Metasurface Research Articles

Controlling Radiation Beams by High-Efficiency Metasurfaces with Enhanced Refractive Index Elements.

The efficient manipulation of the electromagnetic wavefront using metasurfaces (MSs) has attracted a continuous amount of attention in recent years. MSs with arbitrarily controllable amplitude and phase are highly desirable in many emerging fields. However, in previous research, the resonance effect has mainly been used to accomplish the task. This article presents a nonresonant metamaterial element with stronger capacitive coupling. The broadband sandwich-structured meta-atom achieves an enhanced refractive index ranging from 2.44 to 8.90. By using the enhanced refractive index elements, the proposed transmissive MS has an ultrathin thickness (0.23 free-space wavelength). It consists of a specially designed gradient index MS placed on top of the transmitting antenna. The MS is designed to convert the spherical surface wave into a plane wave with high efficiency. One core layer (CL) gradient index MS and two impedance matching layers (IMLs) have been designed to transform the magnitude and phase of the radiation beam from the transmitting antenna. An efficient transmissive MS has been designed and experimentally evaluated with a maximum measured efficiency of over 85%. The proposed control strategy increases the design flexibility of the MS and shows great potential for broadband applications in the realization of efficient functional devices.

Multimaterial Additively Manufactured Transmissive Spin‐Decoupled Polarization‐Maintaining Metasurfaces

AbstractTaking advantage of multimaterial additive manufacturing, this work demonstrates a kind of polarization‐maintaining metasurface (MS) for spin‐decoupled beam shaping. The wavefronts of the two spin states, i.e., the left‐hand circular polarization (LHCP) and the right‐hand circular polarization (RHCP), can be independently manipulated while the output spin state remains the same as the input. The meta‐atom is implemented with dual‐polarized antennas at the top and the bottom with phase delay lines connecting them. The dual‐polarized antenna on the receiving side is fixed to receive the LHCP/RHCP incident waves, while the dual‐polarized transmitting antenna is rotated, providing the opposite phase shifts for the LHCP and RHCP. The absolute value of the phase shift is the same as the transmitting antenna's rotation angle. Meanwhile, varying the length of phase delay lines introduces 2π phase shifts for both LHCP and RHCP inputs. Thus, combining the two phase‐shifting degrees of freedom, the LHCP and RHCP wavefronts can be independently controlled while the polarization is not flipped after transmitting through the MS. One MS produces vortex beams with different topological charges for LHCP and RHCP. The other achieves spin‐decoupled focusing. The multilayered MSs are conveniently printed using multimaterial additive manufacturing without postprocessing.

Dispersion and Polarization Control in Below-Cutoff Circular Waveguides Using Anisotropic Metasurface Liners

This article explores the application of metasurface (MTS) liners in perfect-electric-conducting (PEC) circular waveguides for cutoff manipulation and the introduction of controllable chiral properties. A dispersion equation is derived to predict the propagation characteristics of a circular waveguide lined with an MTS exhibiting a general full tensor surface admittance response. A design procedure for the MTS liner is presented and validated with three design examples. Two of the examples use a capacitively loaded grid topology akin to a Jerusalem cross structure aligned with the principal coordinate system of the waveguide, resulting in a diagonal susceptance tensor. These two examples are used to demonstrate how the dispersion properties of the waveguide can be significantly modified based on the geometry of the MTS-lined waveguide system and the surface admittance exhibited by the MTS. The third design consists of a rotated fully printed Jerusalem cross structure with a general tensor susceptance, which is found to produce a separation in the dispersion curves of the left-and right-hand circularly polarized modes in the waveguide while also producing below-cutoff propagation. Theoretically predicted dispersions are validated with full-wave electromagnetic simulations.

Low‐profile multi‐polarization and pattern reconfigurable antenna based on metasurface

AbstractIn this letter, a low‐profile metasurface (MS) antenna with multi‐polarization and pattern reconfigurability is presented. The antenna mainly consists of two parts, including a loop antenna fed by balun, and a MS antenna excited by the coupled aperture. When switching the PIN diode and rotating the MS, the multi‐polarization and pattern reconfigurable ability of the antenna is obtained. While the loop antenna is excited, a horizontally linear polarized (HLP) wave and omnidirectional pattern are obtained. Besides, when the MS is operated, due to the differential phase between the orthogonal surface impedance components, the antenna can be switched between vertical linear polarization (VLP), left‐hand circular polarization (LHCP), and right‐hand circular polarization (RHCP). Furthermore, the antenna could achieve a directional pattern. As the simulated and measured results shown, the proposed antenna has an overlapped impedance bandwidth (IMBW) of 24.7% for all polarization modes and a 3‐dB axial ratio (AR) bandwidth of 11.8% for L/RHCP modes. Moreover, the antenna owns a low profile of 0.06 λ (under the operating centre frequency).

