Two-photon photoluminescence excitation spectroscopy of single quantum dots

We present an experimental and theoretical study of single semiconductor quantum dots excited by two nondegenerate, resonantly tuned, variably polarized lasers. The first laser is tuned to excitonic resonances. Depending on its polarization it photogenerates a coherent single-exciton state. The second laser is tuned to biexciton resonances. By scanning the energy of the second laser for various polarizations of the two lasers, while monitoring the emission from the biexciton and exciton spectral lines, we map the biexciton photoluminescence excitation spectra. The resonance-rich spectra of the second photon absorption are analyzed and fully understood in terms of a many-carrier theoretical model which takes into account the direct and exchange Coulomb interactions between the quantum confined carriers.