  Understanding and manipulating spin polarization and transport in the
vicinity of semiconductor-hosted defects is a problem of present technological
and fundamental importance. Here, we use high-field magnetic resonance to
monitor the relaxation dynamics of spin-3/2 nuclei in semi-insulating GaAs. Our
experiments benefit from the conditions created in the limit of low
illumination intensities, where intermittent occupation of the defect site by
photo-excited electrons leads to electric field gradient fluctuations and
concomitant spin relaxation of the neighboring quadrupolar nuclei. We find
indication of a heterogeneous distribution of polarization, governed by
different classes of defects activated by either weak or strong laser
excitation. Upon application of a train of light pulses of variable repetition
rate and on/off ratio, we uncover an intriguing regime of mesoscale nuclear
spin diffusion restricted by long-range, non-uniform electric field gradients.
Given the slow time scale governing nuclear spin evolution, such
optically-induced polarization patterns could be exploited as a contrast
mechanism to expose dark lattice defects or localized charges with nanoscale
resolution.
