High Density Exciton Dynamics of CuBr Nanocrystals Embedded in PMMA

When CuBr nanocrystals (NCs) embedded in poly methyl methacrylate are strongly excited, there appears a new luminescence band at the low energy side of exciton luminescence peak. The luminescence band is associated with biexcitons in the CuBr NCs. As the excitation intensity increases, the biexciton luminescence band shows obvious broadening, while the exciton band does not. Interaction among more than three excitons may cause this broadening effect. Optical gain has also been found by pump-probe measurement around the biexciton luminescence band at 77K. The decay time of the optical gain 15ps coincides with that of the biexciton luminescence band, which means that the lifetime of the biexciton is 15ps.