Multiple light-coupling modes of confined excitons observable in photoluminescence spectra of high-quality CuCl thin films

We report the observation of multiple light-coupling modes of excitons confined in CuCl thin films with thicknesses of a few hundred nanometers beyond the long-wavelength approximation in photoluminescence spectra. Due to a remarkably long coupling length between light and multinode-type excitons resulted from very high crystalline quality of thin films, photoluminescence signals from the excitonic states corresponding to not only odd but also even quantum numbers, which are optically forbidden in the long-wavelength approximation, are clearly observed. The full width at half maximum of the excitonic state deduced qualitatively from the corresponding photoluminescence band shows almost the same dependence on the quantum number as the theoretical prediction.