A high-efficiency and ultrathin transmission-type circular polarization converter based on surface structure

In this paper, we propose, design and fabricate a kind of ultrathin and high-efficiency circularly polarization converter based on artificially engineered surfaces in the transmission mode. The converter is composed of double-layer periodic surface structures with cross slots. The top and bottom layers are printed on both sides of the F4B substrate and connected by metallic via holes. The proposed converter can transform the right-handed circularly polarized incident electromagnetic (EM) wave to a left-handed circularly-polarized one with near-unity efficiency in the transmission mode, or vice versa. We explain the conversion mechanism based on numerical simulations and equivalent circuit (EC) theory. The measured result has a good agreement with the simulated one in the working frequency band. Such ultrathin polarization converters can be used in wireless microwave communication, remote sensing, and EM imaging where circularly polarization diversity is needed.

Paper-based flexible metamaterial for microwave applications

Metamaterial has become a hotspot in many research fields, including electromagnetism, thermodynamics and mechanics, as it can offers additional design freedom for material to obtain novel properties. Especially for the electromagnetic devices, various interesting electromagnetic properties which cannot be found in nature materials can be realized, such as negative refraction, invisible cloak, etc. Herein, we provide an overview of paper-based metamaterial for microwave application. This work reviews the metamaterial realized on paper substrate, including the fabrication techniques, application fields, as well as the outlook on future directions of the paper-based metamaterial for the readership.

Significant enhancement of magnetic shielding effect by using the composite metamaterial composed of mu-near-zero media and ferrite

The magnetic shield plays an important role in magnetic near-field control. However, the requirements of efficiency, ultra thinness, lightness and cheapness are still the challenges. Here, we firstly propose a composite metamaterial in which the mu-near-zero media is covered with a ferrite slab. We verify that this structure can enhance the shielding effectiveness (SE) in a small area. Furthermore, we optimize the magnetic path by changing the bulk ferrite slab into a patterned slab. In this way, significant SE enhancement can be achieved in a large area. Experimental results show that the maximum SE of the composite metamaterial with a patterned ferrite is 20.56 dB, which is nearly 19 dB higher than that of a single ferrite slab with the same thickness of the composite metamaterial. The results on the composite metamaterial would be very useful in the applications involving magnetic shielding.

Estimating the effective conductivity for ellipse-inclusion model with Kapitza thermal resistance

The ellipse assemblage model with imperfect interface has quite complex microstructure, that can be considered an extension of the circle assemblage model with imperfect interfaces. The paper introduces an approximate method for computing the effective conductivity of isotropic composites with imperfect interfaces in two-dimensional space. Based on the coated-ellipse assemblage model and the equivalent inclusion approximation, one can determine the effective thermal conductivity of the composites. The polarization approximation is given in an explicit form (PEK) and this method will be applied to calculate the effective conductivity of the composite with Kapitza thermal resistance model. The PEK result will have compared with the Fast Fourier Transform (FFT) simulation and Hashin-strikman bounds (HS).

Vertical interface augmented tunability of scattering spectra in ferromagnetic microwire/silicone rubber metacomposites

Previously, we have demonstrated a viable approach based on microstructural and topological modulation of periodically arranged elements to program wave scattering in ferromagnetic glass-coated microwire metacomposites. In order to fully exploit the intrinsic structure of the composite, here, we implement the concept of composites plainification by an in-built vertical interface on randomly dispersed short-cut microwires allowing the adjustment of electromagnetic properties to a larger extent. Such interface was modified through arranging wires with different internal structures in two separated regions and by alternating these regions through wire concentration variations associated with polarization differences across the interface. When the wire concentration was equal in both regions, two well-defined transmission windows with varied amplitude and bandwidth were generated. Wire concentration fluctuations resulted in strong scattering changes ranging from broad passbands to pronounced stopbands, demonstrating the intimate relationship between wire content and space charge variations at the interface. This provides a new method to rationally exploit interfacial effects and microstructural features of microwire metacomposites. Moreover, the advantages of enabling tunable scattering spectra by merely 0.053 vol.% of fillers and simple structure make the proposed plainification strategy instrumental to designing filters with broadband frequency selectivity.

