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 (69)Ga 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 approximately 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 (69)Ga 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.