Nonlinear metasurfaces and photonic crystals provide a significant way to generate and manipulate nonlinear signals owing to the resonance- and symmetry-based light-matter interactions supported by the artificial structures. However, the nonlinear conversion efficiency is generally limited by the angular dispersion of optical resonances especially in nonparaxial photonics. Here, we propose a metagrating realizing a quasi-bound-state in the continuum in a flat band to dramatically improve the third harmonic generation (THG) efficiency. A superior operating angular range is achieved based on the interlayer and intralayer couplings, which are introduced by breaking the mirror symmetry of the metagrating. We demonstrate the relation of angular dispersion between the nonlinear and linear responses at different incident angles. We also elucidate the mechanism of these off-axis flat-band-based nonlinear conversions through different mode decomposition. Our scheme provides a robust and analytical way for nonparaxial nonlinear generation and paves the way for further applications such as wide-angle nonlinear information transmission and enhanced nonlinear generation under tight focusing.

由于人造结构支持基于共振和对称的光与物质相互作用，非线性超表面和光子晶体提供了一种生成和操纵非线性信号的重要方法。然而，非线性转换效率通常受到光学谐振角色散的限制，尤其是在非近轴光子学中。在这里，我们提出了一种在平带连续谱中实现准束缚态的元光栅，以显着提高三次谐波生成（THG）效率。基于层间和层内耦合实现了优越的工作角度范围，这是通过打破元光栅的镜像对称性引入的。我们证明了不同入射角下非线性和线性响应之间的角色散关系。我们还通过不同的模式分解阐明了这些离轴平带非线性转换的机制。我们的方案为非傍轴非线性生成提供了一种稳健的分析方法，并为广角非线性信息传输和紧聚焦下增强非线性生成等进一步应用铺平了道路。