Anomalous saturation effects due to optical spin depolarization in nitrogen-vacancy centers in diamond nanocrystals

The photoluminescence of the negatively charged nitrogen-vacancy (NV^-) center displays anomalous saturation behavior at high excitation pulse energies. Where the luminescence is expected to approach a maximum value asymptotically, we have found that it instead drops by as much as a factor of two. In this report, we present evidence that these effects are caused by optical spin depolarization of the NV^- center. We show that the presence of an external magnetic field results in a marked decrease in the anomalous character of the NV^- centers' luminescence and demonstrate that low-energy pulses applied after a strong depolarizing pulse can repolarize the spin of the center. We also offer a model and derive important parameters describing dynamics in the NV center and its interaction with light, which quantitatively explain all the observations.