Scattering and dephasing of excitonic polaritons in CuCl

We study the energy and intensity dependence of the dephasing time T2 of the polarization in CuCl inside an excitonic resonance by a femtosecond four-wave mixing technique at 5 K. We first compare results obtained on bulk material and on epitaxial films of different thicknesses for a backward-scattering configuration. This configuration is most sensitive to the region close to the surface of the samples. In bulk material, the coherence properties of the polarization are mainly limited by recombination processes and scattering with acoustic phonons while they are in addition influenced by scattering with impurities and/or imperfections in the epitaxial films. In these films, in a transmission configuration, the polarization dephases less rapidly than it appears in retrodiffusion, indicating the importance of surface recombination processes and of the polariton propagation.