# Frequency Selective Surfaces: A Review
## 1 Introduction  
Metasurfaces are broadly named as planar metamaterials with subwavelength thickness and they can be easily fabricated while using lithography and nano-printing techniques. Both metamaterials and metasurfaces are rapidly growing research directions, and with their use, spatially varying EM or optical responses can be achieved at will, with scattering phase, amplitude, and polarization. Through a good selection of materials and design, the ultra-thin structure of MSs can considerably suppress the detrimental and undesirable losses in the wave propagation direction. When considering the polarization response, all metasurfaces can be categorized based on the operating principle of array element, i-e their functionalities (frequency selective surfaces (FSS), high impedance surfaces, perfect absorbers, reflecting surfaces, etc.). According to the definition in [1], FSSs are metasurfaces that merely exhibit an electric response [2; 3]. Since, in order to tailor the frequency selectiveness in transmission/reflection characteristics, only electrical polarization may be sufficient. From the theory of antenna and microwave engineering, these surfaces are made by planar and periodic arrays of metallic patches or strips with different shapes. The patch is of negligible thickness as compared to the wavelength, although it is larger enough in contrast to the metal's skin depth. Consequently, such a structure can impeccably be estimated as a minuscule thin array of perfect conducting resonant elements. This approximation is also applicableto the complementary FSSs structures i-e apertures. However, aperture-type FSSs face a limitation when the area of the cavity/aperture becomes equal to the unit cell (a wire-mesh type) [2; 3]. Square and hexagonal wire-mesh unit cells have typically been used and are also termed as the capacitive grid. The existence of the resonating size of array element causes the emergence of the side lobes in the transmitted and reflected fields, which are the defining feature of FSSs. However, as compared to the FSSs, the resonating element and unit cell of metasurface is relatively much smaller than the wavelength and it helps to eliminate the grating lobes in the frequency response. Therefore, FSSs in the terahertz domain are usually termed as metasurfaces [4]. Frequency selective surface (FSS) is a robustly studied topic of electromagnetic (EM) science, which are two-dimensional periodic structures having planar metallic array elements (patch or apertures) on a dielectric substrate, exhibiting transmission and reflection at certain resonant frequency [2]. Depending on the array element design, an incoming plane wave will be either transmitted or reflected, completely or partially. This happens when the frequency of the plane wave matches the resonance frequency of the FSS elements. Therefore, an FSS can pass or block the EM waves with certain frequencies in free space; so they are best identified as spatial filters. Traditionally, the element size, shape, and periodicity of an FSS result in the resonance. FSSs have extensively been investigated over six decades and a range of microwave and optical FSSs structures have been evolved. In past, they were frequently used in reflector antennas [5], including diplexers for quasi-optical microwave devices, resonant beam splitters, and antenna radomes [6; 7]. Nowadays, FSSs have been employed in dichroic sub-reflectors [8], radio frequency identification (RFID) [9], lenses antennas [10], and protection from electromagnetic interference [11]. Currently, the most famous applications of FSSs are as antennas radomes and controlling radar cross-section (RCS). Their performance is limited by the design complications, including the requirement of compact size and insensitivity to the incidence angle, as well as polarization of EM wave, consequently stipulating to improve their design features. Traditional FSSs are narrow band and they do not provide adequate spatial filtering response.