Nonlinear Broadband THz Generation from NV Centers in Bulk Diamond Crystals

Diamond single crystals are promising nonlinear THz sources due to their high damage threshold, transparency, and small dispersion linear dispersion over THz-NIR which enables relaxing the need for additional phase-matching engineering . However, the centrosymmetry of a diamond's lattice prohibits even-order nonlinear effects, including second harmonic generation and optical rectification. We demonstrate broadband THz emission via optical rectification in an NV-doped diamond, where NV centers break inversion symmetry and induce a nonlinear susceptibility in the lattice. THz time-domain spectroscopy reveals single-cycle emission spanning over 4 THz bandwidth, enabled by a high NV density (~200 ppm) and lattice strain. Density functional theory (DFT) confirms the emergence of finite second-order nonlinear susceptibility, directly linking symmetry breaking to THz generation. The wide bandgap and defect-induced strain support efficient THz emission without crystal damage, establishing NV-diamond as a robust platform for high-field ultrabroadband THz generation.