Merging of Accidental Bound States in the Continuum in Symmetry and Symmetry-Broken Terahertz Photonic Crystal Slabs

Recently, the merging of accidental bound states in the continuum (BICs) has attracted significant attention due to the enhanced light-matter interactions. Here, we theoretically demonstrate the merging of accidental BICs in perturbed all-silicon terahertz photonic crystal (PhC) slabs with <i>C</i>₂ and <i>C</i>₂ broken-symmetry structures. The PhC slabs consist of an array of four cylindrical holes and support a TM symmetry protected (SP) vector BIC at the Γ point. Our results indicate that the merging and band transition of accidental BICs can be achieved by varying the diameter of diagonal holes in a <i>C</i>₂-symmetry structure. Notably, in a <i>C</i>₂ broken-symmetry PhC slab, the SP BIC will first convert to a quasi-BIC, then transit to a new accidental BIC, which are well displayed and interpreted by tracing the accidental BICs in momentum space. We believe that the results presented in this work show potential for the design and application of BICs in both symmetric and asymmetric PhC slabs.