Bulk Nuclear Polarization Enhanced at Room-Temperature by Optical Pumping

Bulk 13C polarization can be strongly enhanced in diamond at room-temperature based on the optical pumping of nitrogen-vacancy color centers. This effect was confirmed by irradiating suitably aligned single-crystals at a ~50 mT field promoting anti-crossings between electronic excited-state levels, followed by shuttling of the sample into a custom-built NMR setup and by subsequent 13C detection. A nuclear polarization of ~0.5% - equivalent to the 13C polarization achievable by thermal polarization at room temperature at fields of ~2000 T - was measured, and its bulk nature determined based on line shape and relaxation measurements. Positive and negative enhanced polarizations were obtained, with a generally complex but predictable dependence on the magnetic field during optical pumping. Owing to its simplicity, this 13C room-temperature polarizing strategy provides a promising new addition to existing nuclear hyperpolarization techniques.