Electron spin resonance of NV$^{(-)}$-centers in synthetic fluorescent diamond microcrystals under conditions of optical spin polarization

Electron paramagnetic resonance (EPR) spectra of synthetic diamond microcrystals with NV$^{(-)}$-centers have been studied. It is shown that under the conditions of irradiation of the material with the light of a xenon lamp at low temperatures $\sim$ 100 K, the intensities of the EPR signals corresponding to the “forbidden” ($\Delta m_s$ = 2) and low field allowed ($\Delta m_s$ = 1) transitions are amplified several times, while the EPR signals from paramagnetic nitrogen and impurity nickel in the charge state -1 practically do not change. This is due to a change in the population of levels of the ground triplet state of the NV$^{(-)}$-center and the optical polarization of spins in the state $m_s$ = 0 of the triplet level.