Photocurrent Control in a Magnetic Field through Quantum Interference.

摘要

Quantum-mechanical interference between excitation pathways can be used to inject photocurrents optically in semiconductors, the properties of which can be coherently controlled through the phases and polarizations of the optical pulses. In this thesis, coherent photocurrent control is investigated theoretically for two-dimensional semiconductor systems in a perpendicular magnetic field. The semiconductor systems are subjected to optical pulses with centre frequencies o 0 and 2o0, which excite interband transitions through one- and two-photon processes, selection rules for which are determined from envelope wave functions. It is shown using time-dependent perturbation theory that the interference between one- and two-photon pathways connecting a particular valence Landau level to two different but adjacent conduction Landau levels manifests itself as electron currents that rotate counterclockwise, while interference between pathways connecting two adjacent valence Landau levels to a particular conduction Landau level manifests itself as hole currents that rotate clockwise. The initial directions of the currents can be controlled by adjusting the polarizations and a relative phase parameter of the pulses.;The analysis is performed for a GaAs quantum well, monolayer graphene and bilayer graphene. For GaAs, the equally spaced Landau levels in each band lead to electron currents rotating at a single frequency and hole currents rotating at a different frequency. Monolayer and bilayer graphene allow currents with multiple frequency components as well as other peculiarities resulting from additional interference processes not present for GaAs.;The photocurrents in all of these systems radiate in the terahertz regime. This radiation is calculated for realistic experimental conditions, with scattering and relaxation processes accounted for phenomenologically.;Finally, the effect of Coulomb interactions on the coherent control process is considered for an undoped GaAs quantum well in a magnetic field. The interaction is treated in a simple first-order perturbation model, and it is determined that the main effect of Coulomb interactions between electrons and holes on the injected photocurrents is to produce quantitative differences through the energy levels of the system, but the main qualitative features remain unchanged.