Far-Field Excitation of a Photonic Flat Band via a Tailored Anapole Mode

The photonic flat band, defined by minimal dispersion and near-zero group velocity, has facilitated significant advances in optical technologies. The practical applications of flat bands, such as enhanced light-matter interactions, require efficient coupling to far-field radiation. However, achieving controlled coupling between flat bands and their corresponding localized modes with far-field radiation remains challenging and elusive. Here, we achieve the tunable far-field excitation of a flat band in the near-infrared spectral range by coupling it to a photonic anapole mode. Distinct from conventional multipolar resonances in both its physical nature and unique radiation dynamics, the anapole mode offers highly localized field distributions and tunable emission characteristics, enabling the realization of a photonic flat band and precise control over its transition from a nonradiative to a radiative state. We directly observed the flat band within ±25° experimentally by angle-resolved far-field transmissivity spectroscopy. Simulation results extending to 90° confirm the persistence of the band's flatness across all incident angles, validating the inherent flatness of the band. Our findings not only provide a viable approach to accessing photonic flat bands but also significantly advance the field of nanoscale photonic manipulation, offering broad potential applications in optical technologies.