Epsilon-negative media from the viewpoint of materials science

A comprehensive review of the fundamentals and applications of epsilon-negative materials is presented in this paper. Percolative composites, as well as homogeneous ceramics or polymers, have been investigated to obtain the tailorable epsilon-negative properties. It's confirmed the anomalous epsilon-negative property can be realized in conventional materials. Meanwhile, from the perspective of materials science, the relationship between the negative permittivity and the composition and microstructure of materials has been clarified. It's demonstrated that the epsilon-negative performance is attributed to the plasmonic response of delocalized electrons within the materials and can be modulated by it. Moreover, the potential applications of epsilon-negative materials in electromagnetic interference shielding, laminated composites for multilayered capacitance, coil-less electric inductors, and epsilon-near-zero metamaterials are reviewed. The development of epsilon-negative materials has enriched the connotation of metamaterials and advanced functional materials, and has accelerated the integration of metamaterials and natural materials.

Carbon fiber skeleton/silver nanowires composites with tunable negative permittivity behavior

With the development of periodic metamaterials, more attention has been paid to negative permittivity behavior due to great potential applications. In this paper, silver nanowires (AgNWs) were introduced to the porous carbon fibers (CFS) by an impregnation process to prepare CFS/AgNWs composites with different content of AgNWs and the dielectric property was investigated. With the formation of conductive network, the Drude-like negative permittivity was observed in CFS/AgNWs composites. With the increase of AgNWs, the connectivity of conductive network became enhanced, the conductivity gradually increases, and the absolute value of the negative dielectric constant also increases to 8.9 × 104, which was ascribed to the enhancement of electron density of the composite material. Further investigation revealed that the inductive characteristic was responsible for the negative permittivity.

Progress and perspective on advanced cloaking metasurfaces: from invisibility to intelligent antennas

Among the different cloaking applications proposed in the literature, the antenna framework has emerged as one of the most fruitful and mature field. In particular, mantle cloaking approach has proven to be a powerful tool for enabling unprecedented possibilities in antenna design. Here, we provide a review of the most significant works in the field of electromagnetic invisibility for antenna applications, demonstrating the versatility of cloaking metasurfaces in antenna scenarios. We also discuss our recent results and investigations on the design of advanced cloaking metasurfaces equipped with electronic components and circuits and able enriching the antenna intelligence.

Metasurface virtual absorbers: unveiling operative conditions through equivalent lumped circuit model

Virtual absorption concept has been recently introduced as a new phenomenon observed in electromagnetics and optics consisting of theoretically unlimited accumulation of energy within a finite volume of material without dissipation. The anomalous behaviour is achieved by engaging the complex zero scattering eigenmodes of the virtual absorbing system by illuminating it with a proper complex frequency ω = ω r + jω i , whose value is strictly determined by the system characteristics. In this paper, we investigate on the position of the zero-pole scattering pairs in the complex frequency plane as a function of the input impedance of the metasurface-based lossless virtual absorber. We analytically derive the conditions under which a properly modulated monochromatic plane wave can be virtually absorbed by the system and stored within its volume. The analysis is developed by modelling the propagation of a normally impinging plane wave through its equivalent transmission line model terminated in an arbitrary reactive load, which in turn models the input impedance of the metasurface-based system under consideration. The study allows to determine a priori whether the metasurface-based system can support the virtual absorption or not by evaluating the time-constant from its equivalent circuit.

Tailoring the scattering properties of coding metamaterials based on machine learning

Diverse electromagnetic (EM) responses of coding metamaterials have been investigated, and the general research method is to use full-wave simulation. But if we only care its scattering properties, it is not necessary to perform full-wave simulation, which is usually time-consuming. Machine learning has significantly impelled the development of automatic design and optimize coding matrix. Based on metamaterial particle that has multiple response and genetic algorithm which is coupled with the scattering pattern analysis, we can optimize the coding matrix quickly to tailor the scattering properties without conducting full-wave simulation a lot of times for optimization. Since the coding matrix control of each particle allow modulation of EM wave, various EM phenomena can be achieved easier. In this paper, we proposed two reflective unitcells with different reflection phase, and then a semi-analytical model is built up for unitcells. To tailor the scattering properties, genetic algorithm normally based on binary coding, is coupled with the scattering pattern analysis in order to optimize the coding matrix. Finally, simulation results are compared with the semi-analytical calculation results and it is found that the simulation results agree very well with the theoretical values.