Observation of charged few-particle states in the optical spectra of single semiconductor quantum dots

A detailed discussion of the optical properties of single n-type modulation-doped semiconductor quantum dots is presented. We use the photo-depletion/back-hopping mechanism to optically control the number of surplus electrons in the dot. Comparison of the experimental data with calculated luminescence spectra, obtained within a direct-diagonalization approach for calculating electron-hole states, allows the identification of luminescence from the decay of up to fivefold charged excitons. Finally we study the influence of fluctuating local electric fields, produced by ionized impurities in the surrounding of the dots, on the optical selection rules and on the linewidth of the emission peaks.