Spin dynamics and electronic states of N-Vcenters in diamond by EPR and four-wave-mixing spectroscopy

A new phase-modulation technique for nonlinear laser spectroscopy is applied with a relative resolving power in the sub-Hz range to measure fundamental relaxation processes of the N-V center in diamond. Complementary EPR experiments versus temperature establish the spin character of the ground state in the absence of optical illumination and show that spin-lattice decay occurs via two-phonon processes involving the dominant vibrational mode. The combined results permit deduction of reliable fine-structure assignments for three states of the center and accurate values for zero-field intersystem crossing and spin-lattice relaxation rates from linewidths of individual resonances in the four-wave-mixing spectrum.