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This paper presents a detailed design, simulation, measurement, and validation of an Inductive Intertwined Frequency Selective Surface (IIFSS) employing a 2.5D configuration (2.5DIIFSS). The proposed unit cell achieves high miniaturization, with dimensions as small as $0.0096\lambda _{0} \times 0.0096\lambda _{0}$ at 0.268 GHz, using 4 vias per unit cell. The compact design delivers a fractional bandwidth of 87% for the passband associated with the fundamental harmonic. Additionally, the unique 2.5D configuration introduces extra inductance and capacitance, effectively shifting higher harmonics to higher frequencies, thus maintaining a stable stop band beyond the fundamental harmonic. The angular stability analysis reveals minimal variation up to an incidence angle of 60° for both TE and TM modes across the fundamental harmonic. To elucidate the underlying physics, an equivalent circuit model was developed, accurately capturing the fundamental harmonic behavior of the structure. To further validate the design and demonstrate its scalability, a prototype was designed and fabricated for operation at 2.4 GHz, addressing the measurement challenges associated with the original 0.268 GHz design. This prototype was rigorously tested in a transmission regime, with measurement results showing good agreement with simulation data, thereby confirming the efficacy and practicality of the proposed design.

This study introduces a reflective polarization converter based on metasurface, designed to offer both Linear-to-Linear polarization (LLP) and Linear-to-Circular polarization (LCP). The design comprises two periodically arranged split ring resonators with a slotted stripe that operate in the X, Ku, and K bands. The three discrete frequency bands demonstrate over 90% Polarization Conversion Rate for LLP for oblique incidence waves up to 45°. Additionally, the proposed converter achieves Left-Hand Circular Polarization (LHCP) in two sub-bands and Right-Hand Circular Polarization (RHCP) in two sub-bands for oblique incidence waves up to 45°. The metasurface design introduced in this study exhibits significant potential for satellite applications in 6G networks, owing to its versatile LLP and LCP conversion properties in the X, Ku, and K bands.

We present a 1-bit reconfigurable intelligent surface (RIS) operating at millimeter-wave frequencies that suppresses the undesired grating lobes encountered in binary phase modulation schemes and achieves high resolution beam steering. We incorporate fixed, random phase delays at each unit cell of the surface which breaks the periodicity of the phase quantization error and suppresses side lobes. Additionally, the random phase delays reduce the beam pointing error – a limitation of binary RISs - which can be beneficial in applications that require high resolution beam steering. The proposed topology allows for scalable RIS apertures that are compatible with printed circuit board (PCB) fabrication technology. It consists of four metasurface tiles of 256 radiating elements ( $16\times 16$ ) connected on a separate control board that houses the control unit and power supply. The prototype is designed to operate at 27.2 GHz and perform electronic beam steering in ±60° in both azimuth and elevations planes. A quantization lobe reduction of more than 10 dB is achieved with the proposed technique and the surface is well suited for mmWave 5G communication scenarios to enhance signal coverage and signal-to-noise ratio.

Line-waves are one-dimensional modes propagating at the interface between two planar complementary surfaces, characterized by tight transversal confinement of the field. Despite their unique guiding properties, their use in real microwave devices is still in the early stages, lacking a comprehensive design procedure and comparative analysis with conventional guiding structures. To fill this gap, here we report a simple and straightforward workflow for designing waveguides supporting 1D modes propagation. The design is based on the analytical relations existing between the surface impedance of the propagating modes and the sheet impedance of the metasurfaces, which allow quick retrieval of the geometrical parameters of the complementary metasurfaces sustaining the line-wave propagation. This approach is used to design several waveguiding layouts and compare their transmission performance through full-wave simulations accounting for dielectric and ohmic losses. Finally, experimental results for some selected designs in the microwave regime are provided, and a thoughtful comparison between a line-wave waveguide and an equivalent microstrip transmission line is carried out to assess the suitability of these devices for efficient high-frequency waveguiding.

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.

Reconfigurable intelligent surface (RIS)-aided near-field communications are investigated. First, the necessity of investigating RIS-aided near-field communications and the advantages brought about by the unique spherical wave-based near-field propagation are discussed. Then, the family of patch array-based RISs and metasurface-based RISs is introduced along with respective near-field channel models. A pair of fundamental performance limits of RIS-aided near-field communications, namely, their power scaling law and effective degrees of freedom (EDOF), are analyzed for both patch array-based and metasurface-based RISs, which reveals the potential performance gains that can be achieved. Furthermore, associated near-field beam training and beamforming design issues are studied, where a two-stage hierarchical beam training approach and a low-complexity element-wise beamforming design are proposed for RIS-aided near-field communications. Finally, a suite of open research problems is highlighted for motivating future research. [ABSTRACT FROM AUTHOR]

The latest discussions on upcoming 6G wireless communications are envisioning future networks as a unified communications, sensing, and computing platform. The recently conceived concept of the smart radio environment, enabled by reconfigurable intelligent surfaces (RISs), contributes toward this vision, offering programmable propagation of information-bearing signals. Typical RIS implementations include metasurfaces with almost passive unit elements capable of reflecting their incident waves in controllable ways. However, this solely reflective operation induces significant challenges for RIS optimization from the wireless network orchestrator. For example, RISs lack information to locally tune their reflection pattern, which can be acquired only by other network entities and then shared with the RIS controller. Furthermore, channel estimation, which is essential for coherent RIS-empowered communications, is challenging with the available RIS designs. This article reviews the emerging concept of hybrid reflecting and sensing RISs (HRISs), which enables metasurfaces to reflect the impinging signal in a controllable manner while simultaneously sensing a portion of it. The sensing capability of HRISs facilitates various network management functionalities, including channel parameter estimation and localization, while giving rise to potentially computationally autonomous and self-configuring metasurfaces. We discuss a hardware design for HRISs and detail a full-wave electromagnetic (EM) proof of concept. The distinctive properties of HRISs, in comparison to their solely reflective counterparts, are highlighted, and a simulation study evaluating HRISs’ capability for performing full and parametric channel estimation is presented. Future research challenges and opportunities arising from the HRIS concept are also included. [ABSTRACT FROM AUTHOR]

