DIRECTLY DETECTED NUCLEAR-MAGNETIC-RESONANCE OF OPTICALLY PUMPED GAAS QUANTUM-WELLS

Optical pumping strongly enhances nuclear spin polarizations in an n-doped GaAs/AlGaAs multiple quantum well, allowing NMR signals of the GaAs wells to be investigated by direct, radio-frequency detection in a 7.05 T field. The optically pumped NMR signals exhibit unanticipated dependences on the pumping wavelength, polarization, and power. The temperature dependence of the Ga-71 NMR Knight shift is consistent with a g* value of -0.49 +/- 0.02 for electrons in the wells, with no exchange enhancement. We present evidence that the doped and photoexcited electrons do not reach a common spin temperature under optical pumping.