A Phenomenological Study of High-Field Optically Pumped 13C NMR in Diamond
Nuclear magnetic resonance (NMR) is a powerful spectroscopic technique capable of probing the local electronic environment in a wide array of materials. The sensitivity of an NMR experiment is proportional to a net nuclear spin polarization that is often generated by placing the sample of interest in a static external magnetic field and allowing the spin state populations to come to thermal equilibrium. Unfortunately these thermal polarizations are exceedingly small. Even with a 23.5 Tesla magnet, the strongest currently available, the room temperature 1H polarization is only 0.0081%. Significant sensitivity enhancements can be achieved by hyperpolarizing the nuclear spin system. Transferring polarization from electrons to nuclei, also known as dynamic nuclear polarization (DNP), is one method by which this hyperpolarization is achieved. Optically pumped NMR (OPNMR) is a form of DNP, which uses laser light to athermally polarize a reservoir of electron spins which in turn athermally polarize coupled nuclei.
For much of its history solid state OPNMR involved transferring polarization from photo-excited conduction electrons to hyperfine coupled nuclei in zincblende semiconductors at temperatures ≤ ~80 K. Within the last decade it was found that OPNMR of 15N, 14N, and 13C in single crystal diamond is possible via optically polarized negatively charged nitrogen vacancy (NV-) defects. The best characterized form of OPNMR in diamond is a hyperfine-mediated phenomenon that takes advantage of the NV- excited state level anti-crossing that occurs when the external field is set to ~50 mT. A high-field form of 13C optical pumping has been observed at 7.05 and 9.4 T, well beyond the level anti-crossing. Polarization rates, lifetimes, and magnitudes are influenced by the concentrations of NV- and P1 defects in the diamond. Two of the samples characterized in this study have defect concentrations that allow for the generation of room temperature 13C polarizations up to 200 times that of thermal equilibrium. Both positive and negative polarizations are observed. The sign and magnitude of the polarization exhibit an extraordinary sensitivity to the orientation of the crystal with respect to the polarization of the electric field vector of the optical illumination incident on the sample. For example, the sign of the polarization can flip with as little as a 0.5° change in the orientation of the crystal.
The mechanism responsible for this high-field pumping process remains unknown. Progress in developing a theoretical model is hindered by the simultaneous presence of four defect orientations for every orientation of the crystal.