Metagratings for Efficient Wavefront Manipulation

Recently, it was revealed that conventional gradient metasurfaces are fundamentally limited on their overall efficiency to reroute the impinging waves towards arbitrary directions in reflection and transmission. Their efficiency is particularly limited for extreme wavefront transformations. In addition, due to the fastly varying impedance profiles that these surfaces require, they usually need high-resolution fabrication processes, limiting their applicability and overall bandwidth of operation. To address these issues, the concept of metagrating was recently introduced, enabling engineered surfaces capable of manipulating light with unitary efficiency even in the limit of extreme wavefront manipulation. Metagratings are periodic or locally periodic arrays operated in the few-diffraction order regime. Their unit cell contains carefully designed scatterers that, in contrast with conventional metasurfaces, do not need to support a continuous gradient of surface impedance, and consequently are far simpler to fabricate and can afford broader bandwidths because of the larger footprint of the constituent elements. Tailoring the coupling towards the few available diffraction channels, metagratings enable wavefront transformations with high efficiency. In this paper, we review the physical mechanisms behind the functionality of metagratings and provide an overview and outlook of the recent progress in this field of science and technology.