Multimodal antireflective coatings for perfecting anomalous reflection from arbitrary periodic structures

Metasurfaces and metagratings possess vast wave-manipulation capabilities, including reflection and refraction of a plane wave into nonstandard directions. These require meticulously-designed subwavelength meta-atoms in each period of the metasurface which guarantee unitary coupling to the desired Floquet-Bloch mode or, equivalently, suppression of the coupling to other modes. Herein, we propose an entirely different scheme to achieve such modal control. Extending the concept of antireflective coatings to enable simultaneous manipulation of multiple modes, we show theoretically and experimentally that a simple superstrate consisting of only several uniform dielectric layers can be modularly applied to arbitrary periodic structures to yield perfect anomalous reflection. This multimodal antireflective coating (MMARC), designed based on an analytical model, presents a conceptually and practically simpler paradigm for wave control across a wide range of physical branches, from electromagnetics and acoustics to seismics and beyond.