Flexible Metamaterial Absorber Research Articles

Ultrawideband Flexible and Intensity-Tunable Metamaterial Absorber Based on Lossy Stepped Impedance Resonator

Ultrawideband Flexible and Intensity-Tunable Metamaterial Absorber Based on Lossy Stepped Impedance Resonator

Synthesis and characterization of flexible CaCoAlFe2O4 based microwave substrate for triband polarization-insensitive metamaterial absorber

This study investigated CaxCo(0.90-x)Al.01Fe2O4 nanoparticle synthesized using the sol–gel synthesis method to achieve a flexible substrate with a high dielectric constant value. The XRD, FESEM, and dielectric characteristic is analyzed to explore the performance of the suggested Ox Wheels-shaped (OWS) metamaterial absorber unit cell. The X-ray diffraction (XRD) analysis found significant peaks at 30° (318), 33° (176), 35° (794), 43° (232), 53° (116), 57° (256), 62° (304), and 74° (126) planes. The dielectric constant values of the prepared nanomaterial materials are 4.52 (X = 90 %), 4.73 (X = 60 %), and 5.07 (X = 30 %) due to the deviations in raw material compositions. The prepared, flexible substrate is used in the anticipated OWS metamaterial absorber unit cell, and analysis has been widely considered and continuously concerned due to their excellent ultra-thin thickness, lightweight, and high absorbance resonance features. The predicted structure reached S21 response at 3.47 GHz, 7.05 GHz, and 10.76 GHz with the magnitude of −24.11 dB, −17.07 dB, and −19.19 dB, respectively, using CST simulation software. The absorber prototype is measured for further investigation within the frequency range of 2.25–2.76, 5.84–6.15 GHz, and 9.63–9.76 GHz, where maximum magnitude is achieved at 2.50 GHz, 6.02 GHz, and 9.70 GHz, respectively. This specific resonance frequency reached the highest 99.61 %, 98.03 %, and 98.79 % absorption, which covers the S, C, and X band frequency spectrum. Various electrical, magnetic, mechanical, and multi-parameter responses are examined to achieve almost perfect absorption (PA). The enormous range of possible uses of metamaterial, such as absorber wave modification, infrared imaging, radiative cooling, energy harvesting, and chemical detection, makes it appreciated. The CaCoAlFe2O4 ferrite-based flexible metamaterial absorber structure has ultra-thin, flexible, and non-corrosive advantages.

Large-Scale Fabrication of Customized, Tunable, Ultrathin, and Flexible Metamaterial Absorbers Based on Laser-induced Graphene

Developing various 3D graphene microwave absorbers is a hotspot strategy to address the issue of continuous electromagnetic pollution, but remains great challenges in realizing high-performance microwave absorption capability at thin thickness. Herein, a series of flexible metamaterial absorbers (MAs) composed of periodical laser-induced graphene (LIG) surface, dielectric substrate and metallic ground fabric from top to bottom are fabricated. The LIG meta-surface is customized by means of regulating parameters including laser power density (LPD), laser engraving rate (LER) and input patterns to well balance impedance matching and electromagnetic loss. LIG MAs exhibit minimum reflection loss (RL) of −39.7 dB (with effective absorption bandwidth (EAB) of 5.5 GHz) and maximum EBA of 12.1 GHz (with RL of −18.9 dB) at thickness less than 1.1 mm, and the simulated and experimental results are highly consistent. In addition, radar cross section (RCS) simulation of LIG MA with multi-angles endows its potential for prospective applications in in-orbit satellites electromagnetic protection.

Dual-band flexible metamaterial absorber working in 5G band

This article presents a thin and flexible artificial electromagnetic absorber with two absorption peaks at frequencies of 28 and 39 GHz for 5G communication. Copper and a flexible FR4 substrate are used in a pyramid-shaped layered configuration, achieving absorption rates exceeding 99% at 28 and 39 GHz. The thickness of the proposed structure is only 0.34 mm, which is ∼1/32 and 1/23 of the corresponding free-space wavelengths at the absorption frequencies. Due to its symmetric structure, the absorber is polarization-insensitive. Furthermore, the proposed structure exhibits over 90% absorption within the incident angle range of −45° to 45°. To investigate its absorption mechanism, the electric field and magnetic field distributions at the absorption frequencies were analyzed. Then a 150 × 150 mm2 sample was fabricated using printed circuit board manufacturing techniques. This sample was placed on flat or curved surfaces and tested under normal or oblique incidences. The experimental results were consistent with the simulated results, confirming the feasibility of the design. This absorber can be applied in 5G communication and millimeter-wave imaging.

