In this letter, the photon-induced charge conversion dynamics of a single Nitrogen-Vacancy (NV) center in nanodiamond between two charge states, negative (NV-) and neutral (NV0), is studied by the auto-correlation function. It is observed that the ionization of NV- converts to NV0, which is regarded as the dark state of the NV-, leading to fluorescence intermittency in single NV centers. A new method, based on the auto-correlation calculation of the time-course fluorescence intensity from NV centers, was developed to quantify the transition kinetics and yielded the calculation of transition rates from NV- to NV0 (ionization) and from NV0 to NV- (recharging). Based on our experimental investigation, we found that the NV--NV0 transition is wavelength-dependent, and more frequent transitions were observed when short-wavelength illumination was used. From the analysis of the auto-correlation curve, it is found that the transition time of NV- to NV0 (ionization) is around 0.1 μs, but the transition time of NV0 to NV- (recharging) is around 20 ms. Power-dependent measurements reveal that the ionization rate increases linearly with the laser power, while the recharging rate has a quadratic increase with the laser power. This difference suggests that the ionization in the NV center is a one-photon process, while the recharging of NV0 to NV- is a two-photon process. This work, which offers theoretical and experimental explanations of the emission property of a single NV center, is expected to help the utilization of the NV center for quantum information science, quantum communication, and quantum bioimaging.
Keywords: NV centers; auto-correlation; blinking; ionization; nanodiamond.