High-Q Quasi-BIC metasurfaces with flat response to structural perturbations using Brillouin zone folding

Traditional high-Q metasurfaces driven by bound states in the continuum (BICs) rely on slight symmetry-breaking or nanoscopic control of structural parameters, inevitably requiring stringent and precise fabrication. Here, we propose a high-Q qBIC metasurface which is quite robust to dimensions of nanorods using Brillouin zone folding (BZF). The BZF can fold bound states at the edge of the first Brillouin zone (FBZ) into the radiation continuum, thereby converting them into improved high-Q quasi-BICs (qBICs). By continuously tuning the size of the nanorod in the unit cell to induce BZF and its unfolding, the expected BIC-qBIC-BIC transition is accessible. Consequently, regardless of the nanorod shapes, the achievable Q factors of generated qBICs are impressively flat in the middle of the transition zone and concurrently exhibit a high lower limit. Compared with traditional high-Q qBICs, our method can not only achieve a stable performance but also further relax the fabrication. Simulations confirmed the remarkable robustness and high flexibility of such design. This study establishes a foundation for designing robust, high-Q, and fabrication-friendly metasurfaces.