Design and characterization of a lightweight and flexible carbon fiber-loaded silicone foam-based broadband metamaterial absorber for microwave applications

This study reports a simple fabrication technique for creating a flexible, organic, and microwave-absorbing material using carbon fiber-loaded silicone foam. The "particle-template" elaboration method, using sugar crystals as sacrificial particles, is conducted. The material's density is adjusted by varying the sugar crystal concentration while ensuring effective dispersion of carbon fibers in the silicone foam matrix. The dielectric characterization of a composite with a density of 0.33 g.cm−3 and loaded with 2 wt.% of 12 mm carbon fibers results in a permittivity ranging from 7 to 1.1 and a dielectric loss ranging from 1.9 to 0.7 across the frequency range of 2–18 GHz. These properties are used in a parametric study to optimize the overall dimensions of a structured absorber to achieve wideband absorption performance. The resulting prototype, with a total thickness of 16 mm, demonstrates promising performance by exhibiting broadband absorption (4–18 GHz), as predicted by simulation.

Flexible bandwidth-enhanced metamaterial absorbers with epoxy/graphene nanoplatelets-silver nanowire polymer composites as substrates

The realm of flexible devices has seen considerable advancement in recent technology due to their ability to bend and conform to particular shapes. Flexible metamaterial absorbers offer the advantage of combining the features of conventional metamaterials with the attributes of conformal systems, thereby opening new avenues in electromagnetic technology. This work involves the realization of flexible metamaterial absorbers with epoxy/graphene nanoplatelets-silver nanowire (GnP-AgNW) polymer composites as substrates; with a bandwidth enhancement achieved by incorporating losses in the substrate and combining resonances in the unit cell. A novel in-situ technique is employed to synthesize the nanohybrid GnP-AgNW (GA) with different weight percentages of AgNWs grown on GnP (GAx; x = 10, 15, 20 wt%). The dielectric measurements of the polymers showed that an increase in weight percentages of AgNWs in the composite enhanced the dielectric constant as well as losses of the polymer (εr՛: 7.7 to 8.8 and εr՛՛: 0.62 to 1.12). The numerical study on the designed flexible metamaterial absorbers with these substrates of thickness 1.4 mm showed a maximum absorptivity of 99% with a bandwidth (having absorption >90%) varying between 1.2 and 1.86 GHz. The experimental measurements on the fabricated metamaterial absorber were found to match the simulation results closely. The polymer composites demonstrated in this work are attractive candidates as substrates for metamaterial absorbers for conformal applications. This paper approaches the problem of bandwidth enhancement of metamaterial absorbers by material engineering along with the resonant structure optimization leading to a synergetic effect in the properties of absorber.

Polarization-insensitive, frequency- and amplitude-tunable transparent microwave absorber based on graphene metasurface

Transparent and flexible metamaterial absorbers can provide solutions for electromagnetic compatibility and electromagnetic radiation pollution. They also play key roles in platforms with optical windows that require low detectability and immunity to electromagnetic interference. Due to the lack of active adaptability to complex electromagnetic environments, early metamaterial absorbers need to be redesigned and manufactured once the application scenarios change. The new two-dimensional material graphene provides a practical technical approach for the realization of transparent tunable absorbers. In this work, we propose a polarization-insensitive dual-tunable microwave absorber based on graphene metasurface. By controlling two separate bias voltages applied to patterned graphene or continuous graphene, independent dynamic regulation of electromagnetic wave absorption frequency and amplitude is achieved. It also possesses the features of optical transparency and flexibility. The measurement results show that the absorption frequency can be shifted from 9.8 to 11.3 GHz, the peak absorption rate at each absorption frequency remains above 99.6%, and the reflection amplitude modulation can be achieved simultaneously at each absorption frequency. The proposed absorber integrates multiple modulation functions for electromagnetic waves, and the overall structure is optically transparent and flexible. Therefore, it can contribute to many practical wave absorbing applications.

Flexible broadband metamaterial absorber in long-wave infrared with visible transparency fabricated by laser direct writing.

Absorption of the long-wave infrared from human beings and the surroundings is a key step to infrared imaging and sensing. Here we demonstrate a flexible and transparent broadband infrared absorber using the photoresist-assisted metamaterials fabricated by one-step laser direct writing. The photoresist is patterned by the laser as an insulator layer as well as a mask to build the complementary bilayer metamaterials without lithography. The average absorptivity is 94.5% from 8 to 14 μm in experiment due to the broadband destructive interference of the reflected beam explained by the Fabry-Perot cavity model. The proposed absorber is applicable to various substrates with additional merits of polarization insensitivity and large angle tolerance, which offers a promising solution for thermal detection and management.

