Metamaterial Absorber for Electromagnetic Waves in Periodic Water Droplets

Comparative Studies of the Dielectric Properties of Polyester Imide Composite Membranes Containing Hydrophilic and Hydrophobic Mesoporous Silica Particles

In this paper, comparative studies of hydrophilic and hydrophobic mesoporous silica particles (MSPs) on the dielectric properties of their derivative polyester imide (PEI) composite membranes were investigated. A series of hydrophilic and hydrophobic MSPs were synthesized with the base-catalyzed sol-gel process of TEOS, MTMS, and APTES at a distinctive feeding ratio with a non-surfactant template of D-(-)-Fructose as the pore-forming agent. Subsequently, the MSPs were blended with the diamine of APAB, followed by introducing the dianhydride of TAHQ with mechanical stirring for 24 h. The obtained viscous solution was subsequently coated onto a copper foil, 36 μm in thickness, followed by performing thermal imidization at specifically programmed heating. The dielectric constant of the prepared membranes was found to show an obvious trend: PEI containing hydrophilic MSPs > PEI > PEI containing hydrophobic MSPs. Moreover, the higher the loading of hydrophilic MSPs, the higher the value of the dielectric constant and loss tangent. On the contrary, the higher the loading of hydrophobic MSPs, the lower the value of the dielectric constant with an almost unchanged loss tangent.

Broadband Bi-Directional All-Dielectric Transparent Metamaterial Absorber

Water-based absorbers have shown great development potential in the past few years. In this paper, an all-dielectric transparent bi-directional water-based broadband metamaterial absorber is designed. The simulation results indicate that absorptance of the absorber is over 90% in 5.7-41.6 GHz, and its fraction bandwidth is 151.8%. The experimental results are greatly consistent with the simulations. The designed absorber has excellent performances of polarization insensitivity, oblique incidence stability and thermal stability. When the absorptance is more than 0.8, the maximum incident angle reaches 40° in TE mode and is over 60° in TM mode. In 0-80 °C, absorptance of the absorber is hardly changed. Because of the optical transparency of the designed absorber, it can be extensively used in stealth window weapons and electromagnetic compatibility equipment.

Biosensors based on novel nonlinear delta-function photonic crystals comprising weak nonlinearities

In this research, we propose a novel nonlinear delta-function photonic crystal for detecting sodium iodide (NaI) solution of different concentrations. The suggested structure comprises 50 delta stacks of GaP in an aqueous solution of NaI. These stacks are considered to have weak defocusing nonlinearity in the order of 10^-6 (V/m)^-2. Due to nonlinearity of the design, a defect-like resonance is formed within the photonic band gap. Thus, the detection of NaI with different concentrations can be easily investigated without the inclusion of a defect through the photonic crystal structure. The effects of both the linear part of the refractive index of GaP layers and nonlinear coefficient on the transmittance value are thoroughly discussed. The numerical findings investigate that the resonant peak begins to split at some critical nonlinearity. In our proposed structure, splitting occurs at about - 12 × 10^-6 (V/m)^-2. In this regard, the suggested sensor provides a high sensitivity of 409.7 nm/RIU and a wonderful detection limit of 0.0008.

Reconfigurable multi-band water-graphene cascade metamaterial perfect absorbers loaded with vanadium dioxide

We demonstrate the perfect trapping of electromagnetic fields over multi-band frequencies through all-dielectric terahertz absorbers using water graphene cascade metamaterials. More specifically, the coating water layer greatly enhances the higher-order Fabry-Pérot resonant absorbing modes and can achieve more than 8 absorbing peaks with the absorptions exceeding 99% in the spectrum below 3 THz. Especially such multiple perfect absorbing bands can readily be reset when the proposed water-graphene metamaterial absorbers integrate with thermal controlled vanadium dioxide. Such a perfect absorbing capacity would also be valid for the wide angular illuminations with different polarizations, and the reconfigurable characteristics of graphene can also enable the dynamically tuning of the absorbing frequencies, offering great freedom of extensive applications in energy harvesting and wave manipulation.

