Dark state photophysics of nitrogen-vacancy centres in diamond

Nitrogen-vacancy (NV) colour centres in diamond are attractive fluorescence emitters owing to their unprecedented photostability and superior applicability to spin manipulation and sub-diffraction far-field optical microscopy. However, some applications are limited by the co-occurrence of dark state population and optical excitation. In this paper, we use fluorescence microscopy and correlation spectroscopy on single negatively charged NV centres in type IIa bulk diamond to unravel the population kinetics of a >100 s long-lived dark state. The bright-dark state interconversion rates show a quadratic dependence on the applied laser intensity, which implies that higher excited states are involved. Depopulation of the dark state becomes less effective at wavelengths above 532 nm, resulting in a complete fluorescence switch-off at wavelengths >600 nm. This switch is reversible by the addition of shorter wavelengths. This behaviour can be explained by a model consisting of three dark and three bright states of different excitation levels, with the most efficient interconversion via the respective higher excited states. This model accounts for the nonlinear dark state and photoswitching kinetics, as well as for the decrease of the NV's fluorescence lifetime with excitation intensity and the strong dependence of fluorescence emission on excitation intensity. Unfortunately, our data do not give enough insight to allow us to assign the different states to specific electronic states known from the literature. Nevertheless, our observations allowed us to improve the recording of fluorescence images of single NV centres with sub-diffraction spatial resolution but they also have important implications for studying their spin states. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.