Energy-relaxation dynamics of photogenerated excitons observed from time-resolved photoluminescence of exciton-exciton scattering in CuI thin films

We have investigated the excitation-photon-energy dependence of time-resolved photoluminescence spectra of the P emission due to exciton-exciton scattering at 10 K in CuI thin films from the viewpoint of energy-relaxation dynamics of photogenerated excitons. The ultrashort time resolution of 0.4 ps using an optical-Kerr-gating method enabled us to obtain precise information of the relaxation dynamics under various excitation-photon-energy conditions. It is found that the onset time of the P emission, which corresponds to the time required for the appearance of the P emission after the irradiation of an excitation pulse, increases with increasing excitation-photon energy, while the rise and decay times of the P emission are independent on the excitation-photon energy. These results indicate that the onset time is connected with an energy-relaxation time of the photogenerated excitons toward the bottleneck region of the exciton-polariton. On the basis of a Fröhlich-interaction model, we have quantitatively analyzed the energy-relaxation time, taking account of interactions between the excitons and longitudinal optical (LO) phonons, the so-called cascade process of LO-phonon scattering, in momentum space.