Rapid customized design of a conformal optical transparent metamaterial absorber based on the circuit analog optimization method

In this paper, a conformal optical transparent metamaterial absorber (COTMA) is proposed based on the circuit analog optimization method (CAOM), which can effectively enhance the optimization speed in the metamaterial absorber structure design by quantifying the equivalent circuit parameters. The operating frequency band can be customized at any band through CAOM, such as microwave, terahertz, and near-infrared frequencies. Here, a five-square-patch structure absorber with transparency and flexible properties is achieved. The simulated and measured incident electromagnetic (EM) wave absorptions of COTMA can reach above 90% in 15.77 - 38.69 GHz band. Meanwhile, COTMA exhibits excellent conformal EM absorption, a thinner substrate (0.078 wavelength at 15.77 GHz), lower structure complexity and polarization independence, and it can also be adapted to the EM absorption of different curved screens. This design is expected to have potential applications for wearable electronics, curved surface screens and OLED displays.

Flexible Metamaterial Absorber with Tailored Bandwidth and High Absorption Performance

Different from previous works which have focused on broadening the bandwidth, we propose an electromagnetic absorber with a medium and tailored bandwidth absorption, which avoids the unnecessary absorption caused by the too-wide bandwidth. Nevertheless, absorption is extremely high to be more than 99% and 97% for the normal and even oblique (45°) incidence, respectively, in a tailored frequency range of 5.8 ± 0.25 GHz even for both TE and TM polarization. This means the absorber is insensitive to the polarization of incident electromagnetic wave. Furthermore, it is flexible, since the main portion of sample is soft and flexible polyimide. The same properties are also realized in a range of 10 ± 0.5 GHz through adjusting the parameters of structure. The center frequencies of 5.8 and 10 GHz are very useful in our daily life, and the cost of absorber is relative low. Therefore, we believe the absorber can be used in many practical fields such as vehicle high-pass applications and radars.

Material-structure integrated design for ultra-broadband all-dielectric metamaterial absorber

Material and structure are the essential elements of all-dielectric metamaterials. Structure design for specific dielectric materials has been studied while the contribution of material and synergistic effect of material and structure have been overlooked in the past years. Herein, we propose a material-structure integrated design (MSID) methodology for all-dielectric metamaterials, increasing the degree of freedom in the metamaterial design, to comprehensively optimize microwave absorption performance and further investigate the contribution of material and structure to absorption. A dielectric metamaterial absorber with an ultra-broadband absorption from 5.3 to 18.0 GHz is realized. Theoretical calculation and numerical simulation demonstrate that the symphony of material and structure excites multiple resonance modes encompassing quarter-wavelength interference cancellation, spoof surface plasmon polariton mode, dielectric resonance mode and grating mode, which is essential to afford the desirable absorption performance. This work highlights the superiority of coupling of material and structure and provides an effective design and optimization strategy for all-dielectric metamaterial absorbers.

Polarization and Incidence Angle Independent Low-Profile Wideband Metamaterial Electromagnetic Absorber Using Indium Tin Oxide (ITO) Film

We use indium tin oxide (ITO), one of the representative resistive materials, for the implementation of a metamaterial electromagnetic (EM) absorber with a high absorbance in a wide frequency range. Highly symmetrical split ring resonators made of ITO film are deposited on the polyethylene terephthalate with transparent and flexible features, and such a configuration causes the proposed absorber to be insensitive to polarization and incidence angles. The proposed absorber, with a profile of only 0.171λ, exhibits a wideband absorbance of 7.2 GHz to 27 GHz, with a 90% absorption criterion. A prototype is built, and all the computed expectations from the full-wave EM simulations in this work are verified experimentally.

