Excitonic optical nonlinearities in semiconductors and semiconductor microstructures.

Abstract

This dissertation describes the study of excitonic optical nonlinearities in semiconductors and semiconductor microstructures. The main emphasis is placed on the evolution of optical nonlinearities as one goes from bulk to quantum-confined structures. Included are experimental studies of molecular-beam-epitaxially-grown bulk GaAs and ZnSe, GaAs/AlGaAs multiple-Quantum-Wells (MQW's), and finally, quantum-confined CdSe-doped glasses. The microscopic origins and magnitudes of the optical nonlinearities of bulk GaAs and ZnSe were investigated and the exciton recovery time in ZnSe was measured. A comparison with a plasma theory indicates that in GaAs, band filling and screening of the continuum-state Coulomb enhancement are the most efficient mechanisms, while in ZnSe, exciton screening and broadening are the dominating mechanism for the nonlinearity. The maximum nonlinear index per excited electron-hole pair of ZnSe at room temperature is comparable to that of bulk GaAs and the exciton recovery times are of the order of 100 ps or less. A systematic study of the dependence of the optical nonlinearities on quantum well thickness for GaAs/AlGaAs MQWs and the results of nonlinear optical switching and gain in a 58 A GaAs/AlGaAs MQW are reported and discussed. The maximum change in the refractive index is greatest for the MQWs with the smallest well size and decreases with increasing well size, reaching a minimum for bulk GaAs. The maximum index change per photoexcited carrier increases by a factor of 3 as the well size decreases from bulk to 76 A MQW. A differential energy gain of 0.2 and the contrast of 4 are measured for a 58 MQW using 3 ns laser pulses. The linear and nonlinear optical properties of CdSe semiconductor microcrystallites grown under different heat treatments in borosilicate glasses are investigated. Pump-probe spectroscopic techniques and interferometric techniques were employed to study size quantization effects in these microcrystallites (quantum dots). Nonlinear optical properties due to the transitions between quantum confined electron and hole states are reported for low temperature and room temperature. A relatively large homogeneous linewidth is observed. Single beam saturation experiments for quantum confined samples were performed to study the optical nonlinearities as a function of microcrystallite size. Results indicate that the saturation intensity is larger for smaller size quantum dots.