It is possible to polarize (i.e., orient) the electron spins in semiconductors by irradiating them with circularly polarized light near the band gap. The extent to which the electrons can be oriented depends on the details of the band structure, the type of optical transitions allowed, the relaxation processes, and various other external factors. By changing the wavelength, the photon energy, of the excitation laser, different parts of the band structure may be accessed. Coupling between the oriented electrons and nuclear spins results in enhanced NMR signals, termed “optically polarized NMR.”

In probing the band structure, we have observed 69Ga shifts in semi-insulating GaAs by OPNMR. We are currently exploring the shift dependence on photon energy, on the polarization of the laser light, and on the laser power. These observations are important for understanding the mechanism of NMR signal enhancement in semiconductors, including Fermi contact hyperfine interactions with the electron spin system and other possible mechanisms. We observe a shift in the resonance at different illumination times, indicating that the signal arises from a combination of the hyperfine-coupled nuclear spins and from regions where nuclear spin-diffusion plays a dominant role in the NMR signal intensity. These results will be discussed in the context of a match to previously reported spin diffusion coefficients.