Metasurfaces: Theoretical Basis and Application Overview

In a book dealing with a definite topic like hybrid flatland metastructures, it is fundamental to put each eventual reader with a general knowledge in physics and material science in a position to fully comprehend the content of the following chapters. This is the aim of this first chapter. A gradual description is utilized to bring the reader from the wide framework of metamaterials to the more actual definition of hybrid metastructures. A general introduction briefly explains the need to reduce the typical size of bulk metamaterials to more efficient, virtually 2D systems, indicated as metasurfaces. Besides their negligible thickness, metasurfaces efficiently reproduce most of the metamaterials functions, though introducing much smaller losses and fewer hindrances for eventual applications requiring ultracompact dimensions. Metasurfaces typically comprise subwavelength scatterers for which the classical laws of refraction are not applicable due to discontinuities in the phase introduced in an incoming electromagnetic wave. For this reason, the theoretical approach utilized to predict the behavior of metasurfaces exploits generalized laws of refraction. This approach is valid for both metallic and dielectric metasurfaces. Another convenient and largely utilized theoretical approach useful to model the effect of metasurfaces on the wavefront of an impinging wave is based on the Pancharatnam–Berry phase. This phase value is usually calculated in the presence of systems where 18 July 2023 15:10:15 1-18 Hybrid Flatland Metastructures Principles the phase is spatially variant and allows designing of beam-shaping meta-arrays that can be operated in transmission (transmitarrays) or reflection (reflectarrays). After having dealt with the physical grounds of metasurfaces, it is appropriate to consider those systems where this theoretical background has been effectively applied. Hence, a series of examples begins detailing several representative applications of metasurfaces. The order of appearance of these examples is mainly chronological and shows how research slowly drifted from plasmonic and strongly sub-λ systems to dielectric and near-λ ones toward implementation in the visible spectral range. The transition to dielectric metasurfaces has invaluable advantages because of low absorption losses and much larger individual resonators, which are easier to fabricate when scaling to visible frequencies. Once some of the outstanding and exotic possibilities offered by metasurfaces have been illustrated, the concept of hybrid metastructure is introduced. Being ideally a system based on the combination of different metamaterials, a metastructure is expected to access a richer scenario of functionalities, including reprogrammable features and reconfigurable geometry. With this motivation in mind, it is not difficult to imagine several basic configurations that open a wide scenario of possibilities for fundamental and applied research in different fields, including engineering, photonic, biomedical, and security applications.