Metamaterial Absorbers: From Tunable Surface to Structural Transformation

MXene-Enabled Pneumatic Multiscale Shape Morphing for Adaptive, Programmable and Multimodal Radar-infrared Compatible Camouflage

Achieving radar-infrared compatible camouflage with dynamic adaptability has been a long-sought goal, but faces significant challenges owing to the limited dispersion relations of conventional material systems operating in different wavelength ranges. Here, this work proposes the concept of pneumatic multiscale shape morphing and design a periodically arranged pneumatic unit consisting of MXene-based morphable conductors and intake platforms. During gas actuation, the morphable conductor transforms centimeter-scale 2D flat sheets into 3D balloon shapes to enhance microwave absorption behavior, and also reconfigures micrometer-scale MXene wrinkles into smooth planes in combination with cavity-induced low heat transfer to minimize infrared (IR) signatures. Through theory-guided reverse engineering, the final pneumatic matrix shows remarkable frequency tunability (2.64-18.0 GHz), moderate IR emissivity regulation (0.14 at 7-16.5 µm), rapid responsiveness (≈30 ms), wide-angle operation (>45°), and excellent environmental tolerance. Additionally, the multiplexed pneumatic matrix enables over 14 programmable coding sequences that independently alter thermal radiation without compromising radar stealth, and allows multimodal camouflage switching between three distinct compatible states. The approach may facilitate the evolution of camouflage techniques and electromagnetic functional materials toward multispectral, adaptability and intelligence.

Strongly plasticized gelatin-based hydrogel for flexible encapsulation of complex-shaped electronic devices

The growth of environmentally sensitive complex-shaped electronic devices (ECEDs) has led to a surging demand for flexible electromagnetic wave (EMW) absorbers. Herein, the water loss property of hydrogel was ingeniously applied for the flexible encapsulation (FE) of ECEDs. To be specific, saturated state (SGT) hydrogels were prepared by chemical cross-linking, and the hydrogen bond dissipation network promoted FE. Additionally, SGT has an effective absorption bandwidth (EAB) of 6.04 GHz at 1.65 mm due to the presence of dipole polarization. With the loss of water, SGT transitions to its natural state (NGT), and the decreasing conductivity leads to better impedance matching. NGT exhibited a broader EAB (9.20 GHz at 2.65 mm) and also strength and lightness (density of 0.3 g cm-3). Furthermore, the semi-automatic reversible cyclic transformation between SGT and NGT gels further broadens application scenarios. GT gel combines self-encapsulation and self-optimized performance as a potential EMW absorber for FE.

Multifunctional integrated metamaterials for radar-infrared-visible compatible multispectral stealth

Metamaterials offer exciting opportunities for developing multispectral stealth due to their unique electromagnetic properties. However, currently transparent radar-infrared-visible compatible stealth metamaterials typically involve complex hierarchical designs, leading to thickness and transparency limitations. Here, we propose an integrated metamaterial for multispectral stealth with high transparency. Our design features an ITO/dielectric/ITO sandwich structure, with the upper-layer ITO acting as a resonator for broadband microwave absorption while maintaining a high filling ratio to suppress infrared (IR) radiation. Experimental results demonstrate excellent performance, with over 90% microwave absorption in 8-18 GHz, an IR emissivity of approximately 0.36 in 3-14 µm, an average optical transmittance of 74.1% in 380-800 nm, and a thickness of only 2.4 mm. With its multispectral compatibility, the proposed metamaterial has potential applications in stealth and camouflage fields.

