Angularly robust terahertz third-harmonic generation empowered by all-dielectric nonlocal metasurfaces

All-dielectric metasurfaces have emerged as a premier platform for enhancing light-matter interactions via high-quality-factor resonances, particularly through bound states in the continuum. However, conventional BICs are typically restricted to fixed frequencies and specific points in momentum space, limiting their operation to narrow angular ranges. Here, we propose a fabrication-friendly all-dielectric nonlocal metasurface to enhance third-harmonic generation in the terahertz range, achieving robust ultrahigh-Q resonances over a wide incident angle of up to 30°. This robustness is realized by manipulating quasi-guided modes and bound states in the continuum via Brillouin zone folding. We systematically elucidate the physical origins, far-field characteristics, and the mode-switching mechanism governed by symmetry breaking. Leveraging these robust resonances, we demonstrate a high third-harmonic generation conversion efficiency exceeding 10^-4. These results provide a viable strategy for robust harmonic generation and establish a paradigm for designing high-Q devices for sensing, detection, and nonlinear applications.