Flatband electromagnetically induced transparency via bound states in the continuum

Photonic crystals (PhCs) supporting bound states in the continuum (BIC) offer a promising platform for realizing electromagnetically induced transparency (EIT) analogs, enabling slow light and coherent control via modal interference. Their practical performance, however, is constrained by finite size effects and angle sensitivity. Here, we overcome this challenge by engineering a photonic flatband with a symmetry-protected BIC at the Γ point. Subsequently, a surface lattice mode (SLM) is precisely aligned with the flatband BIC via fine-tuning of structural parameters, forming a Flat-EIT response. Simulations show that at incidence angles of 4° and 10°, the Flat-EIT PhC exhibits group delays of 60 ps and 18.2 ps, respectively, which represent a significant enhancement in slow-light performance compared to previously reported non-flatband structures. Our results may facilitate the pathway towards the practical application of photonic devices.