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We apply low temperature confocal optical microscopy to spatially resolve, and spectroscopically study, a single self-assembled quantum dot. By comparing the emission spectra obtained at various excitation levels to a theoretical many body model, we show that (a) single exciton radiative recombination is very weak, and (b) sharp spectral lines are due to optical transitions between confined multiexcitonic states among which excitons thermalize within their lifetimes. Once these few states are fully occupied, broadbands appear due to transitions between states which contain electrons in the continuum.
Dekel et al. (Mon,) studied this question.