Ultrafast Processes in Semiconductor Structures

We review the dynamics of some of the more relevant optical processes in semiconductor quantum wells. We concentrate on the linear regime and study the time evolution of the light emission, using time-resolved photoluminescence spectroscopy. In intrinsic materials, excitonic effects determine their optical properties. Here we describe the formation and recombination of excitons, and the dependence of these processes on lattice temperature, exciton density, and energy of the excitation light pulses. We also describe the dynamics of the exciton's spin by optical orientation experiments. We discuss the principal mechanisms responsible for the spin flip of the excitons and clarify the role of the exciton localization. Finally, we will show that exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. In doped quantum wells, we show that the two spin components of an optically created two-dimensional electron gas are well described by the Fermi-Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.