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This paper proposes miniaturized, lightweight and ultrasensitive planar metamaterial sensor for relative permittivity measurement of nondispersive materials. The proposed sensor is designed using a thin-substrate microstrip line loaded with tapered sectorial Complementary Split Ring Resonator (CSRR). Compared to similar state-of-the-art sensors, the proposed one is at least (25)% more sensitive with a wide dynamic range of the sensing related frequency. Moreover, unlike previously proposed sensors, the relative permittivity of a dielectric sample can be estimated using the variation of the minimum transmission frequency as well as the variation of the 10-dB sensor’s bandwidth which increases the integrity and accuracy of the obtained results. The minimum transmission frequency of the proposed sensor shifts by almost 7.6 GHz with a percentage change of 61% when the relative permittivity of the material under test (MUT) changes from 1 to 10. In addition, the 10-dB bandwidth is reduced by almost 7.7 GHz for the same MUT relative permittivity changes. Experimental measurements are in good agreement with the numerical findings. The paper includes a comprehensive sensitivity analysis that investigates the effect of resonator’s split length as well as its path width on the sensitivity and dynamic range of CSRR based sensors. Finally, the proposed sensor was used for microfluidic sensing to further demonstrate its practicality using different samples with different electrical properties. The sensor was able to provide distinct features for three different eye drops. The proposed sensor can be utilized as an effective permittivity sensor for various sensing applications such as displacement, nondestructive and biomedical sensing. more...

This article presents a reconfigurable unit cell for transmitarrays at $10 \,\mathrm{GHz}$ based on the fishnet metamaterial structure. The proposed design achieves a phase shift of up to $180 \mathrm{^{\circ }}$ between different states and additional tunability in each phase state by switching the electric response using p-i-n diodes and simultaneously tuning the magnetic response by employing liquid crystal. Thus, it offers the potential to overcome the limitations of switchable unit cells with few discrete phase states in terms of achievable antenna gain and side lobe level. With an insertion loss of $1.8$–$2.5 \,\mathrm{dB}$ in combination with its high phase tunability it exhibits much higher performance than designs using only liquid crystal. A lumped-element model for prediction of the unit cell behavior is proposed, allowing to reduce the number of full-wave simulations during performance analysis. The biasing network for the liquid crystal is designed for integration in an active-matrix configuration, greatly reducing the complexity of the control circuit. To verify the proposed design, a single unit cell prototype is manufactured and measured inside a waveguide simulator, demonstrating good agreement with full-wave simulations. more...

Recent advancements in terahertz wave applications for sensing, imaging, and high-speed communication have spurred a substantial demand for low-cost, easily accessible optical components capable of manipulating terahertz wave polarization. In this study, terahertz circular dichroism spectroscopic measurements of micrometer-sized metallic chiral helix arrays, fabricated using computer numerical control (CNC) machining, are presented. Experimental results demonstrate a giant circular dichroism of ±0.6 in an axial mode of operation for left- and right-handed arrays within a sub-terahertz frequency range. The helix array converts linearly polarized light into circularly polarized light from 120 GHz to 200 GHz, with a 3 dB axial ratio relative bandwidth approaching 50%. The polarization extinction ratio is in excess of 20 dB. These outstanding features, combined with their low cost, high accessibility, as well as their potential for mass production, pave the way for development of diverse range of optical components such as polarizers, filters, absorbers, and isolators operating in the terahertz frequency region. more...

Self-adaptive camouflage skin found in chameleon provide survival protection in the presence of predators, offering inspiration for intelligent security module of future transportation vehicles facing adversarial radar detection scenarios. This work presents a real-time self-adaptive electromagnetic wave suppressor module system that operates within a frequency band of 3.39 GHz to 4.34 GHz. In a hypothetical radar surveillance scenario, an electro detector senses the frequency of the scanning electromagnetic wave, triggering a control logic that adjusts the absorber’s properties to match the detected frequency. Reflectivity measurement results demonstrate that this self-adaptive wave suppressor system can automatically respond to incoming electromagnetic waves with arbitrarily preset frequencies within the specified frequency band. This electromagnetic wave suppressor system has significant application value for transportation carriers operating in threatening environments, including vessels, vehicles, and aircraft. Furthermore, this scene-material interaction module offers insights into the predator-prey interaction mechanisms found in nature. more...

This research investigates the process of designing, practical implementation, and modification of features in a sectoral antenna for mobile communication technology. The study employs the principles of a magneto-electric dipole and metamaterials to achieve its objectives. The antenna comprises a pair of magneto-electric curved strip dipoles arranged in a cross configuration for feeding. The antenna superstructure is constructed using a metamaterial consisting of three layers of metallic rods and a U-shaped reflector plane. The feeding component is oriented orthogonally to the ground plane at angles of ±45 degrees and is powered by two crossed-shaped strips to achieve dual polarization. Circular polarization can be attained through a three-layer metamaterial, providing control over the electric field’s magnitude and phase. Additionally, the U-shaped reflector plane reflects waves, effectively increasing the sectoral antenna’s beamwidth. The sectoral antenna achieves a peak gain of 15.35 dBi, representing an improvement over the initial gain of 9.8 dBi achieved solely through the use of magneto-electric curved strip dipoles. This proposed methodology can be adapted to enable circular polarization for mobile communication. more...

To have uninterrupted wireless connectivity, higher throughput, and latency rate down to nanoseconds; future networks will rely heavily on higher frequency bands, yet these signals suffer from attenuation and can be easily blocked by objects. Hence, those signals lose strength over longer distances significantly compared to their low-frequency counterparts. To overcome these constraints, reconfigurable intelligent surfaces (RIS) can increase the wireless network capacity and coverage by consciously altering the wireless propagation environment, which makes them one of the key enablers of the 6G communication networks. The potential benefits of RIS-assisted communication systems can be effectively increased using machine learning (ML) to pave the path towards Wireless 2.0, having a vision of digitization, programmability, and intelligence. This paper reviews recent developments in RIS technology with a special emphasis on tuning techniques, hardware concepts, RIS empowered by artificial intelligence techniques, and applications for next generation of networks. Finally, a viewpoint on the difficulties and potential directions of intelligent metasurfaces are also presented. more...

