Evidence for a spontaneous coherent phase transition of polaritons in CdTe microcavities

We report on the non linear laser-like regime occuring in CdTe microcavities under strong non-resonant excitation. We find that the sharp peak responsible for the nonlinearities is dispersionless. This feature is discussed in terms of coherent polariton-polariton scattering. A phase transition on the coherence of the emission is demonstrated experimentally: firstly, images of the luminescence in the Fourier plane exhibits a contrasted speckle patterning that doesn't exist under weak excitation. Secondly, a direct measurement of the transverse coherence is realized. It shows that the coherence is strongly enhanced at the transition toward the nonlinear regime. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)