  We present a comprehensive study of the optical transitions and selection
rules of variably charged single self-assembled InAs/GaAs quantum dots. We
apply high resolution polarization sensitive photoluminescence excitation
spectroscopy to the same quantum dot for three different charge states: neutral
and negatively or positively charged by one additional electron or hole. From
the detailed analysis of the excitation spectra, a full understanding of the
single-carrier energy levels and the interactions between carriers in these
levels is extracted for the first time.
