Quasi-flatband resonances and bound states in the continuum in coupled photonic topological defects for boosting light–matter interactions

In this Letter, we explore the interplay between topological defects and resonant phenomena in photonic crystal slabs, focusing on quasi-flatband resonances and bound states in the continuum (BICs).We identify anisotropic quasi-flatband resonances and isotropic quasi-flatband symmetry-protected BICs that exist in coupled topological defects characterized by nontrivial 2D Zak phases, originating from monopole, dipole, and quadrupole corner modes within second-order topological insulator systems.These topological defect modes, whose band structures are described using a tight-binding model, exhibit distinctive radiative behavior due to their symmetry and multipolar characteristics.Through far-field excitation analysis, we demonstrate the robustness and accessibility of these modes in terms of angular and spectral stability.Furthermore, we investigate potential applications of the quasi-flatband resonances in light-matter interactions, including optical forces, second-harmonic generation, and strong coupling, which exhibit robust performance under varying illumination angles.These findings offer new opportunities for precise control over light-matter interactions.