A low-profile metamaterial absorber with ultrawideband reflectionless and wide-angular stability

An ultrawideband reflectionless metamaterial absorber(MA)is proposed by replacing the metallic ground with the complementary split-ring resonator(CSRR)structure.The proposed MA exhibits-10 d B reflectivity spectrum from 1 GHz to 20 GHz,which maintains more than 90%absorption from 1.5 GHz to20 GHz.Furthermore,it achieves angle stability for TE and TM polarization at oblique incident angles up to 40°and 65°,respectively.To achieve broadband absorption spectrum,we have adopted a single-layer high-impedance surface(HIS)loaded with a double-layer magnetic material(MM)structure.To further realize the RCS reduction into a lower frequency range,we have employed the scattering cancellation technology into the traditional metallic ground.Finally,we have fabricated a sample exhibiting the 10 d B RCS reduction from 1 GHz to 20 GHz with a thickness of 10 mm.Measurement and simulation results confirm that the proposed MA exhibits excellent comprehensive performance,making it suitable for many practical applications.

A low-frequency pure metal metamaterial absorber with continuously tunable stiffness

To address the incompatibility between high environmental adaptability and deep subwavelength characteristics in conventional local resonance metamaterials,and overcome the deficiencies in the stability of existing active control techniques for band gaps,this paper proposes a design method of pure metal vibration damping metamaterial with continuously tunable stiffness for wideband elastic wave absorption.We design a dual-helix narrow-slit pure metal metamaterial unit,which possesses the triple advantage of high spatial compactness,low stiffness characteristics,and high structural stability,enabling the opening of elastic flexural band gaps in the low-frequency range.Similar to the principle of a sliding rheostat,the introduction of continuously sliding plug-ins into the helical slits enables the continuous variation of the stiffness of the metamaterial unit,achieving a continuously tunable band gap effect.This successfully extends the effective band gap by more than ten times.The experimental results indicate that this metamaterial unit can be used as an additional vibration absorber to absorb the low-frequency vibration energy effectively.Furthermore,it advances the metamaterial absorbers from a purely passive narrowband design to a wideband tunable one.The pure metal double-helix metamaterials retain the subwavelength properties of metamaterials and are suitable for deployment in harsh environments.Simultaneously,by adjusting its stiffness,it substantially broadens the effective band gap range,presenting promising potential applications in various mechanical equipment operating under adverse conditions.

A wideband metamaterial absorber based on a magnetic resonator loaded with lumped resistors

A wideband metamaterial absorber （MA） based on a magnetic resonator loaded with lumped resistors is presented. It is composed of a one-dimensional periodic array of double U-shaped structured magnetic resonators loaded with lumped resistors, a dielectric substrate, and a metal plate. We simulated, fabricated, measured, and analyzed the MA. The experimental results show that the reflectance （S11） is below -10 dB at normal incidence in the frequency range of 7.7 GHz 18 GHz, and the peak value is about -20 dB. Simulated power loss density distributions indicate that wideband absorption of the MA is mainly attributable to the lumped resistors in the magnetic resonator. Further investigations indicate that the distance between two unit cells along the magnetic field direction significantly influences the performance of the MA.

Multiband terahertz metamaterial absorber

This paper reports the design of a multiband metamaterial （MM） absorber in the terahertz region. Theoretical and simulated results show that the absorber has four distinct and strong absorption points at 1.69, 2.76, 3.41 and 5.06 THz, which are consistent with ＇fingerprints＇ of some explosive materials. The retrieved material parameters show that the impedance of MM could be tuned to match approximately the impedance of the free space to minimise the reflectance at absorption frequencies and large power loss exists at absorption frequencies. The distribution of the power loss indicates that the absorber is an excellent electromagnetic wave collector： the wave is first trapped and reinforced in certain specific locations and then consumed. This multiband absorber has applications in the detection of explosives and materials characterisation.

