Efficient radiative recombination of multinode-type excitons up to room temperature in CuCl thin films

We report the observation of anomalous temperature dependences of degenerate four-wave mixing spectra in CuCl thin films with high crystalline quality. The observed temperature dependence is in good agreement with the phase-decay-constant dependence of calculated induced-polarization spectrum. An excitonic state with large radiative width can be observed at high temperatures as superradiance is faster than the dephasing process. We succeeded in observing the DFWM signal up to room temperature based on the extremely large radiative width peculiar to the thickness region beyond the long-wavelength approximation regime.