High-resolution nonlinear laser spectroscopy of exciton relaxation in gallium arsenide semiconductors.

This dissertation presents a systematic study of exciton relaxation in GaAs/AlGaAs quantum well (QW) structures and in GaAs crystals. New methods of nonlinear laser spectroscopy based on frequency domain four wave mixing (FWM) are developed. In these measurements excitons are created resonantly at a given energy, and subsequent decay of the exciton population as well as decay of the induced polarization are measured through the third order nonlinear optical response of the material. The FWM response is first described using a two level model. Asymmetric and interference FWM line shapes are predicted for systems with excitation migration. Nonlinear optical interactions in semiconductors are described by the effective Bloch equations (EBE), which include many-body interactions such as the electron-hole Coulomb attraction and effects of the Pauli exclusion principle. The EBE resemble the density matrix equation for an atomic system, and indicate line shape theories based on a two level model can provide a qualitative description for nonlinear measurements in QW structures. High resolution nonlinear measurements in a 100 A GaAs/AlGaAs QW show that excitons below the absorption line center are localized. At very low temperature, relaxation of these excitons is dominated by migration among localization sites on a time scale of 100 ps. The steady state redistribution of the exciton population as a result of the migration is obtained for the first time along with the exciton homogeneous linewidth. The temperature dependence of the exciton relaxation rate confirms the recent model of phonon assisted migration. Above the absorption line center, excitons are shown to be weakly delocalized and decay quickly to localized states at lower energy. Rapid diffusion of the exciton is observed. The exciton dephasing time is of order 1 ps and is presumably due to exciton-acoustic phonon and exciton-interface disorder scattering. The nonlinear measurements in a GaAs crystal yields the true radiative life time for free excitons in GaAs. Non-diffusive transport of the free exciton is observed. Additional room temperature measurements demonstrate the presence of excitonic states at room temperature, and yields the carrier recombination time and the ambipolar diffusion coefficient.