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This work introduces a wideband metamaterial (MM) absorber designed to operate effectively across a wide reception angle and be polarization-insensitive within ISM band (2.4 GHz) applications. The proposed absorber unit cell comprises four copper sectors loaded with lumped resistors and a full copper ground plane hosted onto two FR4 substrates. Furthermore, an air layer suspended between the ground plane and a FR4 substrate is applied to achieve wideband absorption. In addition, the simulation results show that particular design factors, such as lumped resistors and unit cell geometry, can be optimized to improve the efficiency of the absorber. The simulations demonstrate that the proposed absorber achieves a wideband absorption, exceeding 90%, over a broad frequency range from 1.94 GHz to 2.98 GHz. The designed absorber was fabricated and tested, and the simulation and measurement results were agreed well.

In this paper, a broadband, polarization-independent metamaterial absorber for millimeter waves is proposed. Operating across the frequency range of 22.7 GHz to 37.2 GHz, our broadband, polarization-independent absorber demonstrates exceptional efficacy by capturing over 90% of incident wave power. Distinguished by an array of conductive square patches on a dielectric substrate, the proposed structure boasts a remarkably low thickness, measuring less than a quarter of the free space wavelength of the central point within its operational spectrum (excluding the ground layer thickness). Leveraging a combination of circuit modeling and transmission line theory, this paper presents the design of this absorber and also elucidates its underlying operating principles. The combination of broadband operation, low thickness, and systematic design is rarely seen in previous works. To validate the proposed approach, a physical prototype is realized using 3D printing technology, employing conductive polylactic acid (C-PLA) patches on a light polyvinyl chloride (L-PVC) substrate. The proposed absorber has various potential applications in communication, electromagnetic shielding, and satellite technologies. Finally, a comparison with recent works is performed to demonstrate the feasibility of the proposed structure for millimeter-wave applications. [ABSTRACT FROM AUTHOR]

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]

The present study proposes a strong and wide microwave absorption of multilayered metastructure (SWMAMM) capable of achieving an ultra-wideband absorption of −20 dB and 60° oblique incidence. SWMAMM primarily consists of metasurface-Ⅰ (MS-Ⅰ), metasurface-Ⅱ (MS-Ⅱ), and a top absorption-enhanced skin, which are separated by three support dielectric slabs. Based on impedance matching theory and the interference model, MS-Ⅰ serves as the core functional layer for wideband absorption; whereas MS-Ⅱ not only provides ultra-wideband impedance matching but also enhances absorptivity, distinguishing it from most designs. To further improve impedance matching, different hole arrays are incorporated into the substrates of MS-Ⅰ, MS-Ⅱ, and the top absorption-enhanced skin by modifying their equivalent reactance. The measurement results demonstrate that the −10 dB and −20 dB reflection bands are separately in the range of 3.97–23.19 GHz and 5.24–21.81 GHz when the oblique incidence angle reaches 5°; the −10 dB reflection band can cover the range of 5.99–25.00 GHz when the oblique incidence angle reaches 60°. Our approach, which involves impedance matching design of MS-Ⅱ and optimizing impedance matching by incorporating different hole arrays into the substrates of the MS-Ⅰ, MS-Ⅱ, and the top absorption-enhanced skin, can be applied to various unit cells and dielectric materials. This approach offers significantly enhanced convenience and efficiency compared to existing designs, thereby facilitating further optimization and development of Electromagnetic absorbers.

Multi-band terahertz (THz) absorber based on a non-symmetric double-sided graphene comb resonator array is designed and simulated by the finite element method (FEM) in CST Software. Then, an equivalent circuit model (ECM) based on admittance with a fast MATLAB code is proposed to analyze the absorber in the THz region. The admittance-based ECM approach could be used for any metamaterial absorber containing one layer of resonators sandwiched between two dielectric slabs and backed by a metal layer consisting of a layer of resonators with a thickness much smaller than the minimum wavelength in the considered wavelength range. The proposed absorber is dynamically tunable with a one-layered resonator array. It has strong linear dichroism (LD) response of 98% and the frequency range of 0.7-5 THz with absorption $>$ 96%: two absorption bands for TE mode and three for TM mode. The proposed absorber can be used in polarization-sensitive devices and systems in the THz region. The ECM model of the metastructure was derived to provide an efficient approach to analyzing the performance of the absorber. The FEM simulation results are in good agreement with the ECM ones.

