Photonic Crystal Defect Cavities Enable Air-Stable and Enhanced SHG from NbOCl₂

Layered ferroelectric NbOCl₂ exhibits intrinsically strong second-order nonlinear susceptibility, making it a promising candidate for second-harmonic generation (SHG) application in ultrathin nonlinear optical devices. In practical implementations, such devices require not only high SHG efficiency but also robust operation under ambient conditions. Therefore, a vertical 1D photonic crystal (PhC) microcavity integrated design is proposed, which could simultaneously enhance SHG from NbOCl₂ flakes and promote long-term environmental stability. By embedding NbOCl₂ between TiO₂/SiO₂ PhC mirrors, the cavity supports optical resonance that matches the excitation wavelength, achieving a 12-fold enhancement in SHG through resonant field confinement. Furthermore, the encapsulated structure maintains its nonlinear performance after 10 months in ambient, highlighting its potential as a robust platform for ultrathin nonlinear materials in integrated photonic and quantum optics.