Microwave-Assisted Spectroscopy Technique for Studying Charge State in Nitrogen-Vacancy Ensembles in Diamond

We introduce a microwave-assisted spectroscopy technique to determine the relative ratio of fluorescence emitted by nitrogen-vacancy (N-V) centers in diamond that are negatively charged (N-V-) and neutrally charged (N-V-0) and present its application to studying spin-dependent ionization in N-V ensembles and enhancing N-V-magnetometer sensitivity. Our technique is based on selectively modulating the N-V- fluorescence with a spin-state-resonant microwave drive to isolate, in situ, the spectral shape of the N-V- and N-V-0 contributions to an N-V-ensemble sample's fluorescence. As well as serving as a reliable means to characterize the charge state, the method can be used as a tool to study spin-dependent ionization in N-V ensembles. As an example, we apply the microwave technique to a high-N-V-density diamond sample and find evidence for an additional spin-dependent ionization pathway, which we present here alongside a rate-equation model of the data. We further show that our method can be used to enhance the contrast of optically detected magnetic resonance (ODMR) on N-V ensembles and may lead to significant sensitivity gains in N-V magnetometers dominated by technical noise sources, especially where the N-V-0 population is large. With the high-N-V-density diamond sample investigated here, we demonstrate an up to 4.8-fold enhancement in the ODMR contrast. We also propose a second postprocessing method of increasing the ODMR contrast in shot-noise-limited applications. The techniques presented here may also be applied to other solid-state defects, as long as their fluorescence can be selectively modulated by means of a microwave drive. We demonstrate this utility by applying our method to isolate room-temperature spectral signatures of the V2-type silicon vacancy from an ensemble of V1 and V2 silicon vacancies in 4H silicon carbide.