Broadband RCS Reduction by a Quaternionic Metasurface

A quaternionic metasurface consisting of two pairs of units with destructive phase difference is proposed to extend the bandwidth of radar cross section (RCS) reduction. The two pairs of units are designed to have complementary phase-different bandwidth, which extends the bandwidth of RCS reduction. The overlaps of their bandwidth enhance the RCS reduction, resulting in a metasurface having broadband and strong RCS reduction. This design and the wideband RCS reduction of the quaternionic metasurface were verified by analytical calculation with superposition principle of electric field, numerical simulation with commercial software package CST Microwave Studio and experiment in microwave anechoic chamber. The scattering mechanism and the angular performance of the quaternionic metasurface were also investigated.

Multiband metamaterial selective absorber for infrared stealth

Nanostructured selective absorbers have widespread applications ranging from artificial color to thermophotovoltaics and radiative cooling. In this paper, we propose a metamaterial selective absorber with a metal-insulator-metal structure for infrared stealth. It can realize multiband absorption, and one sharp peak is at 1.54 µm, which can be used to reduce the scattering signals in laser-guided missiles. The other two relatively broad absorption peaks are at 2.83 µm and 6.11 µm, which can match the atmospheric absorption band. It can reduce up to 90 % of the detected infrared signals while maintaining a relatively high level of thermal emission capability. The dependence of the spectral characteristics on the incident angle is studied. The infrared signatures of the structure could be suppressed across a wide temperature range.

Ultra-broadband and tunable saline water-based absorber in microwave regime

We present a saline water-based absorber that can offer an extremely wide bandwidth of electromagnetic absorption in the microwave frequency regime. The designed absorber is a hybrid of saline water and Poly Tetra Fluoro Ethylene (PTFE) dielectric material, which exhibits an absorption level greater than 90% across the frequency band from 1.4 to 3.3 and 4.3 to 63 GHz, with a relative absorption bandwidth as high as 180%. Meanwhile, the sensitivity of the absorption against oblique incidence is studied for both TE and TM polarizations. Additionally, the absorption performance of this saline water-based absorber could be tunable by changing the salinity and the combination of different temperatures and salinity. To explain the mechanism, interference theory is employed to investigate the designed absorber, and the theoretically calculated results are well in agreement with the simulation. This design offers an effective and feasible way to construct tunable and ultra-broadband absorber in low-cost stealth technology.

Electrically Small Water-Based Hemispherical Dielectric Resonator Antenna

Recently, water has been proposed as an interesting candidate for use in applications such as tunable microwave metamaterials and dielectric resonator antennas due to its high and temperature-dependent permittivity. In the present work, we considered an electrically small water-based dielectric resonator antenna made of a short monopole encapsulated by a hemispherical water cavity. The fundamental dipole resonances supported by the water cavity were used to match the short monopole to its feed line as well as the surrounding free space. Specifically, a magnetic (electric) dipole resonance was exploited for antenna designs with a total efficiency of 29.5% (15.6%) and a reflection coefficient of −24.1 dB (−10.9 dB) at 300 MHz. The dipole resonances were effectively excited with different monopole lengths and positions as well as different cavity sizes or different frequencies in the same cavity. The overall size of the optimum design was 18 times smaller than the free-space wavelength, representing the smallest water-based antenna to date. A prototype antenna was characterized, with an excellent agreement achieved between the numerical and experimental results. The proposed water-based antennas may serve as cheap and easy-to-fabricate tunable alternatives for use in very high frequency (VHF) and the low end of ultrahigh frequency (UHF) bands for a great variety of applications.

Ultra-broadband microwave metamaterial absorber with tetramethylurea inclusion

The absorption region of a water-based absorber was expanded by introducing tetramethylurea (TMU) into the inclusion, whose dielectric properties are tunable through the concentration of TMU. The dielectric spectroscopy of a TMU/water mixture was deconstructed using a Debye model. We designed a four-layer ultra-broadband microwave absorber with a supernatant micro-structure. Simulation and experiment results indicate that the absorber can achieve 90% perfect absorption, covering a broad frequency range of 4-40 GHz. The concentration dependence of the absorber was also studied experimentally and numerically. The concentration control provides a more practical and large frequency-region modulation of perfect absorption.

