Nonlinear Optical Fingerprints of Defects in Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) defects are promising building blocks for quantum photonics and sensing, yet their nonlinear optical signatures remain poorly understood. We use first-principles calculations to determine the polarization-resolved linear, second-order, and third-order susceptibilities of multilayer h-BN containing vacancies and carbon-based defect complexes. Defects weakly affect the linear response but strongly enhance nonlinear susceptibilities through symmetry breaking and resonant defect transitions. Carbon complexes produce orders-of-magnitude increases in second-harmonic generation and distinct polarization-dependent spectra that differentiate defects with identical composition but different geometry. Third-harmonic generation is also enhanced, although it is less sensitive to local structure. Measurements on carbon-doped h-BN qualitatively support these trends. These results establish nonlinear spectroscopy as a powerful probe of defect identity and geometry in h-BN and provide a route to defect-engineered nonlinear quantum photonics.