Optically induced cross relaxation via nitrogen-related defects for bulk diamond C13 hyperpolarization

Abstract

In this Rapid Communication we utilize nuclear magnetic resonance to investigate the hyperpolarization effect of negatively charged nitrogen vacancy (NV) centers on bulk $^{13}\mathrm{C}$ nuclei in a diamond single crystal. We were able to identify several polarization peaks of a different sign at different magnetic fields in a region of some tens of Gauss centered around $50\phantom{\rule{0.28em}{0ex}}\mathrm{mT}$. The bulk $^{13}\mathrm{C}$ hyperpolarization in the investigated field range is usually attributed to the excited state level anticrossing of the NV center. However, we found that this bulk hyperpolarization is caused by optically induced cross relaxation and that it takes place in the NV center ground state. The four-spin coupling between the polarized NV electron spin, the electron spin of a substitutional nitrogen impurity (P1), as well as its $^{14}\mathrm{N}$ nuclei and the $^{13}\mathrm{C}$ nuclear spin have to be considered. We introduce a simple theoretical model which completely fits with the experimental data and which clearly shows that the P1 centers are involved in the polarization process. We expect that the current work has a significant impact on future NV-based polarization applications.