Quasi-flatband bound states in the continuum in coupled photonic crystal defects

We demonstrate a simple photonic crystal slab design that supports quasi-flatband bound states in the continuum (BICs) through the introduction of periodic defect sites within a supercell lattice. The structure hosts four distinct defect modes—quadrupole, monopole, and two dipoles, where the quadrupole mode forms a nearly dispersionless band with a diverging Q-factor at the Γ point, indicative of a quasi-flatband BIC. Using a tight-binding model, we show that long-range interactions up to sixth-nearest neighbors are essential for accurately capturing the band behavior. These results provide a robust framework for engineering flatband BICs in planar photonic systems such as sensors and lasers.