Coherent optical manipulation of electron spins in semiconductor nanostructures
Oleary, Shannon, 1977-
Electron spin coherence can arise through a coherent superposition of two spin states in the conduction band of a semiconductor and can persist over remarkably long time and length scales. The robust nature of electron spin coherence makes it an excellent model system for exploring coherent quantum phenomena in semiconductors. This dissertation presents both spectral- and time-domain nonlinear optical studies of electron spin coherence through Λ-type three-level systems in two- and zero-dimensional semiconductor systems. The spectral domain study focuses on the experimental realization of electromagnetically induced transparency (EIT), a phenomenon that exploits destructive interference induced by the spin coherence. Coherent Zeeman Resonance (CZR), a precursor to EIT, is demonstrated in two 2D systems, a GaAs mixed-type quantum well (MTQW) and a modulation doped CdTe quantum well (QW). For these studies, Λ-type three-level systems are formed via dipole coupling of a trion to two electron spin states. The CZR response can be described qualitatively by effective density matrix equations. In addition, effects of manybody Coulomb interactions on CZR are investigated by varying the electron density in the MTQW via optical carrier injection. Time-domain studies based on transient differential transmission (DT) are carried out to explore the excitation, manipulation, and detection of electron spin coherence and to better understand how manybody interactions affect coherent nonlinear optical processes in semiconductors. While electron spin coherence can be formed and detected via resonant excitation of excitons or trions, a surprising observation is that injecting excitons into the 2D electron gas in a modulation doped CdTe QW can significantly alter the oscillatory nonlinear response of the electron spin coherence, while the response remains qualitatively unchanged when trions are injected. These behaviors are attributed to an interplay between manybody effects and carrier heating generated by trion formation from excitons. Finally, donor-bound electrons in GaAs are used as a model of localized electron spins. Spin decoherence of order 10 ns, limited by nuclear hyperfine interactions, is observed. Electron spin rotation induced by a nearly resonant laser pulse is also observed, opening the door for further work on mitigating electron spin decoherence time through optical spin echoes.