Optical Properties of Gaas Coupled Quantum Wells and Superlattices: Electric and Magnetic Field Effects

In this thesis the effects of electric and magnetic fields on the optical transmission and reflection spectra of GaAs/AlGaAs coupled quantum wells and superlattices are investigated and their potential application in optical modulation demonstrated. Quenching and enhancement of excitonic absorption peaks is achieved at low bias fields of the order of 10-60 kV/cm for the asymmetric GaAs/AlGaAs coupled quantum wells studied. This is interpreted in terms of energy level anti-crossing and switching between intrawell and interwell transitions. The presence of an additional magnetic field, oriented parallel to the plane of the quantum well layers, shifts the electric field at which energy level anti-crossing set in. Electric field induced enhancement of excitonic absorption peaks was also observed in the superlattice structure studied. A sharpening of the excitonic peak was seen when the inplane magnetic field was increased to 7 T and is attributed to the onset of magnetic localization. The considerable changes in the absorption spectra over moderately low external electric fields demonstrate the possibility of optical modulators consisting of coupled quantum wells and superlattice structures. A GaAs coupled quantum well reflection modulator grown on Si is demonstrated and an enhanced contrast ratio is attained by taking advantage of resonant cavity effects in conjunction with the quantum confined Stark effect.