Origin of enhanced dynamic nuclear polarization and all-optical nuclear magnetic resonance in GaAs quantum wells

Time-resolved optical measurements of electron-spin dynamics in a (110) GaAs quantum well are used to study the consequences of a strongly anisotropic electron g tensor, and the origin of previously discovered all-optical nuclear magnetic resonance. All components of the g tensor are measured, and a strong anisotropy even along the in-plane directions is found. The amplitudes of the spin signal allow the study of the spatial directions of the injected spin and its precession axis. Surprisingly efficient dynamic nuclear polarization in a geometry where the electron spins are injected almost transverse to the applied magnetic field is attributed to an enhanced nonprecessing electron spin component. The small absolute value of the electron g factor combined with efficient nuclear spin polarization leads to large nuclear fields that dominate electron spin precession at low temperatures. These effects allow for sensitive detection of all-optical nuclear magnetic resonance induced by periodically excited quantum-well electrons. The mechanism of previously observed $\ensuremath{\Delta}m=2$ transitions is investigated and found to be attributable to electric quadrupole coupling, whereas $\ensuremath{\Delta}m=1$ transitions show signatures of both quadrupole and electron-spin induced magnetic dipole coupling.