E singlet state of NV centers in diamond

The nitrogen-vacancy (NV) center in diamond is valued for its accessible quantum properties and for quantum sensing applications. The NV’s basic energy level diagram and optical cycle explain the NV’s key characteristics and are well-understood. For applications, there are significant efforts to improve optical and electrical readout by controlling ionization and recombination between NV^- and NV⁰ charge states. In this work, we use photoionization spectroscopy to determine the ionization energy of the long-lived ¹E singlet state, the final energy level in the optical cycle yet to be determined.  In our measurements, we modulate the populations of the optical cycle states by applying and removing a 100 mT magnetic field, while observing the resulting amplitude changes in the NV⁰ and NV ^- photoluminescence spectra.  The extensive measurements cover a region of parameter space spanning 488 nm to 560 nm in excitation wavelength, 1.5 µW to 40 µW in excitation power, and 1.6 K to 295 K in temperature.  Ionization from ¹E indicated by field-induced increases in NV⁰ photoluminescence is observed for excitation wavelengths 532 nm and below but not 550 nm and above.  From this data, we conclude that the ionization threshold from the singlet is between 2.25 eV and 2.33 eV, in agreement with theoretical predictions based on singlet-triplet transition rates. Subtle narrowing of the NV⁰ zero-phonon line with the applied field is consistent with a reduction in charged defects.  At the same time, a strong, zero-field, spin-mixing phenomenon between 10 K and 100 K is revealed.