Structural Engineering of Hierarchical Magnetic/Carbon Nanocomposites via In Situ Growth for High-Efficient Electromagnetic Wave Absorption

Materials exhibiting high-performance electromagnetic wave absorption have garnered considerable scientific and technological attention, yet encounter significant challenges. Developing new materials and innovative structural design concepts is crucial for expanding the application field of electromagnetic wave absorption. Particularly, hierarchical structure engineering has emerged as a promising approach to enhance the physical and chemical properties of materials, providing immense potential for creating versatile electromagnetic wave absorption materials. Herein, an exceptional multi-dimensional hierarchical structure was meticulously devised, unleashing the full microwave attenuation capabilities through in situ growth, self-reduction, and multi-heterogeneous interface integration. The hierarchical structure features a three-dimensional carbon framework, where magnetic nanoparticles grow in situ on the carbon skeleton, creating a necklace-like structure. Furthermore, magnetic nanosheets assemble within this framework. Enhanced impedance matching was achieved by precisely adjusting component proportions, and intelligent integration of diverse interfaces bolstered dielectric polarization. The obtain Fe3O4-Fe nanoparticles/carbon nanofibers/Al-Fe3O4-Fe nanosheets composites demonstrated outstanding performance with a minimum reflection loss (RLmin) value of - 59.3 dB and an effective absorption bandwidth (RL ≤  - 10 dB) extending up to 5.6 GHz at 2.2 mm. These notable accomplishments offer fresh insights into the precision design of high-efficient electromagnetic wave absorption materials.

A Polarization-Insensitive, Vanadium Dioxide-Based Dynamically Tunable Multiband Terahertz Metamaterial Absorber

A tunable multiband terahertz metamaterial absorber, based on vanadium dioxide (VO2), is demonstrated. The absorber comprises a three-layer metal-insulator-metal (MIM) configuration with a split ring and slots of VO2 on the uppermost layer, a middle dielectric substrate based on silicon dioxide (SiO2), and a gold reflector on the back. The simulation results indicate that, when VO2 is in the metallic state, the proposed metamaterial exhibits nearly perfect absorption at six distinct frequencies. The design achieves an average absorption of 98.2%. The absorptivity of the metamaterial can be dynamically tuned from 4% to 100% by varying the temperature-controlled conductivity of VO2. The proposed metamaterial absorber exhibits the advantages of polarization insensitivity and maintains its absorption over 80% under different incident angle conditions. The underlying physical mechanism of absorption is explained through impedance matching theory, interference theory, and the distribution of electric fields. The ability to achieve multiband absorption with tunable characteristics makes the proposed absorber a promising candidate for applications in terahertz sensing, imaging, communication, and detection. The polarization insensitivity further enhances its practicality in various scenarios, allowing for versatile and reliable performance in terahertz systems.

Microwave-Absorbing Foams with Adjustable Absorption Frequency and Structural Coloration

Microwave-absorbing materials with regulatable absorption frequency and optical camouflage hold great significance in intelligent electronic devices and advanced stealth technology. Herein, we present an innovative microwave-absorbing foam that can dynamically tune microwave absorption frequencies via a simple mechanical compression while in parallel enabling optical camouflage over broad spectral ranges by adjusting the structural colors. The vivid colors spanning different color categories generated from thin-film interference can be precisely regulated by adjusting the thickness of the conformal TiO2 coatings on Ni/melamine foam. Enhanced interfacial and defect-induced polarizations resulting from the introduction of TiO2 coating synergistically contribute to the dielectric attenuation performance. Consequently, such a foam exhibits exceptional microwave absorption capabilities, and the absorption frequency can be dynamically tuned from the S band to the Ku band by manipulating its compression ratio. Additionally, simulation calculations validate the adjustable electromagnetic wave loss behavior, offering valuable insights for the development of next-generation intelligent electromagnetic devices across diverse fields.

Tunable Microwave Absorbing Devices Enabled by Reversible Metal Electrodeposition

Tunable microwave absorbers have gained significant interest due to their capability to actively control microwaves. However, the existing architecture-change-based approach lacks flexibility, and the active-element-based approach is constrained by a narrowband operation or small dynamic modulation range. Here, a novel electrically tunable microwave absorbing device (TMAD) is demonstrated that can achieve dynamic tuning of the average reflection amplitude between -13.0 and -1.2 dB over a broadband range of 8-18 GHz enabled by reversible metal electrodeposition. This reversible tunability is achieved by electrodepositing silver (Ag) layers with controlled morphology on nanoscopic platinum (Pt) films in a device structure similar to a tunable Salisbury screen, employing Ag electrodeposited on Pt films as the modifiable resistive layer. Furthermore, this TMAD possesses a simple device architecture, excellent bistability, and multispectral compatibility. Our approach offers a new strategy for dynamically manipulating microwaves, which has potential utility in intelligent camouflage and communication systems.

