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In this work, the design of broadband, wide-angle, direction-independent metamaterial (MM) electromagnetic wave (EM) absorber for K-band frequency application is investigated and validated experimentally. The unit cell of the metamaterial absorber consists of four 90° rotated L-shaped metallic patches imprinted on a dielectric substrate backed by a metallic sheet. The structure yield absorption in the broad frequency ranges from 22.5 to 29.3 GHz for both TE and TM polarized waves with more than 90 % absorptivity having a wide fractional bandwidth of (6.8 GHz) 25.8 %. The structure is four-fold symmetric and hence yields polarization insensitivity for different angles of polarization under both TE and TM polarized waves. The structure is also investigated under oblique incidence where the 80 % absorptivity holds up to 45° incident angles for both TE and TM waves. The absorption mechanism is explained with the help of top and bottom surface current distribution, induced electric field, and parametric analysis. To verify the resonance in the structure, characteristic mode, and equivalent circuit analysis have been carried out and presented. A prototype of the absorber has been fabricated and simulated results are validated with measured results. Measured results are showing good agreement with the simulated responses. The novelty of the proposed absorber lies in its unique metallic pattern on a λ0/8 (concerning the canter frequency of absorption bandwidth) thin FR-4 substrate while showing the wide absorption bandwidth and direction independence to normal and oblique incidence. The compact nature of the absorber and broadband response with good polarization insensitivity at normal and oblique incidence makes it commercially suitable for the reduction of radar cross section (RCS) in stealth applications at the K-band. [ABSTRACT FROM AUTHOR]

Broadband optical absorbers are increasingly in demand in various applications, including solar cells and radiative cooling systems. Among various types of structures, absorbers based on metamaterial structures have attracted much attention. However, they generally suffer from the issues of narrow bandwidth, high-cost fabrication, and high sensitivity to polarization changes. This paper presents a broadband, polarization-independent metamaterial absorber working in both infrared and visible frequency regimes. This structure is composed of a continuous phase-change material film ( Ge 2 Sb 2 Te 5 ) separated between two thin spacer layers of SiO 2 , and an array of amorphous Silicon particles located on the top of the structure. The proposed device was shown to exhibit remarkable absorptivity (more than 90 percent) within a broad range of frequencies starting from 250 to 1050 THz. In addition, thanks to the structural symmetry, the strong absorbance shows a considerable overlap between transverse magnetic (TM) and transverse electric (TE) modes over an extensive range of incident angles. The designing procedure for this absorber can be used as a guideline for designing similar metamaterial absorbers in any desirable frequency band. [ABSTRACT FROM AUTHOR]

An ultra-thin, multiband, micro-slot based metamaterial absorber is presented in this paper. The proposed unit cell is compact which is in the form of single-layer gold patch-gallium arsenide-ground package, involving four identical micro-slots. Using the micro-slots, the fundamental circular patch gains a multiband resonation skill, and the absorber operates at 1.16, 2.73, and 4.57 THz bands with near-perfect absorptivity. The absorption mechanism is discussed based on the electric and surface current distributions, and all outcomes are also validated by the effective medium approach. The proposed absorber is ultra-thin with a thickness of 2.6 µm, corresponding to ∼λ/100 at its lowest operation frequency. The unit cell indicates polarization-independent, and wide incident angle absorption spectra up to 40°, hence, the proposed absorber is a promising candidate as an absorbing platform for THz band imaging and sensing applications. [ABSTRACT FROM AUTHOR]

Devices designed to dynamically control the transmission, reflection, and absorption of terahertz (THz) radiation are essential for the development of a broad range of THz technologies. A viable approach utilizes materials which undergo an insulator‐to‐metal transition (IMT), switching from transmissive to reflective upon becoming metallic. However, for many applications, it is undesirable to have spurious reflections that can scatter incident light and induce noise to the system. We present an electrically actuated, broadband THz switch which transitions from a transparent state with low reflectivity, to an absorptive state for which both the reflectivity and transmission are strongly suppressed. Our device consists of a patterned high‐resistivity silicon metamaterial layer that provides broadband reflection suppression by matching the impedance of free space. This is integrated with a VO2 ground plane, which undergoes an IMT by means of a DC bias applied to an interdigitated electrode. THz time domain spectroscopy measurements reveal an active bandwidth of 700 GHz with suppressed reflection and more than 90% transmission amplitude modulation with a low insertion loss. We utilize finite‐difference time domain (FDTD) simulations in order to examine the loss mechanisms of the device, as well as the sensitivity to polarization and incident angle. This device validates a general approach toward suppressing unwanted reflections in THz modulators and switches which can be easily adapted to a broad array of applications through straightforward modifications of the structural parameters. [ABSTRACT FROM AUTHOR]

