Superdirective space-to-surface wave transformations with nonlocal metasurfaces

In this work, we demonstrate theoretically and numerically that propagating waves can be fully converted into surface waves, and vice versa, using an aperiodic metasurface composed of simple, electrically small metallic elements loaded with reactive components. The proposed approach achieves nearly 100% conversion efficiency and even enables operation in the superdirective regime. We show that these metasurfaces can be designed in a modular fashion, allowing for straightforward cascading and the realization of multifunctional responses without the need for global optimization. Specifically, employing this modular methodology, we design a metasurface that converts an arbitrary incident propagating wave into a surface wave, guides it along the surface, and reradiates the received power in any desired direction with a tailored wave-front shape. The proposed technique can be used in antenna engineering and in advanced designs of reconfigurable intelligent surfaces.