Suppression of nuclear polarization near the surface of optically pumped GaAs

We measured the spin dependence of polarized neutrons reflected by a GaAs sample as it was optically pumped. This dependence was correlated with the helicity of the circularly polarized light and found to oscillate with neutron wave vector transfer. The data provide definitive evidence that optically induced nuclear polarization in GaAs is not uniform with depth. Quantitative analysis of the data shows that nuclear polarization is suppressed for tens of nanometers near the surface of GaAs.