Ground-State Depletion Nanoscopy of Nitrogen-Vacancy Centres in Nanodiamonds

The negatively charged nitrogen-vacancy (NV ^-) centre in nanodiamonds (NDs) has been recently studied for applications in cellular imaging due to its better photo-stability and biocompatibility if compared to other fluorophores. Super-resolution imaging achieving 20-nm resolution of NV ^- in NDs has been proved over the years using sub-diffraction limited imaging approaches such as single molecule stochastic localisation microscopy and stimulated emission depletion microscopy. Here we show the first demonstration of ground-state depletion (GSD) nanoscopy of these centres in NDs using three beams, a probe beam, a depletion beam and a reset beam. The depletion beam at 638 nm forces the NV ^- centres to the metastable dark state everywhere but in the local minimum, while a Gaussian beam at 594 nm probes the NV ^- centres and a 488-nm reset beam is used to repopulate the excited state. Super-resolution imaging of a single NV ^- centre with a full width at half maximum of 36 nm is demonstrated, and two adjacent NV ^- centres separated by 72 nm are resolved. GSD microscopy is here applied to NV ^- in NDs with a much lower optical power compared to bulk diamond. This work demonstrates the need to control the NDs nitrogen concentration to tailor their application in super-resolution imaging methods and paves the way for studies of NV ^- in NDs’ nanoscale interactions.