Symmetry-protected bound states in the continuum in terahertz metasurfaces

Symmetry-protected bound states in the continuum (BICs), which appear at the Γ-point of the Brillouin zone of periodic lattices of scatters, are modes that confine electromagnetic field with virtual no loss. These lattices constitute perfect optical cavities. Symmetry-protected BICs are robust modes under modifications in the lattice if the C₂ symmetry (two-fold rotational symmetry) is preserved. Here, we focus on the manipulation of these symmetry-protected BIC and quasi-BIC modes in a lattice formed by two metallic rods per unit cell by adjusting their lateral separation and displacement. For non-symmetric systems under 180° rotation, we investigate two different coupling mechanisms resulting from changes in the lateral separation: the coupling of two half-wavelength (λ/2) resonances and the coupling of two surface lattice resonances. In particular, symmetric structures with minimal lateral separation cannot sustain BIC modes due to the near-field coupling between the rods. However, when the lateral separation is sufficiently large, the existence of a BIC mode supported by a surface lattice resonance remains robust even to position shifts. Furthermore, in asymmetric systems, increasing the lateral separation and the displacement between the rods induces a red shift and a blue shift, respectively, in the quasi-BIC mode. This shift is attributed to the near-field coupling effect between two rods, allowing the resonance with a high quality factor to be tuned.