Optically detected magnetic resonance in negatively charged nitrogen-vacancy centers in diamond under resonant optical excitation at cryogenic temperatures

We study optically detected magnetic resonance (ODMR) in diamond ${\mathrm{NV}}^{\ensuremath{-}}$ centers at cryogenic temperatures. We find that when we use resonant optical excitation at zero phonon line wavelength, turning on microwave radiation leads to an increase in fluorescence. This is different from the conventional case of nonresonant optical pumping where microwave radiation instead leads to a decrease in fluorescence. In addition, we observe a significant increase in contrast compared to the nonresonant ODMR. To explain this effect, we propose a theoretical model based on the interaction of the resonant optical radiation with different groups from the inhomogeneously broadened ensemble of ${\mathrm{NV}}^{\ensuremath{-}}$ centers. We find that the effect is temperature dependant and can only be observed at temperatures below 35 K.