Bose-Einstein Condensation of Microcavity Exciton-Polaritons in Thermal Equilibrium*

Strong coupling of cavity photons and quantum-well excitons gives rise to new bosonic quasiparticles called exciton-polaritons or polaritons. In this experiment, we measure the distribution function of polaritons inside an annular optical trap in a GaAs microcavity under non-resonant pumping for different detunings and bath temperatures. The annular optical trap forms a repulsive potential originating from the interactions of polaritons with the excitonic reservoir, hence trapping the polaritons inside. We explore the conditions needed for equilibration of the polaritons and the sensitivity to the density of states of the trap. We also report the observation of critical fluctuations of the polariton photoluminescence near the critical density of condensation.