Manifestation of Landau level effects in optically-pumped NMR of semi-insulating GaAs

Oscillations in the magnitude of optically generated nuclear spin polarization as a function of photon energy have been observed in bulk semiconductors using optically-pumped NMR (OPNMR). We compare experimental measurements of ⁶⁹Ga OPNMR with experimental measurements of magneto-absorption in bulk GaAs at low temperatures (6 K) and multiple magnetic fields. These data show that the photon energy dependence of both the OPNMR signal intensities and the magneto-absorption coefficients are well correlated over the studied energy regime ≈1.515-1.568 eV, i.e., at and above the bandgap energy. Using the magneto-absorption coefficients as an input to a previously reported model of OPNMR, we are able to capture many of the oscillatory features in the ⁶⁹Ga signal, demonstrating that the origin of these oscillations are magnetic in nature, namely the formation of Landau levels. The nuclear spins as probed by OPNMR provide a measure of the circularly-polarized optical interband transitions between Landau levels. This correlation shows the importance of magneto-absorption measurements for understanding the complex photon energy dependence profile of OPNMR.