Metamaterials and Metasurfaces

Metamaterials and metasurfaces are one of the most popular topics in the realm of electromagnetics. Metamaterials are defined as artificial structures that are designed to exhibit specific electromagnetic properties that are not commonly found in nature [118]. Metamaterials have drawn great attention because of their ability to provide new materials with tunable permittivity, permeability, and refractive index. These materials can be used to redirect the beam in different directions [119]–[121], and hence, achieve a simple form of beam-steering. This beam-tilting can be understood through Snell’s law and is described in detail within the Supplementary Information.
Metasurfaces are two-dimensional metamaterials composed of an array of meta-atoms and can be used to control the electromagnetic environment and redirect beam in different directions [122]. Frequency selective surfaces (FSS) can be considered as one of the primitive examples of such metasurfaces, and they are popularly seen to be used for a full 360° control of beam-control by placing small FSS sections across an omni-directional antenna.
Tunable FSS are used as reconfigurable filters, and when in the form of a superstrate or a radome and coupled with antennas, they can act as a frequency window and hence allow the beam to radiate in selected directions [123], [124], see Figure 7. Work presented in [125], [126] used hexagonal structures for dielectric resonator antennas with FSS that allowed six beams to be generated in the azimuth plane and two beams in the elevation plane, and hence, giving a 2D beam-steering mechanism. The design used a cantilever mechanism to generate a tunable FSS, where a change in the height of cantilever introduced variation in capacitance and hence the band-pass frequency. This change would only allow the beam to radiate through the FSS in a specific direction, while blocking it in every other direction. Superstrates consisting of reconfigurable FSS and partially reflective surfaces (PRS) with active components can also provide a similar behavior [127]–[131]. PRS can also be used to create a Fabry-Perot cavity and generate beam-reconfigurability [132], [133].
Besides tunable FSS, the metasurface community has been growing significantly to create reconfigurable intelligent surfaces (RIS) that can act as either transmit-arrays or reflect-arrays. Such RIS can be reconfigured using a pre-defined codebook to change the beam-direction of an incident wave with minimal losses. This change in beam direction occurs because of the change in surface currents of the incident electromagnetic wave induced by the phase profile of the metasurface. RIS as a superstrate to the antennas can be used to achieve both beam-steering as well as multi-beam generation with a single feeder. Several such designs have been presented in the literature that use intelligent metasurfaces as either a reflector or a transmit-array to attain reconfigurability [134]–[140], see Figure 7.