Angle-insensitive nonlinear enhancement in an all-dielectric metasurface via Brillouin-zone folding

Metasurfaces have become a powerful platform for strengthening light-matter interactions. Recent research has shown that all-dielectric metasurfaces supporting high-quality ($Q$) factor resonances, such as anapole modes and quasibound states in the continuum (BIC), can significantly enhance third harmonic generation (THG) efficiency. Nevertheless, a key challenge remains: precisely controlling the $Q$ factor of these resonances is difficult, and maintaining a high-$Q$ factor across a wide range of incident angles is even more demanding. Here, we achieve substantial enhancement of THG by introducing perturbations within the metasurface. The guided modes below the light line can be folded into the light cone, resulting in high-$Q$ resonant modes. Compared with previous BICs, this folded resonance exhibits a persistent high-$Q$ characteristic in momentum space, which makes it possible to achieve angular insensitive nonlinear enhancement. The angular robustness of the proposed metasurface ensures consistent high nonlinear performance, making it a viable candidate for practical applications in nonlinear optics, such as frequency conversion and photonic signal processing.