Tailoring bound states in the continuum in resonant metasurfaces with a monoclinic lattice

Bound states in the continuum (BICs) provide an alternative way of realizing ultrahigh-Q resonances via free space excitation, thus holding great promise in enhancing light-matter interaction. However, most published studies so far focus on BICs in resonant metasurfaces with either hexagonal lattices or square/rectangular lattices. In this work, we report the BICs in a resonant metasurface with a monoclinic lattice. We demonstrate that both the included angle and size ratio of the monoclinic lattice play a critical role in manipulating the BICs in momentum space. We start by considering a metasurface with a rhombus-shaped lattice with an included angle θ of 60°. It can be found that by varying the included angle multiple accidental BICs at the off-Γ point merge into a single one while the center BIC with a topological charge q = −2 is split into two accidental BICs with q = −1 at the off-Γ point. Also, we find that further decreasing or increasing θ allows for the observation of BICs’ repulsion since they share the same topological charges q = −1. Besides, we demonstrate that multiple BICs can be engineered to move and merge into a super-BIC by simultaneously changing θ and the thickness. Furthermore, by replacing the circular air hole with a rotated elliptical hole, much more freedom is provided in tailoring the BICs. Simulation results indicate that some BICs can merge at specific off-Γ points by choosing the appropriate sets of structural parameters. Finally, we confirm that BICs can also be dynamically controlled by the size ratio of the monoclinic lattice. Our results provide an effective approach of engineering BICs in momentum space.