Strong interlayer coupling and chiral flat-band cascades in twisted bilayer gratings.

From atomic crystals to macroscopic material structures, twisted bilayer systems have emerged as a promising route to control wave phenomena. In few-layer van der Waals materials, however, the intrinsically weak interlayer coupling typically demands precise control of small twist angles to reach magic-angle conditions. Here, we show that twisted one-dimensional photonic crystal bilayers can overcome this limitation by accessing a regime of strong interlayer coupling-comparable to intralayer coupling. This strong coupling enables ultrawide flat-band formation even at large twist angles. We experimentally realize this regime by stacking tungsten disulfide gratings using a two-step lithography method, resulting in ultrawide chiral flat-band cascades in magic-angle twisted bilayer gratings. Our work not only provides a platform for designing photonic applications with a tunable photonic band but also explores a previously unidentified regime of physics that is unattainable in conventional solid-state-based moiré systems.