Performance of a microwave absorbing material can be increased by moulding it into a resonator structure. These tuned structures made up of lossy materials like dielectric cavities trapped the electromagnetic waves and dissipate them. However, resonator structures are narrowband, and to make them wideband absorbers, a technique has been proposed here. In the presented work, an array of dielectric cavity resonators has been proposed having cavities of different physical dimensions. Due to change in physical dimensions, all cavities have unique resonating peaks. Arranging these cavities into two-dimensional array leads to overlapping of all the resonating peaks, and accordingly, a broadband dielectric cavity resonator (DCR) array absorber is designed. Proposed array structure has − 22.6 dB reflection coefficient and 8 GHz (6 to 14 GHz) absorption bandwidth makes it a broadband absorber which can be utilized to build an anechoic chamber for testing the electromagnetic devices. [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]

A multi-band electromagnetic absorber is proposed for sensing the adulterant concentration in biofuels based on refractive index variations at room temperature (300 K). The proposed absorber structure consists of a gold based metallic resonating geometry and metallic ground plane on the top and bottom layer of the cavity loaded dielectric material. The unit cell has volume of 24 μ m × 24 μ m × 3. 57 μ m (0. 1088 λ L × 0. 1088 λ L × 0. 01618 λ L where λ L is wavelength at lowest operating frequency), five absorption peaks at 1.36, 3.349, 5.338, 5.887 & 9.307 THz with peak absorptivity of 96.15%, 97.55%, 95.63%, 83.60% & 96.13% and Full Width at Half Maxima (FWHM) of 0.098, 0.275, 1.01 & 0.417 THz respectively. For solid substrate, the absorption performance is polarization insensitive and with cavity inside the substrate, the absorption performance is polarization sensitive in normal incidence. The absorber's Refractive Index (RI) and biofuel adulteration sensing performance is also analyzed by loading analyte on its top surface and inside the substrate cavity. Peak sensitivity of ∼ 3 THz/RIU with a maximum Figure of merit (FOM) of ∼ 5. 69 RIU − 1 and maximum ∼ 0.16% error in ethanol based biofuel adulteration estimation are achieved. The proposed absorber has advantages of ultracompact dimensions, very high sensitivity and very less estimation error in adulteration estimation over other absorber based sensors. • Compact unit cell volume of 0. 10724 × 0. 10724 × 0. 015952 λ L 3 , where λ L = λ at 1.36 THz. • %A ≈ 96.15, 97.55, 95.63, 83.6 and 96.13 at 1.36, 3.349, 5.338, 5.887 and 9.307 • Can be used as a Biofuel adulteration sensor with S m a x ∼ 3 THz/RIU and FOM ∼ 6 RIU − 1 • Unknown refractive indices (avg. calc. RI) estimation with max.{% error} ∼ 0.16% • Substrate cavity & container lead to efficient sensing of solid and liquid analytes [Display omitted] [ABSTRACT FROM AUTHOR]

This work presents an ultrawideband and high-absorption structure (UHAS) obtained by a fast design method. The proposed analytic approach can simply design single-FSS-layer absorber with ideal performance stability under oblique incidence. With this method, a composite material reinforced circuit-analog (CA) absorber is designed, which unit cell of CA sheet is a square ring loaded with chip resistors. At normal incidence, both TE and TM polarization simulation results demonstrate that the reflection coefficient below −10 dB is from 5.8 to 22.2 GHz; meanwhile, the reflection coefficient less than −20 dB covers a bandwidth of 7.4–19.1 GHz; also, there is a −30 dB absorption band in 8.6–17.2 GHz. Furthermore, under TE polarization, experimental results indicate that UHAS can maintain a −20 dB absorption band from 7.4 to 18.0 GHz within 40° oblique angle of incidence; −10 dB absorption is from 6.1 to 18.0 GHz within 50° oblique angle of incidence. The good agreement between simulation and measurement validates the proposed method and UHAS. Finally, the UHAS, with a protective layer, will be more stable in practical applications. [ABSTRACT FROM AUTHOR]

