Temperature dependence of dynamical processes of photoluminescence from exciton-exciton scattering in CuI thin films

We have investigated the time-resolved photoluminescence spectra of the P emission due to exciton-exciton scattering in CuI thin films at various temperatures by using an optical-Kerr-gating method with a time resolution of 0.4 ps. It is found that the rise time of the P emission hardly changes with temperature. Since the rise time reflects collision processes of excitons in the lowest (n=1) exciton state, the temperature dependence demonstrates that the exciton-collision rate is hardly influenced by temperature. On the other hand, the decay time of the P emission becomes faster from 8 to 5 ps with increasing temperature from 10 to 150 K. This is due to the enhancement of the contribution of the photon state away from the polariton-bottleneck region to the photon-like polariton state leading to the P emission owing to thermal broadening in momentum space; namely the photon nature of the photon-like polariton becomes considerable.