Flexible dielectric spacer with tunable dielectric properties for metamaterial absorber application

The development of a flexible and robust metamaterial absorber is a great motivation for the new generation of microwave absorbers covering any conformal shape. Ceramic–polymer composites with variable dielectric constant within the microwave frequencies are proposed as a flexible middle dielectric spacer (MDS) layer to provide an additional degree of design freedom for metamaterial absorber applications. By varying the dielectric constant of the MDS layer, the resonance band of the metamaterial absorber can be tuned while retaining a low profile in addition to enhancing its electromagnetic performance for transverse magnetic (TM) polarized waves. To formulate these composites, tetragonal structured barium titanate (BaTiO3) powder is selected as an excellent candidate when mixed with elastomer polydimethylsiloxane (PDMS), resulting in high mechanical strength substrates. Experimentally, samples with a range of dielectric permittivity from 2.81 to 5.67 were obtained for a 0%–20% volume fill fraction of BaTiO3 in PDMS. The obtained dielectric permittivities are consistent with the Bruggeman effective medium theory. Utilizing an optimized composition (10% BaTiO3), resulting in = 4 substrate, an X-band flexible absorber is designed and fabricated to absorb more than 90% of incident electromagnetic waves within a frequency band of 8.16–12.12 GHz.

A flexible metamaterial absorber with temperature-insensitive design at microwave frequencies

Developing a metamaterial absorber (MA) with both flexibility and temperature insensitivity continues to be a challenge in the field of radar stealth. In this paper, we propose a compensation method for designing flexible and temperature-insensitive MA. This method is revealed through the relationship between the square resistance of the resistive film, the permittivity of the substrate, and the temperature. Importantly, the compensation method is applicable to both the MA in the plane state and the bending state. By utilizing the benefits of the compensation mechanism and the flexible designability of the bilayer cross-shaped structure, the flexible MA proposed in this paper can achieve temperature-insensitive absorption across a broad frequency range. Experimental results indicate that the absorption peaks achieve an absorptivity greater than 90% within the frequency of 7.2–9.4 GHz, exhibiting excellent temperature stability from 25 °C to 300 °C. In comparison to previous studies on flexible MAs, this design offers a distinct advantage in high-temperature environments and provides valuable guidelines for the design of integrated multi-functional absorbers in practical applications.

Ultrathin optically transparent and flexible wideband absorber based on ANN and DGCNN

Optically transparent and structurally flexible metamaterial absorbers (MMAs) are widely used in many practical applications. However, the realization of such MMAs requires both thickness reduction and bandwidth enhancement, which remains a challenge task. As a multi-objective optimization problem, it is a time-consuming and resource-demanding process. Guided by prior-knowledge, two degrees of freedom are taken into account in the design, namely the structure of unit-cells and their spatial arrangement. In this way, the multi-objective optimization problem is simplified into a two-step problem. A general procedure based on machine-learning (ML) method is proposed to solve this problem. First, an artificial neural network (ANN) is used to map the configuration parameters of a type of meta-atom onto its reflection coefficients; then, a dynamic graph convolutional neural network (DGCNN) is trained to realize the synthesis of quasi-periodic distributed MMA array; finally, combined with the differential evolution (DE) algorithm, the optimized configuration of the meta-atoms and their optimum distribution are obtained. Based on the proposed procedure, an ultrathin optically transparent and flexible MMA is designed, fabricated and further verified by experiments. The designed MMA realizes 87% absorption bandwidth covering 6.22 GHz–19.42 GHz (103% relative bandwidth) with the thickness only 3.3mm(0.068λL), which approaches the theoretical limit 1/17λL. And its averaged transparency is about 63% at the wavelength range 500–800mm. The whole design process is achieved 50 times faster than the conventional full-wave simulation, which convincingly demonstrates the superiority of the proposed method.

