Coupling mechanisms for optically induced NMR in GaAs quantum wells

Optical pumping can increase the polarization of nuclear spins in semiconductors such as GaAs by many orders of magnitude, improving the sensitivity in conventionally detected nuclear magnetic resonance (NMR) experiments. Optical detection of these NMR transitions provides an additional increase in sensitivity, and furthermore, can distinguish signal contributions from different quantum wells in multiple quantum well samples. In this article we study the coupling mechanisms for all-optical NMR experiments, where modulation of the cw optical excitation at the nuclear Larmor frequency induces transitions between the nuclear spin states. We find clear evidence for two different types of interaction between the photogenerated carriers and the nuclear spins: the hyperfine interaction and the coupling between the electric field of the electron and the nuclear quadrupole moment. While the former induces only $\Delta m_I=\pm 1$ transitions, the latter also causes (single photon-) $\Delta m_I=\pm 2$ transitions.