  Shallow, negatively-charged Nitrogen-Vacancy centers (NV-) in diamond have
been proposed for high-sensitivity magnetometry and spin-polarization transfer
applications. However, surface effects tend to favor and stabilize the less
useful neutral form, the NV0 centers. Here, we report the effects of green
laser irradiation on ensembles of nanometer-shallow NV centers in flat and
nanostructured diamond surfaces as a function of laser power in a range not
previously explored (up to 150 mW/{\mu}m2). Raman spectroscopy,
optically-detected-magnetic-resonance (ODMR) and charge-photoconversion
fluorescence detection are applied to characterize the properties and dynamics
of NV- and NV0 centers. We demonstrate that high laser power strongly promotes
photoconversion of NV0 to NV- centers. Surprisingly, the excess NV- population
is stable over a time scale of one hundred milliseconds after switching off the
laser, resulting in long-lived enrichment of shallow NV-. The beneficial effect
of photoconversion is less marked in nanostructured samples. Our results are
important to inform the design of samples and experimental procedures for
applications relying on ensembles of shallow NV- centers in diamond.