Quantum Inspired Broadband Photonic Absorber: Potential Application as Solar Sail along with Mobility Analysis

Electromagnetic wideband absorption is still perceived as a critical and formidable challenge to address with an unambiguous photonic absorber. Subwavelength metamaterial (MM) unit cells with unique and controlled features have recently gained considerable interest. However, meta‐atoms, generated using a quantum‐inspired pattern distribution, are underwhelming in existing literature to design photonic absorbers and their potential application to manufacture solar sails is still quite uncommon. In this article, to create a flexible, polarization‐insensitive, ultrathin, and broadband MM absorber, quantum interference pattern‐inspired design is utilized. Herein, a novel approach to fabricating solar sails for the space exploration incorporates the proposed broadband photonic absorber rather than conventional reflectors. The quantum‐inspired meta‐absorber (QIMA) exhibits an absorption of over 91% for the visible domain, i.e., 380–800 nm under a conventional plane‐polarized source. It is shown in the study that broadband absorbers are almost equivalent to excellent reflectors to design the solar sails in terms of the time‐averaged force calculated by utilizing the Maxwell stress tensor method. Thus, the QIMA has the potential to be a viable alternative to reflectors in the design of futuristic solar sails for space exploration. The interference theory model is also utilized to assure the dependability of calculated data, and additionally, the standard AM1.5 solar spectrum is utilized to demonstrate the QIMA's solar‐harvesting potentiality.

Flexible Terahertz Metamaterials Absorber based on VO2

Terahertz (THz) metamaterials have attracted great attention due to their widely application potential in smart THz devices; however, most of them are fabricated on rigid substrate and thus limit the exploration of flexible THz electronics. In this paper, a flexible THz metamaterial absorber (MMA) incorporated with phase change material vanadium dioxide (VO2) is proposed. The simulation results indicate that two absorption peaks at around 0.24 THz (marked as A) and 0.46 THz (marked as B) can be observed by designing a I-shaped metamaterial combined with split ring structure. The strong absorption over 92% at 0.24 THz is bending-insensitive, but the absorption at 0.46 THz is bending-sensitive, across the bending angle in the range of 0–50 degrees. Moreover, dynamic modulation of the absorption can be achieved across the insulator-metal phase transition of VO2. Particularly, the absorption of the A-peak can be tuned from 99.4% to 46.9%, while the absorption of the B-peak can be tuned from 39.6% to 99.3%. This work would provide significance for the design of flexible THz smart devices.

Ultra-wideband, polarization-insensitive flexible metamaterial absorber base on laser printed graphene using equivalent circuit design method

In this paper, an ultra-wideband, angle and polarization-insensitive laser assisted graphene metamaterial absorber is presented. The design consists of double graphene resistive layers with the combination of three dielectric substrates. The graphene imbedded nano-magnetic material Fe3O4 has been synthesisedby laser direct writing technology, which demonstrates a high degree of uniformity with deviations less than 6% for different designed values of sheet resistance. Numerical simulation and analytical studies were carried out in order to analyze the absorption mechanism. The measured results confirm that the absorber operates from 1.32 to 17.9 GHz with over 90% absorptivity. The absorber achieves 173% fractional absorption bandwidth with a thickness of 0.068λL.

Analysis and characterization of structural, dielectric and magnetic properties of MgxCo(0.90−x)Zn0.10Fe2O4 nanoparticles based flexible metamaterial absorber for wireless communication

The anisotropic flexible metamaterial absorber (FMA) structure comprises a resonating layer, a ferrite-based flexible substrate layer, and a back layer. The sol-gel method is used to prepare the ferrite-based nanomaterial for enhancing absorbability. The solution prepared material is according to the formula of MgxCo(0.90−x)Zn0.10Fe2O4 and specified as X25, X50, X75 for X = 25%, 50%, and 75%, respectively. Then the proposed ferrites materials are characterized by XRD, and FESEM, respectively. Moreover, the magnetic and dielectric properties of the proposed MgxCo(0.90−x)Zn0.10Fe2O4 ferrites samples were explored for microwave applications. The dielectric constants and loss tangents of the fabricated, flexible substrates are measured at a frequency range of 3–7 GHz using the DAK 3.5. The computed relative permittivity values are 2.95 for X25, 3.55 for X50, and 4.05 for X75, respectively, while the loss tangents values are 0.00295 for X25, 0.00495 for X50, and 0.00695 for X75. Whereas the permeability and magnetic loss tangent values varied from 0.875 to 1.00 and 0.00225–0.00585, respectively. Finally, the resonators of the absorber consist of a Y-style shape, square-shaped metallic plate resonator, square ring resonator (SRR), and cross-liked shape. The proposed flexible metamaterial absorber (FMA) structure is developed on MgxCo(0.90−x)Zn0.10Fe2O4 nanomaterial-based flexible substrate with dimensions of 0.110λ x 0.110λ x 0.017λ to validate the design experimentally. The electrical dimension is calculated at a 5.5 GHz resonance frequency. The simulated results from the CST Microwave Studio simulator reveal that absorption peaks at 5.5 GHz with a 99.99% absorption that includes the C band frequency that can be used in satellite applications. The proposed FMA structure achieved a 60° incident angle response for TE and TM modes. The structure has superiority with compact dimensions, lightweight, and flexibility and can be used for C band wireless communications.

