Microscopic interpretation of optically pumped NMR signals in GaAs.

By numerical modelling and least squares fitting to the field, time and polarization dependence of optically pumped 69Ga NMR data in semi-insulating GaAs at low temperature and high field, a microscopic picture of this phenomenon emerges. Numerical values can be obtained for the key optical pumping parameters: the correlation time for the electron-nuclear contact interaction, and the electronic g-factor. The values of these parameters, which are in close agreement with previously reported values, can be combined with literature estimates for the average hyperfine interaction and spin diffusion coefficient D to generate numerical solutions for the Zeeman nuclear spin order, (r,t). The modelling permits the influence of variations with any other relevant parameter to be predicted. Experimentally, the optical polarization and time dependence data are presented. Least squares fits to the optical polarization dependence yield a value for the optically induced electron spin polarization at the given field, temperature, and excitation intensity. With the numerical solutions (r,t) that best fit the available data, the hyperfine frequency shift can be used to simulate the time evolution of the 69Ga NMR line shape. These simulations provide guidance on how to optimize the experimental conditions to most effectively observe hyperfine effects near shallow donors in this particular material.