Carrier-carrier correlations in an optically excited single semiconductor quantum dot

We applied low-temperature diffraction-limited confocal optical microscopy to spatially resolve and spectroscopically study photoluminescence from sin gle self-assembled semiconductor quantum dots. Using selective wavelength i maging we unambiguously demonstrated that a single photoexcited quantum dot emits light in a few very narrow spectral lines. The measured spectrum and its dependence on the power of either cw or pulsed excitation are explaine d by taking carrier correlations into account. We solve numerically a many- body Hamiltonian for a model quantum dot, and we show that the multiline em ission spectrum is due to optical transitions between confined exciton mult iplexes. We furthermore show that the electron-electron and hole-hole excha nge interaction is responsible for the typical appearance of pairs in the p hotoluminescence spectra and for the appearance of redshifted new lines as the excitation power increases. The fact that only a few spectral lines app ear in the emission spectrum strongly indicates fast thermalization. This m eans that a multi-exciton relaxes to its ground state much faster than its radiative lifetime.