  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.
