Bulk nuclear polarization enhanced at room temperature by optical pumping

Bulk C13 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 single crystals at a ∼50 mT field promoting anticrossings between electronic excited-state levels, followed by shuttling of the sample into an NMR setup and by subsequent C13 detection. A nuclear polarization of ∼0.5% - equivalent to the C13 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 C13 room temperature polarizing strategy provides a promising new addition to existing nuclear hyperpolarization techniques.