Ultra-Thin and Broadband P-Band Metamaterial Absorber Based on Carbonyl Iron Powder Composites

The field of P-band (0.3-1 GHz) absorption has witnessed rapid development in metamaterial absorbers due to their exceptional designability and the absence of restrictions imposed by the one-fourth wavelength rule. In this study, we combined carbonyl iron powder (CIP) composites with a periodic structure composed of metal capacitive patterns and employed a genetic algorithm (GA) to optimize the electromagnetic parameters of the CIP substrate. By selecting the appropriate shape and material for the units of pattern based on transmission line theory, as well as regulating relevant structural parameters, we successfully designed an ultra-thin broadband metamaterial absorber for the P-band. Experimental results demonstrate that within the range of 0.3-0.85 GHz, the reflection loss of our absorber remains below -5 dB, with a maximum value of -9.54 dB occurring at 0.45 GHz. Remarkably, this absorber possesses a thickness equivalent to only 1/293 of its working wavelength. Then, we conducted analyses on electric field distribution, magnetic field distribution, and energy loss density. Our findings suggest that high-performance absorption in metamaterials can be attributed to λ/4 resonant or coupling effects between structural units or diffraction phenomena. This absorber offers several advantages, including broad low-frequency absorption capability, ultra-thin profile, and convenient fabrication process, thus providing valuable theoretical insights for designing metamaterial structures.

An Integrated Approach for Piezo-Electrochemical Nanoenergy Generation, Storage, and Real-Time Electromagnetic Interference Shielding Control

The extensive utilization of high-end wireless electronic equipment in medical, robotics, satellite, and military communications has created a pressing challenge for real-time electromagnetic interference (EMI) control. Herein, a piezo-powered self-chargeable supercapacitor (PPSC) architecture based on an iron-doped graphitic nitride (Fe-g-C3N4: FGN) electrode with a solid piezoelectrolyte is devised, which can provide real-time controlled EMI shielding through piezo-powered self-charging voltage (SCV). This PPSC device along with real-time SCV-controlled EMI shielding also integrates additional features like nanoenergy generation and storing capability. The results demonstrate that the PPSC device is capable of exhibiting a piezo-tuned self-charging ability of up to 669.2 mV under 9.47 N of dynamic pressing for 180 s. The SCV electrostatically modifies the PPSC device that causes destructive interference and governs the absorption of electromagnetic radiation (EMR) and controls the absorption-dominated EMI shielding up to 59.2 dB at 500 mV. Additionally, the SCV-led electrification of the PPSC device also controls a unique functional transition from the EMR reflector to the EMR absorber at ∼90 mV. Hence, this strategy of tailored absorption and reflection adjustments of EMR could also potentially contribute toward the advancement of stealth technology for military armaments with externally controlled stealth capabilities.

High-Precision Printing of Flexible MXene Patterns for Dynamically Tunable Electromagnetic Interference Shielding Performance

Smart electromagnetic interference (EMI) shielding materials are of great significance in coping with the dynamic performance demands of cutting-edge electronic devices. However, smart EMI shielding materials are still in their infancy and face a variety of challenges (e.g., large thickness, limited tunable range, poor reversibility, and unclear mechanisms). Here, we report a method for controllable shielding electromagnetic (EM) waves through subwavelength structure changes regulated by the customized structure via a direct printing route. The highly conductive MXene ink is regulated with metal ions (Al3+ ions), giving superb metallic conductivity (∼5000 S cm-1) for the printed lines without an annealing treatment. The reversible tunability of EMI shielding effectiveness (SE) ranging from 8.2 dB ("off" state) to 34 dB ("on" state) is realized through the controllable modulation of subwavelength structure driven by stress. This work provides a feasible strategy to develop intelligent shielding materials and EM devices.