Optically-transparent opportunistic electromagnetic skins (OTO-EMSs) are proposed to enable outdoor-to-indoor (O2I) millimeter-wave (mmW) wireless communications with existing windows/glass-panels. More in detail, static passive EMSs consisting of optically-transparent conducting patterned layers attached to standard glass-panels are designed. Towards this end, both the phase coverage and the optical transparency of a meshed copper-based meta-atom printed on a non-dedicated insulated glass substrate are optimized. Successively, the feasibility of OTO-EMSs able to support mmW high-efficiency O2I transmissions along non-Snell refraction directions is numerically demonstrated also through full-wave simulations. more...

In this paper we have presented a novel Intelligent Reflecting Surface (IRS) for future 6G communications. IRS is suitable for compensating the power loss of wireless communications, such as Satellite based communications, through redirecting the reflected signals. We have analyzed a novel saw-tooth shape unit-cell within C-band and X-band. A full 360° phase variation of an IRS is required in some applications such as beam steering. We have investigated the IRS using three different methods: 1) full wave simulations of the IRS, 2) analysis of the equivalent circuit model (ECM) of the unit-cell, and 3) measurements of the reflection coefficients of ‘shorted’ and ‘open’ unit-cells. Two sets of ‘shorted’ and ‘open’ unit-cells were tested using waveguide based method. Tested results show that the unit-cell can achieve up to 340° phase variation at a particular frequency. It is possible to redirect the reflected signal towards a desired direction through different combination of ‘shorted’ and ‘open’ unit-cells. Phase differences of the tested results at different frequencies (e.g. 8.2-8.4 GHz, 9.1 GHz, and 9.56 GHz) suggest that the designed unit-cells can be utilized in beam redirection purposes. We have presented one such scenario of beam redirection of the IRS at 10 GHz. more...

This paper introduces an innovative $4 \times 4$ dual-band Butler matrix (BM) characterized by compactness and an enhanced frequency ratio (K). The design employs meandered lines and an interdigital capacitor (IDC) unit-cell-based composite right/left-handed transmission-line (CRLH-TL) metamaterial (MTM) structure. The BM integrates compact dual-band 3 dB branch-line couplers (BLC), a 0 dB crossover, and dual-band ±45° phase shifters on a single Rogers RT5880 substrate having relative permittivity $\varepsilon _{r}$ of 2.2 and thickness $h$ of 0.787 mm. Simulations and measurement results demonstrate reflection and isolation coefficients exceeding −20 dB at all ports, with obtained insertion loss of −6±3 dB over the 0.7 GHz and 3.5 GHz frequency bands. The achieved output phase differences of ±45°, ±135°, ±135°, and ±45° at the designed frequencies indicate a maximum average phase tolerance of ±4.5° concerning the ideal values. Importantly, the BM’s overall dimensions are 143 mm $ \times 186$ mm, resulting in an impressive 78% size reduction compared to traditional T-shaped BM designs. The proposed configuration is designed and simulated using CST Microwave Studio, with the agreement between simulated and measured parameters highlighting design reliability and effectiveness. Additionally, a performance evaluation comparing the proposed BM with existing circuits reveals its suitability for sub-6 GHz 5G dual-band antenna array beamforming networks (BFN) due to its compact size and improved band ratio. more...

This study highlights the potential of employing terahertz metamaterial structures as dual-band biosensors for the early detection of cancerous biological tissue. The fundamental principle leveraged here is the alteration of the effective dielectric constant of biological tissue by cancerous cells. The change in the dielectric constant, in turn, induces a shift in the resonance frequency of the metamaterial sensor. One notable advantage of the terahertz metamaterial sensor is its relatively compact size compared to other sensor types, as its dimensions are independent of the wavelength. This property translates into a requirement for a much smaller biopsy sample, facilitating less invasive testing procedures. Beyond the size advantage, the proposed biosensor demonstrates efficacy in detecting abnormalities within biological tissue. more...

Polarization-dependent dual-band THz metamaterial using a modified Trishul structure is proposed in this paper. The absorber has three layers namely, a conducting ground plane, a dielectric substrate, and a structure top metallic patch. The top and bottom metallic is made up of gold material and the dielectric substrate is polyimide. The characteristics are obtained using the finite element method. The structure has obtained two resonant peaks at 0.78 THz(f1) and 1.65 THz(f2) for the X-polarized wave and at 0.81 THz(f3) and 1.53 THz(f4) for the Y-polarized wave respectively. The simulation result reveals that the absorber has obtained ultra-narrow band absorption with a full width half maximum(FWHM) of 4.8 GHz, 10 GHz, 5 GHz, and 10 GHz for f1, f2, f3, and f4 respectively. In addition, the underlying physical mechanism of the multi-band characteristics is analyzed using electric field and surface current distributions. The proposed absorber design has importance in the emerging THz system for polarization imaging and sensing applications. Also, flexible metamaterials could be useful for soft robotics applications also. more...

This paper presents the numerical modelling and simulation of a 2D vertical well tubular model excited by a coaxial radiofrequency transmission line placed in its annular space for the purpose of downhole RF energy transmission. The study identifies the mechanism of energy losses due to coupling between the transmission line and steel tubing and casing using resonance mode analysis. The design of a left-handed artificial power line is proposed to minimize the losses due to the right-handed properties of the coaxial transmission line. The design utilizes ideal lumped element method to determine the left-handedness of the right-handed coaxial transmission line over the industrial heating radio-frequency band. The model is developed and simulated using COMSOL multi-physics commercial simulation software. The effect of transmission line position within the annular space on the tangential magnetic field and resulting power losses along the tubing is discussed. It is observed that the resulting artificial power line improves RF power transmission over 60 m depths of the vertical oil well within the industrial heating radiofrequency band. more...

