A figure of merit for focusing metasurfaces

We propose and define a figure of merit (FoM) for designing focusing metasurfaces by establishing a reference case. We emphasize the important role of the focal point concept for obtaining the desired functionality. We suggest several design considerations along with the FoM for a careful design and performance evaluation of the prototype, without which the focusing property of a designed metasurface may be only slightly better than that of a planar scattering surface. (C) 2015 AIP Publishing LLC.