The next generation of infrared (IR) sensors may enable unprecedented applications in fields like spectroscopy, health monitoring, and communication systems. For instance, metasurface-enhanced micro-electromechanical system (MEMS)-based IR sensors have demonstrated excellent performance in terms of responsivity and spectral-selectivity. However, it is burdensome to experimentally determine the performance limits of this and other IR sensing technologies as it requires time-consuming and expensive systematic testing not always feasible in research settings. To address this challenge, an automated solution for characterizing miniaturized sensing devices is presented in this paper and applied to experimentally determine the performance limits of MEMS-based IR sensors. The system offers low-cost, rapid, and automated characterization of on-chip IR sensors, determining key performance metrics such as noise, responsivity, and noise-equivalent power. The platform is flexible, easily adapted to different types of devices, chip layouts, and light sources, and is designed to test a large number of sensors within the same wafer −spending ~20 seconds per device− using a combination of optical and radiofrequency interrogation techniques. The system has been applied to test over 1500 MEMS-based IR sensors. Collected data revealed hidden trade-offs between responsivity and noise with respect to the device thickness and allowed a statistical analysis of sensing response versus device geometrical dimensions. The best-performing devices exhibit a quality factor, responsivity, fluctuation induced noise, and noise-equivalent power of 2391, 164 Hz/nW, 0.257 Hz/ $\sqrt {\mathbf {Hz}}$ , and 5.01 pW/ $\sqrt {\mathbf {Hz}}$ respectively. The proposed automated platform provides an efficient and cost-effective solution for characterizing the next generation of IR sensing devices.

Resonant cavity antennas (RCAs) produce reasonably high gains (15-20 dBi) with thin form factors, and find strong applications in wireless communication. A recently proposed RCA achieves a broad 3dB bandwidth of over ${20\%}$ using a spherically modified ground (SMG), but its curved metallic surface is very costly to manufacture at high frequencies. In this paper, we planarize this broadband RCA by replacing the SMG with a true-time-delay metasurface (TTD-MS) which mimics the both reflection phase shift and time-delay of the SMG over antenna’s entire operation bandwidth. We verify our proposed method through full-wave simulations and experimental measurements. Our measurements demonstrate an impedance bandwidth $({\rm S_{11}\le -10\:{\mathrm{ dB}}})$ of ${32\%}$ and a 3dB-gain bandwidth of ${21.3\%}$ (from 11.95 GHz to 14.8 GHz), with a peak gain of 17.5 dBi. This performance is comparable to the RCA with the SMG, but we have planarized the structure, leading to dramatic simplification in fabrication. The height of the cavity is also reduced by ${20\%}$ . The proposed TTD-MS RCA exhibits high gain, stable impedance matching, reduced cavity height and simple configuration, and elucidates a new path to RCA design for microwave and mm-wave frequencies.

We propose the graphene film with trapezoid-shaped nanoparticles (GTNAs) to transport particles. In our design, the conversion of plasmon surface resonances can be realized without changing the excitation light source. By sequentially activating three closely packed potential wells, nanoparticles can be transported between adjacent traps in a creeping manner. Three adjacent potential wells form a linearly repeating array structure, forming a nano-optical conveyor belt. When the resonant wavelength is 5.5 μm, and the power density is 0.4 mW/μm2, we verified that the target particle can move along the direction of the hot spots. In addition, the movement of nanoparticles in a liquid environment will be interfered with by viscous resistance and the random Brownian motion process. Since particles produce hysteresis or derailment during transmission, we also analyzed the time interval of switching the Fermi level to manipulate the particle in real-time. The three-dimensional finite-difference time-domain method has been used to verify that the design of this paper provides a conveyor belt in tunable graphene without rotating the polarization angle of the light source and has broad application prospects in biomedical diagnostics.

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.

In the trend of spectrometer miniaturization, the design of combining computational imaging with the metasurfaces has been proved to be an efficient and feasible solution. Nevertheless, determining the optimal filter array design remains a compelling subject for ongoing research. A computational microspectrometer consisting of a planar array of broadband optical filters is proposed, which are formed by low-loss freeform dielectric metasurfaces. Diverse metasurface designs are obtained by using binary graph generation technology, and rich spectral responses are obtained. The final performance of microspectrometer depends on the trade-offs between reconstruction time, relative error, footprint, and resolution. To optimize these trade-offs, we have improved the existing freeform metasurface design scheme and optimized the filter array design. Simulation results show the proposed microspectrometer can successfully reconstruct the spectra over the 380 to 680 nm spectral range with a resolution of 1 nm. In summary, we propose an integrated design including meta-pixel design, filter combinations and simulation test flow, which leads to an impressive performance microspectrometer.

Planar metasurface lens structures based on microstrip technology, optimized to control the propagation of surface waves (SWs), are presented. Two different lenses are studied, i.e., a converging or plane-wave-like lens and a diverging lens. Near-field simulations and measurements are reported to demonstrate the guiding features of the designed lenses, which have also been proposed as a feeding system for planar leaky-wave antennas (LWAs). Far-field results show improvements in terms of reduced cross polarization and sidelobe level as well as increased realized gain when compared without these SW lenses. The proposed structures are also simple to manufacture and are employed herein to control SW fields generated by a TM0 surface-wave launcher (SWL) integrated in the ground plane of the developed prototype demonstrators. Some applications include high-gain radar and remote sensing satellite antennas as well as microwave and millimeter-wave communication systems.