Wideband High-Gain Filtering Vivaldi Antenna Design Based on MS and Herringbone SSPP Structure

This paper presents a wideband high gain filtering Vivaldi antenna design that utilizes metasurface (MS) and spoof surface plasmon polariton (SSPP) metamaterial for gain enhancement and filtering performance, respectively. The design includes a set of slits etched on the external edges of the antenna to reduce diffraction current, the SSPP structure with inherent dispersion characteristic etched in the slot-line to inhibit RFI, and a novel ‘Φ’-shaped MS unit cell for wideband gain enhancement. These functional metamaterials jointly contribute to optimized performances, and the proposed prototype shows immunity to RFI above 27 GHz and high gain performance over the wide frequency range from 15 GHz to 27 GHz. For demonstration, the protype is fabricated and measured. Measured results agree well with simulated ones.

Eigenfunction Expansion (EFE) Analysis of Cylindrical Metasurfaces—Part II: Sectors and Multishells

This article presents a formulation of an eigenfunction expansion (EFE) technique for the analysis of multishell cylindrical metasurface (MS) configurations and tests the validity of zero-thickness sheet models for simulating practical structures. The formulation homogenizes the MS, modeling the surface as a zero-thickness discontinuity via the generalized sheet transition conditions (GSTCs) characterized by surface susceptibilities. Comparison with full-wave simulations in commercial solvers show that, for a thin deep subwavelength unit cell, single-shell arcs with increasing curvatures (up to at least half of a wavelength in radius) are accurately captured by the use of the model. The simulations also show that the edge diffraction ff the finite length arcs are well modeled. This is an important result as it indicates that susceptibilities extracted from full-wave unit-cell simulations assuming a flat periodic infinite surface are appropriate for use in high curvature finite surfaces. A second set of simulations showed that the EFE predicted multishell field distributions accurately when compared with unit-cell-based simulations. A final set of simulations for single-and double-shell configurations is used to demonstrate the limitations of the GSTC approach for unit cells of appreciable thickness.

Wideband differential‐fed metasurface antenna array using characteristic mode analysis

AbstractA wideband differential‐fed (DF) metasurface (MTS) antenna array is proposed in this letter. As the element of the proposed array, a nonuniform MTS is designed step by step using characteristic mode analysis. In the first step, a 2 × 2 rectangular patch‐based MTS is investigated. We observe that its two fundamental modes exhibit even/odd symmetric current distributions, which can be regarded as a pair of c/π modes. Unlike c mode, π mode features nonbroadside radiation, which can be reshaped by modifying MTS to an asymmetric configuration in the second step. In the last step, a direct DF method is adopted to excite c/π modes and suppress other unwanted modes for wideband performance. The optimal DF positions are quantitatively predicted by calculating the impendence bandwidth of MTS fed by virtual probes. Moreover, to further increase the gain and improve the symmetry of radiation patterns, two proposed elements are integrated into a linear array. Finally, the proposed antenna array is fabricated and measured. The measured impedance bandwidth is 22.2% (6–7.5 GHz) with broadside radiation, stable gains varying from 8.5 to 11.3 dBi, and a front‐to‐back ratio better than 19 dB.

Low-profile metasurface lens antenna with flat-top radiation pattern

This paper proposes a low-profile metasurface (MS) lens antenna with flat-top radiation pattern. Specifically, a low-profile slot array fed by the pillbox system serves as the feeding source while an MS lens placed close to the slot array can further shape the beam to flat-top radiation pattern. Compared with the conventional MS lens antennas with a single feeding source, the overall profile is greatly reduced and the fabrication stability is improved. Theoretical analyses, simulations, and measurements are carried out in this study. The proposed MS lens antenna operates at a central frequency of 28.5 GHz, aiming at supporting 5G millimeter-wave (mmW) communications. The measured results agree well with the analyzed and simulated ones. The proposed MS antenna realizes the flat-top pattern of over ± 20° with the measured peak gain of 10.5 dBi. The overall size is 78 mm × 63 mm × 4.4 mm, corresponding to 7.4λ0 × 6λ0 × 0.42λ0 at 28.5 GHz (λ0 is the free-space wavelength).