Vector vortex beams which possess complex intensity and polarisation patterns, and phase or polarisation singularities are important in a variety of applications ranging from high resolution imaging and data transmission to nonlinear optics and quantum technologies. Here, we theoretically analyse the effect of vector vortex beams propagation through strongly anisotropic metamaterials exhibiting epsilon-near-zero behaviour. The interaction of the metamaterial with longitudinal field of the vector beams results in a strong modification of the polarisation distribution, with the metamaterial acting as an azimuthal polariser. The dependence on the local ellipticity of the field is also investigated revealing conversion of spin angular momentum to orbital angular momentum and polarisation structure vorticity, mediated by the longitudinal field. The results show the promise of an anisotropic metamaterial platform for manipulation of polarisation properties of and spin-orbit coupling in vector vortex beams. more...

A planar phase gradient metasurface (PGM) with phase gradients in two in-plane directions will introduce two additional wave vectors to the reflected wave vector. For small incident angles, close to the normal vector, the magnitude of the reflected wave vector is smaller than that of the incident wave vector and the direction of the anomalous reflected wave can be determined from the incident wave vector components and additional phase gradient components. The expanded generalized Snell’s law, which includes diffraction order modes, is combined with array theory to accurately predict the directions of the reflected waves from a planar PGM for incident angles where the magnitude of the reflected wave vector is larger than that of the incident wave vector. The predicted directions of the reflected waves from a planar PGM are compared with simulated RCS results obtained with CST Studio Suite and measured results obtained in a compact range. more...

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

Voice assistants play an important role in facilitating human–machine interactions and have been widely used in audio consumer electronic products. However, it has been shown that they are susceptible to inaudible attacks in which the malicious signals are in the ultrasound regime and cannot be heard by human ears. In this study, we show that a judiciously designed acoustic metamaterial filter can mitigate such attacks by modulating the received signals by the microphones. The metamaterial filter is composed of rigid plates with individual holes which exhibit local resonance phenomena that suppress incoming waves at specific frequencies. The effectiveness of the metamaterial filter is confirmed by experiments that show a combination of the holes can collectively distort the attack signals and protect the smart speakers. Moreover, normal audible signals are not affected by the proposed metamaterial, which adds to the flexibility of the device. The metamaterial filter has a small footprint and can be easily installed on various audio products. Our proposed strategy expands the capacity of acoustic metamaterials and improves the security of devices that use voice assistants. more...

The study of technological materials made by meticulously arranging acoustic elements has received a lot of attention over the past three decades with the goal of generating improved acoustic properties, often going beyond the behavior of materials found in nature. These are frequently referred to as acoustic metamaterials, and because of the way wave propagation phenomena is managed, they exhibit unusual properties. Improvements in noise mitigation techniques of acoustic systems based on metamaterials principles have been made effectively and precisely using combined or hybrid numerical methods and improved numerical formulations. These noise mitigation properties should be optimized by modifying topology and inner elements properties, which requires a huge search space. This work focuses on metamaterials called sonic crystals with Helmholtz resonators, and its innovative insulation properties as noise barriers are optimized with the Particle Swarm Optimization (PSO). This evolutionary algorithm provides a mono objective solution for the multi-dimensional search space inspired in animal behavior looking for food and communicating between each other. In order to assess the results obtained by the proposed approach, the presented PSO algorithm is compared with a Genetic Algorithm (GA): the results show that the PSO algorithm provides a better solution that pursues the objective of satisfying the acoustic comfort without exceeding the imposed practical constraints. more...

The reduction of radar cross-section (RCS) becomes an essential requirement for many military and civilian applications. One of the ways to achieve this is by using artificial magnetic conductors (AMCs) as electromagnetic surfaces. In this research, the reduction of the RCS is realized by adding a novel single layer AMC based metasurface unit cells that depend on polarization to cancel backscattering for Ka applications. The proposed AMC unit cell consists of a rectangular patch with open ended microstrip stubs. This unit cell exhibits a wide band phase difference about $\mathrm {180^{\circ }\pm 38^{\circ }}$ between the two reflected orthogonal field components in the frequency range from 27 to $\mathrm {56.5 GHz}$ (about $\mathrm {71\%}$ relative bandwidth). Four AMC block arrays perpendicular to each other are combined in a chessboard configuration. To validate the capability of achieving a good RCS reduction, a substrate integrated wave guide (SIW) cavity-backed slot antenna is placed in the center of the AMC sheet. Simulation results show that the proposed AMC unit cell significantly reduces backscattered energy from $\mathrm {29.5 GHz to 50 GHz}$ and from 28.5 GHz to 48.5 GHz in both $y-$ and $x-$ polarization, respectively, and covers a $\mathrm {-10 dB}$ backscattering reduction at antenna working band ( $\mathrm {32.2-34 GHz}$ ). The antenna’s bistatic performance at different frequencies is also presented. The fabricated design’s measurements and simulations match well, verifying and validating the proposed design. more...

In this paper, we analyze the operative limits and the conditions characterizing low frequency magnetic metasurfaces used in practical Wireless Power Transfer applications. In the literature, the analytical modeling of similar structures, allowing a control over their frequency response, has been already demonstrated. By starting from this point, practical aspects and differences between a purely theorical approach and real scenario constraints are highlighted and discussed. In particular, accurate and representative numerical simulations are conceived, by firstly analyzing and quantifying the effect on the metasurface performance of the resistive losses characterizing its constitutive conductive material. Then, the metasurface performance deviations produced by the interactions with additional radiofrequency coils, as in Wireless Power Transfer arrangements, are considered. In the latter case, the loading condition at the receiving coil was specifically studied. For both the considered scenarios, detailed conditions and possible solutions to alleviate these detrimental effects, which deviate the metasurface response from the desired behavior, are derived. This work can be useful to guide the design process of low frequency magnetic metasurfaces in a practical environment, achieving the best performance possible for Wireless Power Transfer applications. more...

