Exciton-polariton condensate induced by evaporative cooling in a three-dimensionally confined microcavity

We realize the evaporative cooling of a polariton gas and observe the formation of polariton condensate in a potential trap based on a ZnO microwire at room temperature. Due to the three-dimensional confinement, the system is characterized by a well-defined discrete spectrum of polariton states which allows accurate measurements of the polariton population distribution. With increasing pumping power, the polariton gas system is cooled down very efficiently and relaxes closely to the thermal distribution via the polariton evaporative cooling. At an optimized excitation condition, the effective temperature of the polariton gas falls down to 22 K with the condensate fraction reaching nearly 50% in room temperature experiments.