Interface-induced second-order topological bound states in the continuum in composite photonic crystals

Second-order topological bound states in the continuum (SOTBICs) are counterintuitive topological modes that coexist with extended states. Here, we propose an innovative approach to generate SOTBICs in composite photonic crystal systems. By considering the interface hybridization of two subsystems with distinct topological phases, we discover a broad class of non-BIC corner states and analytically reveal their intrinsic connection to the formation of SOTBICs. Moreover, by introducing chiral symmetry breaking and environmental perturbations, we identify the conditions under which SOTBICs transition into conventional resonant modes. Our work demonstrates a simple way to induce SOTBICs, offering a new, to the best of our knowledge, platform for engineering high-Q photonic modes in complex lattices.