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 NV 0 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/μm 2 ). Fluorescence spectroscopy, optically detected magnetic resonance (ODMR), and charge-photoconversion detection are applied to characterize the properties and dynamics of NV – and NV 0 centers. We demonstrate that high laser power strongly promotes photoconversion of NV 0 to NV – centers. Surprisingly, the excess NV – population is stable over a timescale of 100 ms 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.

Ensembles of negatively charged nitrogen-vacancy centers (NV−) in diamond have been proposed for sensing of magnetic fields and paramagnetic agents, and as a source of spin-order for the hyperpolarization of nuclei in magnetic resonance applications. To this end, strongly fluorescent nanodiamonds (NDs) represent promising materials, with large surface areas and dense ensembles of NV−. However, surface effects tend to favor the less useful neutral form, the NV0 centers, and strategies to increase the density of shallow NV− centers have been proposed, including irradiation with strong laser power (Gorrini in ACS Appl Mater Interfaces. 13:43221–43232, 2021). Here, we study the fluorescence properties and optically detected magnetic resonance (ODMR) of NV− centers as a function of laser power in strongly fluorescent bulk diamond and in nanodiamonds obtained by nanomilling of the native material. In bulk diamond, we find that increasing laser power increases ODMR contrast, consistent with a power-dependent increase in spin-polarization. Conversely, in nanodiamonds we observe a non-monotonic behavior, with a decrease in ODMR contrast at higher laser power. We hypothesize that this phenomenon may be ascribed to more efficient NV−→NV0 photoconversion in nanodiamonds compared to bulk diamond, resulting in depletion of the NV− pool. A similar behavior is shown for NDs internalized in macrophage cells under the typical experimental conditions of imaging bioassays. Our results suggest strong laser irradiation is not an effective strategy in NDs, where the interplay between surface effects and local microenvironment determine the optimal experimental conditions.