In this paper, we propose a metamaterial absorber using 3-D printing technology. To miniaturize the footprint size of a unit cell, we introduce a 3-D printed stair-like Jerusalem cross (JC) structure. It is demonstrated that the absorption frequency can be decreased by building up one and two stairs from the planar JC. In this paper, the unit cell footprint size of the three-stair JC is reduced by 41% compared with the unit cell footprint size of the planar JC. The proposed metamaterial absorber with 9×9 unit cells is fabricated using a 3-D printer. Poly lactic acid is used as a dielectric material for 3-D printing, and a conductive pattern is realized by applying the silver paste to the 3-D printed dielectric structure. The simulated and measured absorptivity are 99.9% and 99.8%, respectively, at 5.18 GHz. Due to the symmetric unit cell structure, the absorptivity does not change, although the incident polarization gets rotated.

This paper proposes a switchable metasurface for broadband and polarization-insensitive absorption, whose absorption spectrum can be fluidically switched using liquid metal. The proposed metasurface consists of a top metallic pattern, a flexible printed circuit board substrate, a polydimethylsiloxane substrate, and a bottom metallic ground plane. The proposed unitcell is inspired by a four-circular-sector structure. To maintain the symmetrical geometry and increase the switching range, a symmetrical microfluidic channel is designed with a circular ring capillary and four isolated patches. To measure the absorptivity of the proposed metasurface, 10 × 10 unitcells are fabricated with 20 inlets and 20 outlets. When the microfluidic channels of the metasurface are empty, the fabricated metasurface achieves an absorptivity of higher than 90%, from 6.23 to 12.14 GHz. When eutectic gallium-indium alloy is injected into the microfluidic channels, the fabricated metasurface achieves higher than 90% absorptivity, from 5.44 to 6.12 GHz. Thus, for both cases, the absorptivity is constant at different incident polarization angles.

This paper specifies a concise review of recent research and development in the context of electromagnetic absorbers. An attempt has been made to justify the need for economical and thin microwave absorbers for achieving good absorption characteristics over a wide range of frequencies within a specified band of electromagnetic spectrum. This paper mainly focuses on the challenges encountered while designing a perfect electromagnetic absorber which can absorb electromagnetic radiations of any polarization that may appear from any direction and may occur over a broader bandwidth. The absorbers which are competent enough to withstand all these challenges are capable to suppress the reflection and transmission of unknown electromagnetic radiations efficiently to the lowest possible level. Several prospects of nanotechnology in flexible absorber design are also described to enhance absorber characteristics. This paper also provides a futuristic glimpse on several applications of such absorbers for commercial and military purposes. [ABSTRACT FROM AUTHOR]

In this paper, a polarization-insensitive, metamaterial absorber with more than 91 % absorptivity is investigated numerically and experimentally for radar cross-section reduction at X, Ku, and K bands. The unit cell structure of the absorber is comprised of a symmetrically arranged defective L-shaped patch made of copper metal, which is imprinted on a metal-backed FR-4 lossy dielectric substrate. The proposed absorber yields multiband absorption response at distinct absorption peaks at 11.1, 15.5, and 16.3 GHz along with the broadband response of 4.8 GHz from 18.6 to 23.4 GHz (absorption peaks at 18.9, 20, 21.9, and 23 GHz) in the absorption spectra. The Characteristic mode analysis has been carried out and presented to verify the modes and resonance in the structure. The physical absorption phenomenon is clarified by presenting the induced electric field, top and bottom surface current distribution, and various retrieved constitutive electromagnetic parameters. The structure is studied for various polarization angles for both transverse electric (TE) and transverse magnetic (TM) waves at normal incidence which shows strict insensitivity to the direction of arrival of electromagnetic(EM) signal. The structure is also investigated under oblique incidence for both TE and TM waves where the response holds good up to 50° incident angles in both cases. The novelty of the proposed absorber lies in its multiband response at X, Ku, and K bands through a unique metallic pattern on a thin FR-4 substrate along with characteristics of broadband, polarization insensitiveness, wide incidence angle independence, and compactness incorporated in a single and simple design without using external components. A prototype of the absorber has been fabricated and simulated results are validated with measured results. All the above-mentioned attributes in a simple design make it commercially suitable for radar cross-section (RCS) reduction and energy harvesting applications at X, Ku, and K bands. [ABSTRACT FROM AUTHOR]

