Optical detection of NMR in high-purity GaAs: Direct study of the relaxation of nuclei close to shallow donors

Optical detection of NMR has two outstanding features: (i) selective observation of the nuclei in the regions of electronic localization and (ii) extremely high sensitivity. This technique can be applied to semiconductors under circularly-polarized light excitation and involves monitoring the change at resonance of the degree of circular polarization of the luminescence. We have detected optically the resonance of nuclei close to shallow donors in high-purity GaAs at 1.7 K. These nuclei are extremely scarce, of total number of the order of ${10}^{11}$. We have studied their spin-lattice relaxation due to hyperfine contact interaction with the photoelectrons trapped on the donors. We show the very fast modulation of this interaction, of characteristic time of several ${10}^{-11\mathrm{}}$ s, which is very likely produced by spin exchange between free and trapped electrons. Furthermore, we demonstrate in a very striking way the occurrence of transport of nuclear magnetization by spin diffusion. This is done by switching off the light excitation, since the evolution of the nuclear spin system in the absence of photoelectrons is entirely due to spin diffusion. The analysis of the experimental results yields the value of the diffusion constant, which is comparable with its theoretical estimate.