra e - population creates extra scattering gain is sensitive to broadening C. Ciuti et al. PRB 62 R4825 (2)
Indirect / Oriented Polaritons Oriented polaritons with enhanced dipole moment Could provide additional control over polariton-polariton interactions G. Christmann et al, APL 98, 81111 (211)
Photoluminescence (arb. units) Indirect / Oriented Polaritons Peak position (ev) Energy (ev) 1 5 1 4 1 3 V 1.42 1.418 1.416 1.414 1.412 1 2 1 1 1 1.41 1.4 1.398 1.396 1.394 1.392 Field (arb. units) IX DX cavity 868 872 876 88 Wavelength (nm) 1.39 1.388 1 2 Field (ev) 3 4x1 3
Credits University of Crete PG Savvidis NT Pelekanos Tingge Gao Peter Eldridge Simos Tsintzos Panos Tsotsis Device Fabrication Dr. G. Kostantinidis Dr. G. Deligeorgis MBE Growth Z. Hatzopoulos FORTH UOC Collaborations University of Cambridge J. J. Baumberg G. Christmann University of Southampton PG Lagoudakis A. Grundy Funding: EU, ITN CLERMONT 4 & ICARUS projects, Greek Research Council
Summary GaAs polariton LED device operating up to RT Polariton lasing under nonresonant CW excitation up to 5K promising for the electrically pumped polariton lasers Direct control of parametric gain sharp and dramatic gain modulation Evidence of oriented polaritons in ADQW microcavities Thank you