Robustness and Tunability of Symmetric‐Protected Bound States in the Continuums and Quasi‐Bound States in the Continuums in Terahertz Metasurfaces

Abstract

Symmetry‐protected bound states in the continuum (BICs), emerging at the point in the Brillouin zone of periodic lattices of scatters, are robust optical modes under modifications in the lattice if the C2 symmetry (two fold rotational symmetry) is preserved. In this study, the manipulation of these symmetry‐protected BIC and quasi‐BIC modes in a system comprising two metallic rods per unit cell by adjusting their lateral separation and displacement is focused on. With non‐symmetric systems under 180° rotation, two distinct coupling mechanisms resulting from changes in the lateral separation are investigated: the coupling of two half‐wavelength (λ/2) resonances and the coupling of two surface lattice resonances (SLRs). Notably, symmetric structures with minimal lateral separation cannot sustain BIC modes owing to the near‐field coupling between the rods. However, when the lateral separation is sufficiently large, the existence of a BIC mode supported by an SLR remains robust even with positional shifts. Furthermore, increasing the lateral separation and the shift displacement between the rods in asymmetric systems induces a redshift and a blueshift in the quasi‐BIC mode, respectively. This shift is attributed to the near‐field coupling effect between two rods, enabling the tunability of the resonance frequency with a high‐quality factor.