Efficiently coupling a guiding wave to a plane wave with a cascaded aperiodic metasurface

The investigation of plane wave to surface wave (or guiding wave) conversion reveals that a highly efficient converter can only be designed as an aperiodic structure. This requirement arises from the momentum mismatch between the plane wave and the surface wave (or guiding wave). Because of this aperiodicity, the design of such highly efficient converters becomes inevitably complex, particularly when the radiation aperture is large, posing a significant challenge to structural engineering. In this work, we propose a straightforward and effective approach for designing a highly efficient converter, which consists of a cascaded metasurface assembled from successively and individually designed subunits. Unlike previous methods, which typically attempt to design the entire structure in a single step, our method designs the structure in two separate steps, thereby significantly reducing design complexity. As a demonstration, a plane-wave-to-guiding-wave metasurface for acoustic waves with 12 units in length is designed. The high efficiency of the structure, operating in a reciprocal state (i.e., functioning as a guiding-wave-to-plane-wave radiator), is validated through both numerical simulations and experimental measurements. This work not only provides a design guideline for a plane-wave-to-guiding-wave converter but also presents a general understanding of the coupling mechanisms between uniform and nonuniform wave fronts.