This research proposes a quadruple-cluster leaf-shaped metasurface (MTS)-based circularly-polarized (CP) stacked-patch antenna array with hybrid coupler feed network for sub-6 GHz 5G applications. In the study, the leaf-shaped MTS-based CP stacked-patch antenna is characterized by characteristic mode analysis (CMA). In the antenna design, one cluster of the quadruple-cluster leaf-shaped MTS-based antenna array consists of $4\times4$ leaf-shaped MTS elements; and the hybrid coupler feed network is used to enhance impedance bandwidth (IBW), axial ratio bandwidth (ARBW), and antenna gain. Simulations are carried out and an antenna prototype is fabricated and experiments undertaken. The measured IBW, ARBW, and maximum gain at the center frequency (4 GHz) are 62.5% (3.4– 5.9 GHz), 21% (3.8– 4.54 GHz), and 9.04 dBic at 3.9 GHz. The novelty of this research lies in the use of: (i) the CMA concept to design and develop the leaf-shaped wideband MTS-based stacked-patch antenna with CP radiation pattern; and (ii) a low-complexity hybrid coupler feed network to enhance the IBW, ARBW and gain.

This paper provides new designs and realizations of leaky-wave (LW) metasurface (MTS) antennas fed at multiple points. The design technique is based on the direct integral-equation solution by the Method of Moments (MoM). The unknown is no longer the current distribution, but the impedance profile itself. The method can be used to generate any shaped radiation pattern in amplitude, phase, and polarization, provided that the antenna area and the feeder illumination are consistent with the shape of the desired radiation pattern. The algorithm can also be used to design multi-functional antennas (multibeam, multiband, dual-polarizion, etc). While multiple feeds are traditionally used to implement multiple fonctionnalities, they may also be required for the efficient generation of a single shaped beam, thus leading to a higher surface-wave (SW) to LW conversion efficiency. The present paper shows for the first time, two realizations of MTS design with the integral equation formalism, which require the usage of multiple feeds: a circularly polarized sectoral conical beam and a dual-polarized broadside beam MTS antenna. The good comparison between measurements and numerical predictions shows the effectiveness of the design method.

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.

A deep learning approach based on a diffusion model is proposed to yield metasurface unit cell designs. This method takes desired two-port scattering parameters along with the frequency of operation in an attempt to synthesize three-layered metasurface unit cells. The core of this approach lies in casting the three-layered unit cell synthesis process as conditional three-channel binary image synthesis. The conditions are governed by the desired scattering parameters at a given frequency whereas the binary nature implies the presence and absence of metallic traces. Once synthesized, these unit cells are placed beside each other, without any further tuning, to form the final metasurface. The performance of the whole metasurface, for three different design scenarios, is then tested against full-wave simulation and/or experimental data.

In this research, a multi-shaped metasurface broadband circularly polarized (CP) patch antenna with parasitic elements is proposed for 5G new radio (NR) applications. The proposed metasurface CP patch antenna comprises triple-layered substrates without air gap. The upper layer sits with multi-shaped metasurface elements and parasitic patches. The middle layer consists of an L-shaped slot functioning as the ground plane, and the lower layer contains a microstrip and a fan-shaped stub functioning as the feed line. The proposed metasurface CP patch antenna with parasitic elements is evaluated using characteristic mode analysis (CMA). The CMA results indicate that the modal significance of Modes 1 and 2 of the multi-shaped metasurface CP antenna are orthogonal, giving rise to circular polarization. The non-CP radiation of Modes 3 and 4 are suppressed by using the multi-shaped metasurface elements and parasitic patches. The measured impedance bandwidth and axial ratio bandwidth are 42.85% (3.4 – 4.9 GHz) and 38% (3.27 – 4.6 GHz), achieving the maximum gain of 7.23 dBic at 3.7 GHz. The experiments demonstrate that the multi-shaped metasurface CP patch antenna with parasitic elements is suitable for 5G NR wireless applications. The novelty of this study is attributed to its utilization of multi-shaped metasurface elements and parasitic patches, which effectively suppress non-circularly polarized radiation modes.

We propose dual-band anomalous reflectors operating at 220 GHz and 293.3 GHz for 6G applications. The anomalous reflectors consist of paired patch elements on single-layer and via-free substrates. Based on a local reflection phase design and optimization of paired patch elements, higher-order diffraction modes can be controlled simultaneously at the two harmonic frequencies to achieve highly efficient dual-band anomalous reflections in identical directions with wide reflection angles. Two reflectors are designed and fabricated on cyclo-olefin polymer (COP) substrates for the reflection angles of 45° and 50° and the incident angles of 0° and −10°, respectively. To evaluate their anomalous reflection performances under plane wave illumination in the J-band, we develop a compact reflector characterization system in the J-band exploiting a compact antenna test range (CATR) technique consisting of an offset Gregorian antenna. The developed CATR-based system generates a quasi-plane wave with a beam diameter of approximately 250 mm at 300 GHz at a position 900 mm from the antenna, enabling precise far-field bistatic radar cross section (BRCS) measurements with a compact system size. The measured BRCSs of the prototypes are in good agreement with the simulated results and highly efficient dual-band anomalous reflection operations are experimentally demonstrated with the efficiencies of over 80 % excluding the material losses in both the designed bands.

