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 photoexcited 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, nonuniform 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. © 2013 American Physical Society.

Low detection sensitivity stemming from the weak polarization of nuclear spins is a primary limitation of magnetic resonance spectroscopy and imaging. Methods have been developed to enhance nuclear spin polarization but they typically require high magnetic fields, cryogenic temperatures or sample transfer between magnets. Here we report bulk, room-temperature hyperpolarization of (13)C nuclear spins observed via high-field magnetic resonance. The technique harnesses the high optically induced spin polarization of diamond nitrogen vacancy centres at room temperature in combination with dynamic nuclear polarization. We observe bulk nuclear spin polarization of 6%, an enhancement of ∼170,000 over thermal equilibrium. The signal of the hyperpolarized spins was detected in situ with a standard nuclear magnetic resonance probe without the need for sample shuttling or precise crystal orientation. Hyperpolarization via optical pumping/dynamic nuclear polarization should function at arbitrary magnetic fields enabling orders of magnitude sensitivity enhancement for nuclear magnetic resonance of solids and liquids under ambient conditions.