Optical polarization of nuclear spins in GaAs

The photon energy, irradiation time, and sample temperature dependences of laser-enhanced NMR spectra from semi-insulating GaAs at 9.4 T were measured. These data were used to test the existing model for the mechanism of optical nuclear polarization—namely, optical excitation of shallow donor states and subsequent spin diffusion of polarization throughout the bulk. Features of the present results, such as their uniqueness to semi-insulating GaAs and pumping data obtained via laser irradiation above the band gap, are inconsistent with this model. An additional model is proposed that includes localized paramagnetic centers as storage sites for electron spin polarization, but relies upon delocalized electron states to accomplish the bulk nuclear spin polarizations. The delocalized states could be free excitons.