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 >100s 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 532nm, resulting in a complete fluorescence switch-off at wavelengths >600nm. 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