Resonant excitation and anti-Stokes luminescence of GaAs single quantum dots

We have investigated micro-photoluminescence ($\mu$-PL) and excitation ($\mu$-PLE) spectra of a single GaAs/AlGaAs quantum dot grown on a GaAs (411)A surface. We observed sharp resonant lines in both $\mu$-PL and $\mu$-PLE spectra, corresponding to the discrete energy levels of the dot. When the sample was excited at one of the resonant lines, resonant luminescence lines appear not only in Stokes side but also in anti-Stokes side. We discuss the possible origins of the anomalus anti-Stokes luminescence.