Multispin-Assisted optical pumping of bulk C 13 nuclear spin polarization in diamond

One of the most remarkable properties of the nitrogen-vacancy (NV) center in diamond is that optical illumination initializes its electronic spin almost completely, a feature that can be exploited to polarize other spin species in their proximity. Here we use field-cycled nuclear magnetic resonance to investigate the mechanisms of spin-polarization transfer from NVs to C13 spins in diamond at room temperature. We focus on the dynamics near 51 mT, where a fortuitous combination of energy-matching conditions between electron and nuclear spin levels gives rise to alternative polarization transfer channels. By monitoring the C13 spin polarization as a function of the applied magnetic field, we show C13 spin pumping takes place via a multispin cross-relaxation process involving the NV-spin and the electronic and nuclear spins of neighboring P1 centers. Further, we find that this mechanism is insensitive to the crystal orientation relative to the magnetic field, although the absolute level of C13 polarization-reaching up to ∼3% under optimal conditions-can vary substantially depending on the interplay between optical pumping efficiency, photogenerated carriers, and laser-induced heating.