Penetration depth model for optical alignment of nuclear spins in GaAs

Abstract

A framework for predicting bulk NMR quantities in semi-insulating GaAs under optical alignment conditions is developed by combining literature penetration depth data with simple kinetic equations. The model accounts for the major photoconductivity and NMR intensity variations with photon energy, including the peak near 1.5 eV. With the fitting parameters fixed, the photon-flux dependence of the NMR intensity is predicted quantitatively up to an energy shift. At the highest fluxes, we see the NMR intensity level off, suggesting that it is possible to completely fill the relevant electronic reservoir. The model also quantitatively fits the time dependence of the light-induced hyperfine shift. The experimental and modeling results have implications toward understanding the microscopic mechanism, and are consistent with nuclear polarization via bound electrons. Finally, the model makes experimentally testable predictions for the photon energy and flux dependences of the spin temperature and hyperfine shift of the NMR line.