Dynamics of excitons and biexcitons in CuCl nanocrystals embedded in NaCl at 2 K

In CuCl nanocrystals we have observed two luminescence bands associated with free and bound biexcitons in a high-density excitation at 2 K. We also observed, in subpicosecond pump-and-probe measurements, an optical gain and induced absorption at the energy position of the bound biexciton luminescence band, and another optical gain at the free biexciton luminescence band. The decay time of the change in exciton absorption was found to be about 1 ns in nanocrystals with an effective radius of 4.3 nm, which is two times greater than that at 77 K. Taking time responses of the luminescence spectrum and change in the absorption into account, it was clarified that a dipole transition occurs between a bound biexciton state and a bound exciton state at 2 K.