Transparent and Ultra-Thin Flexible Checkerboard Metasurface for Radar-Infrared Bi-Stealth

This paper proposed a single-layer checkerboard metasurface with simultaneous wideband radar cross-section (RCS) reduction characteristics and low infrared (IR) emissivity. The metasurface consists of an indium tin oxide (ITO)-patterned film, a polyethylene terephthalate (PET) substrate and an ITO backplane from the top downwards, with a total ultra-thin thickness of 1.6 mm. This design also allows the metasurface to have good optical transparency and flexibility. Based on phase cancellation and absorption, the metasurface can achieve a wideband RCS reduction of 10 dB from 10.6 to 19.4 GHz under normal incidence. When the metasurface is slightly cylindrically curved, an RCS reduction of approximately 10 dB can still be achieved from 11 to 19 GHz. The polarization and angular stability of the metasurface have also been verified. The filling rate of the top ITO-patterned film is 0.81, which makes the metasurface have a low theoretical IR emissivity of 0.24. Both simulation and experimental results have verified the excellent characteristics of the proposed checkerboard metasurface, demonstrating its great potential application in radar-IR bi-stealth.

Chimera metasurface for multiterrain invisibility

Invisibility, a fascinating ability of hiding objects within environments, has attracted broad interest for a long time. However, current invisibility technologies are still restricted to stationary environments and narrow band. Here, we experimentally demonstrate a Chimera metasurface for multiterrain invisibility by synthesizing the natural camouflage traits of various poikilotherms. The metasurface achieves chameleon-like broadband in situ tunable microwave reflection mimicry of realistic water surface, shoal, beach/desert, grassland, and frozen ground from 8 to 12 GHz freely via the circuit-topology-transited mode evolution, while remaining optically transparent as an invisible glass frog. Additionally, the mechanic-driven Chimera metasurface without active electrothermal effect, owning a bearded dragon-like thermal acclimation, can decrease the maximum thermal imaging difference to 3.1 °C in tested realistic terrains, which cannot be recognized by human eyes. Our work transitions camouflage technologies from the constrained scenario to ever-changing terrains and constitutes a big advance toward the new-generation reconfigurable electromagnetics with circuit-topology dynamics.

Multifunctional Graphene Metasurface for Highly Flexible Control of Microwave Absorption

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.

Bioinspired Double-Broadband Switchable Microwave Absorbing Grid Structures with Inflatable Kresling Origami Actuators

Tunable radar stealth structures are critical components for future military equipment because of their potential to further enhance the design space and performance. Some previous investigations have utilized simple origami structures as the basic adjusting components but failed to achieve the desired broadband microwave absorbing characteristic. Herein, a novel double-broadband switchable microwave absorbing grid structure has been developed with the actuators of inflatable Kresling origami structures. Geometric constraints are derived to endow a bistable feature with this origami configuration, and the stable states are switched by adjusting the internal pressure. An ultra-broadband microwave absorbing structure is proposed with a couple of complementary microwave stealth bands, and optimized by a particle swarm optimization algorithm. The superior electromagnetic performance results from the mode switch activating different absorbing components at corresponding frequencies. A digital adjusting strategy is applied, which effectively achieves a continuously adjusting effect. Further investigations show that the proposed structure possesses superior robustness. In addition, minimal interactions are found between adjacent grid units, and the electromagnetic performance is mainly related to the duty ratio of the units in different states. They have enhanced the microwave absorbing performance of grid structures through a tunable design, a provided a feasible paradigm for other tunable absorbers.

