Ultrafast processes in semiconductor structures

We review the dynamics of some of the more relevant optical processes in se miconductor quantum wells. We concentrate on the linear regime and study th e time evolution of the light emission, using time-resolved photoluminescen ce spectroscopy. In intrinsic materials, excitonic effects determine their optical properties. Here we describe the formation and recombination of exc itons, and the dependence of these processes on lattice temperature, excito n density, and energy of the excitation light;pulses.:We also describe the dynamics of the exciton's spin by optical orientation experiments. We discu ss the principal mechanisms responsible for the spin hip of the excitons an d clarify the role of the exciton localization. Finally, we will show that exciton-exciton interaction produces a breaking of the spin degeneracy in t wo-dimensional semiconductors. In doped quantum wells, we show that the two spin components of an optically created two-dimensional electron gas are w ell described by the Fermi-Dirac distributions with a common temperature bu t different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energ y but accelerated by thermal spreading of the carriers.