NV$^-$-centers in diamond and silicon carbide as the basis for room-temperature masers

The successful realization of coherent microwave amplification (maser effect) at room temperature (300 K) based on optically aligned triplet spin sublevels of negatively charged nitrogen-vacancy centers (NV$^-$) in diamond marked a new milestone in the development of solid-state masers. In this paper, we present a comparative analysis of the spin-optical properties of NV$^-$ centers in diamond and NV$^-$ centers in silicon carbide (SiC) with a reduced content of the magnetic isotope $^{29}$Si ($I$ = 1/2), which are used to create masers operating at room temperature. The similarity of the optical pumping mechanisms that form the inverse population of the ground triplet state in both systems is demonstrated. The transverse spin relaxation times $T_2^*\approx$ 1.5 $\mu$s for NV$^-$ centers in isotopically modified $^{28}$SiC significantly exceed the corresponding values for diamond ($T_2^*\approx$ 0.3 $\mu$s). The longitudinal relaxation times are comparable with the requirements for maintaining the population inversion: about 1.5 ms for NV$^-$ in diamond and 100 $\mu$s for NV$^-$ in $^{28}$SiC. The combination of the ability to grow large SiC single crystals and the high permissible concentration of active centers opens up prospects for creating a scalable and technologically efficient platform for solid-state masers operating at room temperature.