Gangqiang Liu


Allen G12 Desk #4
(412) 624-5870
Postdoctoral Fellow


Title: Bose-Einstein Condensation and Quantized Flow of Microcavity Polaritons with Long Lifetime
Over the last two decades, exciton-polaritons (polaritons) in a semiconductor microcavity has become an important platform for studying the physics of quantum fluid in solid state system. Polaritons are formed by the strong coupling between photons and the sharp electronic resonance (exciton resonance) in quantum wells embedded in a microcavity. They are weakly interacting bosonic particles with a small effective mass due to its half-light and half-matter nature. Spontaneous coherence phenomena, such as superfluid transition and Bose-Einstein condensation (BEC) have been observed in polariton systems at temperatures in the range from several kelvin to room temperature. This dissertation focuses on new methods of trapping polaritons and the BEC and superfluidity of polaritons in these new traps. The first part of this dissertation describes experiments on trapping polaritons with an optically generated potential barrier and the thickness gradient of the micorcavity. When the polariton density increases, we first observed a transition from ballistic motion to coherent motion of polaritons over hundreds of micrometers. At even higher particle density, we observed a very sharp transition from the coherent motion state to the ground state of the trap. This sharp transition is very similar to the phase transition in equilibrium system. The second part of this dissertation explores the superfluid properties of polaritons in a ring-shaped trap. This ring trap is formed by combining a stress-induced harmonic trap with an optically created barrier at the trap center. This trapping method enables fine control of the trap profile as well as the properties of the polaritons in the trap. The formation of a polariton ring condensate is observed in this trap. The phase and polarization measurement of the ring condensate reveals that it is in a half-quantized circulation state which features a phase shift of π and a polarization vector rotation of π of the polaritons around a closed path in the ring. The direction of circulation of the flow around the ring fluctuates randomly between clockwise and counter-clockwise from one shot to the next. In contrast, the rotation of the polarization of polaritons are very stable. This property is experimentally studied, and it is found that the stable spatial polarization pattern may relate to the optical spin Hall effect. In the last part of this dissertation, I will present some preliminary results on generating polariton flow in the ring trap with a spatial light modulator.
Keywords: Microcavity Polaritons, Bose-Einstein Condensation, Exciton Barrier, Stress Trap, Ring Trap, Quantized Flow. 

Postdoctoral Affiliates

Michael Hatridge

Graduate Advisor

David W Snoke