A thermal-electric invisibility cloaking is designed by a nonlinear transformation, which can guide both electric current and heat flux around the concealed region and restore their original diffusion directions. Compared with previous thermal-electric cloaks designed by linear transformation, the proposed thermal-electric cloak by nonlinear transformation can introduce some parameters during the coordinate transformations, thus providing more degrees of freedom for subsequent material design and selection. As an example, we further design a 2N-layered structure consisting of four isotropic homogeneous materials for implementing the proposed cloak, which shows expected good cloaking effect for both electric current and heat flux by numerical simulations. The simulated results notably shows that the proposed thermal-electric cloak can manipulate the waves without any agitation either the highly conductive material placed in a vacuum or waves can generate from any other direction, which verify the expected thermal-electric invisibility cloaking simultaneously in the steady-state situation. more...

We report a class of all-dielectric, additively-manufactured polarization converters with tailored temporal frequency responses within the Ku and Ka microwave bands (15– 40 GHz). These multi-layer devices consist of cascaded, subwavelength, high-contrast gratings with different fill fractions and orientations, providing control over the effective anisotropic properties of each layer. In design, the subwavelength gratings are modeled as homogeneous anisotropic layers. This allows the overall metastructure to be treated as a stratified dielectric medium. Therefore, it can be analyzed and optimized using plane-wave transfer matrix techniques that fully account for multiple reflections between layers. Using this cascaded grating geometry, a variety of high-efficiency microwave polarization converters can be realized with broadband, multiband, or multifunctional behavior. The transmissive metastructures do not require anti-reflection layers since impedance matching is incorporated into their design. Three example devices based on alumina/air gratings have been monolithically fabricated using ceramic stereolithography: a broadband reflective half-wave plate, a broadband isotropic polarization rotator, and a dual-band linear-to-circular polarization converter. more...

A specially designed metamaterial was investigated wherein the top metasurface was comprised of MgF2-graphene periodic nanopillers formed over an InSb nanolayer. The transmission characteristics of metamaterial was artificially controlled, and the effects due to altering external stimuli, such as thermal ambience, graphene chemical potential and magnetostatic bias, were studied in the 6–15 THz regime. The structure exhibits vanishing transmission with sharp (transmission) minima in certain frequencies, the positions of which essentially depend on the external stimuli. The angular dependence and polarization-insensitive properties were also discussed, apart from the effect of using different numbers of graphene layers in the nanopillers. The results show potential of the metamaterial as sensor due to the thermal, electrical and magnetic variations. [ABSTRACT FROM AUTHOR] more...

In this paper, an active metamaterial (MM) absorber loaded with PIN diodes is proposed. By adjusting the bias currents of PIN diodes to change the value of the introduced equivalent resistance, the Q factors of multiple resonances of the whole structure are altered. Therefore, multiple broadband absorption states with moderate absorption performance at different frequencies are obtained. These states cover continuously tunable absorption frequencies in an ultrabroad frequency band. Bias circuits for PIN diodes are also specially designed to obtain polarization-insensitive absorption performance. Optimizing works were carried out by analyzing the operating mechanism based on the simulated field distribution and power loss distribution. The presented absorber covers an ultrawideband absorption bandwidth with a reflection coefficient below -10 dB from 0.78 GHz to 4.62 GHz (142.2% in relative bandwidth) by adjusting the PIN diodes. It is worth noting that the absorption bandwidth of the low-resistance state reaches 21.0% in the P band, which greatly improves performance in the low-frequency regime. The absorber also shows good angular stability under oblique incident angles from 0° to 30°. In addition, the total thickness of the absorber is only 1/16 of the wavelength at the lowest operating frequency with an areal density of 2.06 kg/m2. more...

A terahertz metamaterial refractive index/thickness sensor with flexible substrate, exhibiting low-frequency Fano resonance and high-frequency electromagnetically induced transparent (EIT) resonance, is proposed. The physical formation mechanisms of Fano and EIT resonances are investigated by calculating the electromagnetic field. Simulated results demonstrate that the refractive index sensing sensitivity based these two resonances are up to 60 and 100 GHz/RIU, and the thickness sensing sensitivity are up to 1 and 1.7 GHz/μm, respectively. Furthermore, the proposed sensor was fabricated using femtosecond laser etching technology, and its sensing performance was verified by the experimental results that it can distinguish different liquids and detect the polyimide film with different thicknesses less than 180 μm. The remarkable performances make the proposed metamaterial sensor has feasible capability for biological and chemical sensing in terahertz range. more...

On-body transmission represents a challenge due to human body shadowing. This paper proposes a Sommerfeld-Goubau single-wire transmission line (SWTL) implemented using electronic textiles for low-loss on-body links up to millimeter-wave frequencies, overcoming the spherical spreading loss and on-body absorption. The SWTL is fabricated using a conductive thread suitable for embroidery on textiles. A compact tapered launcher is implemented on a flexible polyimide substrate to excite the surface mode along the SWTL. In space, a 3 m-long line maintains a forward transmission over −10 dB between 1 and 3 GHz. The SWTL link is characterized for different body parts showing under 20 dB insertion loss with a 1 cm air gap. Across the torso, a forward transmission over −20 dB is maintained from 0.5 to 2.5 GHz, which represents at least 20 dB improvement over two antennas, of larger dimensions, over-the-air. Directly on-skin, the SWTL can be used around 1 GHz with an $S_{21}$ over −25 dB, over 50 dB improvement over two on-skin antennas. At 50 GHz, the shielded SWTL exhibits an ultra-low on-body attenuation around 0.11 dB/mm, a four-fold improvement over a microstrip line on the same substrate. It is concluded that SWTLs can enable ultra high-speed future body area networks. more...

