Spectroscopy of photoionization from the ^1E singlet state in nitrogen-vacancy centers in diamond

The ^1E-^1A_1 singlet manifold of the negatively charged nitrogen vacancy (NV^-) center in diamond plays a central role in the quantum information and quantum sensing applications of the NV^- center. However, the energy of this manifold within the diamond bandgap and with respect to the ^3A_2-^3E triplet manifold has not been measured directly. Using field-quenching effects on photoluminescence (PL) spectra, we report on the energy gap between the ^1E-^1A_1 singlet manifold and the ^3A_2 and ^3E ground and excited triplet states of the NV^- as a function of excitation wavelength and power, temperature, and applied magnetic field in a heavily nitrogen-doped sample. Increased PL and decreased zero-phonon line width from the NV^0 were observed in the presence of an applied magnetic field, indicating ionization from the long-lived ^1E singlet state. A temperature-dependent ionization threshold between 532 nm and 550 nm was found, locating the singlet states within the diamond band gap.