Radio-frequency control of optically-induced dynamic nuclear polarization in diamond

Optical spin pumping of nitrogen-vacancy (NV)-hosting diamonds has attracted increasing attention as a means for a more broadly accessible DNP implementation, primarily for the ability to operate under convenient experimental conditions of room temperature and low magnetic fields. However, the mechanisms describing the generation and transport of polarization within the crystal host are still only partly understood. In this work, we combine field-cycling experiments with selective radio-frequency (RF) excitation to probe transitions between hybrid nuclear and electronic spin states near energy-matching conditions between the NV and surrounding paramagnetic impurities. We analyze and model the dependence of the nuclear polarization on the applied magnetic field, the RF frequency, and the RF power. Our results indicate that the observed nuclear polarization can be understood as originating from a reduced, specific set of strongly-hyperfine-coupled nuclear spins. This kind of hyperfine spectroscopy sheds light on intriguing pathways to generate nuclear polarization without the use of optical or microwave excitation.