A deep learning neural network model in conjunction with a method to incorporate auxiliary surface waves is developed for the macroscopic design of transmitting metasurfaces. The main input to the neural network model is the user-defined desired far-field specifications. This network is used to calculate the required tangential electromagnetic fields on the metasurface. These fields will then be augmented by incorporating auxiliary surface waves along the metasurface for power redistribution to satisfy the requirement for having lossless and passive metasurfaces. The designs will then be evaluated using full-wave simulations of metasurfaces with three-layer unit cell topology in both 2D and 3D scenarios.

Metasurface absorber-based terahertz biosensors are one of the emerging fields for sensing devices because of their quick response to changes in the background refractive index, small size, easier fabrication and measurement. In this article, a simple design and highly sensitive metasurface absorber is presented for the detection of the change in background refractive index. The proposed device provides maximum sensitivity of 2.37 THz/RIU, Q-factor of 574.46, and FOM of 540 RIU−1 for the refractive index ranges from 1.0 to 1.37. The meta-atom consists of a bow-tie metallic patch that exhibits dual-band absorptance at 1.93 THz and 2.7 THz for the x-polarized incident wave. The surface resonances at 2.7 THz result in narrow bandwidth with a very high Q-factor which enable the sensing ability of the proposed device. The sensing performance of the proposed metasurface is demonstrated by detecting the tuberculosis bacteria in the blood sample. The extremely good traits of the proposed metasurface absorber-based sensor can be utilized to detect other viruses and bacteria present in the blood cells for diagnosis applications. [ABSTRACT FROM AUTHOR]

Ultramassive multiple-input, multiple-output (MIMO) is one of the key enablers in the forthcoming 6G networks to provide high-speed data services by exploiting spatial diversity. In this article, we consider a new paradigm, termed holographic radio for ultramassive MIMO, where numerous tiny and inexpensive antenna elements are integrated to realize high directive gain with low hardware cost. We propose a practical way to enable holographic radio by a novel metasurface-based antenna, i.e., reconfigurable holographic surface (RHS). Specifically, RHSs incorporating densely packed tunable metamaterial elements are capable of holographic beamforming. Based on the working principle and hardware design of RHSs, we conducted full-wave analyses of RHSs and built an RHS-aided point-to-point communication platform supporting real-time data transmission. Both simulated and experimental results show that the RHS has great potential to achieve high directive gain with a limited size, thereby substantiating the feasibility of RHS-enabled holographic radio. Moreover, future research directions for RHS-enabled holographic radio are also discussed. [ABSTRACT FROM AUTHOR]

In the presented study, a graphene metasurface-based refractive index sensor (RIS) has been developed and built, which is highly beneficial in urine observation. Pregnant and nonpregnant urine samples were simulated and analyzed using effective refractive indices (RIs) of 1.335, 1.34, 1.342, and 1.343. Analysis of the sensor’s sensitivity, resonance frequency, resolution, detection limit (DL), quality factor (QF), figure of merit (FOM), and other parameters has been done. In addition, the numerical investigations employ the finite-element method (FEM). In addition to this, a geometrical variation to attain a highly performing sensor performance is also carried out to demonstrate the effect of various structural parameters on sensitivity, and in a process, optimal parameters are also identified. With this investigation, we were able to obtain the greatest possible sensitivity response value of 200 GHz/RIU, as well as the highest QF and an FOM, and the highest resolution and DL of 11.97, 5.12 RIU $^{-{1}}$ , 0.15 THz, and 0.88 RIU, respectively. Along with that, an innovative realization of the NOR gate has been demonstrated by employing graphene with various graphene chemical potential (GCP) values. [ABSTRACT FROM AUTHOR]

This letter presents a dual-polarized low-profile linear phased array antenna design achieving one-dimensional wide-angle scanning with high polarization isolation. The dual-polarized antenna is constructed based on a metasurface serving different purposes for two polarization cases to achieve high polarization isolation while providing a low profile as a whole. It works as artificial magnetic conductor for x-polarization and radiator for y-polarization. To achieve wide-angle scanning, surface wave effect is exploited for x-polarization while for y-polarization widebeam active element patterns is implemented with a small array spacing of 0.42 λ0 applied. To verify the design, a 1×8 linear array was fabricated and tested. Measured and simulated results both demonstrate that, within a wide operating band, the proposed dual-polarized antenna has a high port-to-port polarization isolation over 29 dB with a low profile of 0.06 λ0. The measured results show that for x-polarization, the linear array can steer its main beam up to 81°, while for y-polarization, it can steer the main beam up to 70°. The cross-polarization levels can maintain below −20 dB during scanning in the general. [ABSTRACT FROM AUTHOR]

A strategy for folded reflectarray with low detectability and enhanced gain is presented. For this purpose, the folded reflectarray is engineered by combining a polarization-conversion focusing metasur face, a frequency selective rasorber (FSR), and a diffusion metasurface. The FSR is composed of a lossy layer and a single-polarized I-shaped slotted frequency selective surface (FSS) between which the diffusive metasurface is inserted for in-band and out-of-band cross-polarized low radar cross-sectional (RCS) reduction. Therein, the antenna exhibits a transmission window and two out-of-band absorptions due to the FSR for copolarized radiation. In contrast to detection, the system operated for both wave incidence. For cross-polarized wave incidence, a wideband RCS reduction is enabled due to the wave-diffusive function of the inserted metasurface. The half profile is guaranteed since the wave bounces two times between the FSS and focusing metasurface. The numerical and experimental results coincide well, both indicating that the low-RCS folded reflectarray achieves a peak gain of 23.5 dB at 10.2 GHz, corresponding to an aperture efficiency of 37.8%. Besides, the antenna achieves 10 dB monostatic RCS reduction within 4.6–8.8 GHz and 9.6–13.8 GHz under copolarization detection and a 10 dB monostatic RCS reduction from 8.4 to 19 GHz under cross-polarization detection. Moreover, good invisibility is also achieved for bistatic RCS reduction. [ABSTRACT FROM AUTHOR]