Reflection phase modification by metamaterial interface: an understanding of design criteria for ultrathin multispectral absorber

We introduce a metasurface to modify the reflection phase for multispectral microwave absorbers. General quantitive design criteria are proposed by carrying out field analysis, so that design of multispectral microwave absorber can be effectively realized. Optimal design process is discussed to develop an understanding of the absorbing mechanism. Experiment results show that the absorber having only 0.015 times the wavelength at the center frequency can simultaneously achieve high absorption in the microwave, visible light and near-infrared light bands. Multispectral absorption comes with added features of flexibility, ultrathin thickness and a light weight that make it a powerful candidate in advanced stealth application.

Thermally controllable Mie resonances in a water-based metamaterial

Active control of metamaterial properties is of great significance for designing miniaturized and versatile devices in practical engineering applications. Taking advantage of the highly temperature-dependent permittivity of water, we demonstrate a water-based metamaterial comprising water cubes with thermally tunable Mie resonances. The dynamic tunability of the water-based metamaterial was investigated via numerical simulations and experiments. A water cube exhibits both magnetic and electric response in the frequency range of interest. The magnetic response is primarily magnetic dipole resonance, while the electric response is a superposition of electric dipole resonance and a smooth Fabry–Pérot background. Using temporal coupled-mode theory (TCMT), the role of direct scattering is evaluated and the Mie resonance modes are analyzed. As the temperature of water cube varies from 20 °C to 80 °C, the magnetic and electric resonance frequencies exhibit obvious blue shifts of 0.10 and 0.14 GHz, respectively.

A Perspective of Non-Fiber-Optical Metamaterial and Piezoelectric Material Sensing in Automated Structural Health Monitoring

Metamaterials are familiar in life sciences, but are only recently adopted in structural health monitoring (SHM). Even though they have existed for some time, they are only recently classified as smart materials suitable for civil, mechanical, and aerospace (CMA) engineering. There are still not many commercialized metamaterial designs suitable for CMA sensing applications. On the other hand, piezoelectric materials are one of the popular smart materials in use for about 25 years. Both these materials are non-fiber-optical in nature and are robust to withstand the rugged CMA engineering environment, if proper designs are adopted. However, no single smart material or SHM technique can ever address the complexities of CMA structures and a combination of such sensors along with popular fiber optical sensors should be encouraged. Furthermore, the global demand for miniaturization of SHM equipment, automation and portability is also on the rise as indicated by several global marketing strategists. Recently, Technavio analysts, a well-known market research company estimated the global SHM market to grow from the current US $ 1.48 billion to US $ 3.38 billion by 2023, at a compound annual growth rate (CAGR) of 17.93%. The market for metamaterial is expected to grow rapidly at a CAGR of more than 22% and the market for piezoelectric materials is expected to accelerate at a CAGR of over 13%. At the same time, the global automation and robotics market in the automotive industry is expected to post a CAGR of close to 8%. The fusion of such smart materials along with automation can increase the overall market enormously. Thus, this invited review paper presents a positive perspective of these non-fiber-optic sensors, especially those made of metamaterial designs. Additionally, our recent work related to near field setup, a portable meta setup, and their functionalities along with a novel piezoelectric catchment sensor are discussed.

Realization of switchable EIT metamaterial by exploiting fluidity of liquid metal

Novel manipulation techniques for the propagation of electromagnetic waves based on metamaterials can only be performed in narrow operating bands, and this drawback is a major challenge for developing metamaterial-based practical applications. We demonstrate that the scattering of metamaterials can be switched and that their operating band can be tuned by introducing liquid metal in the design of functional metamaterials. The proposed liquid metal-based metamaterial is composed of a copper wire pair and a tiny pipe filled with a liquid metal, namely eutectic gallium-indium. The interference of the sharp magnetic resonance of the copper wire pair and the broad dipolar mode of the liquid metal rod lead to an electromagnetically induced transparency (EIT)-like spectrum. We experimentally demonstrate that this EIT-like behavior can be switched on or off by exploiting the fluidity of the liquid metal, which is useful for multi-frequency modulators. These findings will hopefully promote the development of fluid matter-based metamaterials for extending the operating band of novel electromagnetic functions.

