Electron-induced state conversion in diamond NV centers measured with pump-probe cathodoluminescence spectroscopy

Nitrogen-vacancy (NV) centers in diamond are reliable single-photon emitters, with applications in quantum technologies and metrology. Two charge states are known for NV centers: NV$^0$ and NV$^-$, with the latter being mostly studied due to its long electron spin coherence time. Therefore, control over the charge state of the NV centers is essential. However, an understanding of the dynamics between the different states still remains challenging. Here, conversion from NV$^-$ to NV$^0$ due to electron-induced carrier generation is shown. Ultrafast pump-probe cathodoluminescence spectroscopy is presented for the first time, with electron pulses as pump, and laser pulses as probe, to prepare and read out the NV states. The experimental data is explained with a model considering carrier dynamics (0.8 ns), NV$^0$ spontaneous emission (20 ns) and NV$^0$ $\rightarrow$NV$^-$ back transfer (500 ms). The results provide new insights into the NV$^-$$\leftrightarrow$NV$^0$ conversion dynamics, and into the use of pump-probe cathodoluminescence as a nanoscale NV characterization tool.