  Coherent light sources emitting in the terahertz range are highly sought
after for fundamental research and applications. THz lasers rely on achieving
population inversion. We demonstrate the generation of THz radiation using
nitrogen-vacancy (NV) centers in a diamond single crystal. Population inversion
is achieved through the Zeeman splitting of the $S=1$ state in $15\ \text{T}$,
resulting in a splitting of $0.42\ \text{THz}$, where the middle $S_z=0$
sublevel is selectively pumped by visible light. To detect the THz radiation,
we utilize a phase-sensitive THz setup, optimized for electron spin resonance
measurements (ESR). We determine the spin-lattice relaxation time up to $15\
\text{T}$ using the light-induced ESR measurement, which shows the dominance of
phonon-mediated relaxation and the high efficacy of the population inversion.
The THz radiation is tunable by the magnetic field, thus these findings may
lead to the next generation of tunable coherent THz sources.
