THz time-domain spectroscopy of coherent cyclotron phenomena in gallium arsenide heterostructures

The recent development of optoelectronic techniques for the generation and detection of sub-picosecond (THz), freely propagating electromagnetic transient pulses, has resulted in new opportunities for temporally resolving the ballistic dynamics of high-mobility charge carriers in semiconductors. In this thesis we demonstrate several applications of THz spectroscopy to the study of these systems under moderate magnetic fields and cryogenic temperatures.; The high-frequency coherent response of an equilibrium two-dimensional electron gas is investigated in the time domain upon intraband excitation by a single-cycle electric field oscillation. Under magnetic field, we resolve in real time multiple oscillations of the induced current due to the ballistic cyclotron motion of the electrons.; Alternately, the dynamics of a non-equilibrium system may be observed by injecting a femtosecond interband optical excitation. The photogenerated coherent polarizations of the conduction and valence bands radiate an oscillatory electric field at their cyclotron frequencies via a Landau-level- and surface-depletion field-enhanced second-order nonlinear susceptibility; the cyclotron frequency is found to depend on the laser excitation energy in a manner consistent with the known non-parabolicity of the associated bands. Evidence is also found for strain-induced anisotropy of the valence bands.; Coupling of the cyclotron radiation with other dynamic modes of {dollar}rm Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}As{dollar} heterostructures is investigated, including the interaction between the photoexcited carriers and a quasi-3D electron gas. The latter system exhibits a dramatic resonance as the photoelectron cyclotron frequency coincides with the equilibrium magnetoplasmon.; Future prospects include studies of THz microcavities, and temporally--and spatially--resolved measurements of nanostructures.