A broadband circularly polarized metasurface antenna with low radar cross section (RCS) is proposed. Four metasurface elements are arranged orthogonally in a 2×2 array and fed with a 90° phase difference to achieve circular polarization radiation. Each metasurface element is a nonuniform subarray of 4×4 metallic patches. Attributed to the nonuniform property of the metasurface element, two characteristic modes with different resonant frequencies but with similar surface current distributions are formed. By optimizing the geometric parameters of the nonuniform element, the two modes are merged with each other, resulting in bandwidth (BW) broadening of the proposed antenna. Simulated and measured results show that the designed antenna has a −10 dB impedance BW of 49.60%, 3 dB axial ratio BW of 33.13%, and peak gain of 13.17 dBic. Moreover, the RCS reduction is higher than 6 dB in the entire operation frequency band. Additionally, the antenna has a low profile of 0.08λ (λ is the free-space wavelength at an operation frequency of 8.25 GHz). [ABSTRACT FROM AUTHOR]

For the issue of stealth and electromagnetic protection of weapons and equipment in strong electromagnetic pulse (EMP) environment, a transparent double-sided ultrawideband metasurface is proposed to realize wave absorption/absorption-scattering integrated function in two directions. The interactions between backscattered waves produced by the meta-atoms are manipulated and optimized simultaneously by hybrid far-field scattering pattern analysis and genetic algorithm to achieve ultra wideband destructive interference. Both simulation and experimental results demonstrate that the polarization-independent −10 dB radar cross section (RCS) reduction performance can be achieved for more than 128% of the fractional bandwidth (4–18.25 GHz) at forward incidence, and the absorption more than 90% in the range of 2.75–12.75 GHz (fractional bandwidth more than 129%) at backward incidence. The scattering and absorbing characteristics of the metasurface are still well maintained in the range of 0°–50° oblique incidence. The method of this letter has an important prospect in related applications of strong EMP stealth and protection technology under the integration of information equipment in the future. [ABSTRACT FROM AUTHOR]

This letter presents a thermal-switchable absorbing metasurface (TSAM) based on the phase-change material vanadium dioxide (VO2). A thin film of VO2 was deposited on a sapphire substrate by a magnetron sputtering and then sliced into tiny chips, which can be regarded as thermal-switchable resistors. The surface structure of the TSAM element is realized by loading four VO2 chips into a square metallic loop. Polytetrafluoroethylene is used as the substrate due to its high thermal conductivity compared with foam. The TSAM is on the “absorbing” mode when it is heated underneath and is switched into the “reflecting” mode at room temperature. The absorbing mode is characterized by a wideband absorption (8–22 GHz). A TSAM prototype was fabricated and measured to verify this design. [ABSTRACT FROM AUTHOR]

Metasurfaces (MTSs) have emerged in the last years as a promising platform for next generation planar devices in a broad range of frequencies, thanks to their unique field manipulation capability. In particular, in the microwave range this solution is characterized by low-cost, lightweight, and simple integration with electronic circuits, since MTSs can be realized in PCB technology by printing electrically small patches over a dielectric slab. The MTS behavior can then be conveniently described in terms of homogenized boundary conditions of impedance type. Spatial modulation of these boundary conditions allows one to implement a broad class of functionalities, involving space wave or surface wave control, as well as conversion from surface waves to leaky waves. This paper reviews numerical and analytical models, design procedures and microwave applications of modulated MTSs, with particular emphasis on surface wave manipulation, and it discusses expected future developments of this technology.

Metasurfaces have been widely used in the manufacture of various optical components in recent years. Based on all-dielectric metasurfaces, we proposed broadband, high-efficiency (over 83%) quarter-wave plates (QWP) and half-wave plates (HWP), respectively. The structure of the HWP we designed is equivalent to the superposition of two QWPs. The proposed QWP and HWP have high transmission efficiency, with bandwidths of 1376–1624 nm and 1411–1584 nm, respectively. Furthermore, based on the three-layer all-dielectric metasurfaces structure, we proposed a reversible bifunctional metasurface, which can realize the function of QWP and the effect of light deflection. This work may be conducive to the miniaturization and integration of polarization conversion devices.

The development of the next generation of wireless systems is bringing renewed attention to the physical layer of the communication link. Indeed, the new low-latency requirements cannot be satisfied relying only upon the extreme virtualization of the hardware functions. Moreover, conventional wireless systems with fixed characteristics and functionalities are not appropriate to modern electromagnetic environments, which undergo continuous and fast variations. Embedded smartness is a required feature for widening the range of the possible electromagnetic responses to support advanced and novel functionalities. In this paper, we show how the combination of 2D metastructures with conventional antennas and reflectors enables an unprecedented electromagnetic behavior, which is at the basis of new properties, capabilities, and applications. In particular, we propose an emerging design approach based on the shifting of the reconfigurability, adaptivity, sensing, and power management of a wireless system at the physical level, thanks to the use of properly designed 2D metastructures to be coupled with standard antennas and reflectors.

