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 single self-assembled semiconductor quantum dots. Using selective wavelength imaging 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 explained by taking carrier correlations into account. We solve numerically a many-body Hamiltonian for a model quantum dot, and we show that the multiline emission spectrum is due to optical transitions between confined exciton multiplexes. We furthermore show that the electron-electron and hole-hole exchange interaction is responsible for the typical appearance of pairs in the photoluminescence spectra and for the appearance of redshifted new lines as the excitation power increases. The fact that only a few spectral lines appear in the emission spectrum strongly indicates fast thermalization. This means that a multiexciton relaxes to its ground state much faster than its radiative lifetime.