Conductive polymer for ultra-broadband, wide-angle, and polarization-insensitive metamaterial perfect absorber

We numerically and experimentally investigate a broadband, polarization-independent and wide-incident-angle metamaterial perfect absorber (MPA) based on conductive polymer. By optimizing the electrical conductivity of the polymer, a 16.7 GHz broadband MPA is observed with the absorptivity greater than 80% for both transverse magnetic and electric polarization. The measurement results performed in the range 8-18 GHz show a diametrical concatenation with simulation results and theoretical analysis. The absorption mechanism is explained by demonstrating the influence of polymer conductivity on the dissipated power, the equivalent impedance, and the induced electric field. Our work may contribute to further studies on broadband MPA using for various applications.

Printable all-dielectric water-based absorber

The phase interplay between overlapping electric and magnetic dipoles of equal amplitude generated by exclusively alldielectric structures presents an intriguing paradigm in the manipulation of electromagnetic energy. Here, we offer a holistic implementation by proposing an additive manufacturing route and associated design principles that enable the programming and fabrication of synthetic multi-material microstructures. In turn, we compose, manufacture and experimentally validate the first demonstrable 3d printed all-dielectric electromagnetic broadband absorbers that point the way to circumventing the technical limitations of conventional metal-dielectric absorber configurations. One of the key innovations is to judicially distribute a dispersive soft matter with a high-dielectric constant, such as water, in a low-dielectric matrix to enhance wave absorption at a reduced length scale. In part, these results extend the promise of additive manufacturing and illustrate the power of topology optimisation to create carefully crafted magnetic and electric responses that are sure to find new applications across the electromagnetic spectrum.

Ultra-subwavelength thickness for dual/triple-band metamaterial absorber at very low frequency

An integrated model utilizing external parasitic capacitors for a dual-band metamaterial perfect absorber (DMPA) is proposed and demonstrated in the UHF radio band. By adjusting the lumped capacitors on a simple meta-surface, the thickness of absorber is reduced to be only 1/378 and 1/320 with respect to the operating wavelength at 305 and 360.5 MHz, respectively. The simulations and the experiments confirm that the DMPA can maintain an absorption over 91% in a wide range of incident angle (up to 55°) and independent of the polarization of incident radiation. Additionally, we examine the integrated model for smaller dual-band absorber and absorption performance at higher frequencies (LTE band). Finally, we consolidate our approach by fabricating an ultrathin triple-band perfect absorber miniaturized to be only 1/591 of the longest operating wavelength. Our work is expected to contribute to the actualization of metamaterial-based devices working at radio frequency.

3D-Printed Low-Cost Dielectric-Resonator-Based Ultra-Broadband Microwave Absorber Using Carbon-Loaded Acrylonitrile Butadiene Styrene Polymer

In this study, an ultra-broadband dielectric-resonator-based absorber for microwave absorption is numerically and experimentally investigated. The designed absorber is made of the carbon-loaded Acrylonitrile Butadiene Styrene (ABS) polymer and fabricated using the 3D printing technology based on fused deposition modeling with a quite low cost. Profiting from the fundamental dielectric resonator (DR) mode, the higher order DR mode and the grating mode of the dielectric resonator, the absorber shows an absorptivity higher than 90% over the whole ultra-broad operating band from 3.9 to 12 GHz. The relative bandwidth can reach over 100% and cover the whole C-band (4–8 GHz) and X-band (8–12 GHz). Utilizing the numerical simulation, we have discussed the working principle of the absorber in detail. What is more, the absorption performance under different incident angles is also simulated, and the results indicate that the absorber exhibits a high absorptivity at a wide angle of incidence. The advantages of low cost, ultra-broad operating band and a wide-angle feature make the absorber promising in the areas of microwave measurement, stealth technology and energy harvesting.