A tri-band terahertz (THz) absorber for sensing the adulteration percentage in mustard oil is proposed in this paper. It consists of a gold based metallic resonating geometry and metallic ground plane on the top and bottom layers of the dielectric material respectively. It has three absorption peaks with peak absorptivity of 99.74%, 99.96% & 99.94% and Full Width at Half Maxima (FWHM) of 0.57, 0.862 & 1.112 THz at 4.6296, 8.295 & 12.6309 THz respectively. The proposed absorber's unit cell volume has an overall volume of 10 μ m × 10 μ m × 1. 61 μ m (0.15432 λ L × 0. 15432 λ L × 0. 024846 λ L where λ L is wavelength at lowest operating frequency i.e. 4.6296 THz). It has polarization insensitive absorption performance under normal incidence and incident angle (θ ∘) stability up to 50° for A ≥ 90%. The proposed structure is simulated through by using CST & HFSS. The results are validated by developing an equivalent circuit model (ECM). The results of CST, HFSS and ECM are found to be in good agreement. A sensitivity varying from ∼ 0.23 THz/RIU to ∼ 1.62 THz/RIU is achieved for Refractive Index (RI) sensing case, whereas a high sensitivity in the range of ∼ 0.4 THz/RIU to ∼ 2 THz/RIU is achieved for mustard oil adulteration sensing case by mounting the absorber's top surface with a polyimide based container filled with an analyte (either refractive index material or mustard oil). During the validation, a maximum ∼ 0.16% error in mustard oil adulteration estimation is achieved. The proposed sensor has advantages of simple & ultra compact dimensions, high sensitivity and minimal error in impurity concentration estimation with respect to other absorber-based sensors. • Unit cell volume 0.15432 λ L × 0. 15432 λ L × 0. 024846 λ L where λ L = λ at 4.6296 THz • Peak absorption of ∼ 99.9% at 4.6296, 8.295 and 12.6309 THz • Polarization independent and Incident angle (θ °) stability up to 50°for A ⩾ 90% • Used as Mustard oil adulteration sensor with maximum sensitivity of ∼ 2 THz/RIU. • Maximum 0.16% error in estimation of unknown refractive indices (avg. calc. RI) [Display omitted] [ABSTRACT FROM AUTHOR]

With the effect of technological advances, the use of electrical and electronic devices has increased dramatically in recent years. Wireless technologies and related applications are mostly preferred for the communication of these devices with each other. Thus, people are easily exposed to electromagnetic waves in daily life. The extensive global use of these devices raises the question of their possible biological effects on human health. Also, electromagnetic waves influence the functioning of a nearby device. In this study, an electromagnetic absorber based on boric acid (5, 10, 20, and 30 wt%) added wastepaper was developed. Copper (Cu) and aluminum (Al) were also used as mineral additives for comparison. Three different kinds of wastepaper namely, office paper, newsprint, and cardboard paper were selected for the experimental study. The effect of varying boric acid contents on the electromagnetic absorption of the boards manufactured was evaluated and compared to Cu (30 wt%) and Al (30 wt%) added boards. The results show that newsprint has better absorption effectiveness than office and cardboard paper and absorption up to 40 dB was achieved. The absorption effectiveness of Al, Cu, and boric acid added boards was achieved at approximately 40, 30, and 20 dB, respectively. As a result, the electromagnetic absorption effectiveness of boric acid added the board is acceptable levels. Also, apart from the use of boric acid as a powder, it has been determined that the application of the surface as a layer is effective in absorption. [ABSTRACT FROM AUTHOR]

