Microwave-Assisted Spectroscopy Technique for Studying Charge State in Nitrogen-Vacancy Ensembles in Diamond
D.P.L. Aude Craik, P. Kehayias, A.S. Greenspon, X. Zhang, M.J. Turner, J.M. Schloss, E. Bauch, C.A. Hart, E.L. Hu, R.L. Walsworth
Abstract
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 ($\mathrm{N}\text{\ensuremath{-}}{V}^{\ensuremath{-}}$) and neutrally charged ($\mathrm{N}\text{\ensuremath{-}}{V}^{0}$) 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 $\mathrm{N}\text{\ensuremath{-}}{V}^{\ensuremath{-}}$ fluorescence with a spin-state-resonant microwave drive to isolate, in situ, the spectral shape of the $\mathrm{N}\text{\ensuremath{-}}{V}^{\ensuremath{-}}$ and $\mathrm{N}\text{\ensuremath{-}}{V}^{0}$ 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 $\mathrm{N}\text{\ensuremath{-}}{V}^{0}$ 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.