Tunable broadband metamaterial absorber consisting of ferrite slabs and a copper wire

A tunable broadband metamaterial absorber is demonstrated at microwave frequencies in this paper. The meta- material absorber is composed of ferrite slabs with large resonance beamwidths and a copper wire. The theoretical analysis for the effective media parameters is presented to show the mechanism for achieving the perfect absorptivity characteristic. The numerical results of transmission, reflectance, and absorptivity indicate that the metamaterial ab- sorber exhibits a near perfect impedance-match to free space and a high absorptivity of 98.2% for one layer and 99.97% for two layers at 9.9 GHz. The bandwidth with the absorptivity above 90% is about 2.3 GHz. Moreover, the absorption band can be shifted linearly in a wide frequency range by adjusting the magnetic bias. This metamaterial absorber opens a way to prepare perfectly matched layers for engineering applications.

Design of Broadband Metamaterial Absorbers for Permittivity Sensitivity and Solar Cell Application

A broadband and ultra-thin absorber for solar cell application is designed. The absorber consists of three layers, and the difference is that the four split ring resonators made of metal gold are encrusted in the gallium arsenide （GaAs） plane in the top layer. The simulated results show that a perfect absorption in the region from 481.2 to 684.0THz can be obtained for either transverse electric or magnetic polarization wave due to the coupling effect between the material of GaAs and gold. The metamaterial is ultra-thin, having the total thickness of 56nm, which is less than one-tenth resonance wavelength, and the absorption coefficients at the three resonance wavelengths are above 90%. Moreover, the effective medium theory, electric field and surface current distributions are adopted to explain the physical mechanism of the absorption, and the permittivity sensing applications are also discussed. As a result, the proposed structure can be used in many areas, such as solar cell, sensors, and integrated photodetectors.

An ultrathin wide-band planar metamaterial absorber based on a fractal frequency selective surface and resistive film

We propose an ultrathin wide-band metamaterial absorber （MA） based on a Minkowski （MIK） fractal frequency selective surface and resistive film. This absorber consists of a periodic arrangement of dielectric substrates sandwiched with an MIK fractal loop structure electric resonator and a resistive film. The finite element method is used to simulate and analyze the absorption of the MA. Compared with the MA-backed copper film, the designed MA-backed resistive film exhibits an absorption of 90% at a frequency region of 2 GHz-20 GHz. The power loss density distribution of the MA is further illustrated to explain the mechanism of the proposed MA. Simulated absorptions at different incidence cases indicate that this absorber is polarization-insensitive and wide-angled. Finally, further simulated results indicate that the surface resistance of the resistive film and the dielectric constant of the substrate can affect the absorbing property of the MA. This absorber may be used in many military fields.

A metamaterial absorber with direction-selective and polarisation-insensitive properties

This paper reports the design of a metamaterial absorber with direction-selective and polarisation-insensitive property. Both theoretical and simulated results reveal that the absorber has a distinct absorption point with direction selectivity at 7.48 GHz, which is related to the resonance of the metamaterial and is not influenced by the polarisation. The retrieved impedance indicates that the impedance of the absorber can be tuned to approximatively match the impedance of the free space on one side and not to match the impedance of the free space on the other side. This design can result in the minimal reflectance, the minimal transmission and the highest absorbance at the absorption frequency. The distribution of the power loss indicates that the absorber is an excellent electromagnetic wave collector： the wave is first trapped and reinforced in certain specific locations, and then mostly consumed. The distribution of the surface current is consistent with the design, the retrieved impedance and the distribution of the power loss. This absorber may have applications in many scientific and technological areas.

Multi-band microwave metamaterial absorber based on coplanar Jerusalem crosses

The influence of the gap on the absorption performance of the conventional split ring resonator(SRR) absorber is investigated at microwave frequencies. Our simulated results reveal that the geometry of the square SRR can be equivalent to a Jerusalem cross(JC) resonator and its corresponding metamaterial absorber(MA) is changed to a JC absorber. The JC MA exhibits an experimental absorption peak of 99.1% at 8.72 GHz, which shows an excellent agreement with our simulated results. By simply assembling several JCs with slightly different geometric parameters next to each other into a unit cell, a perfect multi-band absorption can be effectively obtained. The experimental results show that the MA has four distinct and strong absorption peaks at 8.32 GHz, 9.8 GHz, 11.52 GHz and 13.24 GHz. Finally, the multi-reflection interference theory is introduced to interpret the absorption mechanism.

