Exciton saturation dynamics and spin gratings in multiple quantum well semiconductors

Abstract

The dynamics of exciton saturation in GaAs/AlGaAs multiple quantum wells are investigated using picosecond optical pulses. The main contributions to exciton saturation have been shown to be phase space filling. Coulomb screening and exciton lineshape broadening. The spin dependent nature of the phase space filling nonlinearity has allowed its separation from the effects due to Coulomb screening and broadening. The effect of lineshape broadening on exciton saturation has been investigated through its density dependence for a number of quantum well widths. The results show that the effects due to broadening are important in narrow wells of high quality. An investigation into electron spin relaxation at room temperature has been carried out as a function of the well width. The observed decrease in the electron spin relaxation time with decreasing well width is in good agreement with previously reported results and suggests the D'Yakonov-Perel mechanism is the dominant spin-flip mechanism for electrons in quantum wells at room temperature. The first demonstration of an electron spin grating in a quantum well semiconductor is reported by utilising the optical selection rules for quantum wells and optical polarisation gratings. Time resolved studies of the grating decay have allowed the first optical measurement of the in-plane electron diffusion coefficient in a quantum well semiconductor.