We present the end-to-end design of compact passive and lossless metasurface antennas with integrated feeds. The complete low-profile system consists of a single-layered reactive impedance metasurface on top of a grounded dielectric substrate, and is fed by sources which are embedded inside the substrate. The top impedance layer is implemented with an array of printed metallic wires, each of which is periodically loaded with subwavelength reactive elements (e.g. printed capacitors). An accurate and efficient volume-surface integral equation-based model of the device is developed, and used as the basis for the rapid optimization of the wire impedances, with the goal of producing the desired radiation characteristics. It is found that the optimized designs leverage tailored surface waves to facilitate the realization of extreme field transformations. In particular, we present surfaces capable of wide-angle beamforming up to 60° off-broadside with nearly 100% aperture efficiency. We also demonstrate a multi-input, multi-output, antenna with two embedded sources emitting independent beams at ±20°. The output beams each exhibits an aperture efficiency of around 90%, despite sharing the same physical aperture. Our design framework is supplemented by several feasibility-related constraints, which can significantly enhance the power efficiency as well as the bandwidth of the metasurface antennas when they are implemented in practice. Utilizing these constraints, a Chebyshev pattern antenna with a side lobe level of −20 dB is designed with realistic loaded wires and validated with full-wave simulations. The obtained radiation pattern confirms the ability of the developed framework for arbitrary beam-shaping. The realized power efficiency (limited by copper and dielectric losses) is over 93% and the 3-dB directivity bandwidth slightly over 6%.

Recently, it was revealed that conventional gradient metasurfaces are fundamentally limited on their overall efficiency to reroute the impinging waves towards arbitrary directions in reflection and transmission. Their efficiency is particularly limited for extreme wavefront transformations. In addition, due to the fastly varying impedance profiles that these surfaces require, they usually need high-resolution fabrication processes, limiting their applicability and overall bandwidth of operation. To address these issues, the concept of metagrating was recently introduced, enabling engineered surfaces capable of manipulating light with unitary efficiency even in the limit of extreme wavefront manipulation. Metagratings are periodic or locally periodic arrays operated in the few-diffraction order regime. Their unit cell contains carefully designed scatterers that, in contrast with conventional metasurfaces, do not need to support a continuous gradient of surface impedance, and consequently are far simpler to fabricate and can afford broader bandwidths because of the larger footprint of the constituent elements. Tailoring the coupling towards the few available diffraction channels, metagratings enable wavefront transformations with high efficiency. In this paper, we review the physical mechanisms behind the functionality of metagratings and provide an overview and outlook of the recent progress in this field of science and technology.

Metasurfaces, which are ultrathin planar artificial materials flexibly controlling the phase, amplitude, and polarization of the light, have facilitated the high integration and miniaturization of hyperspectral imaging systems. However, the pushbroom hyperspectral imaging systems based on metasurfaces usually face the large smile and keystone, which reduces the spectral uniformity and the fidelity of the spectral information. Moreover, the employment of many metasurfaces would cause manufacturing difficulties. In this paper, a planar hyperspectral imager only using two metasurfaces is proposed to achieve a spectral resolution of 3.5 nm in 850-1000 nm band with the near diffraction-limited focusing capability, and its maximum of smile and keystone reach to 124.4 μm and 33.6 μm for the image plane size of 4.75 mm × 1.35 mm and pixel size of 26 μm × 26 μm, respectively. This imager can provide a promising avenue for portable environmental monitoring systems and wearable electronic consumer products.

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.

Fifth-generation (and beyond) networks are characterized by ever more demanding requirements in terms of speed, bandwidth, and number of servable users. Fast and reliable access to the main network is mandatory, requiring technologies and procedures that ensure high performing cell search and initial access (IA). Existing phased array antennas (PAAs) are limited by the single beam scanning approach and complex feeding systems. In this paper, a beamforming metasurface that shifts the field manipulation from an electric level to an electromagnetic one is proposed for speeding up the IA procedure with respect to a traditional system using PAAs. The main advantage is given by the simultaneous transmission of multiple signals in different directions. The numerical results demonstrate that a much faster IA with similar success probability can be reached. Our system provides high gain, parallel computation, and scalability for larger systems, becoming a relevant candidate in the new radio and smart electromagnetic environment context.

Meta-lenses are advanced optical devices composed of artificial nano-antenna arrays. Its flat, light-weight, ultra-thin, compact, customizable, and easy-to-integrate advantages enable widely potential usages in new demands. We demonstrate a GaN-based polarization-independent meta-lens-based camera on a drone. The diameter of the meta-lens is 2.6 mm, and the measured focal length is 5.03 mm under the 532 nm light incident. An array of the 750 nm height cylindrical nano-antennas with various sizes of the meta-lens provides the $2\pi $ phase modulation of the focusing phase distribution. The meta-lens is integrated with an image sensor and mounted on the drone to realize the aerial photography and landing assistance. By taking images of the specific pattern on the ground at different heights through the meta-lens, the flying height of the drone can be detected for landing and flying. We trust meta-lens-camera can reduce the weight burden for prolonging flight time. We believe the meta-lens-based optical devices for imaging and sensing is an important key for micro/nano-robots, micro air vehicles, and intelligent sensing devices in the future.

Using machine learning (ML) techniques like deep learning (DL) for accelerated design (forward and inverse) of metasurfaces has attracted great interest. However, most studies are focused on using relatively regular and less complex patterns for specific photonics applications. In this paper, we report significant improvements of our prior developed DL model tested on complex and random metasurfaces by combining the the Network Architecture Search (NAS) method and the spatial symmetry information of the complex metasurfaces. It is found that a shallow and wide neural network will provide better performance for the complex and physics based metasurfaces problem, which is in contrast to the deep trend in existing DL models. Our method can now accurately identify the EM response locations from arbitrary random and complex metasurfaces while the conventional models fail to accomplish. It can also accurately predict the EM response curve by injecting correct symmetry information into the architecture design step. Thus this paper offers a platform to distill the influence of different fundamental operations for complex metasurface design problems. In future, it may play an essential role in determining the most suitable neural network for the complex metasurface problems. Finally, we are sharing this home-generated physics-based dataset [SUTD polarized reflection of complex metasurfaces (SUTD-PRCM)] for future testings from the research community that we believe the best DL model is yet to be found.