Metasurfaces analogues of Fano resonances provide a powerful platform for high sensitivity sensing, nonlinear optics, and light manipulation. However, previous Fano-resonant metasurfaces usually are not compatible with silicon complementary metal-oxide semiconductor circuits due to their hybrid material structures and large non-radiative loss. Herein, we theoretically demonstrate a silicon-on-insulator metasurface (SOIM) enhancing Fano resonances by using a tandem neural network design. Multiple Fano resonances with high Q-factor have been observed in the symmetry-breaking SOIM. The Fano-resonant mechanism of the SOIM is analyzed. Additionally, the spectral features of the Fano-resonant SOIM as a function of the symmetry tuning factor of the double silicon nanobars and the environment refractive index are also investigated. The result shows that the Fano-resonant SOIM as a methanol sensor with a sensitivity of 310 nm/RIU can achieve an overall figure of merit of 195 in the near-infrared spectral regime. The designed Fano-resonant SOIM shows enormous potential applications in highly sensitive sensors and light-matter interaction enhancement. more...

In this work, a highly efficient metasurface-based multi-scale wireless power transfer (MSWPT) system is presented. The MSWPT is designed to work as a near-field WPT at 6.78 MHz (AirFuel Alliance standard frequency) and far-field WPT at 433 MHz (ISM band for household devices) according to its power transfer distance (PTD). This property that can function in both near-field scale and far-field scale is defined as multi-scale property in this paper. The metasurface slabs which have an electromagnetic (EM) wave focusing characteristic for both frequency bands are integrated with the transmitter (Tx) and receiver (Rx) coils to enhance the power transfer efficiency (PTE) of the MSWPT. The metasurface-based MSWPT system is characterized and measured for both near-field and far-field scales. The measured results exhibit that the PTE of the MSWPT is improved from 5.4 % to 50.1 % with the metasurface slabs at a PTD of 50 cm (near-field scale at 6.78 MHz), i.e. 9.3 times improvement while the PTE at a PTD of 140 cm (far-field scale at 433 MHz) is improved from 2.3 % to 9.4 % with the metasurface slabs, i.e. 4.1 times improvement. In addition, the PTE measurements with the lateral and angular misalignment conditions in the MSWPT system are investigated for practical applications. Results show that the metasurface slabs enhance the PTE of the MSWPT even in the misaligned condition. more...

Optical metasurfaces, i.e., thin planar structures with subwavelength metal or dielectric unit cells, have attracted great interest due to their intriguing capabilities of shaping light in both linear and nonlinear regimes. However, their saturable absorption properties and corresponding applications have so far been rarely exploited. Here we report on passively mode-locked ytterbium-doped fiber lasers utilizing saturable metasurfaces made of periodically arranged gold nanorods. Three 400-nm-period metasurfaces with gold nanorods of different lengths (from 210 to 240 nm) and plasmonic resonances (from 968 to 1044 nm) are fabricated and found to exhibit nonlinear absorption with decent modulation depths, facilitating the formation of mode-locking states at 1060 nm. The corresponding lasers generate typically mode-locked pulses with the duration of ∼ 62.3 ps, repetition rate of 10.33 MHz, and signal-to-noise ratio of ∼74 dB. Our experimental results demonstrate that the gold nanorod-based metasurfaces can be used as relatively broadband mode-lockers in the 1-μm-wavelength range. more...

The growth of the nanotechnology and intense studies on subwavelength nanophotonics has facilitated research in the field of metamaterials and metasurfaces. Optical waveguides has been a proper platform for metasurface applications because of its versatility and numerous infrastructure. In this paper, a metasurface interconnection method via attaching antiresonant waveguide is introduced. The antiresonant waveguide allows all-waveguide type inline metasurface schematic. Numerical calculations verified that the antiresonant fiber is capable of metasurface interconnection. In addition, an inline optical transmission modulator has been proposed by filling liquid crystal inside the hollow region. A liquid crystal device with on- state transmittance of 93.64%, and off-state transmittance of 0.96% with a smooth transient region are achieved. more...

We present electrically small, multi-band, metamaterial-inspired antennas with adequate radiation characteristics and isolation enhancement. The antenna element consists of a complementary split-ring resonator (CSRR) embedded in a small monopole that has a size of $\lambda /8\,\,\times \,\,\lambda /10$ at the lowest frequency band, while rectangular patches are placed underneath it to further improve the performance. The antenna operates at the 2.4-2.5/2.9-4.8/5.1-6.5 GHz frequency bands. Moreover, we propose a systematic, metamaterial-based approach in order to improve the isolation between two of these small, closely spaced antenna elements at the lowest and highest frequency bands. The proposed techniques reduce the coupling by up to 29 dB without increasing the size of the structure. In particular, the isolation enhancement at the highest frequency band of interest is remarkably wideband. The cable effect, which is a common concern during the measurements of small antennas, is examined as well. The proposed antennas are not only small but also densely packed and can be easily integrated with modern, compact communication devices with advanced functionality. Simulations along with experimental results validate the effectiveness of our design. more...

The equivalent circuit model (ECM) is developed by using a MATLAB code to analyze graphene-based multi-band chiral metamaterial absorbers composing graphene-based square split-ring resonator arrays in the terahertz (THz) range. The absorbers are simulated numerically by the finite element method (FEM) in CST Software to verify the ECM results. Our introduced multi-band absorbers can be used as suitable platforms in polarization-sensitive devices and systems in the THz range. We have designed four tunable graphene-based chiral metamaterial absorbers containing one, two, three, and four gaps in their arms, respectively. The absorber with one gap has four absorption bands (two for TE and three for TM, one band of both modes approximately overlaps) with absorption >50%. The absorber with two gaps has three absorption bands (two for TE and two for TM, one band of both modes approximately overlaps). The absorber with three gaps has four absorption bands (three for TE and two for TM, one band of both modes approximately overlaps). The absorber with four gaps has three absorption bands (three for TE and two for TM, two bands of both modes approximately overlap). They work in the 1-5.5 THz with maximum linear dichroism (LD) responses of 98, 99, 89, and 77%, respectively. The designed absorbers are dynamically tunable. Additionally, by a 90° rotation of the incident electromagnetic fields, it is possible to switch between the number and/or location of absorption bands making these absorbers a promising candidate for future THz systems. ECM results are following the FEM ones. The proposed ECM procedure is a simple and fast way to recognize the characteristics of the designed absorbers. Our proposed absorbers could be promising enablers in future THz systems. more...

