Metasurfaces are two-dimensional or surface counterparts of metamaterials. Just like metamaterials, it is possible to characterise their response through their electric and magnetic polarizabilities. They are also referred to in the literature as metafilms [
14]. Metamaterials control the propagation of light due to their bespoke permittivity and permeability values; however, they still use the propagation effect to manipulate the electromagnetic waves. This can result in a complicated relatively bulky structure whereas metasurfaces try to manipulate the wave over a single extremely thin layer [
15,
16]. The two-dimensional nature of metasurfaces, therefore makes them less bulky and offers the possibility of lower loss structures [
1]. Due to their 3D nature, it is also difficult to fabricate metamaterials. Metasurfaces offer an extremely promising alternative. Due to their planar structure, metasurfaces can be easily fabricated using planar fabrication tools [
17,
18]. The planar fabrication process is also very cost-effective in comparison to the manufacturing of the complex 3D metamaterials [
19]. Metasurfaces, being two-dimensional materials, can, therefore, be easily integrated into other devices which can make them a salient feature for nanophotonic circuits; this property will also allow them to be a part of “lab on chip” photonics [
20].
The negative index of the metamaterials is due to the resonance of the individual meta-atoms. This property makes the metamaterials inherently dispersive, thus the electromagnetic properties of such materials are highly sensitive to the changes in the operating frequency, thus making such materials bandwidth limited. It has been shown in [
21] that by using extremely thin metasurfaces with deep sub-wavelength notches in a two-layered fishnet structure, the dispersion characteristics can be engineered. This technique was then used to make a broadband metasurface filter. The (in-band) transmission and (out of band) rejection was achieved by respectively matching and mismatching the impedance of this metasurface (to the free space). The dispersion characteristics were controlled by tailoring the primary (and secondary) magnetic resonances, and the plasma wavelengths for permittivity. Both these properties (of the metasurface) were highly dependent on the design of the sub-wavelength deep notches. The design was optimized by the help of a genetic algorithm. This broadband metasurface also had a very low insertion loss in the transmission band [
21]. Due to the variety of advantages offered by metasurfaces over metamaterials, the scientific community has shown a keen recent interest in this area. This has led to rapid development in the underlying physics which govern the behaviour of metasurfaces and their potential applications.