Electromagnetic metasurfaces have attracted significant interest recently due to their low profile and advantageous applications. Practically, many metasurface designs start with a set of constraints for the radiated far-field, such as main-beam direction(s) and side lobe levels, and end with a non-uniform physical structure for the surface. This problem is quite challenging, since the required tangential field transformations are not completely known when only constraints are placed on the scattered fields. Hence, the required surface properties cannot be solved for analytically. Moreover, the translation of the desired surface properties to the physical unit cells can be time-consuming and difficult, as it is often a one-to-many mapping in a large solution space. Here, we divide the inverse design process into two steps: a macroscopic and microscopic design step. In the former, we use an iterative optimization process to find the surface properties that radiate a far-field pattern that complies with specified constraints. This iterative process exploits non-radiating currents to ensure a passive and lossless design. In the microscopic step, these optimized surface properties are realized with physical unit cells using machine learning surrogate models. The effectiveness of this end-to-end synthesis process is demonstrated through measurement results of a beam-splitting prototype.

This research proposes a low-profile Z-shaped metasurface (MTS)-based wideband circularly polarized (CP) Fabry–Pérot antenna for C-band satellite communication. The proposed low-cost and low-complexity CP Fabry–Pérot antenna is realized by using three substrate layers: upper, middle, and lower. The substrates are of FR-4 type with a dielectric constant of 4.3 and loss tangent of 0.025. The upper substrate contains $9\times $ 9 periodically-arranged Z-shaped MTS unit cells functioning as the partially reflecting surface and circular polarization conversion, and at the center of the middle substrate sits a corners-truncated square patch. The lower substrate consists of a copper plate with an H-shaped slot at the center of the ground plane and a microstrip feed line. The lower and middle substrates function as the source antenna. The periodic Z-shaped MTS unit cells are utilized to enhance the impedance bandwidth (IBW) and gain of the source antenna and also to convert linearly polarized into CP wave. The antenna dimension is $1.5\lambda _{0} \times 1.5\lambda _{0} \times 0.51\lambda _{0}$ . Simulations are performed and experiments carried out. The measured IBW and axial ratio bandwidth are 64% (4.4– 7.6 GHz) and 18% (4.4– 5.3 GHz) at the center frequency of 5 GHz. In addition, the proposed antenna scheme achieves a measured 3-dB boresight gain bandwidth of 30% (4.3– 5.8 GHz) with the maximum gain of 12.88 dBic at 4.7 GHz, rendering the proposed Z-shaped MTS-based CP Fabry–Pérot antenna operationally suitable for satellite communication. In essence, the novelty of this research lies in the use of the low-cost and low-complexity Z-shaped MTS unit cell to effectively enhance the antenna gain and convert LP into CP wave.

Compared with traditional phase-shifting metasurface design methods, computer-aided optimization has attracted significant interest due to its advantages of high efficiency and a wide modulation range, resulting in the ability to generate various random-shaped meta-atoms to maximize their phase modulation capabilities. However, these meta-atoms often have sharp boundaries, chamfers, microscopic particles, and small holes, which are unsuitable for micro-/nano-fabrication. We propose a method to generate 2D meta-balls, which can generate random-shaped meta-atoms with smooth boundaries free of small particles or holes and suitable for micro-/nano-fabrication. This method can easily control the characteristics of generated shapes, such as boundary continuity and symmetry. It is simple, straightforward, and time-saving, and it helps in computer-aided optimization phase-shifting metasurfaces design.

In this paper we present a high-performance compact phased array antenna which is easy to integrate into mobile devices for 5G-and-beyond wireless telecommunications. The proposed design features high efficiency and wide-scan capabilities. The linear array consists of eight elements realized using substrate integrated waveguide technology in combination with two rows of metasurfaces that are used to optimize the transition towards free space for enhanced impedance matching characteristics. The integrated metasurface structure also enables a larger half-power beamwidth and wide-angle scanning at array level. A prototype has been realized using a dielectric substrate of Rogers RO4003C with relative permittivity of 3.55. The array is designed with an inter-element spacing of half-wavelength at 29.5 GHz and is characterized using dedicated millimeter-wave anechoic and reverberation chambers. The measurement results show that the proposed antenna array can scan from $\phi = -55^{\circ }$ to $\phi = 55^{\circ }$ with a gain fluctuation less than 3 dB in the frequency band of operation from 27 GHz to 29.5 GHz, and a measured total efficiency above 70 % with an uncertainty of 10% (95% confidence interval). Furthermore, when compared to the state-of-the-art, the proposed antenna provides a much wider scanning range while occupying a significantly smaller and compact volume.

A 1-bit folded reflectarray (FRA) based on characteristic modes-inspired polarization twisting metasurface element is proposed. The element is inspired by a 2×2 patch-based metasurface where a corner patch is removed and a pair of the patch is connected to break the symmetry for polarization twist and reduce the number of switches with the assistance of the characteristic mode analysis. On this basis, only two switches are required in each element for 1-bit phase encoding. To verify the proposed metasurface element, two 1-bit FRA in the Ka-band with beam directions of 0° and 60° are designed and fabricated where the metal stubs/open circuits are employed instead of real switches. The measured peak gains reach 27.4 dBi at 30 GHz with an aperture efficiency of 21.2% and a 3 dB bandwidth of 16.2%. This FRA exhibits good beam scanning characteristics with high gain and wide fractional bandwidth, which shows great potential in the application of low-cost electronically beam-steerable antennas. [ABSTRACT FROM AUTHOR]

