Coherent population oscillations with nitrogen-vacancy color centers in diamond

We present results of our research on two-field (two-frequency) microwave spectroscopy in nitrogen-vacancy ($\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$) color centers in a diamond. Both fields are tuned to transitions between the spin sublevels of the $\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$ ensemble in the ${}^{3}{A}_{2}$ ground state (one field has a fixed frequency while the second one is scanned). Particular attention is focused on the case where two microwave fields drive the same transition between two $\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$ ground state sublevels (${m}_{\mathrm{s}}=0\ensuremath{\leftrightarrow}{m}_{\mathrm{s}}=+1$). In this case, the observed spectra exhibit a complex narrow structure composed of three Lorentzian resonances positioned at the pump-field frequency. The resonance widths and amplitudes depend on the lifetimes of the levels involved in the transition. We attribute the spectra to coherent population oscillations induced by the two nearly degenerate microwave fields, which we have also observed in real time. The observations agree well with a theoretical model and can be useful for investigation of the NV relaxation mechanisms.