# A 60-GHz Ultra-Thin and Flexible Metasurface for Frequency-Selective Wireless Applications
## 1 Introduction  
In the past decades, metamaterials and metasurfaces have gathered an incredible consideration due to their unprecedented potency of tuning the electromagnetic (EM) characteristics and to achieve such EM properties not easily feasible in nature [1]. Metamaterials are three-dimensional assemblies of repeated patterns of multiple elements designed at scales that are smaller than the wavelengths of the design frequency range [2; 3; 4]. These structures are developed from composite materials such as metals or plastics, however, their behavior is different from the conventional materials due to the newly designed geometry. Metasurfaces, on the other hand, are two-dimensional artificial materials with precise geometry, size and orientation which enable smart properties to manipulate EM waves by restricting, absorbing, or bending the waves that cannot be achieved from the traditional materials [5; 6; 7]. For instance, metasurfaces are deployed as frequency selective surfaces (FSSs) to offer either band-pass or band-stop shielding [8; 9]. Metasurfaces possess a planar structure with a thickness much smaller than the wavelength of incident light and allow full control over the phase, amplitude, and polarization of the EM-wave beams [10]. Metasurface has produced spectacular optical effects through which it can guide the light along an arbitrary direction, and can cause negative refraction, optical mirages to cloaking, super/hyper-lensing etc. [11; 12; 13; 14; 15]. With exceptional wave-steering capability, the metasurfaces designed for acoustic applications are capable to establish the distinctive features of realizing an abnormal reflection, demonstrated by the Snell's law and ultrathin planar lenses [16]. These remarkable properties of metasurfaces have established profoundresearch interest in the development of high-frequency metasurfaces for future wireless applications with preferably conformal construction to enhance the ease of integration. The frequency response of the metasurface depends on the current distribution on the unit-cell, as the current density on the FSS is influenced by the shape of the designed unit-cell [17]. FSSs are usually designed as periodic arrays of metallic structures, such as conventional patches, loops, dipole, crosses and fractals printed on dielectric substrates, or perforated apertures on thin conductive sheets, deployed to filter out incident plane waves at certain bandwidth [17; 18]. The EM characteristics of a metasurface are entirely dependent on the type of metallic geometry, size, substrate type as well as the inter-cell spacing [19]. Structural modifications can enhance the performance to a considerable extent, for instance, a high-impedance metasurface can realize electrically thin EM absorbers with better bandwidth and absorption for the TM polarization by means of metallic vias to connect patches with the ground plane [20]. It has also been investigated that the loop structures offer more angular stability than the solid metallic patches due to lesser deviation in resonant frequency, generally for TM-polarized incident plane waves [21]. In addition, circular ring metasurface can provide stability to incidence wave angles and polarizations, with lower coupling between the elements [22]. The advantages of such loop-based unit-cell structures have encouraged this research to come up with a new design combining circular and square loops in the proposed geometry of the metasurface. In comparison with the metamaterial, metasurface can conveniently avoid the fabrication complexities involved in the synthesis of the 3D-structures and can be easily manufactured on a large scale by using conventional planar technologies. However, the performance of metasurfaces can be further enhanced by the design of novel elements/unit-cells as well as the precise and low-cost fabrication methods.