Origin of enhanced dynamic nuclear polarization and all-optical nuclear magnetic resonance in GaAs quantum wells - art. no. 195304

Time-resolved optical measurements of electron-spin dynamics in a (110) GaA s quantum well are used to study the consequences of a strongly anisotropic electron g tensor, and the origin of previously discovered all-optical nuc lear magnetic resonance. All components of the g tensor are measured, and a strong anisotropy even along the in-plane directions is found, The amplitu des of the spin signal allow the study of the spatial directions of the inj ected spin and its precession axis. Surprisingly efficient dynamic nuclear polarization in a geometry where the electron spins are injected almost tra nsverse to the applied magnetic field is attributed to an enhanced nonprece ssing electron spin component. The small absolute value of the electron g f actor combined with efficient nuclear spin polarization leads to large nucl ear fields that dominate electron spin precession at low temperatures. Thes e effects allow for sensitive detection of all-optical nuclear magnetic res onance induced by periodically excited quantum-well electrons. The mechanis m of previously observed Deltam = 2 transitions is investigated and found t o be attributable to electric quadrupole coupling, whereas Deltam=1 transit ions show signatures of both quadrupole and electron-spin induced magnetic dipole coupling.