Observation of a laserlike transition in a microcavity exciton polariton system

We present experimental evidence of spontaneous buildup of coherent exciton polariton population in a microcavity, i.e., an exciton polariton laser. The laser phase transition was confirmed by the increased differential quantum efficiency and decreased linewidth of the lasing polariton mode due to onset of final-state stimulations and the decreased differential quantum efficiency of a nonlasing polariton due to onset of gain clamping at threshold. The exciton polariton laser is distinctly different from an optical laser because of its density-dependent scattering mechanism. The rate equations, taking into account phonon-assisted polariton emission and the polariton-reservoir exciton scattering rate, explain the measurement results well.