In its negatively-charged state, the nitrogen vacancy center in diamond (NV-) can be used as an optically-read-out spin sensor of nanoscale magnetic fields with exciting applications ranging from imaging fields in living cells to extracting information about the formation of our solar system from paleomagnetic early-Earth rocks. In contrast, the neutral charge state of the defect (NV0) offers no optical spin readout, producing only a spin-independent fluorescence background under the 532-nm illumination typically used to read out NV- ensembles. Hence, to maximize sensitivity of ensemble-based magnetometers, we would like to understand how to produce diamond samples in which the NVs are predominantly negatively charged.
There is evidence that both material parameters and illumination intensity can be tuned to maximize the NV- to NV0 charge state ratio in a diamond sample. However, the mechanisms behind these effects are poorly understood and a reliable technique for in-situ charge-state determination is required to study them.
In this talk, I'll present a microwave-assisted spectroscopy technique for determining the relative concentrations of NV- and NV0 defects in diamond. Our method is based on selectively modulating the NV- fluorescence with a microwave drive to isolate, in-situ, the spectral shape of the NV- and NV0 contributions to an NV-ensemble sample's fluorescence. The sample-specific nature of this method accounts for sample-to-sample variations in the spectral shape of the NV- and NV0 fluorescence and can be applied with any illumination conditions of interest, as long as they produce a fluorescence contrast between the NV- spin states.
I will present applications of our method to determining the behavior of charge state in an NV ensemble as a function of illumination power and will discuss this technique's use as a new tool to study the spin-dependence of NV- ionization (to NV0) under different illumination and material regimes.
