High- <i>Q</i> Chiral Flat-Band Guided Resonances in Nonlocal Metagratings

Photonic flat bands are central to modern nanophotonics due to their intrinsically large photonic density of states and pronounced slow-light effect. Here, we theoretically propose and experimentally demonstrate a paradigmatic strategy for realizing high- Q, chiral flat-band resonances in a nonlocal metagrating composed of an array of silicon waveguides with bilateral semicircular edge grooves. Driven by tight-binding model in the limit of weak interwaveguide coupling, the flat-band guided modes emerge at a straight silicon waveguide array. By introducing the edge grooves and precisely shifting their lateral positions, the flat-band guided modes evolve into chiral flat-band guided resonances, with ultrahigh Q -factors exceeding 10 3 over incident angles as wide as ±10° and exhibiting reversible circular dichroism with a sign switch up to ±0.85. Crucially, all experimental results show excellent agreement with those of full-wave simulations. The wide-angle, high- Q chiral flat-band guided resonances will find utility in chiral light sources, polarization-selective detection, and nonlinear frequency conversion.