A Polarization-Insensitive Broadband Metamaterial Absorber at the Optical Regime

We present a polarization-insensitive broadband absorber which has a feature of metal-insulator-metal structures. The top metal layer consists of four-fan-rings-shaped gold. Simulations show that the absorber exhibits an absorption of nearly unity at the wavelength of 386.1 nm and a relative absorption bandwidth of 0.548, which refers to the ratio of the full absorption bandwidth over an absorption of 0.9 to the central wavelength. Meanwhile, the absorption is nearly independent of the polarized direction of the incident wave. This absorption bandwidth with insensitive polarization is widely reported to date for such metal-insulator-metal structures. Such a structure offers a way of realization of a polarization-insensitive broadband absorber ranging in ultraviolet-to-visible wavelengths.

Graphene-Based Tunable Polarization Insensitive Dual-Band Metamaterial Absorber at Mid-Infrared Frequencies

A graphene-based tunable dual-band metamaterial absorber which is polarization insensitive is numerically pro- posed at mid-infrared frequencies. In numerical simulation the metamaterial absorber exhibits two absorption peaks at the resonance wavelengths of 6.246 μm and 6.837μm when the Fermi level of graphene is fixed at 0. 6 eV. Absorption spectra at different Fermi levels of graphene are displayed and tuning functions are discussed in detail. Both the resonance wavelengths of the absorber blue shift with the increase in Fermi level of graphene. Moreover, the surface current distributions on the gold resonator and ground plane at the two resonance wavelengths are simulated to deeply understand the physical mechanism of resonance absorption.

Bridging the terahertz near-field and far-field observations of liquid crystal based metamaterial absorbers

Metamaterial-based absorbers play a significant role in applications ranging from energy harvesting and thermal emitters to sensors and imaging devices.The middle dielectric layer of conventional metamaterial absorbers has always been solid.Researchers could not detect the near field distribution in this layer or utilize it effectively.Here,we use anisotropic liquid crystal as the dielectric layer to realize electrically fast tunable terahertz metamaterial absorbers.We demonstrate strong,position-dependent terahertz near-field enhancement with sub-wavelength resolution inside the metamaterial absorber.We measure the terahertz far-field absorption as the driving voltage increases.By combining experimental results with liquid crystal simulations,we verify the near-field distribution in the middle layer indirectly and bridge the nearfield and far-field observations.Our work opens new opportunities for creating high-performance,fast,tunable,terahertz metamaterial devices that can be applied in biological imaging and sensing.

A flexible ultra-broadband metamaterial absorber working on whole K-bands with polarization-insensitive and wide-angle stability

For potential military applications, a flexible metamaterial absorber(MMA) working on whole K-bands with totalthickness of 3.367 mm, ultra-broadband, polarization-insensitive, and wide-angle stability is presented based on frequencyselective surface(FSS). The absorber is composed of polyvinyl chloride(PVC) layer, polyimide(PI) layer, and poly tetra fluoro ethylene(PTFE) layer, with a sandwich structure of PVC–PI–PTFE–metal plate. Periodic conductive patterns play a crucial role in the absorber, and in traditional, it is designed on the upper surface of PI layer to form LC resonance. Different from commonly absorber, all the patterns are located on the lower surface of the PI layer in this work, and hence the impedance matching and absorptivity are improved in this purposed absorber. The flexible absorber with patterns on lower surface of the PI layer is compared with that on upper surface of the PI layer, the difference and the reasons are explained by absorption mechanism based on equivalent circuit model, and surface current density and electric field distribution are used to analyze resonance peaks. Absorptivity is greater than 90% in a frequency range of 10.47 GHz–45.44 GHz with relative bandwidth of 125.1%, covering the whole Ku, K, Ka, and some of X, U bands, especially containing the whole K bands from 12 GHz to 40 GHz. Radar cross section(RCS) is reduced at least 10 dB in 11.48 GHz–43.87 GHz frequency ranges,and absorption remained about 90% when the incident angle changed from 0°to 55°. The purposed absorber is fabricated,measured, and experiment results show good agreement with theoretical analysis and numerical simulation. After bonded on outer surface of different cylinders with diameters of 200 mm and 100 mm, the absorption of MMA is approximately reduced 10% and 20% respectively, which shows good conformal character with surface of various curvatures. Due to the attractive performance on strong absorption in the whole K-bands, flexible and easy conformal, our design exhibits broad potential application in radar stealth and sensors.

