Résumé
Nuclear spin dynamics in semiconductors has been studied extensively by optical means for more than forty years. Yet generally the existing techniques require the injection of out of equilibrium carriers in the semiconductor in order to probe the nuclear polarization, which in turn disturbs the nuclei to be measured. This is the case for photoluminescence experiments, but also for time resolved Faraday/Kerr rotation, in which a shift in the electron Larmor frequency is induced by the nuclear field.1 In this work, we demonstrate a non-destructive detection of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation of an off-resonant light beam. We use a two-step protocol, as proposed by Artemova and Merkulov2, where preparation and measurement are separated in time. The nuclear spins are first cooled down by optical pumping, then the pump beam is switched off and the subsequent evolution of nuclear polarization is detected by the Faraday rotation of a below band gap probe beam. This experimental protocol applied to metallic GaAs just above the metal-insulator transition reveals a non- monotonous evolution of the nuclear induced Faraday rotation. We interpret this behavior as a consequence of the existence of two contributions in the Faraday rotation, which are related to optical transitions involving either localized or delocalized states. Therefore, this detection scheme turns out to be sensitive to nuclear spins situated within the localization radius of bound electron states, as well as to all other nuclear spins in the sample. The contributions to the Faraday rotation of different nuclear species with different localizations in the sample are identified in nuclear magnetic resonance experiments. Our results suggest that the nuclear relaxation in metallic n-GaAs is dominated by diffusion towards fast relaxing centers, rather than by the Korringa mechanism.