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 ⁷¹Ga 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.