Photonic Crystal Defect Cavities Enable Air‐Stable and Enhanced SHG from NbOCl ₂

Abstract Layered ferroelectric NbOCl 2 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 NbOCl 2 flakes and promote long‐term environmental stability. By embedding NbOCl 2 between TiO 2 /SiO 2 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.