Optically detected magnetic resonance of nitrogen-vacancy centers in diamond under weak laser excitation

As promising quantum sensors, nitrogen-vacancy (N-$V$) centers in diamond have been widely used in frontier studies in condensed-matter physics, material science, and life sciences. In practical applications, weak laser excitation is favorable as it reduces the side effects of laser irradiation, such as phototoxicity and heating. Here we report a combined theoretical and experimental study of (near) zero-field optically detected magnetic resonance (ODMR) of N-$V$-center ensembles under weak 532-nm laser excitation. In this region, both the width and splitting of the ODMR spectra decrease with increasing laser power. This power dependence is reproduced with a model that accounts for the laser-induced charge neutralization of $\mathrm{N}\text{\ensuremath{-}}{V}^{\ensuremath{-}}$--${\mathrm{N}}^{+}$ pairs, which alters the local electric field environment. These results are useful for the understanding and development of N-$V$-based quantum sensing in light-sensitive applications.