The paper investigates possibilities for studying nitrogen-vacancy (NV) centers in diamonds and NV^- → NV⁰ conversion dynamics with a novel CL technique called ultrafast pump-probe cathodoluminescence spectroscopy. In this technique, a material is studied using a combination of ultrafast electron and light pulses of which the relative arrival time can be tuned with a high degree of accuracy.

Diamond NV centers have shown great potential for being used in quantum technologies and metrology due to their single-photon emission, photostability, quantum yield and brightness properties, as well as robustness against decoherence. Understanding and controlling NV state dynamics is important for the development of efficient quantum optical systems based on diamond.

Previous experiments have demonstrated the possibilities of NV centers conversion with laser irradiation, and by shifting Fermi level chemically or with external voltage. In this paper the authors study state conversion dynamics with  pump-probe CL spectroscopy. For these experiments an ultrafast SEM was used, in which picosecond electron pulses were used to pump the diamond sample while simultaneously optically probing the NV state. The results of the experiments show that high-energy (5 keV) electron irradiation of the material leads to state conversion from the NV^- to the NV⁰ state. Moreover, this conversion is reversible (back transfer time is 500 ms). 

The pump-probe CL approach presented in the paper can be used to study dynamical behaviour of other complex solid-state emitter systems. If you would like to read the full paper, please click here.