Wide-Angle Second-Harmonic Generation via Flat-Band BICs in 3R-MoS 2 Photonic Crystals

The integration of nonlinear photonic devices requires efficient frequency conversion at subwavelength scales, where bound states in the continuum (BICs) offer a promising approach through their ultrahigh quality factors and strong field localization. However, conventional BICs typically exhibit strong angular sensitivity, limiting their practical applications in nonlinear frequency conversion. Here, we numerically demonstrate wide-angle second-harmonic generation (SHG) enabled by a flat-band BIC (FB-BIC) in an all-3R-MoS2 one-dimensional photonic crystal. By optimizing the grating filling factor, two off-Γ Friedrich-Wintgen quasi-BIC (FW-qBIC) resonances progressively approach the Γ-point symmetry-protected BIC, accompanied by suppressed band curvature and an extended high-Q region in momentum space. The resulting FB-BIC exhibits a resonant wavelength drift of less than 1 nm over a 10° incident-angle range, while the SHG conversion efficiency remains above 10-3 across a 14° angular span at 1 mW pump power, more than three times the 4° span of the symmetry-protected BIC under identical conditions. Our results establish a design paradigm that bridges flat-band engineering, BIC topology, and two-dimensional nonlinear materials, offering a route toward angle-robust, highly efficient, and integrable nonlinear nanophotonic devices.