Design, characterization and fabrication of a flexible broadband metamaterial absorber based on textile

Design, characterization and fabrication of a flexible broadband metamaterial absorber based on textile

Ultra-broadband and flexible metamaterial absorber based on MoS2 cuboids with Mie resonances

Ultra-broadband and flexible metamaterial absorber based on MoS2 cuboids with Mie resonances

A Tunable and Wearable Dual-Band Metamaterial Absorber Based on Polyethylene Terephthalate (PET) Substrate for Sensing Applications.

Advanced wireless communication technology claims miniaturized, reconfigurable, highly efficient, and flexible meta-devices for various applications, including conformal implementation, flexible antennas, wearable sensors, etc. Therefore, bearing these challenges in mind, a dual-band flexible metamaterial absorber (MMA) with frequency-reconfigurable characteristics is developed in this research. The geometry of the proposed MMA comprises a square patch surrounded by a square ring, which is mounted over a copper-backed flexible dielectric substrate. The top surface of the MMA is made of silver nanoparticle ink and a middle polyethylene terephthalate (PET) substrate backed by a copper groundsheet. The proposed MMA shows an absorption rate of above 99% at 24 and 35 GHz. In addition, the absorption features are also studied for different oblique incident angles, and it is found that the proposed MMA remains stable for θ = 10-50°. The frequency tunability characteristics are achieved by stimulating the capacitance of the varactor diode, which connects the inner patch with the outer ring. To justify the robustness and conformability of the presented MMA, the absorption features are also studied by bending the MMA over different radii of an arbitrary cylinder. Moreover, a multiple-reflection interference model is developed to justify the simulated and calculated absorption of the proposed MMA. It is found that the simulated and calculated results are in close agreement with each other. This kind of MMA could be useful for dual-band sensing and filtering operations.

Electromagnetic shielding using flexible embroidery metamaterial absorbers: Design, analysis and experiments

Protecting human body from electromagnetic radiation and enhancing the robustness of wearable devices in complex environments become increasingly important. To address this, a flexible embroidery-based metamaterial absorber (MA) which can absorb electromagnetic waves of specific frequency was proposed in this paper. The MA consists of embroidered frequency selective surface (FSS), scuba knitting fabric and metallized fabric. We firstly propose a simplified simulation model to precisely represent the real-world prototype. Then, the parametric effects on the absorption performance were analyzed using equivalent circuit model and full wave simulation. By comparing the power loss density and the power loss rate of each layer, we discover that the resistive loss of the embroidered FSS plays a dominate role in consuming electromagnetic energy. For in-depth investigations, three prototype samples with high-to-low embroidery densities were fabricated and tested. It was found that the peak absorptivity of second sample (with a medium density of 0.70 mm) can reach up to 99% at 2.39 GHz, which verifies the effectiveness of embroidery-based MAs. Moreover, the comparable absorption performance of diverse prototypes shows that a suitable embroidery density is the premise for effective electromagnetic energy absorption, which provides a measurable design guideline for the future research in this area.

Ultra broadband electromagnetic wave absorbing and scattering properties of flexible sandwich cylindrical water-based metamaterials

Water-based electromagnetic wave absorbers have attracted significant attention due to their potential applications for conformal absorbing structures. In this paper, a flexible broadband electromagnetic wave metamaterial absorber (MMA) based on sandwiched cylindrical water resonators is designed and prepared via the 3D printing, which realizes a low and high frequency cooperative electromagnetic absorption in 4–40 GHz. Additionally, the proposed absorber shows a comprehensive performance with temperature insensitivity and broad absorption characteristics at wide incidence angles. The broadband perfection electromagnetic absorption is discussed based on the improved impedance matching induced by the sandwiched water resonators, and the enhanced electromagnetic wave loss introduced by connected water resonators would further improve the absorption efficiency. Particularly, the radar cross section (RCS) of the designed flexible MMA with different curvature also shows excellent conformal absorption in the evaluated frequency range. Therefore, the proposed flexible ultra-broadband MMA based optimized 3D patterned cylindrical water resonators shows excellent comprehensive performances, which could be widely utilized in electromagnetic protection applications for conformal devices.