A tunable broadband terahertz MoS2 absorber using series-parallel hybrid network design

A method for designing a broadband absorber using a series-parallel hybrid network is proposed. The performance of the broadband absorber is improved by using frequency-selective surface patterns based on a series-parallel hybrid equivalent circuit. The results indicate that the tunable single-layered terahertz MoS2 absorber has excellent broadband characteristics. Between 0.84 and 2.34 THz, the absorption and relative absorption bandwidth exceed 90% and 94.3%, respectively. Also, the absorption level can be adjusted from 90% to 10% by applying a bias voltage on the electrodes. The effects of different types of MoS2 layers and surface fluctuations in monolayered MoS2 on the properties of the absorber are demonstrated. In the 60° (TM) and 50° (TE) ranges, the polarization of the terahertz absorber is insensitive to the incidence angle. Overall, this method enables the single-layered absorber to exhibit excellent broadband characteristics comparable to those of multilayered structures, as well as simplifies the structure. Consequently, this method significantly broadens the usefulness of tunable single-layered absorbers for radar stealth, terahertz imaging, and electrically tunable modulation.

An ultra-broadband frequency-agile terahertz perfect absorber with perturbed MoS2 plasmon modes

Multidomain dynamic manipulations for terahertz (THz) absorbers usually necessitate the orchestrated actions of several active elements, inevitably complicating the structural design and elongating the modulation time. Herein, we utilize the coupling between the total reflection prism and electrically-driven MoS2 to activate a tight field confinement in a deep-subwavelength interlayer, ultimately achieving frequency-agile absorption adjustments only with a gate voltage. Theoretical and simulation analysis results indicate that the redistributed electric field and susceptible dielectric response are attributed to the limited spatial near-field perturbation of surface plasmon resonances. We also demonstrate that perturbed MoS2 plasmon modes promote the formation of dual-phase singularities, significantly suppressing the attenuation of the absorption amplitude as large-scale frequency shifts, thereby extending the relative tuning range (WRTR) to 175.4%. These findings offer an efficient approach for expanding the horizon of THz absorption applications that require ultra-broadband and swift-response capabilities.

Microwave absorbing characteristics of porphyrin derivates: a loop of conjugated structure

Microwave absorbing architectures have gained a great deal of attention due to their widespread application in diverse fields, especially in refining electromagnetic pollution. The aim of this study is to investigate the metamaterial characteristics of porphyrin derivatives as conjugated rings in the microwave region and evaluate the influence of electron-withdrawing and donating groups on microwave attenuating performance. Initially, an innovative microwave curing procedure was applied to synthesize the derivates; following that, the phenyl, aniline, and nitrophenyl-coupled structures were identified by XRD, FTIR, FESEM, and DRS analyses. The optical features illustrated that the characteristic band gap of the conjugated loops is obtained and that the optical performance can be manipulated by coupling the functional groups. Eventually, the achieved results demonstrated that the best microwave absorbing performance is related to aniline-coupled porphyrin with a maximum reflection loss (RL) value of -104.93 dB at 10.09 GHz with 2.80 mm in thickness attaining an efficient bandwidth (EB) (RL ≤ 10 dB) higher than the X-band. Noticeably, polyethylene (PE) was applied as an absorbing matrix presenting a meaningful idea for the development of practical microwave absorbers as a new generation of electromagnetic refining and stealth materials. The presented research provides precious inspiration to tailor novel microwave absorbing materials with metamaterial capability to promote their microwave absorbing performance.

Broadband Solar Metamaterial Absorbers Empowered by Transformer-Based Deep Learning

The research of metamaterial shows great potential in the field of solar energy harvesting. In the past decade, the design of broadband solar metamaterial absorber (SMA) has attracted a surge of interest. The conventional design typically requires brute-force optimizations with a huge sampling space of structure parameters. Very recently, deep learning (DL) has provided a promising way in metamaterial design, but its application on SMA development is barely reported due to the complicated features of broadband spectrum. Here, this work develops the DL model based on metamaterial spectrum transformer (MST) for the powerful design of high-performance SMAs. The MST divides the optical spectrum of metamaterial into N patches, which overcomes the severe problem of overfitting in traditional DL and boosts the learning capability significantly. A flexible design tool based on free customer definition is developed to facilitate the real-time on-demand design of metamaterials with various optical functions. The scheme is applied to the design and fabrication of SMAs with graded-refractive-index nanostructures. They demonstrate the high average absorptance of 94% in a broad solar spectrum and exhibit exceptional advantages over many state-of-the-art counterparts. The outdoor testing implies the high-efficiency energy collection of about 1061 kW h m^-2 from solar radiation annually. This work paves a way for the rapid smart design of SMA, and will also provide a real-time developing tool for many other metamaterials and metadevices.

