An Overview of the Theory and Applications of Metasurfaces: The Two-Dimensional Equivalents of Metamaterials

Metamaterials are typically engineered by arranging a set of small scatterers or apertures in a regular array throughout a region of space, thus obtaining some desirable bulk electromagnetic behavior. The desired property is often one that is not normally found naturally (negative refractive index, near-zero index, etc.). Over the past ten years, metamaterials have moved from being simply a theoretical concept to a field with developed and marketed applications. Three-dimensional metamaterials can be extended by arranging electrically small scatterers or holes into a two-dimensional pattern at a surface or interface. This surface version of a metamaterial has been given the name metasurface (the term metafilm has also been employed for certain structures). For many applications, metasurfaces can be used in place of metamaterials. Metasurfaces have the advantage of taking up less physical space than do full three-dimensional metamaterial structures; consequently, metasurfaces offer the possibility of less-lossy structures. In this overview paper, we discuss the theoretical basis by which metasurfaces should be characterized, and discuss their various applications. We will see how metasurfaces are distinguished from conventional frequency-selective surfaces. Metasurfaces have a wide range of potential applications in electromagnetics (ranging from low microwave to optical frequencies), including: (1) controllable “smart” surfaces, (2) miniaturized cavity resonators, (3) novel wave-guiding structures, (4) angular-independent surfaces, (5) absorbers, (6) biomedical devices, (7) terahertz switches, and (8) fluid-tunable frequency-agile materials, to name only a few. In this review, we will see that the development in recent years of such materials and/or surfaces is bringing us closer to realizing the exciting speculations made over one hundred years ago by the work of Lamb, Schuster, and Pocklington, and later by Mandel'shtam and Veselago.

I. Introduction

In recent years, there has been a great deal of attention devoted to metamaterials [1]–[10]. Metamaterials are novel synthetic materials engineered to achieve unique properties not normally found in nature. In the context of electromagnetics, early examples of these were artificial dielectrics. Later, we will see that the terms metamaterial and metasurface do not refer to classical periodic structures, such as photonic bandgap (PBG) structures or frequency-selective surfaces (FSS). One particular class of metamaterial that is being studied extensively consists of the so-called double-negative (DNG) materials [1]–[10] (also known as negative-index materials (NIM), backward-wave (BW) media, or left-handed materials (LHM)). Such materials have the property that their effective permittivity and effective permeability are simultaneously negative in a given frequency band. Another property not normally found in nature that can be achieved with metamaterials is that of near-zero refractive index. In this type of material, either the permittivity or permeability is designed to have its real part close to zero. Materials with unique properties such as these have a wide range of potential applications in electromagnetics at frequencies ranging from the low microwaves to optical, including shielding, low-reflection materials, novel substrates, antennas, electronic switches, “perfect lenses,” and resonators, to name only a few.