A thorough review of metasurfaces (2D planar counterpart of metamaterials) based on electromagnetic (EM) classification i.e., $\varepsilon $ -negative (ENG), $\mu $ -negative (MNG), and double-negative (DNG) materials is presented in this paper. The concept of left-handedness in metasurfaces which comprise of ENG, MNG, and DNG is demonstrated through basic Maxwell’s equations. This concept is then explained by designing unit cells of each category along with numerical extractions of parameters through Kramers-Kronig rule (for ENG) and Nicholson-Ross-Weir (NRW for MNG and DNG), thereby verifying unit cell designs. This helps the reader to have complete insight regarding the design, simulation, and mathematical verification of ENG, MNG, and DNG-based metasurfaces used in antenna applications for gain enhancement, absorbers to reduce radar cross-section and beam forming. Moreover, the paper also provides a thorough review of the state-of-the-art research based on these metasurfaces in antenna applications delineated above. A comparison of the state of the art is drawn at the end of this review. more...

Many previous VO2-based perfect metamaterial absorbers can enhance light-matter interaction when VO2 is in metal phase but with weak light-matter interaction when VO2 is in insulator phase. In this work, an actively tunable terahertz (THz) absorber with the ability to switch between dual-band absorption and single ultra-narrowband absorption is theoretically proposed and numerically demonstrated. The electric dipole resonance, localized surface plasmon resonance, Fabry-Perot resonance and surface lattice resonance are utilized to enhance light-matter interaction. The simulation results show that an absorption peak has an absorptance of 98.6% and the Q-factor reaches 351.5 when vanadium dioxide (VO2) is in the insulating phase. Then, two absorption peaks with the absorptance of 96.2% and 99.5% are presented at 2.30 THz and 3.31 THz, respectively, when VO2 is in the metallic phase. To better understand the physical mechanisms of this absorber, the influence of parameters is further investigated. The proposed absorber with the dynamically tunable characteristic between such distinct states can pave numerous promising applications in sensing, imaging, active switch and modulator. more...

A metamaterial (MM) absorber and its application in liquid sensing are proposed in this article. The unit cell of the sensor structure consists of two quartz substrates with metallic patterns and sheets printed on their surfaces. The liquid tunnel with optimal thickness is created by placing the glass micro-pearls between the quartz substrates. By incorporating dual semitoroidal resonators, the enhanced spectral shift per refractive index unit (RIU) improves the sensitivity of the proposed design effectively, and sensitive frequency detection with 52.50 GHz/RIU is numerically demonstrated. Moreover, we experimentally demonstrate its application for determining ethanol and sucrose solutions with different concentrations and three commonly used oils. A good agreement with the experiments and simulations is obtained. In addition, the proposed sensor has the advantages such as compact size, easy fabrication, and reusability, which offers excellent potential in food industry applications. [ABSTRACT FROM AUTHOR] more...

An extraction technique is proposed for electromagnetic characterization of metal-backed multilayer metamaterial (MM) structures demonstrating reflection-asymmetric property. It uses recursive scattering parameters for direct and reversed configurations of a multilayer structure. The algorithm was validated by comparing the extracted electromagnetic properties of multilayer structures composed of different MM unit cells with those determined by one-layer-only S-parameter methods. Two-layer and three-layer multilayer structures composed of MM slabs with C-shaped and $\Omega $ -shaped rings and an FR4 material were fabricated and measured for experimental validation. A sensitivity analysis was also conducted to quantitatively evaluate the performance of our method against a small air gap between the multilayer structures and the short-circuit termination and between adjacent MM slabs within these structures. Strong agreement was observed between the resulted obtained using our method and measurements. [ABSTRACT FROM AUTHOR] more...

This article focused on broadening the azimuth plane scanning range of surface-mounted linear arrays. First, 14 kinds of basic radiation models were theoretically analyzed, and the basic model with a 180° covered main lobe was pointed out. Afterward, a wide-beam surface-mounted antenna was designed using an in-phase-reflection electromagnetic bandgap structure based on the selected element model. Finally, an azimuth plane wide-angle scanning linear array with wide-beam surface-mounted elements was arranged, decoupled, and evaluated. This array can scan its beam in the range of 90° ± 85° in the azimuth plane with a 3 dB gain fluctuation, which verified the effectiveness of the proposed theoretical model. [ABSTRACT FROM AUTHOR] more...

This article presents the design and experimental characterization of microwave structures based on parity time-reversal duality symmetric bifilar edge waveguides (PTD-BEWs) realized through a parallel plate waveguide (PPW) loaded by a metasurface. The analyzed structures include transmission lines with bends and multiple line arrangements. Due to their unique symmetry properties, these structures are robust against backscattering, thus resembling the behavior of topological WGs, despite the fact they are reciprocal. This makes it possible to guide the electromagnetic (EM) waves along the edge with low insertion losses and unique matching properties. Measurements, performed in the frequency range between 24 and 32 GHz, have confirmed the feasibility of the theoretical concept. [ABSTRACT FROM AUTHOR] more...

