Nonlinear Optical Imaging of Single and Ensemble Nitrogen-Vacancy Centers in Diamond

Controlling local symmetry and charge-state dynamics in nitrogen-vacancy (NV) centers is vital for advancing quantum sensing and information technologies. We report broadband second-harmonic generation (SHG) studies revealing how symmetry breaking, defect clustering, and charge-state conversion shape the nonlinear response of NV⁻ centers in diamond. Polarization-resolved SHG uncovers distinct two- and four-fold symmetries linked to isolated and ensemble NVs, while a strong resonance near 685 nm coincides with the NV⁻ phonon sideband, indicating coupling to electronic–phonon hybrid states. Under femtosecond excitation, ultrafast spectral shifts and SHG amplitude modulations arise from photoionization-driven charge conversion and transient NV⁻ population enhancement. At higher NV densities, clustering-induced distortions reveal local structural and electronic inhomogeneity. These results position SHG as a powerful probe of ultrafast defect dynamics and symmetry control in solid-state quantum materials.