Effects of optical absorption on ⁷¹Ga optically polarized NMR in semi-insulating GaAs: Measurements and simulations

The intensity and the hyperfine shift of optically polarized NMR (OPNMR) signals of ⁷¹Ga in semi-insulating GaAs have been found to depend on the photon energy and the helicity of light used for optical pumping. Single-crystal GaAs wafers of two different thicknesses, 400 and 175 μm, were examined. The maximum intensity of the OPNMR signals was observed well below the band gap energy, and this maximum OPNMR signal shifted to higher photon energies for the thinner sample. In the range of photon energies for which the maximum OPNMR signals are obtained, there is little or no hyperfine shift of the ⁷¹Ga OPNMR resonance. Hyperfine shifts with the largest magnitude are recorded for photon energies at or above the band gap. At a given photon energy, asymmetric OPNMR signals were observed, with σ⁺ light producing a more intense emissive signal than the corresponding absorptive signal coming from σ⁻ light. We developed a model that accounts for optical absorptivity of GaAs in order to simulate the observed OPNMR intensity, the change in the OPNMR maximum for the thinner sample, and the hyperfine shift dependence on photon energy. This model also accounts for the asymmetric OPNMR intensity for the two σ⁺ and σ⁻ helicities of light. The intensity dependence of the OPNMR signals arises as a consequence of two competing factors: the optical absorptivity of GaAs which directly impacts nuclear polarization ⟨I_Z⟩, and the number of accessible nuclear spins. The magnitude of the hyperfine shift of the OPNMR signals reflects the probability of occupation of optically relevant defects in the semiconductor, also related to optical absorptivity. Finally, the asymmetry in the OPNMR signals arises from the sign of electron spin polarization produced in the optical pumping process.