Transparent broadband metamaterial absorber enhanced by water-substrate incorporation

In this paper, a transparent metamaterial absorber (MA) loaded with water substrate is presented, which can simultaneously achieve enhanced broadband microwave absorption and tunable infrared radiation. As a proof, the indium tin oxide (ITO) films are first introduced here as a frequency selective surface (FSS) on the top layer and reflective backplane on the ground layer. Next the distilled water combined with the polymethyl methacrylate (PMMA) substrate is employed as a hybrid substrate in the middle. Simulation and experimental measurements show that the transparent water-substrate MA can achieve broadband microwave absorption with efficiency over 90% in the frequency band of 6.4-23.7GHz, and the proposed hybrid substrate has almost no influence on its original transmittance. Moreover, owing to the available water circulation system, the infrared radiation of the proposed MA is also demonstrated to be controlled by the temperature of the injected water. Based on its multifunction and high performance, it is expected that the proposed design may find potential applications, such as glass window of stealth equipment, electromagnetic compatible buildings/facilities, etc.

Broadband microwave absorption utilizing water-based metamaterial structures

In this paper, broadband microwave absorbers utilizing water-based metamaterial structure elements have been proposed and investigated. We employ water into the metamaterial structure unit-cell of the absorber as primary resonant elements such as the water-droplet, or water-tube structure. By investigating the resonant modes and the coupling between the water elements and the surrounding dielectrics, it is found the inherent multi-resonance of the proposed metamaterial structures could result in a broadband microwave absorption. For water-droplets design, 90% microwave absorption has been achieved from 7.5 GHz to 15 GHz, while for water-tube design, a much broader bandwidth from 5 GHz to 15 GHz is obtained for nearly 90% microwave absorption. The broadband absorption performance has been verified by both full wave simulation and experimental measurement. We believe the proposed broadband water-based absorber may find some applications in microwave stealth and electromagnetic compatibility technology.

Thermally Tunable Ultra-wideband Metamaterial Absorbers based on Three-dimensional Water-substrate construction

Distilled water has frequency dispersive characteristic and high value of imaginary part in permittivity, which can be seen as a good candidate of broadband metamaterial absorbers(MAs) in microwave. Here, an interesting idea based on the combination of water-substrate and metallic metamaterial in the three-dimensional construction is proposed, which can achieve outstanding broadband absorption. As a proof, the distilled water is filled into the dielectric reservoir as ultra-thin water-substrate, and then the water-substrates are arranged on the metal backplane periodically as three-dimensional water-substrate array(TWA). Simulation shows that the TWA achieves broadband absorption with the efficiency more than 90% from 8.3 to 21.0 GHz. Then, the trigonal metallic fishbone structure is introduced here between the water-substrate and the dielectric reservoir periodically as three-dimensional water-substrate metamaterial absorber(TWMA). The proposed TWMA could achieve ultra-broadband absorption from 2.6 to 16.8 GHz, which has increase by 64.8% in relative absorption bandwidth. Meanwhile, due to the participation of distilled water, the thermally tunable property also deserves to be discussed here. In view of the outstanding performance, it is worth to expect a wide range of applications to emerge inspired from the proposed construction.

Water metamaterial for ultra-broadband and wide-angle absorption

A subwavelength water metamaterial is proposed and analyzed for ultra-broadband perfect absorption at microwave frequencies. We experimentally demonstrate that this metamaterial shows over 90% absorption within almost the entire frequency band of 12-29.6 GHz. It is also shown that the proposed metamaterial exhibits a good thermal stability with its absorption performance almost unchanged for the temperature range from 0 to 100°C. The study of the angular tolerance of the metamaterial absorber shows its ability of working at wide angles of incidence. Given that the proposed water metamaterial absorber is low-cost and easy for manufacture, we envision it may find numerous applications in electromagnetics such as broadband scattering reduction and electromagnetic energy harvesting.

