The fine structure of the neutral nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center in diamond is a widely-utilized system due to its useful quantum properties. Almost all research focuses on the negative charge state (NV$^-$) and comparatively little is understood about the neutral charge state (NV$^0$). This is surprising as the charge state often fluctuates between NV$^0$, and NV$^-$, during measurements. There are potentially under utilized technical applications that could take advantage of NV$^0$, either by improving the performance of NV$^-$, or utilizing NV$^0$, directly. However, the fine-structure of NV$^0$, has not been observed. Here, we rectify this lack of knowledge by performing magnetic circular dichroism (MCD) measurements that quantitatively determine the fine-structure of NV$^0$. The observed behavior is accurately described by spin-Hamiltonians in the ground and excited states with the ground state yielding a spin-orbit coupling of $\lambda = 2.24 \pm 0.05$ GHz and a orbital $g-$factor of $0.0186 \pm 0.0005$. The reasons why this fine-structure has not been previously measured are discussed and strain-broadening is concluded to be the likely reason