Time resolved spectroscopy of single quantum dot structures

We report on photoluminescence measurements with temporal and spatial resolution on a series of individual quantum dot structures. Excitation is performed resonantly with very small laser intensities corresponding to between 200 and 1 exciton per pulse in the dot. Structures with different lateral confinement are investigated, as a function of excitation intensity and wavelength. In all cases, the luminescence appears rapidly, with a rise time of about 10 ps. The experimental results indicate an important contribution of Coulomb scattering to the energy relaxation process. In the sample of maximum lateral confinement, we observe a strong, systematic variation of the spectrum with the excitation intensity. Additional transitions appear up to more than 30 meV above the fundamental PL line, which show no measurable population transfer to lower energy states. This is attributed to the formation of a degenerated Fermi gas of excitons.