Nonlinear broadband THz and harmonic generation from symmetry broken nitrogen-vacancy centers in diamond crystals

Diamond single crystals are highly promising nonlinear media arising from their extremely high damage threshold, broad transparency window, and potential for phase matching over a wide spectral range. However, in pristine diamond the buildup of even-order nonlinear polarization is forbidden by its centrosymmetric crystal structure. Here, we demonstrate two even-order nonlinear processes from a high-density nitrogen-vacancy (NV)-doped diamond sample: broadband terahertz (THz) emission and harmonic generation (HG). In this system, NV centers break the local inversion symmetry, inducing dipoles in the lattice and enabling the buildup of otherwise forbidden even-order responses. The nonlinear THz temporal and spectral emission properties were characterized using time-domain spectroscopy under near-infrared (NIR) femtosecond (fs) excitation, revealing single-cycle, broadband emission extending beyond 4 THz. The HG response in the NIR was investigated using 2.35 μm fs pulses, with harmonics detected from the 2nd to the 5th order. The NV–diamond cell structure was further analyzed using density functional theory (DFT). Explicit DFT calculations confirm the emergence of a finite second-order nonlinear susceptibility in the NV–diamond system, directly linking the observed THz and HG signals to NV-induced symmetry breaking. This combination of high defect density, internal strain, and the wide bandgap of diamond enables NV-doped diamond to support broadband THz and HG at high intensities without crystal damage.