Optically enhanced high-field NMR of GaAs.

Abstract

The $^{69}\mathrm{Ga}$ NMR signals of differently doped GaAs samples were directly observed under optical pumping conditions in high magnetic field (4.2 T). This technique greatly enhances the sensitivity of the NMR signals and allowed us to study the effect of optical pumping on the nuclear polarization under conditions very different from optically detected NMR. The strongest NMR signals were observed when pumping with a light energy below the band gap, suggesting the involvement of impurities and defects for the most efficient polarization transfer. Inversion of the helicity of the pumping light at the lowest temperatures results in a phase inversion of the NMR signal of a bulk-GaAs and a Si-doped sample, but no phase inversion was observed for a Be-doped sample. \textcopyright{} 1996 The American Physical Society.