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

Symmetry-protected bound states in the continuum (BICs), emerging at the ${\Gamma }_0$ 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.