  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.
