Radiative processes related to superfluorescence generation from multi-excitons confined in CuCl quantum dots

Superfluorescence from semiconductor quantum dots (QDs) is an attractive optical phenomenon because of the spontaneous formation of coherent coupling of the QDs. The key to generate superfluorescence is the radiative transition process of multiple excitons confined in a dot. In this study, we revealed the generation process of superfluorescence from multiple excitons confined in a CuCl QD assembly by the excitation density dependence of the photoluminescence (PL) spectra and their time profiles. In addition to biexcitonic superfluorescence reported in previous studies, superfluorescent pulses from triexcitons were observed for the first time for the best of our knowledge. The triexciton superfluorescence emerged at a high excitation density, and its photon energy was higher than that of biexcitons. The pulsed time profiles of the triexciton superfluorescence exhibited a faster rise and decay compared to those of the biexcitonic superfluorescence. Moreover, the PL excitation (PLE) spectra of biexcitons and triexcitons clearly indicated the radiative relaxation processes of triexcitons, biexcitons, and excitons. The excitation photon energy required to generate the strongest superfluorescent pulses depended on the excitation density, which was related to the inhomogeneous size distribution of the dots and superfluorescence generation process. In conclusion, the radiative relaxation processes of multiple excitons confined in CuCl QDs, which were involved in generating superfluorescence, were clarified. Consequently, our findings are significant to clarify the mechanism involved in generating high power and short pulsed superfluorescent emission from QD assembly for application to future optical devices.