Negative charge enhancement of near-surface nitrogen vacancy centers by multicolor excitation

Nitrogen-Vacancy (NV) centers in diamond have been identified over the past few years as promising systems for a variety of applications, ranging from quantum information science to magnetic sensing. This relies on the unique optical and spin properties of the negatively charged NV. Many of these applications require shallow NV centers, i.e. NVs that are close (a few nm) to the diamond surface. In recent years there has been increasing interest in understanding the dynamics of NV centers under various illumination conditions, specifically under infra-red (IR) excitation, which has been demonstrated to have significant impact on the NV centers' emission and charge state. Nevertheless, a full understanding of all experimental data is still lacking, with further complications arising from potential differences between the photo-dynamics of bulk vs. shallow NVs. Here we suggest a generalized quantitative model for NV center spin and charge state dynamics under both green and IR excitation. We experimentally extract the relevant transition rates, providing a comprehensive model which reconciles all existing results in the literature. Moreover, we identify key differences between the photo-dynamics of bulk and shallow NVs, and use them to significantly enhance the initialization fidelity of shallow NVs to the useful negatively-charged state.