Metasurfaces: From microwaves to visible

We review the basic physics and applications of a special class of planar metamaterials, often called metasurfaces, which are composed of optically thin and densely packed planar arrays of resonant or nearly resonant subwavelength elements. Electromagnetic properties and functionalities of such metasurfaces are defined by the structure and specific features of the subwavelength elements and their coupling type and strength, and they are often influenced by an underlying substrate. Metasurfaces may provide a full control of the reflected and transmitted fields, and they can be designed to possess many required properties replacing bulky optical components. Here, we describe different types of metasurfaces suggested in the past and recent years for a broad range of the operational wavelengths ranging from microwaves to the visible, and emphasize their important functionalities. We demonstrate that, despite a wide functional and structural diversity, all suggested metasurfaces can be associated with only several broad classes depending on intrinsic physical mechanisms of their polarizability. We suggest the functionality-based classification of metasurfaces, and clarify a link between their polarization response and field control capabilities. We also suggest a general approach to an optimal design of metasurfaces for many specific applications.