Elementary excitations in charge-tunable InGaAs quantum dots studied by resonant Raman and resonant photoluminescence spectroscopy

We report on resonant optical spectroscopy of self-assembled InGaAs quantum dots in which the number of electrons can accurately be tuned to N=0,1,2 by an external gate voltage. Polarization, wave vector and magnetic field dependent measurements enable us to clearly distinguish between resonant Raman and resonant photoluminescence processes. The Raman spectra for N=1 and 2 electrons considerably differ from each other. In particular, for N=2, the quantum-dot He, the spectra exhibit both singlet and triplet transitions reflecting the elementary many-particle interaction. Also the resonant photoluminescence spectra are significantly changing by varying the number of electrons in the QDs. For N=1 we observe strong polaronic effects which are suppressed for N=2.