Photoconversion of shallow nitrogen-vacancy centers in flat and nanostructured diamond under high-power laser irradiation

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.