Analysis of the Coupled Electron and Nuclear Spin Systems from Optically Induced Dynamic Nuclear Polarization in GaAs

Gallium arsenide (GaAs) is a semiconductor that possesses a direct band gap whose optical transitions are spin selective. This property is ideal for investigation into optical generation and detection of electron spins, which are of interest for potential applications such as spintronics and quantum information. One such technique is by means of a periodic pulsed optical pump-probe Kerr experiment, where the measured Kerr rotation of the reflected light is proportional to the spin polarization. Previous optical investigations into spin phenomena of a n-type GaAs epilayer observed unexpected distortion in the Kerr signal with respect to the change in external magnetic field. It was suspected that dynamic nuclear polarization contributed to these observations as polarized nuclear spins produce an additional magnetic (Overhauser) field that modify the effective Larmor precession of the electrons. However, the known mechanisms for dynamic nuclear polarization require electron spin polarization along the applied magnetic field direction, and the measurements were conducted with the optical pumping perpendicular to the applied magnetic field. In this work, we model the dynamic nuclear polarization in GaAs under periodic optical pumping. This model is constructed from analytic expressions describing the accumulation of electron spins along the external field from a periodic optical pulse train detuned from the donor bound exciton resonance. These spins couple to the nuclear system through the scalar Fermi contact hyperfine interaction and will facilitate nuclear polarization. In this dissertation, the derivation and justification of these expressions is discussed. Simulations replicating the experiment are performed with numerical calculations using the model with the experimental parameters and conditions. The correspondence between the simulated and experimental results provide credence that the physical mechanisms of the observed behavior have been uncovered. The inhomogeneity of the nuclear system is simulated by implementing phenomenological modifications to the model. These simulations demonstrate further additional behavior unique to an inhomogeneous system, such as the bifurcation of the Overhauser distribution. These results are corroborated with additional experiments. Motivated by the success of these simulations, we propose an experiment with an additional time-delayed pump pulse included with the pump-probe Kerr measurement. We predict that further optical control of the nuclear polarization can be achieved by varying the time delay of the two pump pulses.PHDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/174398/1/mdomingu_1.pd