We investigated various excitation-energy-dependent biexciton generation processes for CuCl quantum dot (QD) ensembles by observing photoluminescence (PL) bands originating from free excitons, bound excitons, and biexcitons as a function of the excitation photon energy. The resonant two-photon absorption (TPA) band of the biexcitons was revealed by the PL excitation spectrum of the biexcitons. Three biexciton generation processes were identified in relation to the range of the excitation photon energy: resonant two-photon excitations, resonant excitations of the bound or free excitons, and nonresonant excitations of the lowest-energy free excitons. Under the resonant two photon excitation, the size selective excitation of the biexcitons in the QDs was confirmed. From the obtained relation between the excitation photon energy and the excited dot radius, the TPA band was found to be affected not only by the average dot radius but also by the size distribution of the dots. In addition, for the relaxation process, we demonstrated that the initial level of the biexciton PL was different from the biexciton level excited directly by the excitation light, indicating the presence of a certain relaxation process of the biexciton before radiative relaxation. These results indicate that it is possible to increase the density of the excited dots with identical PL energy regardless of their size distribution, which is advantageous for generating superfluorescence from QD ensembles.
- Quantum dots
- Two-photon absorption