Nonlinear Broadband THz Generation from NV Centers in Bulk Diamond Crystals

Abstract: 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.