Reflection Phase Compensation for Finite Metasurface Using Notches and Slots on the Backing Conductor

This study described a technique to compensate for the difference in reflection phase characteristics yielded by a finite-size meta-surface (MS) to render them identical to the reflection characteristics analyzed using the unit cell model corresponding to the infinite MS structure. Thus, an MS with a backing conductor having notches and slots to compensate for the reflection phase affected by the complex diffraction at the edge, was proposed. The notches and slots’ mechanism inducing the inductive reactions was discussed based on the current distributions. The proposed and conventional MSs were fabricated to verify the deterioration and compensation effect; the former was consistent with the designed performance based on the unit cell analysis corresponding to the infinite structure.

Photo-Excited Metasurface for Tunable Terahertz Reflective Circular Polarization Conversion and Anomalous Beam Deflection at Two Frequencies Independently.

In this paper, a photo-excited metasurface (MS) based on hybrid patterned photoconductive silicon (Si) structures was proposed in the terahertz (THz) region, which can realize the tunable reflective circular polarization (CP) conversion and beam deflection effect at two frequencies independently. The unit cell of the proposed MS consists of a metal circular-ring (CR), Si ellipse-shaped-patch (ESP) and circular-double-split-ring (CDSR) structure, a middle dielectric substrate, and a bottom metal ground plane. By altering the external infrared-beam pumping power, it is possible to modify the electric conductivity of both the Si ESP and CDSR components. By varying the conductivity of the Si array in this manner, the proposed MS can achieve a reflective CP conversion efficiency that ranges from 0% to 96.6% at a lower frequency of 0.65 THz, and from 0% to 89.3% at a higher frequency of 1.37 THz. Furthermore, the corresponding modulation depth of this MS is as high as 96.6% and 89.3% at two distinct and independent frequencies, respectively. Moreover, at the lower and higher frequencies, the 2π phase shift can also be achieved by respectively rotating the oriented angle (αi) of the Si ESP and CDSR structures. Finally, an MS supercell is constructed for the reflective CP beam deflection, and the efficiency is dynamically tuned from 0% to 99% at the two independent frequencies. Due to its excellent photo-excited response, the proposed MS may find potential applications in active functional THz wavefront devices, such as modulators, switches, and deflectors.

Transmission-Reflection-Integrated Multifunctional Passive Metasurface for Entire-Space Electromagnetic Wave Manipulation.

In recent years, many intriguing electromagnetic (EM) phenomena have come into being utilizing metasurfaces (MSs). However, most of them operate in either transmission or reflection mode, leaving the other half of the EM space completely unmodulated. Here, a kind of transmission-reflection-integrated multifunctional passive MS is proposed for entire-space electromagnetic wave manipulation, which can transmit the x-polarized EM wave and reflect the y-polarized EM wave from the upper and lower space, respectively. By introducing an H-shaped chiral grating-like micro-structure and open square patches into the unit, the MS acts not only as an efficient converter of linear-to-left-hand circular (LP-to-LHCP), linear-to-orthogonal (LP-to-XP), and linear-to-right-hand circular (LP-to-RHCP) polarization within the frequency bands of 3.05-3.25, 3.45-3.8, and 6.45-6.85 GHz, respectively, under the x-polarized EM wave, but also as an artificial magnetic conductor (AMC) within the frequency band of 12.6-13.5 GHz under the y-polarized EM wave. Additionally, the LP-to-XP polarization conversion ratio (PCR) is up to -0.52 dB at 3.8 GHz. To discuss the multiple functions of the elements to manipulate EM waves, the MS operating in transmission and reflection modes is designed and simulated. Furthermore, the proposed multifunctional passive MS is fabricated and experimentally measured. Both measured and simulated results confirm the prominent properties of the proposed MS, which validates the design's viability. This design offers an efficient way to achieve multifunctional meta-devices, which may have latent applications in modern integrated systems.

Textile and metasurface integrated wide-band wearable antenna for wireless body area network applications

Wireless Body Area Network (WBAN) enhances the quality of life through personal health and remote healthcare monitoring. In this work, we present a numerical model, which is based on CST Microwave Studio simulations, and a fabrication process to achieve a wideband, low-profile, and flexible wearable antenna by integrating textiles with a metasurface (MTS). The performance of the developed antenna was investigated in free space and on a human body (chest, arm, and leg) both numerically and experimentally. The results showed that by incorporating a metasurface structure on the antenna, the simulated gain was improved from 6.1 dBi to 9.08 dBi, while the measured gain was increased from 6.63 dBi to 8.49 dBi. The increased gain in a specific direction is related to more power concentration in that direction. The antenna operates in a bandwidth ranging from 6 to 12 GHz, which is preferred for WBAN applications. The antenna with an MTS structure displayed a more uniform radiation pattern with greater directivity and a Front-to-Back Ratio (FBR) of approximately 15 dB from both numerical and measured results, which is essential for WBAN to avoid back radiation from the body while worn and to increase the wireless signal distance. Additionally, numerical analysis of the bending evaluation for different diameters revealed that its reflection coefficient is not much altered. We believe that the developed structure could assist in the fabrication of unique WBANs and healthcare monitoring devices.

