Metasurfaces: Theoretical Basis and Application Overview

Abstract

In the last 20 years, metamaterials have attracted much attention for their exotic physical behaviors not commonly present in nature. However, this class of micro- and nanostructured artificial media, characterized by groundbreaking electromagnetic and photonic properties, has encountered difficulty in entering industrial upscale and end-user device mass production. Indeed, high losses and strong dispersion, related to the use of metallic structures, as well as the difficulty of fabricating micro- and nanoscale 3D structures, have hindered practical applications of metamaterials. On the contrary, 2D metamaterials or metasurfaces with negligible height, made of a single or few layers, offer much lower losses and a natural advantage in terms of fabrication with standard lithography or nanoimprint replication techniques. Moreover, metasurfaces allow a spatially varying optical response in terms of scattering amplitude, phase, and polarization. In the specific case of metalenses, optical wavefronts can be shaped and designed at will and eventually integrated with tunable and functional materials to achieve active control and greatly enhanced nonlinear response. In this introductory chapter, key concepts about metasurfaces physics are introduced evidencing peculiar behaviors. A general classification of metasurfaces follows in terms of constituting materials and their assembly, resulting in metastructures with specific application and functionalities.