Spontaneous Coherent Phase Transition of Polaritons in CdTe Microcavities

We report on evidence for polariton condensation out of a reservoir of incoherent polaritons. Polariton population and first-order coherence are investigated by spectroscopic imaging of the far-field emission of a CdTe-based microcavity under nonresonant pumping. With increasing pumping power, stimulated emission with thresholdlike behavior and spectral narrowing is observed in the strong exciton-photon coupling regime. We show that it comes from a narrow ring in k space, exhibiting enhanced spatial and angular coherence at the stimulation onset.