Dual-functional tunable coding metasurface based on saline water substrate

In this paper, a dual-functional tunable coding metasurface is presented at X band based on water substrate, which can realize two different functions of specific scattering pattern and absorption at two different frequency ranges. Besides, by changing the salinity of the saline water substrate, the absorption performance in high frequency can be tuned, while the scattering pattern in low frequency remains unchanged. A coding element is designed with small water cavity in it. Three coding sequences with different radiation patterns are designed to verify these functions, and one of them is fabricated and measured. Experimental results have good accordance with our simulations, which demonstrates our schemes. We believe this work can not only broaden our design manner of metasurfaces, but also have plenty potential applications in biological and medical detection domain.

Selective dual-band metamaterial perfect absorber for infrared stealth technology

We propose a dual-band metamaterial perfect absorber with a metal–insulator–metal structure (MIM) for use in infrared (IR) stealth technology. We designed the MIM structure to have surface plasmon polariton (SPP) and magnetic polariton (MP) resonance peaks at 1.54 μm and 6.2 μm, respectively. One peak suppresses the scattering signals used by laser-guided missiles, and the other matches the atmospheric absorption band, thereby enabling the suppression of long-wavelength IR (LWIR) and mid-wavelength IR (MWIR) signals from objects as they propagate through the air. We analysed the spectral properties of the resonance peaks by comparing the wavelength of the MP peak calculated using the finite-difference time-domain method with that obtained by utilizing an inductor–capacitor circuit model. We evaluated the dependence of the performance of the dual-band metamaterial perfect absorber on the incident angle of light at the surface. The proposed absorber was able to reduce the scattering of 1.54 μm IR laser light by more than 90% and suppress the MWIR and LWIR signatures by more than 92%, as well as maintain MWIR and LWIR signal reduction rates greater than 90% across a wide temperature range from room temperature to 500 °C.

Multiband coherent perfect absorption in a water-based metasurface

We design an ultrathin water-based metasurface capable of coherent perfect absorption (CPA) at radio frequencies. It is demonstrated that such a metasurface can almost completely absorb two symmetrically incident waves within four frequency bands, each having its own modulation depth of metasurface absorptivity. Specifically, the absorptivity at 557.2 MHz can be changed between 0.59% and 99.99% via the adjustment of the phase difference between the waves. The high angular tolerance of our metasurface is shown to enable strong CPA at oblique incidence, with the CPA frequency almost independent of the incident angle for TE waves and varying from 557.2 up to 584.2 MHz for TM waves. One can also reduce this frequency from 712.0 to 493.3 MHz while retaining strong coherent absorption by varying the water layer thickness. It is also show that the coherent absorption performance can be flexibly controlled by adjusting the temperature of water. The proposed metasurface is low-cost, biocompatible, and useful for electromagnetic modulation and switching.

Reconfigurable all-dielectric metasurface based on tunable chemical systems in aqueous solution

Dynamic control transmission and polarization properties of electromagnetic (EM) wave propagation is investigated using chemical reconfigurable all-dielectric metasurface. The metasurface is composed of cross-shaped periodical teflon tubes and inner filled chemical systems (i.e., mixtures and chemical reaction) in aqueous solution. By tuning the complex permittivity of chemical systems, the reconfigurable metasurface can be easily achieved. The transmission properties of different incident polarized waves (i.e., linear and circular polarization) were simulated and experimentally measured for static ethanol solution as volume ratio changed. Both results indicated this metasurface can serve as either tunable FSS (Frequency Selective Surface) or tunable linear-to-circular/cross Polarization Converter at required frequency range. Based on the reconfigurable laws obtained from static solutions, we developed a dynamic dielectric system and researched a typical chemical reaction with time-varying permittivity filled in the tubes experimentally. It provides new ways for realizing automatic reconfiguration of metasurface by chemical reaction system with given variation laws of permittivity.

