We apply low temperature confocal optical microscopy to spatially reso
lve, and spectroscopically study, a single self-assembled quantum dot.
By comparing the emission spectra obtained at various excitation leve
ls to a theoretical many body model, we show that (a) single exciton r
adiative recombination is very weak, and (b) sharp spectral lines are
due to optical transitions between confined multiexcitonic states amon
g which excitons thermalize within their lifetimes. Once these few sta
tes are fully occupied, broadbands appear due to transitions between s
tates which contain electrons in the continuum.