The development of 5G telecommunication technology has seen a high demand for effective electromagnetic (EM) wave absorbers in the millimeter wave (mmWave) band. Because hexagonal M‐type ferrite is known to have high ferromagnetic resonance (FMR) in the 5G band frequency, it has attracted significant attention as a mmWave absorber. However, study of the relationship between magnetocrystalline anisotropy, FMR frequency, and EM wave absorption performance in the mmWave band remains insufficient. In this study, Ni–Ti‐substituted M‐type barium ferrite (Ba–M ferrite) was synthesized using the citrate sol–gel method, and the change in the magnetic properties caused by Ni–Ti substitution was analyzed. The complex permittivity, permeability, and reflection loss (RL) of the Ni–Ti‐substituted Ba–M ferrite composites in the 8.2–75 GHz broadband frequency range were also studied. The EM wave absorption frequency, which is governed by the FMR frequency, could be tuned by controlling the amount of substituted Ni–Ti ions in the Ba–M ferrite. The prepared EM wave absorber exhibited excellent EM wave absorption properties with an RL of −52 dB at 29.5 GHz and a matching thickness of 0.95 mm. This study provides insights into a frequency‐tunable EM wave absorber with enhanced absorption properties, attained by changing the magnetic properties of Ni–Ti‐substituted M‐type hexaferrites in the mmWave frequency band. [Display omitted] • A single-phase Ni–Ti-substituted Ba–M ferrites were synthesized by citrate sol-gel method. • The electromagnetic properties were studied in a wide frequency range from 8.2 GHz to 75 GHz. • Electromagnetic wave absorption frequency was tuned by controlling the ferromagnetic resonance frequency. • Optimum reflection loss of −52 dB at 29.5 GHz and an EAB of 10.05 GHz were obtained at 0.95 mm thickness. [ABSTRACT FROM AUTHOR]

• The impedance design of metasurfaces is important for this absorber's exceptional electromagnetic absorption capability. • The utilization of a dielectric hole array as an impedance matching layer can significantly enhance absorptivity. • By employing hole-dielectric arrays as substrates for metasurfaces, the impedance matching can be enhanced. The present study proposes a strong and wide microwave absorption of multilayered metastructure (SWMAMM) capable of achieving an ultra-wideband absorption of −20 dB and 60° oblique incidence. SWMAMM primarily consists of metasurface-Ⅰ (MS-Ⅰ), metasurface-Ⅱ (MS-Ⅱ), and a top absorption-enhanced skin, which are separated by three support dielectric slabs. Based on impedance matching theory and the interference model, MS-Ⅰ serves as the core functional layer for wideband absorption; whereas MS-Ⅱ not only provides ultra-wideband impedance matching but also enhances absorptivity, distinguishing it from most designs. To further improve impedance matching, different hole arrays are incorporated into the substrates of MS-Ⅰ, MS-Ⅱ, and the top absorption-enhanced skin by modifying their equivalent reactance. The measurement results demonstrate that the −10 dB and −20 dB reflection bands are separately in the range of 3.97–23.19 GHz and 5.24–21.81 GHz when the oblique incidence angle reaches 5°; the −10 dB reflection band can cover the range of 5.99–25.00 GHz when the oblique incidence angle reaches 60°. Our approach, which involves impedance matching design of MS-Ⅱ and optimizing impedance matching by incorporating different hole arrays into the substrates of the MS-Ⅰ, MS-Ⅱ, and the top absorption-enhanced skin, can be applied to various unit cells and dielectric materials. This approach offers significantly enhanced convenience and efficiency compared to existing designs, thereby facilitating further optimization and development of Electromagnetic absorbers. [ABSTRACT FROM AUTHOR]

Reconfigurable multifunctional electromagnetic absorbers have shown broad application prospects in effectively dealing with a series of problems caused by complex electromagnetic environments due to their dynamic reflection wave control characteristics. In this work, we experimentally propose a multifunctional absorber based on a graphene metasurface. Its absorption mode can be flexibly switched among three modes of dual band, broadband, and single band. The reflection amplitude in each absorption mode can be controlled simultaneously. The measurement results of the prepared graphene metasurface indicate that the absorption modes and amplitudes can be dynamically controlled by changing two independent sets of bias voltages applied to the patterned graphene sandwich structures. The proposed graphene metasurface achieves peak absorption rates above 99.9% in both dual-band and single-band absorption modes. Specifically, in the broadband absorption mode, the bandwidth with an absorption rate greater than 90% reaches 17.8 GHz. In addition, it also integrates many advantages, such as optical transparency, polarization-insensitivity, stability of oblique incidence angles, and conformability to the application targets. Therefore, the proposed graphene metasurface is expected to be applied in platforms with optical windows that require resistance to electromagnetic interference and avoidance of electromagnetic radiation.

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]