A metasurface beam-steering antenna at microwaves is proposed in this letter, which is designed according to holographic theory and has the ability to create multibeam radiation patterns with independent control of the individual beam direction. The holographic design approach of the modulated impedance surface follows the methodology of linear superposition of individual field patterns for each beam. The unit cell of the proposed antenna is tuned with a low-loss varactor diode, and its dispersion diagram is obtained with a transverse resonance technique. A novel multibeam antenna fed by a single source is designed with independent beam steering of each beam at around 7.2 GHz. The dual-beam antenna exhibits an average of 11.6 dBi of realized gain for each independent beam. [ABSTRACT FROM AUTHOR]

Using anisotropic holographic metasurfaces to transform the creeping waves into circularly polarized (CP) beams has initiated the quest for tangible applications in different disciplines. However, the present designs are only limited to the planar impedance surfaces with rare studies concerning the linear-to-circular polarization converting holographic metasurface over the conformal platforms. In this article, we demonstrate the perfect transformation of linearly polarized (LP) surface waves into CP waves through cylindrical anisotropic holographic metasurfaces consisting of gradient H-shaped slits etched on the periodic circular patches. Especially, the linear-to-circular polarization conversion rate can always maintain a very high level for the beam synthesis no matter how the curvature of the metasurface changes, and such high-purity CP beams also demonstrate very good quality of steerable characteristics over conformal surfaces. Our approach, using anisotropic holographic metasurfaces to transform the creeping waves into high-purity CP beams, should pave the way for more advanced functional meta-device designs over the nonplanar impedance surfaces. [ABSTRACT FROM AUTHOR]

This communication proposes a novel dual-band shared-aperture antenna hybridizing Fabry–Perot cavity antenna (FPCA) and folded reflectarray (FRA), which operates at both sub-6 GHz and millimeter-wave bands with a large frequency ratio of 1:4.6. This antenna is composed of two substrates, with metasurface printed on the upper substrate and reflector layer printed on the lower one. The metasurface works as partially reflective surface (PRS) of FPCA at the sub-6 GHz band while acts as polarization grid of FRA at the millimeter-waveband. Thus, the FPCA and the FRA are able to share the same aperture with high aperture reuse efficiency. A composite feed structure combining waveguide with microstrip patch antenna is used to feed at two bands, implementing shared-aperture feed. This antenna achieves high gains at two bands without complicated feeding networks. A prototype operating at 5.4 and 25 GHz bands is designed, fabricated, and measured to verify the idea. [ABSTRACT FROM AUTHOR]

In this article, we propose a shared-aperture metasurface (MS) combining spin-decoupled strategy to generate four coaxially circularly polarized (CP) beams carrying orbital angular momentum (OAM) at Ku-band and Ka-band. First, the frequency selective surface (FSS) is adopted to suppress the mutual coupling between the Ku-band and the Ka-band. Next, at each band, the spin-decoupled meta-atom is designed to realize the independent control for dual circular polarizations. Subsequently, the designed MS illuminated by CP feed source would generate left-hand CP (LHCP) beam with OAM mode +1 and right-hand CP (RHCP) beam with OAM mode −1 at Ku-band; meanwhile, the LHCP beam with OAM mode +2 and the RHCP beam with OAM mode −2 are generated at Ka-band. Finally, the MS is designed, manufactured, and measured. Both simulated and measured near-field and far-field performance verify that the proposed MS can generate four independent CP OAM beams along the same spatial axis at dual bands. On the other hand, the experimental results and the corresponding theoretical analysis verify that the proposed MS illuminated by a linear-polarized (LP) feed source could also generate eight correlative LP OAM beams at dual bands. [ABSTRACT FROM AUTHOR]

In this article, we present the design procedure of modulated metasurface leaky-wave antennas (MMLWAs) to generate coaxial superposition of vortex beams with several orbital angular momentum (OAM) states. Based on the flat optics (FO) technique and aperture field estimation (AFE) method, an analytical framework is proposed to facilitate the implementation of MMLWAs generating multiple topological charges in the OAM regimen. Furthermore, using the spectral analysis that has been derived from the proposed model, we have shown that the symmetry of aperture shape can affect the purity of the mode. Also, the perfectly symmetric circular shape is introduced as an ideal choice for high-purity vortex generation. This single aperture antenna with embedded monopole feed can be an appropriate alternative to more complex vortex beam generators such as spiral phase plates and circular antenna arrays. The validity of the proposed analytical method has been studied by designing two structures with petal-like vortex beams. [ABSTRACT FROM AUTHOR]

In this article, a novel design of broadband circularly polarized (CP) folded reflectarray (FRA) antenna is proposed. The CP FRA consists of two kinds of metasurfaces (MSs) and a linearly polarized (LP) open waveguide antenna as the feeding source. The bottom MS functions as the main reflector, which is capable of realizing cross-polarization conversion and beam focusing simultaneously. The upper MS is utilized as a subreflector, which could reflect LP wave coming from the feeding source and transmit its orthogonal counterpart into CP wave. For demonstration, the MS-assisted CP FRA antenna is fabricated and measured. Numerical results are consistent with experimental ones, illustrating a peak gain of 24.1 dBi and a peak aperture efficiency of 51% at 9.9 GHz. The 3 dB gain bandwidth and 3 dB axial ratio (AR) bandwidth are achieved as 27.5% (8.8–11.6 GHz) and 43.9% (8–12.5 GHz), respectively. The proposed CP FRA provides a new avenue to realize broadband high-efficiency CP antenna, which may find great potentials in modern wireless communication systems. [ABSTRACT FROM AUTHOR]

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]