Computational Investigation of Multiband EMNZ Metamaterial Absorber for Terahertz Applications

This study presents an Epsilon Mu near-zero(EMNZ)nanostructured metamaterial absorber(NMMA)for visible regime applications.The resonator and dielectric layers are made of tungsten(W)and quartz(fused),where the working band is expanded by changing the resonator layer’s design.Due to perfect impedance matching with plasmonic resonance characteristics,the proposed NMMA structure is achieved an excellent absorption of 99.99%at 571 THz,99.50%at 488.26 THz,and 99.32%at 598 THz frequencies.The absorption mechanism is demonstrated by the theory of impedance,electric field,and power loss density distributions,respectively.The geometric parameters are explored and analyzed to show the structure’s performance,and a near-field pattern is used to explain the absorption mechanism at the resonance frequency point.The numerical analysis method describes that the proposed structure exhibited more than 80%absorbability between 550 and 900 THz.The Computer Simulation Technology(CST Microwave Studio 2019)software is used to design the proposed structure.Furthermore,CSTHFSS interference is validated by the simulation data with the help of the finite element method(FEM).The proposed NMMA structure is also exhibits glucose concentration sensing capability as applications.So the proposed broadband absorber may have a potential application in THz sensing,imaging(MRI,thermal,color),solar energy harvesting,light modulators,and optoelectronic devices.

Retrieval of the effective constitutive parameters from metamaterial absorbers

The equivalent medium theory of metamaterials provides a way to obtain their effective constitutive parameters.However,because of its non-reciprocity,the complexity of the electromagnetic coupling,and a metallic bottom layer,it has been challenging to retrieve them from a metamaterial absorber.In this paper,we propose a method without any approximation to obtain them,in which the non-reciprocity and the strong electromagnetic coupling are included.Compared with the three methods such as symmetric metamaterial method,asymmetric metamaterial method and metasurface method,our method can reveal the metamaterial absorber’s electrical and magnetic resonance and show its electromagnetic coupling coefficients.To deal with a metamaterial absorber with a metallic bottom layer,four corners of the metallic bottom layer in the unit cell are removed,making it possible to retrieve the electromagnetic parameters.Surprisingly,these results show that the metamaterial absorber with a metallic bottom layer in our example operates in a negative refraction state at the half absorption frequencies,which helps further understand the absorbing mechanism of these metamaterial absorbers.

A Wide-Spaced Dual-Band Metamaterial Absorber Based on 2.5D Frequency Selective Surfaces and Magnetic Material

By applying meander-line for electrical loss and magnetic material for magnetic loss,we present a metamaterial absorber which is wide-spaced and dual-band(1.35—2.24 GHz and 10.37—12.37 GHz).The novelty of this study mainly lies in a combination of two kinds of losses to consume electromagnetic energy,which can get better dual-band absorption.In the electrical loss layer,meander-line structures are printed on both surfaces of the substrate and the structure series with resistors.Considering the need for miniaturization,we connect eight metallic vias with these meander-line areas to form a compact 2.5-dimensional(2.5D)structure.The dimension of the unit cell is miniaturized to be 5.94 mm×5.94 mm,about 0.035λat the center frequency of the lower absorption band.In the magnetic loss layer,the 0.4 mm thick magnetic material is employed on a metallic ground plane.In addition,the complex permittivity and complex permeability of the magnetic material are given.Finally,we fabricate a prototype of the proposed absorber and obtain a measurement result which is in good agreement with the full-wave simulation result.