A metasurface (MTS)-based reactive impedance surface (RIS) is proposed to load an eccentrically placed monopole above a ground plane for controllable wideband omnidirectional radiation. With the off-center placement, the radiation pattern of the monopole is not omnidirectional anymore, with ripples in the H-plane measured by un-roundness. The RIS is introduced above and in parallel with the finite ground plane and the monopole is placed in the center of the RIS. Characteristic mode analysis (CMA) is used to analyze the RIS-loaded monopole and to reveal the mechanism. It is found that two desired modes determine the bandwidth of the omnidirectional radiation together. The resonant frequency of the RIS mode can be easily tuned by changing the sheet impedance of the RIS. A metal-dielectric-metal constructed MTS is proposed to realize the RIS. The grid impedance of the MTS can be tuned easily by changing the size of the patch in the unit cell. For a single monopole placed off-center above a rectangular ground plane (150 mm $\times300$ mm), the impedance bandwidth is increased from 19.6% to 32.4% with $\vert \text{S}_{11}\vert < -10$ dB at $f_{0} =2.5$ GHz. And the maximum un-roundness is reduced by 66.7% from 4.8 to 1.6 dB over the frequency band of 2.2–2.8 GHz. For a two-monopole antenna system with the same ground plane mentioned above, the loading MTS increases the impedance bandwidth of the monopole from 19.2% to 32.4% with $\vert \text{S}_{11}\vert < -10$ dB at $f_{0} =2.5$ GHz. When only one monopole is loaded with MTS in the system, the maximum un-roundness is reduced by 65.5% from 5.5 to 1.9 dB in the frequency band of 2.2–2.8 GHz. When both monopoles are loaded with MTS in the system, the maximum un-roundness is reduced by 40% from 5.5 to 3.3 dB in the frequency band of 2.2–2.8 GHz. The antenna prototypes are fabricated and measured. [ABSTRACT FROM AUTHOR] more...

The piezoelectric actuator is a kind of actuation device that acts through the inverse piezoelectric effect. Due to advantages of high precision, low power consumption, compact size, and flexible structure design, they have a wide range of applications in optics, robotics, microelectromechanical systems, and so on. Piezoelectric materials are the core materials for piezoelectric actuators. In this review, recent developments in high-performance piezoelectric materials (HPMs) are introduced, including relaxor ferroelectric crystals, textured ceramics, piezoelectric metamaterials, and so on. The advances of piezoelectric actuators are introduced in this review based on the developments of those piezoelectric materials, where the relationship between the figure of merits of materials and the performance of actuators is also discussed. Finally, we present outlooks and challenges for piezoelectric materials and actuators. [ABSTRACT FROM AUTHOR] more...

An air-filled mode selective transmission line (AF-MSTL) consisting of an opened groove gap waveguide (GWG) and an inverted coplanar waveguide (CPW) is proposed in this letter. By using the electromagnetic band gap (EBG) structure, the electrical contacts of AF-MSTL are not required during the assembly process. To verify the design, the AF-MSTL prototype, connected to two designed grounded-CPW (GCPW) to AF-MSTL transitions to build a back-to-back structure, has been fabricated and measured. A measured insertion loss of 0.2857 dB/mm at 110 GHz is achieved, which is lower than the dielectric filled (DF) MSTL of 0.5012 dB/mm at 110 GHz. [ABSTRACT FROM AUTHOR] more...

The papers in this special section focus on recent advancements in this field and provide an overview of the potential applications of this technology in the next generation antenna devices, with emphasis on metamaterial-based enhancements enabling real-time control of their radiation characteristics. [ABSTRACT FROM AUTHOR] more...

In next- and future-generation wireless communication systems, demand for antenna technologies has continued to grow. This demand has fueled the development of design techniques that pursue more desirable configurations with simplification, electrically small size, and low profiles as well as multiple functionalities and higher performance with multiband, broad bandwidth, single/multibeam scanning, polarization control, and dispersionless group delay. [ABSTRACT FROM AUTHOR] more...

We theoretically study the ultraviolet sharp Fano resonances (FRs) produced through the optical diffraction coupling between the narrow collective surface mode supported by the array periodicity and the broad high-order magnetic plasmon (MP) mode in the three-dimensional (3D) metamaterials consist of aluminum vertical split-ring resonator (Al 3D-SRR) array. The linewidth of the second-order MP induced ultraviolet FR is as narrow as 7.3 nm due to the diffraction coupling feature of the magnetic fields. The intensity and position of the ultraviolet sharp FR are easily tuned by varying the array period as well as the base and arm length of the Al 3D-SRR structure. The conventionally defined performance index of optical sensors, sensitivity ($S$) and figure of merit (FoM) of our designed 3D metamaterial in the ultraviolet range, possess quite large values of 88 nm/RIU and 12, respectively. [ABSTRACT FROM AUTHOR] more...

In this letter, a terahertz metamaterials broadband perfect absorber based on molybdenum disulfide (MoS2) is proposed. The absorber is composed of a single layer of MoS2 and a metal plate separated by thin polyimide. The single-layer MoS2 structure consists of four rectangular strips transformed by fillets. Numerical simulations using the full-vector finite element method show that the new hybrid mode can be excited and the electric fields of the modes can be redistributed due to the introduction of fillet MoS2 metamaterial structure, which effectively improves the absorption performance of devices. In the range of 1.8 to 2.2 THz, the absorptivity is close to 100%, and the absorption curve is much flatter, which provide a useful reference for terahertz modulation equipment. [ABSTRACT FROM AUTHOR] more...

A new method for the synthesis of single-bit reconfigurable passive electromagnetic skins (1RP-EMSs) featuring advanced beam shaping capabilities is proposed. By using single-bit unit cells, the multiscale problem of controlling 1RP-EMSs is formulated as a two-phase process. First, the macroscale synthesis of the discrete surface current that radiates the electromagnetic (EM) field fitting the user-designed requirements is performed by means of an innovative quantized version of the iterative projection method (QIPM). Subsequently, the meta-atom states of the 1RP-EMS are optimized with a customized implementation of the System-by-Design paradigm to yield a 1RP-EMS that supports such a feasible reference current. A representative set of numerical results is reported to assess the effectiveness of the proposed approach in designing and controlling single-bit meta-atom RP-EMSs that enable complex wave manipulations. [ABSTRACT FROM AUTHOR] more...

