# Cascaded Frequency Selective Surfaces with Matryoshka Geometry for Ultra-Wideband Bandwidth
## 1 Introduction
### Review
The upper and lower metal layers are coupled through a metal ring layer to form the FSS structure. The equivalent circuit model is derived to explain the operating principle of the proposed FSS. In the design, two transmission zeros (TZs) are generated on both sides of the passband, contributing to the high selectivity performance of the structure. Furthermore, the surface distributions of electric current are provided for the analysis of the TZs generation mechanism. The performance of the proposed FSS is insensitive to the polarization and horizontal angle of incidence of the incident wave. To assess performance, a prototype of the FSS was manufactured, and the obtained results corroborated the simulations. In [29], a metallic circular polarizer based on a band-pass FSS is presented that converts the linear polarization of the incident wave to a circular polarization in the Ka-band. In the case where the wavefront is incident parallel to the surface of the FSS, the polarizer operates in the frequency range of 27.5 GHz to 30.1 GHz, and in the case where the wavefront is incident oblique to the surface of the FSS, the operating frequency range is reduced. An FSS-based electromagnetic energy absorber, insensitive to incident wave polarization and with ultra-wide bandwidth, is proposed in [30] for applications in the S- to K-bands. The absorber is formed by two compensation plates, a lossy FSS layer, and a grounded dielectric plate. The FSS unit cell geometry is a combination of a second-order Chinese knot and a cross. Another promising application of FSS is to aid electronic material detection and identification systems. This task is crucial in many situations, such as waste classification and hazardous materials detection. Although existing systems based on radio frequency (RF) signals have achieved great success, they have limited identification accuracy when RF signals cannot penetrate through the target or when a target has different external and internal materials. In [31], a high-precision material identification system is proposed based on an FSS tag, also called FSS-Tag, which uses both incident signals and the effect of electromagnetic coupling. Specifically, an FSS tag was designed and attached to a target material, and the frequency response of the tag was then used for material detection since different target materials have different frequency responses. FSS is also considered in [32] as a basis for the proposal of a dual-polarization angle-selective surface (ASS) based on a dual-layer FSS to achieve angular selectivity. By properly constructing the single-layer FSS structure, band-pass and band-reject modes of operation are produced by the ASS under normal and oblique incidences, respectively, resulting in angular selectivity. The proposed ASS can play an important role in many applications, such as minimizing the sidelobes of an antenna. A type of electromagnetic shield based on an FSS operating in dual-band and insensitive to the polarization of the incident wave is proposed in [33]. The FSS miniaturized unit cell consists of a modified Jerusalem cross loop and a modified square loop at the cell corners. The proposed configuration is a potential candidate for RF shielding/isolation applications. Regarding manufacturing procedures, in [34], the authors present the design and manufacture of an FSS for the THz band based on the joint application of two distinct technologies, called printable electronics (PE) and three-dimensional (3D) printing. The rotational tuning approach was applied to two identical FSSs to form a reconfigurable FSS called Moire FSS. Based on the numerical results obtained, it was possible to verify that the proposed technique that used the PE-based Moire FSS achieved a modulation depth of 58% at 0.25 THz, while the experimental verification presented a modulation depth of 41% at 0.22 THz, confirming that its adoption is economically viable and effective.