Metasurfaces, a special class of metamaterials, have recently become a rapidly growing field, particularly for thin polarization converters. They can be fabricated using a simple fabrication process due to their smaller planar profile, both in the microwave and optical regimes.
1. Introduction
According to Huygen’s principle
[1], each infinitesimally small area of a confined surface has an associated secondary-field source. This means that fields around the surrounding secondary-field sources can be controlled by either changing the source or by changing the electromagnetic properties of the confined surface. It depends upon whether one is interested in the transmitted properties of the confined surface or reflected properties or both. Thus, any desirable properties can be achieved by the proper design of such surfaces. This process is often termed as shaping electromagnetic waves. By shaping, it means either amplitude shaping or polarization shaping. Dielectric lenses, metallic mirrors, and reflectors are widely known for shaping electromagnetic waves in optics and microwave antenna engineering
[2][3]. However, both solutions for shaping are bulky and often have considerable size, weight, and volume even in a millimeter-wave band. Another way to shape electromagnetic waves into or out of the confined surface is to split the surface into multiple elements, each having a polarizable inclusion with specifically selected parameters. This concept, which had been used in reflect and transmit arrays
[4][5] was applied to Fresnel and chirped lenses, and frequency-selective surfaces
[6].
Recent progress in metamaterials (MMs) revealed that there exists a profound and sophisticated class of materials that can shape electromagnetic waves by a design of subwavelength inclusions arranged in densely packed 3D structures. Such inclusions owing to specific electric and magnetic polarizations can control transmitted and reflected fields which in turn offer intriguing opportunities for microwave, millimeter, and optical regions
[7][8][9]. However, these materials suffer from several disadvantages. Firstly, fabrication of bulk MM is very complex, particularly when they lie in the optical region, as it requires bulk MM to be 3D nanostructures. Moreover, wave propagation in metamaterials covers a substantial distance, therefore, they encounter high ohmic losses.
Recently, it was realized that 2D metamaterials can be a more profound type of metamaterial, having the ability to shape/control electromagnetic waves. They have been termed MSs (MS). They can have subwavelength inclusions to tailor electric and magnetic fields, to control transmitted and reflected fields. They are ideal candidates for many novel microwave and optical devices.
The basic properties of MSs are defined by their elementary subwavelength patterns, which result in unusual resonant behavior. Importantly, MSs are patterned with subwavelength structures, so homogeneous or nearly homogenous MSs transmit and reflect plane waves. Since MSs are thin structured and 2D in nature, they are less lossy and easier to fabricate. Another major advantage of using MSs is their easier integration with existing microwave and nanophotonics systems. Several factors influencing the applications and properties of MSs are the type of structure/pattern, their mutual/cross-couplings between adjacent cells, and the substrate used. The most attractive feature of the MS is its ability to control reflected and transmitted fields, which makes it highly effective for certain applications and replaces conventional bulky equipment. There have been excellent review articles on MSs covering theory, fundamentals and applications
[10][11][12][13][14] and their complex fabrication
[12]. For example, Zhao et al. reviewed optical fiber-integrated MSs for applications, such as telecommunication, sensing, imaging, and biomedicine
[10]. Holloway et al. focused on the development in recent years of such MSs from microwave to optical regimes
[11]. Su et al. reviewed the fabrication and applications of MSs
[12]. Glybovski et al. reviewed different types of MSs for a broad range of the operational wavelengths for wavefront shaping, lenses, and polarization transformation
[13].
2. History of MSs
Before further discussion, researchers clarify here that those planar 2D arrays whose periodicity is not smaller than operating wavelengths will not be called MSs here, because the term ‘MS’ is associated with cells whose periodicity is smaller than a wavelength. Here, the wavelength is a term used for λfreespace. Mesh and wire-based structures have been extensively explored for antenna systems to realize polarizers, which were based on averaged boundary conditions and, later, the first homogenization theory of artificial electromagnetic surfaces emerged. Frequency selective surfaces (FSS) are another type of planar array which exhibit resonant type transmission and reflection characteristics. Dipole and aperture based FSS have been employed for microwave filters operating with plane waves.
MSs were termed nano-islands in the starting period of their explorations in the optical region. Their modeling was carried out using oblate spheroids
[15][16][17] to study their different characteristics, such as frequency and polarization selectivity. For most of the applications, the only electric response was not sufficient, therefore, an accompanying magnetic response was required to be considered in MS
[18][19]. The homogenization model of MS presented by
[20][21][22] does not cover bi-anisotropic MS. Such MSs have been targeted for polarization controlling applications for chiral
[23][24] and omega type anisotropicity
[25]. Chiral bi-anisotropic MSs for polarization conversions take advantage of inclusions which are polarized by electric or magnetic fields. For such MSs, simultaneous parallel electric or magnetic fields can exist. Another type is the omega type inclusions, where electric or magnetic dipoles are directly orthogonal to each other due to applied electric or magnetic fields only. These inclusions also help in achieving polarization conversion.
3. Polarization Manipulation Using MSs
The polarization of electromagnetic waves possesses a very important role in radio and optical communication systems. They have been designed and explored for applications, such as dual-polarized radars, fiber optic communications, and MIMO systems. These devices are called polarization manipulators. Their most important type of polarization is a linear polarization wave which can be rotated from one polarization direction to another, converted from one state to another, or selected depending upon their polarization state for reflection or transmission.
Polarization rotators are also termed polarization twisters or twist-polarizers. Initially, they were used for satellite ground stations in dual-polarized antennas
[26]. In optoelectronics applications, they are used for displays
[27]. Optical activity of natural materials as observed in the Nicol prism, the lenses of polarized sunglasses and proteins are found to be weaker
[27], which means rotation of polarization plane per wavelength is low. Therefore MSs are advantageous in this manner. Polarization rotators were designed for the first time using dense wire grids with a multilayer structure whose wire rotates from one layer to another
[26][28]. They can be designed in such a way that they miniaturize the return losses. However, such devices only work for a particular linear polarization. Chirality in the optical regime was reported in
[29]; the device shows the rotation of 250°/λ. Similarly, another polarization rotator was reported
[30] which had only λ/30 thickness and consisted of a complementary split-ring resonator (SRR). MSs can also be employed to possibly engineer the sensitivity to polarization states, such as linear or circular. Devices in line with such properties to improve the properties of microwave antennas were proposed
[31][32]. These devices control circular polarization, as they allow for one form of circularly polarized waves, termed circularly polarized selective surfaces (CPSS). These surfaces are further categorized as symmetric and asymmetric CPSSs where the symmetric allows one type of CP wave to transmit through them, irrespective of the direction.
Polarization manipulation has also been explored for non-contact Hall measurements by the magneto-optic effect, where
[33][34] study the chiral molecular structure of proteins and DNA
[35][36]. One of the most important functions of MSs under polarization manipulation is linear polarization to circular polarization conversion (LP-to-CP conversion) in transmission mode. In addition to many applications of LP-to-CP converters in optical regimes
[37], these converters are being used in microwave and millimeter-wave systems, including imaging systems and reflectors
[38][39].