Model for optically-induced nuclear spin polarization in gallium arsenide

New technologies and corresponding research fields have recently emerged that aim to develop solid-state devices based on large polarizations of electron and/or nuclear spins. These include spin-based strategies for parallel information processing through quantum entanglement ("quantum computing") and semi-classical electronic devices controlled via the spin degree of freedom ("spintronics"). A new rule of thumb - polarization has application - makes the optically pumped semiconductor an interesting system, as it exhibits both large electron and nuclear polarizations. However, several aspects of the process by which nuclear polarization is generated through optical pumping were not understood prior to this thesis, even for the most well studied semiconductor, GaAs. These include the dependence of the nuclear polarization on laser power, irradiation time, and especially on photon energy, which exhibits a dramatic peak near 1.5 eV.;This thesis presents a quantitative model for optical nuclear polarization in GaAs. The model makes predictions for all quantities observable in a hulk optically pumped NMR (OPNMR) spectrum: the OPNMR signal magnitude, the hyperfine shift of the NMR frequency, and the nuclear spin temperature. The model may help researchers to optimize experimental conditions for maximizing nuclear polarization in spintronics or quantum computing architectures.;A clear correlation is shown between the OPNMR signal and the photoconductivity. A photoconductivity model is developed herein that accounts for the varying penetration depth of the light with photon energy and for the presence of band-to-band and band-to-defect recombination of charge carriers. The model's predictions agree well with the photoconductivity data. The photoconductivity model is then combined with a nuclear polarization model. The resulting picture for near-band-gap (1.495 eV ≲ by ≲ 1.6 eV) optical nuclear polarization is as follows. Optical absorption generates free, non-equilibrium electron spins, whose polarization depends on the light polarization. During their excited-state lifetime, these electrons may relax into shallow-donor-bound states, where they experience a strong hyperfine interaction and can cross-relax with nuclear spins. The nuclear polarization near shallow donor defects then evolves over space and time according to a diffusion equation that accounts for localized generation and loss.;This model predicts the photon-energy dependence and laser-power dependence of the OPNMR signal very well. The peak at 1.5 eV is predicted to arise from an optimal balance between a high nuclear polarization and a large irradiation volume. Both theory and experiment exhibit a deviation from linear growth of OPNMR signal with laser power at high powers and an earlier onset of non-linear growth for higher photon energy. Finally, the model predicts a time-dependent hyperfine shift of the NMR frequency that fits the data with quantitative agreement. All free parameters within the model are constrained through the fitting of these various data sets.;With this model, analytical expressions are derived for helicity asymmetries in OPNMR spectra. These asymmetries are related simply enough to electron spin parameters that they provide a methodology for extracting the initially-excited and steady-state electron spin polarizations from OPNMR spectra, allowing the dependence of these polarizations on temperature and photon-energy to be investigated herein. The OPNMR asymmetry is furthermore used as a local thermometer of the irradiated volume, which provides experimental verification of a quantitative model for laser heating.;Several unusual effects in optically pumped GaAs are characterized. Perhaps the most dramatic is a bi-exponential decay of the photoconductivity with irradiation time, observed at all near-band-gap photon energies. This serves as clear evidence of photoquenching of the deep defect known as EL2 under the conditions typical of OPNMR measurements.;Further experiments are suggested to complete the studies in this thesis. A particularly important one is to measure the localization of optical nuclear polarization through the dark evolution of quadrupolar satellite NMR peaks. This would be the most definitive test, to date, of the nuclear polarization mechanism for bulk OPNMR.