Wideband and low profile multipolarization reconfigurable antenna based on metasurface

AbstractIn this letter, a wideband and low‐profile multipolarization reconfigurable antenna based on metasurface (MTS) is proposed. The antenna mainly contains two parts, an upper layer of MTS and a bottom layer of circular slot antenna. The MTS acts as a polarization controller, by mechanically rotating the MTS layer at a proper angle while keeping the circular slot antenna remain stationary, the polarization mode of the antenna can be switched to linear polarization (LP), left‐hand circular polarization (LHCP), and right‐hand circular polarization (RHCP). The experimented −10 dB impedance bandwidth of 37.8% and 54% for LP and circular polarization (CP) modes, respectively. The measured overlapped 3‐dB AR bandwidth is 22.2% for both CP modes, covering from 3.2 to 4 GHz. Moreover, the measured peak gain of the presented antenna reaches 9 dBi and 8 dBic for LP and CP modes, with a lower profile of 0.05 λ0.

Wideband and Low-Profile Multi-Polarization Reconfigurable Filtering Antenna Based on Nonuniform Metasurface

In this article, a wideband and low-profile multi-polarization reconfigurable filtering antenna based on metasurface (MS) is investigated. The proposed antenna consists of two layers, which are the feeding layer and the MS layer. The filtering function is achieved by utilizing a non-uniform MS and inserting a short via between the feedline and ground plane. Besides, by rotating the MS to change its surface current distribution, the antenna can switch among linear polarization (LP), left-hand circular polarization (LHCP), and right-hand circular polarization (RHCP). To demonstrate the simulated design, the proposed antenna is manufactured and measured. As the measured results show, when the antenna works in LP modes, it obtains an -10-dB overlapped impedance bandwidth of 22.4% and an in-band peak gain of 8.8 dBi. Furthermore, the antenna achieves 14-dB and 19-dB out-of-band radiation suppression levels for LP1 and LP2 modes, respectively. Whereas it operates in CP modes, the measured -10-dB overlapped impedance bandwidth and 3-dB axial ratio (AR) bandwidth of the antenna reach 26.5% and 22%, respectively. Meanwhile, its out-of-band radiation suppression level is more than 17.5 dB, with an in-band peak gain of 7.3 dBic. Hence, there illustrates a pretty good agreement between the simulated and measured results.

Design and analysis of a metasurface-based wideband wide-angle scanning phased array

This paper presents a wideband wide-angle scanning phased array, which employs Φ-shaped unit cells to form a metasurface (MS) for achieving wideband impedance matching. To optimize the impedance matching performance, a design procedure based on an equivalent circuit model is proposed. A quasi-analytical model based on the equivalent circuit model and the permittivity ϵ r and permeability μ r of the MS properly predicts the scan range of the full structure, and allows for an accelerated design process. The extensive analysis confirms the effectiveness of the MS as the wideband angle impedance matching layer for the antenna array. The MS has demonstrated a positive impact on the wideband wide-angle scanning performance. Finally, a prototype is fabricated and measured, and the experimental results agree well with the simulation results. The proposed array exhibits wide-angle scanning across a 3.5:1 impedance bandwidth (2–7 GHz) with ±60° scan volume in both E-plane and H-plane.

Compact conformal metasurface antenna for hyperthermia applications

In this paper, the design, analysis and evaluation of a new compact conformal metasurface (MTS)-based bow-tie slot antenna is presented for hyperthermia treatment of cancer. A bow-tie slot antenna in close proximity with tissue designed at 2.45 GHz was used to excite the MTS layer, which is made of a 4 × 4 uniform array of square unit cells. The proposed MTS-based antenna is compact and provides enhanced penetration depth in the tissue medium as compared to the bow-tie antenna only. To verify the design concept, both the proposed MTS-based antenna and bow-tie antenna were fabricated, and the measured results are well anticipated. Finally, the MTS applicator is terminated with water bolus and tissue to characterize its hyperthermia performance in a real scenario. The key features of the proposed MTS applicator include wide impedance bandwidth (stable hyperthermia performance), enhanced penetration depth and effective field size, single-layer structure and ease of fabrication.

