State-resolved observation in real time of the structural dynamics of multiexcitons in semiconductor nanocrystals

Biexciton processes in semiconductor nanocrystals, e. g. optical gain, are widely treated as arising from structureless biexcitons. We challenge this view by observing the binding energies of hot biexcitons in CdSe nanocrystals. These experiments reveal a previously unobserved spectrum in the biexciton binding energies that correlates with hole configuration. Analysis of this spectrum of binding energies shows that the onset and the decay of optical gain in nanocrystals are dictated by cooling dynamics of the hot biexciton rather than Auger recombination. In addition to the structural dynamics of the biexciton, we show that the capacity to control optical gain bandwidth in these nanocrystals arises from stimulated emission from higher multiexcitons.