For the first time, liquid metal is incorporated with an elastomer platform for designing an electromagnetic bandgap (EBG)-backed wearable antenna that can be placed on human body. Here, the liquid metal (EGaIn) is injected into a soft and stretchable Ecoflex 0030 substrate for making a fluidic slot antenna and a liquid EBG (LEBG) structure. The LEBG is then placed at the back of the slot antenna for mitigating the effects of human body and improving the antenna performance. It has an antenna gain of 5.80 dBi in the boresight and a low specific absorption rate (SAR for 1 g U.S. standard) of 1.25 W/kg. Since liquid metal is highly dense, the ground plane of the LEBG is separated into six segments, which are connected through narrow bridges. Due to the use of such design, the wobbly effect of the dense liquid metal can be reduced significantly. The measured impedance bandwidth (3.23–3.59 GHz) of the proposed antenna is very stable and not affected much by its own structural deformation on human body. Since the elastomer platform is waterproof, it makes the antenna characteristics very stable in humid environment, which is a very desirable feature for wearable antennas. [ABSTRACT FROM AUTHOR] more...

This article presents a novel metamaterial-inspired, miniaturized, vertically polarized (VP), pattern reconfigurable dielectric resonator (DR) antenna/phased array for sub-6 GHz fifth-generation (5G) beam-steering applications. The proposed antenna is miniaturized by creatively loading the mushroom structure on the DR, and the beam steering/scanning function is realized using a pattern reconfigurable configuration. Owing to the extra loading capacitance and inductance from the VP mushroom, the resonance frequency of the DR shifts to the lower region. Based on the classical Huygens radiation principle and image theory, a novel miniaturized Huygens form with a tilted pattern is developed. Then, by exploring a switchable magnetic current source, a pattern reconfigurable Huygens design is proposed and studied. For verification, a pattern reconfigurable DR antenna with eight beams and a low-cost 4 $\times4$ wide-angle beam-scanning phased array using the reconfigurable DR cells for 5G microcell and macro-base station demands are fabricated and measured, respectively. The measured −10 dB impedance bandwidth and peak gain are 3.30–3.75 GHz (12.8%) and 4.8 dBi, respectively. Also, the proposed 2-D phased array can scan up to ±60° and supports a 3 dB beam coverage up to ±80° in each region. The proposed beam-steering antenna/array is a promising candidate for 5G microcell and macro-base station applications. [ABSTRACT FROM AUTHOR] more...

The analysis, modeling, design, simulation, and experimental evaluation of a 400 GHz on-chip antenna is presented, with a novel combination of metastructures, a microstrip patch, a quartz-based dielectric resonator, and a diamond-based antireflex layer—all integrated on a 35 nm InGaAs metamorphic high-electron-mobility transistor (mHEMT) technology. The said combination represents a first-time implementation for all submillimeter-wave-capable semiconductor technologies. Circumventing a substrate-thickness limitation of $4.98 \mu \text{m}$ , a state-of-the-art broadband, efficient, and to-the-broadside radiating on-chip antenna solution is realized. It achieves a measured impedance bandwidth of 100 GHz, 25.6%, spanning from 340 to 440 GHz. A consistent pattern bandwidth of 75 GHz is recorded, with an efficiency of 50%–66% and a directivity of up to 10.4 dBi—or 27 dBi, with the utilization of a polypropylene-based dielectric lens. The theoretical analysis of the proposed on-chip antenna is presented, and two modeling approaches are shown and compared. Between the analytical and the 3-D electromagnetic (EM) model, the latter is chosen as it offers greater precision in defining the metastructure unit cell and enables the inclusion of the remaining components of the proposed antenna setup. The measured reflection coefficient and far-field patterns are compared to simulations via the utilized model, and a strong agreement is observed. These far-field patterns are acquired with the on-chip antenna inserted within a broadband 400 GHz transmitter submillimeter-wave monolithic integrated circuit, processed on the 35 nm mHEMT technology. [ABSTRACT FROM AUTHOR] more...

An innovative process for the design of static passive smart skins (SPSSs) is proposed to take into account, within the synthesis, the electromagnetic (EM) interactions due to their finite (macrolevel) size and aperiodic (microscale) layouts. Such an approach leverages the combination of an inverse source (IS) formulation, to define the SPSS surface currents, and of an instance of the system-by-design paradigm, to synthesize the unit cell (UC) descriptors suitable for supporting these currents. As for this latter step, an enhanced artificial intelligence (IA)-based digital twin (DT) is built to efficiently and reliably predict the relationships among the UCs and the nonuniform coupling effects arising when the UCs are irregularly assembled to build the corresponding SPSS. Toward this end and unlike state-of-the-art approaches, an aperiodic finite small-scale model of the SPSS is derived to generate the training database for the DT implementation. A set of representative numerical experiments, dealing with different radiation objectives and smart skin apertures, is reported to assess the reliability of the conceived design process and illustrate the radiation features of the resulting layouts, validated with accurate full-wave simulations, as well. [ABSTRACT FROM AUTHOR] more...

This article proposes an ungrounded shielding gasket that can be applied in the packages of integrated circuits (ICs). Benefitting from the slow-wave and SNG properties, the gasket with an electrically small transverse length, i.e., 3 mm, can change and mitigate the resonances of the original package, and thus a wideband shielding from 5 to 18 GHz, can be achieved. Meanwhile, the proposed gasket is validated to benefit the heat dissipation capability of the original package, which thus shows advantages in both shielding bandwidth and cooling performance. The proposed gasket is experimentally validated, where a good agreement between simulation and measurement is achieved. Such a shielding technique can serve as a good candidate in the shielding packages of compact and wideband ICs. [ABSTRACT FROM AUTHOR] more...