Multi-Scale Design of Ultra-Broadband Microwave Metamaterial Absorber Based on Hollow Carbon/MXene/Mo2 C Microtube

Developing various nanocomposite microwave absorbers is a crucial means to address the issue of electromagnetic pollution, but remains a challenge in satisfying broadband absorption at low thickness with dielectric loss materials. Herein, an ultra-broadband microwave metamaterial absorber (MMA) based on hollow carbon/MXene/Mo₂ C (HCMM) is fabricated by a multi-scale design strategy. The microscopic 1D hierarchical microtube structure of HCMM contributes to break through the limit of thickness, exhibiting a strong reflection loss of -66.30 dB (99.99997 wave absorption) at the thinnest matching thickness of 1.0 mm. Meanwhile, the strongest reflection loss of -87.28 dB is reached at 1.4 mm, superior to most MXene-based and Mo₂ C-based microwave absorbers. Then, the macroscopic 3D structural metasurface based on the HCMM is simulated, optimized, and finally manufactured. The as-prepared flexible HCMM-based MMA realizes an ultra-broadband effective absorption in the range of 3.7-40.0 GHz at a thickness of 5.0 mm, revealing its potential for practical application in the electromagnetic compatibility field.

Ultra-Broadband Perfect Absorber based on Titanium Nanoarrays for Harvesting Solar Energy

Solar energy is a clean and renewable energy source and solves today's energy and climate emergency. Near-perfect broadband solar absorbers can offer necessary technical assistance to follow this route and develop an effective solar energy-harvesting system. In this work, the metamaterial perfect absorber operating in the ultraviolet to the near-infrared spectral range was designed, consisting of a periodically aligned titanium (Ti) nanoarray coupled to an optical cavity. Through numerical simulations, the average absorption efficiency of the optimal parameter absorber can reach up to 99.84% in the 200-3000 nm broadband range. We show that the Ti pyramid's localized surface plasmon resonances, the intrinsic loss of the Ti material, and the coupling of resonance modes between two neighboring pyramids are highly responsible for this broadband perfect absorption effect. Additionally, we demonstrate that the absorber exhibits some excellent features desirable for the practical absorption and harvesting of solar energy, such as precision tolerance, polarization independence, and large angular acceptance.

Hydro/Organo/Ionogels: "Controllable" Electromagnetic Wave Absorbers

Demand for electromagnetic wave (EMW) absorbers continues to increase with technological advances in wearable electronics and military applications. In this study, a new strategy to overcome the drawbacks of current absorbers by employing the co-contribution of functional polymer frameworks and liquids with strong EMW absorption properties is proposed. Strongly polar water, dimethyl sulfoxide/water mixtures, and highly conductive 1-ethyl-3-methylimidazolium ethyl sulfate ([EMI][ES]) are immobilized in dielectrically inert polymer networks to form different classes of gels (hydrogels, organogels, and ionogels). These gels demonstrate a high correlation between their dielectric properties and polarity/ionic conductivity/non-covalent interaction of immobilized liquids. Thus, the EMW absorption performances of the gels can be precisely tuned over a wide range due to the diversity and stability of the liquids. The prepared hydrogels show good shielding performance (shielding efficiency > 20 dB) due to the high dielectric constants, while organogels with moderate attenuation ability and impedance matching achieve full-wave absorption in X-band (8.2-12.4 GHz) at 2.5 ± 0.5 mm. The ionogels also offer a wide effective absorption bandwidth (10.79-16.38 GHz at 2.2 mm) via prominent ionic conduction loss. In short, this work provides a conceptually novel platform to develop high-efficient, customizable, and low-cost functional absorbers.