OBSERVATION OF A LASER-LIKE TRANSITION IN A MICROCAVITY EXCITON-POLARITON SYSTEM

We present experimental evidence of spontaneous buildup of coherent ex citon polariton population in a microcavity, i.e., an exciton polarito n laser. The laser phase transition was confirmed by the increased dif ferential quantum efficiency,and decreased linewidth of the lasing pol ariton 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 distinc tly different from an optical laser because of its density-dependent s cattering mechanism. The rate equations, taking into account phonon-as sisted polariton emission and the polariton-reservoir exciton scatteri ng rate, explain the measurement results well.