In this paper, we propose an ultra-broadband metamaterial absorber optimized by the particle swarm optimization (PSO) algorithm. A unit cell of the absorber is made of TiN/TiO 2 /TiN square disks, offering a metal-insulator-metal (MIM) configuration, surrounded by a TiN square ring resonator which all are located on a thin stack of TiO 2 /TiN films. The optimized structure shows high average absorption of 91.63% over the wavelength range of 200–4500 nm. The over 90% absorption bandwidth is 1590 nm, extended from 200 nm to 1790 nm. Furthermore, the absorber absorbs more than 80% of the incident light with wavelengths from 200 nm to 4480 nm, which covers the ultraviolet, visible, and near-infrared regions. The absorber indicates high absorptivity of over 75% under an oblique incidence up to 60° for both TM and TE polarizations. The effect of the presence of the square ring resonators as well as each layer of the MIM on the absorption of the absorber is also studied. It is shown that the use of square ring resonators combined with square disks significantly enhances the absorption of the absorber at wavelengths longer than 1100 nm. The structure has high thermal and chemical stability due to the use of TiN and TiO 2. Owing to the outstanding features of the proposed absorber, it can be used in different fields such as imaging, thermal emitting, and solar applications. • Having high thermal and chemical stability due to the use of TiN and TiO 2 Materials. • Having an average absorption of 91.63% in the wavelength range of 200-4500 nm. • Having a high bandwidth of 1590 nm, ranging from 200 nm to 1790 nm for over 90% absorption. • Having a high bandwidth of 4280 nm, ranging from 200 nm to 4480 nm for over 80% absorption. • Having polarization-insensitive and angle-independent performance for oblique incident light up to 60° in the wavelength range of 200-4500 nm. [ABSTRACT FROM AUTHOR]

We derive periodic multilayer absorbers with effective uniaxial properties similar to perfectly matched layers (PML). This approximate representation of PML is based on the effective medium theory and we call it an effective medium PML. We compare the spatial reflection spectrum of the layered absorbers to that of a PML material and demonstrate that after neglecting gain and magnetic properties, the absorber remains functional. This opens a route to create electromagnetic absorbers for real and not only numerical applications and as an example we introduce a layered absorber for the wavelength of 8 $$\upmu \hbox {m}$$ made of $$\hbox {SiO}_2$$ and NaCl. We also show that similar cylindrical core-shell nanostructures derived from flat multilayers also exhibit very good absorptive and reflective properties despite the different geometry. [ABSTRACT FROM AUTHOR]

• A mechanically actuated frequency reconfigurable metamaterial electromagnetic absorber is proposed. • A linear actuator and microcontroller is used for changing air substrate thickness mechanically. • The absorption frequency changes from 6.96 GHz to 5.78 GHz when the air substrate thickness is changed from 17 mm to 26 mm. • A rate of change for the absorption frequency relative to the air substrate thickness is 0.12 (GHz/mm). • The proposed metamaterial absorber can be potentially used for wireless pressure sensor applications. In this paper, a mechanically actuated frequency reconfigurable metamaterial electromagnetic absorber is proposed. The absorber's metamaterial unit cell is designed to exploit LC resonance from inductive and capacitive coupling. Because this inductance and capacitance determine the absorber's resonant frequency, we propose a mechanical tuning method that changes the resonant frequency by changing the overall thickness of the metamaterial unit cell. The proposed unit cell consists of an FR4 dielectric substrate with fixed thickness and an air substrate with tunable thickness. When the air substrate thickness is varied over the range of 17 mm to 26 mm, the absorber's resonant frequency changes from 6.96 GHz to 5.79 GHz in EM simulation. In order to verify the proposed idea, a metamaterial absorber was fabricated as a 17 × 17 array of unit cells and a linear actuator was used to control the thickness of the air substrate. We experimentally demonstrated that the absorption frequency changes from 6.96 GHz to 5.78 GHz with 0.12 (GHz/mm) sensitivity when the air substrate thickness is mechanically changed from 17 mm to 26 mm. [ABSTRACT FROM AUTHOR]