A REVIEW ON THE PURSUIT OF AN OPTIMAL MICROWAVE ABSORBER

Mitigation of the electromagnetic radiations is essential for reliable communication of information. The challenges lie in achieving sufficiently good absorption over a broad range of frequencies. Considering the applications in airborne and handheld devices where light weight, thin, conformable and broadband absorbers are desired, numerous techniques and methods are applied to design broadband absorbers. In this review paper, a detailed analysis on electromagnetic absorbers including evolution, the materials used, and characteristics such as absorption efficiency over the years is presented. Progress on recent research on various polymer- based and metamaterial- based microwave shields are included along with their findings. Several prospects such as broadbanding, flexibility, multibanding are described here. Various material and structural composition offering good absorption performance in different frequency bands are also summarized whose the techniques can be used for suppressing electromagnetic interference and radar signature. The paper specifies the aspects one encounters while designing and realizing a perfect microwave absorber. Explored here are several works of distinguished authors  which are based on various techniques used to achieve good absorption performance with ease of mounting

Recent developments of metamaterials/metasurfaces for RCS reduction

In this paper, recent developments of metamaterials and metasurfaces for RCS reduction are reviewed, including basic theory, working principle, design formula, and experimental verification. Super-thin cloaks mediated by metasurfaces can cloak objects with minor impacts on the original electromagnetic field distribution. RCS reduction can be achieved by reconfiguring scattering patterns using coding metasurfaces. Novel radar absorbing materials can be devised based on field enhancements of metamaterials. When combined with conventional radar absorbing materials, metamaterials can expand the bandwidth, enlarge the angular range, or reduce the weight. Future tendency and major challenges are also summarized

Broadband microwave metamaterial absorber with lumped resistor loading

Narrow absorption bandwidth has been a fundamental drawback hindering many metamaterial absorbers for practical applications. In this paper, by loading lumped resistors, we have successfully designed a microwave metamaterial absorber with multioctave wide absorption bandwidth covering the entire X- and Ku-bands, while keeping the thickness of the absorber less than 1/10 of the working wavelength. The polarization-insensitive absorber shows a good angular stability for both transverse electric (TE) and transverse magnetic (TM) incidences. Prototype has been fabricated and measured to validate the design principle and the simulated results, and good agreements are observed between simulated and measured results. The proposed metamaterial absorber offers an efficient way to realize broadband microwave absorption with stable angular performance, which may find potential uses in many applications, for example, electromagnetic compatibility

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.Comment: Accepted for publication in Reports on Progress in Physic

Artificial Impedance Surfaces and Wire Media for Absorption and Cloaking

The main objective of this dissertation is to investigate the ability of utilizing artificial impedance surfaces and wire media for absorption and cloaking applications. The dissertation includes two parts which focus on the electromagnetic wave propagation in absorbers formed by stacked metasurfaces and structured wire media, and electromagnetic wave interaction with the cylindrical cloaking structures. In the first part, we propose a variety of physical systems that show multiband and wideband absorption properties in the microwave regime. For the multiband absorbers, we propose a simple analytical model to study the absorption properties. Further, using the same circuit model, the physical mechanisms of the observed behavior is clearly explained in terms of the open/coupled Fabry-Pérot resonators. To design wideband absorbers, we first analyze a single-layer wire medium loaded with an arbitrary material (a thin copper patch with finite bulk conductivity and a graphene patch characterized by its complex surface conductivity) at one end and a ground plane at the other. Based on the properties of the single-layer structure (which acts as a narrowband absorber), we next propose a novel multilayered mushroom structure with thin resistive patches at the wire-medium junctions for wideband absorption. To characterize the wideband properties, here, we derive new additional boundary conditions and solve the scattering problem using an analytical model developed particularly for the problem at hand. We also show a methodology to design these absorbers and explain the wideband absorption mechanisms. The second part focuses on the application of various metasurfaces for cloaking dielectric and conducting cylinders for plane-wave incidence and for line sources in close proximity. The cloaking mechanism is based on a mantle cloaking technique, wherein the scattered field produced by the object is cancelled by the cloak. The purpose of this work is to design the mantle cloaks using the metasurfaces, to render the objects invisible. The analysis here is carried out using a rigorous analytical model which employs the Lorenz Mie-scattering theory. Two-sided impedance boundary conditions are applied at the interface of the metasurfaces and analytical grid-impedance expressions derived for the planar cases have been successfully used in tailoring the reactances of the cylindrical surfaces. Further, the analytical results presented in the dissertation are verified using the numerical simulations

Thin flexible multi-octave metamaterial absorber for millimeter wavelengths

The development of radiation-absorbent materials and devices for millimeter and submillimeter astronomy instruments is a research area of significant interest that has substantial engineering challenges. Alongside a low-profile structure and ultra-wideband performance in a wide range of angles of incidence, advanced absorbers in cosmic microwave background (CMB) instruments are aimed at reducing optical systematics, notably instrument polarization, far beyond previously achievable specifications. This paper presents a metamaterial-inspired flat conformable absorber design operating in a wide frequency range of 80–400 GHz. The structure comprises a combination of subwavelength metal-mesh capacitive and inductive grids and dielectric layers, using the magnetic mirror concept for a large bandwidth. The overall stack thickness is a quarter of the longest operating wavelength and is close to the theoretical limit stipulated by Rozanov’s criterion. The test device is designed to operate at a 22.5° incidence. The iterative numerical-experimental design procedure of the new metamaterial absorber is discussed in detail, as well as the practical challenges of its manufacture. A well-established mesh-filter fabrication process has been successfully employed for prototype fabrication, which ensures cryogenic operation of the hot-pressed quasi-optical devices. The final prototype, extensively tested in quasi-optical testbeds using a Fourier transform spectrometer and a vector network analyzer, demonstrated performance closely matching the finite-element analysis simulations; that is, greater than 99% absorbance for both polarizations, with only a 0.2% difference, across the frequency band of 80-400 GHz. The angular stability for up to ±10∘ has been confirmed by simulations. To the best of our knowledge, this is the first successful implementation of a low-profile, ultra-wideband metamaterial absorber for this frequency range and operating conditions