Gain dynamics and excitonic transition in CdSe colloidal quantum dots

Dynamics of the photo-excited excitons in CdSe colloidal quantum dots was investigated by degenerate pump-probe measurement. Under resonant excitation, the lowest energy state of an electron-hole pair dominates two-step decay. The fast initial decay and long-term decay were attributed to the intrinsic Auger recombination and radiative recombination, respectively. As increasing the excited exciton density, a significant change of the later decay was seen before the absorption bleaching. As our nanocrystal is the case of strong confinement, in which an excitonic Bohr radius is smaller than the dot size, the biexciton-involved recombination is suggested to dominate at high carrier densities. Optical gain was measured by the variable stripe length method and broad gain spectrum was obtained due to the recombination of many excited levels between biexciton and exciton states. This result also supports biexcitons play an important role in the strong confinement regime at high carrier densities.