Magnetic-field dependence of the optical Overhauser effect in GaAs

When nuclear spin order is induced by optical excitation near the band gap of a semiconductor such as GaAs, the effect is referred to as optical pumping. This paper presents measurements of the optical pumping rate in semi-insulating GaAs over the magnetic field range of 0-24 T at temperatures of 1.5 K and 4.2 K. The enhanced nuclear polarization was sampled by radio wave detected NMR. The data were recorded using Bitter- type magnets which permitted rapid ramping between the pumping and sampling fields in a time short compared to the nuclear spin lattice relaxation time in the dark. The field dependence has been fitted to a relaxation model which includes spin diffusion and dark relaxation terms. Fits were obtained by fixing the g factor to its literature value. The fitted parameters include the correlation time for electron spin-density fluctuations, the average hyperfine field, and the nuclear spin diffusion coefficient.