Ultrafast and terahertz-frequency semiconductor quantum well optoelectronics

This dissertation presents a theoretical study of coherent picosecond ultrafast opto-electronic properties of semiconductor quantum-well devices. Particular interests are in the optical responses arising from the many-body and the intersubband effects in the coherent regime of excitation by a weak, resonant, femtosecond optical pulse in semiconductor quantum wells. The many-body effects are due to the Coulomb interactions between electrons and holes which primarily create excitons. The excitonic optical responses dominate the optical dynamics for near band-gapped excitation of quantum wells. The many-body Coulomb interactions has also been found to increase the Rabi oscillation frequency in semiconductor quantum wells. The intersubband effect arises from the Coulomb interactions between electronic excitations from each subband, and from the optical transitions via intersubband dipole moments and via interband dipole moments. In the coherent excitation regime, the intersubband effect has been shown to lead to observable transient radiations; this is known as Quantum Beat Phenomena. The quantum beat phenomenon in semiconductor quantum wells can lead to useful applications such as terahertz oscillators and coherent terahertz spectroscopy. To study these phenomena, the optical Bloch equations have been developed using the second quantization formalism from many-body quantum mechanics. The equations take into account both intersubband and many-body effects, and also phenomenologically include the incoherent phase relaxation times. Since under the weak and resonant excitation, excitons play an important role; therefore, the optical Bloch equations are reformulated to explicitly show the excitonic effects. From these equations, the dynamical responses of interband, intersubband polarizations and subband carrier densities under coherent optical excitations of quantum wells are obtained. Both coupled GaAs/Ga...