Recent Progress in Chiral Absorptive Metamaterials

Chiral metamaterial absorbers(CMMAs),a particular class of chiral metamaterials that refuse the transmission of incident radiation and exhibit different optical responses upon interactions with left and right circularly polarized(RCP)light,have gained research traction in recent years.CMMAs demonstrate numerous exotic and specialized applications owing to their achievable compatibility with various physical,chemical,and biomolecular systems.Aside from their well-evolved fabrication modalities for a broad range of frequencies,CMMAs exhibit strong chiroptical effects,making them central to various detection,imaging,and energy harvesting applications.Consequently,within the past decade,studies encompassing the design,optimization,and fabrication,as well as demonstrating the diverse applications of CMMAs have emerged.In this review,the theory,design,and fabrication of CMMAs are discussed,highlighting their top-down fabrication techniques as well as recent algorithmic and machine-learning(ML)-based approaches to the design and optimization.Some of their broad-spectrum applications are also discussed,spanning their roles in enantioselective photodetection,chiral imaging,generation of hot electrons,selective temperature sensing,and active chiral plasmonics.

Design of a varactor-tunable metamaterial absorber

In this paper, we design a varactor-tunable metamaterial absorber （MA）. The tunable MA is based on a mushroom-type high impedance surface （HIS）, in which varactors are loaded between adjacent metal patches to adjust the capacitance and tune the resonance frequency, the primary ground plane is etched as the bias network to bias all of the varactors in parallel, and another ultra-thin grounded film is attached to the bottom. Its absorption characteristics are realized for electrically dielectric loss. The simulated values of a sample indicate that a tunable frequency range from 2.85 GHz to 2.22 GHz is achieved by adjusting the varactor capacitance from 0.1 pF to 2.0 pF, and better than 0.97 absorbance is realized; in addition, the tunable frequency range is expanded from 4.12 GHz to 1.70 GHz after optimization.

A pure dielectric metamaterial absorber with broadband and thin thickness based on a cross-hole array structure

A pure dielectric metamaterial absorber with broadband and thin thickness is proposed,whose structure is designed as a periodic cross-hole array.The pure dielectric metamaterial absorber with high permittivity is prepared by ceramic reinforced polymer composites.Compared with those with low permittivity,the absorber with high permittivity is more sensitive to structural parameters,which means that it is easier to optimize the equivalent electromagnetic parameters and achieve wide impedance matching by altering the size or shape of the unit cell.The optimized metamaterial absorber exhibits reflection loss below-10 dB in 7.93 GHz–35.76 GHz with a thickness of 3.5 mm,which shows favorable absorption properties under the oblique incidence of TE polarization(±45°).Whether it is a measured or simulated value,the strongest absorbing peak reaches below-45 dB,which exceeds that of most metamaterial absorbers.The distributions of power loss density and electric and magnetic fields are investigated to study the origin of their strong absorbing properties.Multiple resonance mechanisms are proposed to explain the phenomenon,including polarization relaxation of the dielectric and edge effects of the cross-hole array.This work overcomes the shortcomings of the narrow absorbing bandwidth of dielectrics.It demonstrates that the pure dielectric metamaterial absorber with high permittivity has great potential in the field of microwave absorption.

Low-Cost Metamaterial Loaded Microstrip Antenna for Defense Applications

We have designed a Metamaterial unit-cell for 9 GHz frequency. Periodic structure was used at 4.25 × 4.25 mm with a thickness of 0.35 mm and giving us the 99.99% of absorbance at 9 GHz in simulated results. We have implemented a rectangular microstrip antenna and loaded it with Metamaterial unit-cells which provided improved results. There were results available for reflection coefficient (s11 parameter) at 9 GHz and also helping for the reduction of the Radar Cross Section of an antenna, which reduced more than 20 dB and not affected its directivity and gain.