The strong spectral- and time-dependent emission of NV center charge state in bulk diamonds

We discuss charge state ratio-dependent spectral and temporal emission properties of nitrogen-vacancy (NV) centers in bulk diamonds. An understanding of the charge state ratio in artificially grown diamonds is important to design them for NV-based magnetometry. We show that the NV charge state ratio strongly correlates with the intensity of the zero-phonon line (ZPL) of NV−, which is supported by an emission dip at 2.87 GHz in spin-selective optical transitions. The Debye–Waller factor of NV centers depends on the charge state ratio, and it increases for NV− with an increase in the charge state ratio. We observe that the energy separation between the second phonon sideband (PSB) and ZPL does not scale multiplicatively with that of the energy separation between first PSB and ZPL. This deviation becomes more pronounced with an increase in charge state ratio, suggesting anharmonicity of vibrational energy levels, rather than the usually reported harmonicity. Further, we discuss the inherent relation between decay rates and charge states as a function of emission wavelengths. The excited state lifetime of NV0 remains constant, whereas the NV− lifetime decreases with an increase in the charge state ratio, with an overall decrease of 37%. Our results show an emission spectra-based approach to estimate the charge state ratio in diamond crystals, which is helpful for quantum-enhanced sensing and magnetometry.