Miniaturization for ultrathin metamaterial perfect absorber in the VHF band

An efficient resolution for ultrathin metamaterial perfect absorber (MPA) is proposed and demonstrated in the VHF radio band (30–300 MHz). By adjusting the lumped capacitors and the through vertical interconnects, the absorber is miniaturized to be only λ/816 and λ/84 for its thickness and periodicity with respect to the operating wavelength (at 102 MHz), respectively. The detailed simulation and calculation show that the MPA can maintain an absorption rate over 90% in a certain range of incident angle and with a wide variation of capacitance. Additionally, we utilized the advantages of the initial single-band structure to realize a nearly perfect dual-band absorber in the same range. The results were confirmed by both simulation and experiment at oblique incidence angles up to 50°. Our work is expected to contribute to the actualization of future metamaterial-based devices working at radio frequency.

Flexible frequency selective metamaterials for microwave applications

Metamaterials have attracted more and more research attentions recently. Metamaterials for electromagnetic applications consist of sub-wavelength structures designed to exhibit particular responses to an incident EM (electromagnetic) wave. Traditional EM (electromagnetic) metamaterial is constructed from thick and rigid structures, with the form-factor suitable for applications only in higher frequencies (above GHz) in microwave band. In this paper, we developed a thin and flexible metamaterial structure with small-scale unit cell that gives EM metamaterials far greater flexibility in numerous applications. By incorporating ferrite materials, the thickness and size of the unit cell of metamaterials have been effectively scaled down. The design, mechanism and development of flexible ferrite loaded metamaterials for microwave applications is described, with simulation as well as measurements. Experiments show that the ferrite film with permeability of 10 could reduce the resonant frequency. The thickness of the final metamaterials is only 0.3mm. This type of ferrite loaded metamaterials offers opportunities for various sub-GHz microwave applications, such as cloaks, absorbers, and frequency selective surfaces.

Large scale generation of micro-droplet array by vapor condensation on mesh screen piece

We developed a novel micro-droplet array system, which is based on the distinct three dimensional mesh screen structure and sintering and oxidation induced thermal-fluid performance. Mesh screen was sintered on a copper substrate by bonding the two components. Non-uniform residue stress is generated along weft wires, with larger stress on weft wire top location than elsewhere. Oxidation of the sintered package forms micro pits with few nanograsses on weft wire top location, due to the stress corrosion mechanism. Nanograsses grow elsewhere to show hydrophobic behavior. Thus, surface-energy-gradient weft wires are formed. Cooling the structure in a wet air environment nucleates water droplets on weft wire top location, which is more "hydrophilic" than elsewhere. Droplet size is well controlled by substrate temperature, air humidity and cooling time. Because warp wires do not contact copper substrate and there is a larger conductive thermal resistance between warp wire and weft wire, warp wires contribute less to condensation but function as supporting structure. The surface energy analysis of drops along weft wires explains why droplet array can be generated on the mesh screen piece. Because the commercial material is used, the droplet system is cost effective and can be used for large scale utilization.

Microwave absorption performance of methylimidazolium ionic liquids: towards novel ultra-wideband metamaterial absorbers

The ultra-wideband absorption of microwaves was achieved by metamaterials based on methylimidazolium ionic liquids for the first time.

Ferrite Film Loaded Frequency Selective Metamaterials for Sub-GHz Applications

Electromagnetic metamaterials are constructed with sub-wavelength structures that exhibit particular electromagnetic properties under a certain frequency range. Because the form-factor of the substructures has to be comparable to the wavelength of the operating frequency, few papers have discussed the metamaterials under GHz frequency. In this paper, we developed an innovative method to reduce the resonant frequency of metamaterals. By integrating the meta-structures with ferrite materials of higher permeability, the cell size of the meta-structure can be scaled down. This paper describes the methodology, design, and development of low-profile GHz ferrite loaded metamaterials. A ferrite film with a permeability of 20 could reduce the resonant frequency of metamaterials by up to 50%. A prototype has been fabricated and the measurement data align well with the simulation results. Because of the lowered operational frequency, the proposed ferrite loaded metamaterials offer more flexibility for various sub-GHz microwave applications, such as cloaks, absorbers, and frequency selective surfaces.