A metasurface-backed planar low-profile dual-band monopole antenna

This paper discusses a low-profile, planar, efficient, and directed CPW-fed monopole antenna with a metasurface (MS) for WLAN as well as modern civil and military applications. A stacked 7 × 7 order metasurface (MS) and CPW-fed slotted patch antenna are sandwiched together to create the metasurface antenna (MSA). The overall size of the proposed prototype is 0.25λ0 × 0.25λ0 × 0.06λ0 only, where λ0 is the free-space wavelength at 2.34 GHz. This antenna has fractional bandwidths of 6.8% and 53.8% at two different frequencies of 2.34 and 8.94 GHz, respectively. With a peak gain of 8.7 dBi and radiation efficiency of 91.1% at 11.51 GHz, the antenna offers directional radiation characteristics in both the E- and H-planes. A comparable circuit model for the proposed prototype has also been described in order to analyze the outcomes of the electromagnetic simulation. The intended prototype has been fabricated, and the experimentally measured results closely resemble the simulated ones.

Interconversion between dual-peak electromagnetically induced transparency and dual-peak electromagnetically induced absorption utilizing metasurface

Utilizing metasurface (MS), the transition between dual-peak electromagnetically induced transparency (EIT) and dual-peak electromagnetically induced absorption (EIA) in four states has been proposed in the terahertz (THz) band with the incidence of a circularly polarized (CP) wave. Additionally, the phase-change material vanadium dioxide (VO2) and photosensitive material silicon (Si) have been adopted. The four state transitions can be achieved by adjusting the pump light and the temperature which is not affected by incident CP waves. With the incidence of the pump light, Si can transform from the insulating state to the metallic state. Similarly, when the temperature exceeds 68 °C, VO2 enters the metallic state. Two different dual-peak EIT (State 1 and State 2) accompanied by slow light effect can be observed by modulating the state of VO2 without the action of Si. However, once Si is in metallic state, two different states of dual-peak EIA (State 3 and State 4) are achieved by changing the state of VO2. Notably, the MS has the property of polarization insensitivity. The theoretical oscillator model and equivalent RLC-circuit model verify the feasibility of dual-peak EIT and EIA.

Wideband reflective half- and quarter-wave plate metasurface based on multi-plasmon resonances

In this article, a single-layer reflective anisotropic metasurface (MS) is proposed, which presents both half- and quarter-wave plate operation in different microwave frequency regimes. The unit cell of the proposed metasurface consists of a tilted rectangular plane with triangular ends accompanied by an equidistant-filled triangle on both sides. The unit cell is printed on a dielectric substrate backed by a metallic plane. The proposed meta-plate transforms horizontal polarization into vertical and vice versa in two wide frequency bands, 7.1-15.3 GHz and 19.8-21.7 GHz. Similarly, a linearly polarized (LP) wave is transformed into a circularly polarized (CP) wave and vice versa at 7.9 GHz and 21.8 GHz. The wide bandwidth is acquired through three plasmonic resonances occurring at 8.2 GHz, 12.7 GHz and 20.8 GHz, where the cross-polarization conversion ratio reaches almost 100%. Moreover, quarter and half-wave plate operations occurring at 7.9 GHz and 7.1-15.3 GHz, respectively, are robust to changes in oblique incidence angle (up to 45°) both for transverse-electric (TE) and transverse-magnetic (TM) polarizations. The physical mechanism behind polarization conversion is also explained through surface current distribution. The proposed meta-plate structure is fabricated and validated through experimental measurements. The wide bandwidth, high efficiency, angular stability, and simple structure make the proposed metastructure incredible for numerous microwave applications such as antennas, radars, and satellite communication.

Wideband RCS reduction based on a simple chessboard metasurface

To avoid being detected by radar, it is necessary to reduce stealthy military platforms' radar cross section (RCS). The operation of overlaying the metasurface (MS) on the targets is a good solution. A simple chessboard MS structure that can achieve low RCS over a large bandwidth is proposed. Only one unit cell is used to construct the MS. First, the unit cell working in 0.5 and 1−λ modes is designed to achieve a stable phase difference of 180° for y- and x-polarized waves. Then, the unit cells and rotated ones are used to form a chessboard structure with different distributions. The compared results show that the chessboard MS with 2 × 2 quadrants can facilitate the widest 10 dB RCS reduction band of 111% and the largest RCS reduction. The proposed structure exhibits excellent RCS reduction even when irradiated by y- and x-polarized waves at an oblique incidence of 30°.