(a) 71Ga NMR spectra of a GaAs/AlGaAs quantum well (40 n-doped GaAs layers, 30 nm thick, separated by 180 nm AlGaAs layers, ∼0.3 cm2 area) at 1.6 K at 7.05 T, with optical pumping at 806 nm wavelength for the indicated times after saturation of 71Ga spin polarization. (b) Dependence of the hyperfine shift K between GaAs and AlGaAs signals on Landau-level filling factor ν, varied by tilting the sample relative to the external magnetic field, at 7.05 T and 9.39 T.