Non-uniform Metasurface Research Articles

Self-complementary metasurfaces for designing terahertz deflecting circular-polarization beam splitters

Metasurfaces provide a broad range of functionalities related to wavefront manipulation in different frequency ranges with the advantage of subwavelength thickness. In this work, we numerically and experimentally investigate a terahertz splitter of circularly polarized beams based on a spatially nonuniform metasurface composed of self-complementary unit cells. When illuminated with circular polarization, the metasurface creates a deflected cross-polarized beam while transmitting a co-polarized beam in the normal direction. Despite the existence of two reflected beams leading to reflective losses, the metasurface is single-layer and is easy to design and fabricate. A required linear phase gradient for the cross-polarized beam can be obtained by adjusting only one design parameter of unit cells.

A Wideband Non-Uniform Metasurface-Based Circularly Polarized Reconfigurable Antenna

This paper presents a low-profile circularly polarized (CP) antenna with wideband and polarization-reconfigurable characteristics. The proposed antenna consists of a truncated-corner patch working as a CP source and a non-uniform metasurface (MS) acting as parasitic elements for performance enhancement. The non-uniform MS is utilized to produce additional operating bands in the higher frequency range, which are then combined with the lower band produced by the driven patch to significantly broaden the overall operating bandwidth. The polarization switching mechanism is based on the ON/OFF states of two p-i-n diodes. The fabricated prototype shows the measured overlapped bandwidth between -10 dB impedance and 3-dB axial ratio is 25.6% and peak broadside gain is 7.3 dBic.

Metasurface-Based Wideband MIMO Antenna for 5G Millimeter-Wave Systems

This paper presents a metasurface based multiple-input multiple-output (MIMO) antenna with a wideband operation for millimeter-wave 5G communication systems. The antenna system consists of four elements placed with a 90 degree shift in order to achieve a compact MIMO system while a 2x2 non-uniform metasurface (total four elements) is placed at the back of the MIMO configuration to improve the radiation characteristics of it. The overall size of the MIMO antenna is 24x24 mm2 while the operational bandwidth of the proposed antenna system ranges from 23.5-29.4 GHz. The peak gain achieved by the proposed MIMO antenna is almost 7dB which is further improved up to 10.44 dB by employing a 2x2 metasurface. The total efficiency is also observed more than 80% across the operating band. Apart from this, the MIMO performance metrics such as envelope correlation coefficient (ECC), diversity gain (DG), and channel capacity loss (CCL) are analyzed which demonstrate good characteristics. All the simulations of the proposed design are carried out in computer simulation technology (CST) software, and measured results reveal good agreement with the simulated one which make it a potential contender for the upcoming 5G communication systems.

Wideband Circularly Polarized Antenna Based on a Non-Uniform Metasurface

This paper presents a non-uniform metasurface (MS)-based circularly polarized (CP) antenna that is able to perform with a wideband operation characteristic. A squared patch with truncated corners was chosen as a radiating CP source. Then, unlike the conventional CP MS antennas with a uniform MS, the proposed design employed a non-uniform MS placed above the driven patch. Apart from increasing the impedance bandwidth, the non-uniform MS was also capable of generating two additional CP bands in the high-frequency region, which contributed to significantly increasing the antenna’s overall performance. For demonstration, an antenna prototype was fabricated and experimentally tested. The measured operating bandwidth of the fabricated antenna was about 30% and the broadside gain within this band was around 6.6 dBic. Compared to the other reported CP MS antennas in the open literature, the proposed design has the advantages of very wideband operation with a comparable size and gain.

Enhanced Efficiency of Asymmetric Wireless Power Transmission Using Defects in 2D Magnetic Metamaterials

In this work, we investigate an asymmetric wireless power transfer (WPT) system constructed by a big transmitter (Tx) and a small receiver (Rx) operated at 13.56 MHz. Because of the high radius ratio between Tx and Rx coils, the efficiency of asymmetric WPT is lower than the symmetric one. Therefore, a compact metasurface placed between the Tx and Rx is used to enhance the efficiency of WPT. To further improve efficiency, we create a nonuniform metasurface by controlling the resonant frequency of the unit cells. This defect with higher resonant frequency than others is easily formed by tuning the external capacitor. This modified metasurface can strongly localize the magnetic field into the defect region at a deep subwavelength scale of 2.3λ × 10−3and is, therefore, suitable for coupling to the small Rx of asymmetric WPT. There is an optimum position of metasurface for achieving the highest efficiency when the couplings between the defect, Tx, and Rx coils are appropriate. By using the optimization approach, the efficiency of the WPT increases significantly from 9.8% to 60.2% with a nonuniform metasurface. The performances of WPT at various locations of defect and receiver are also considered.

A Focusing Circular-Polarization THz Beam Splitter Based on a Self-Complementary Metasurface

Quasi-optical polarization beam splitters are important components of terahertz instrumentation widely used in interferometric and polarimetric measurements. Recently metasurfaces, i.e., two-dimensional periodic or quasi-periodic optically dense structures composed of unit cells with subwavelength dimensions, have been shown to operate as compact and efficient beam splitters. Typically, their design was based on careful optimization of anisotropic metal or dielectric resonant scatterers confined in each unit cell. In this work, we propose and experimentally demonstrate a simple and useful approach to designing circular-polarization beam splitters taking advantage of intrinsically frequency-independent properties of single-layer self-complementary metasurfaces (SCMSs). Theoretically, when illuminated with a circularly polarized beam, any SCMS at any frequency transmits a copolarized beam with a complex transmission coefficient of 1/2. At the same time, a cross-polarized beam of the same magnitude is produced, with a transmission phase that can be controlled at every point of a metasurface aperture. In this work, to split the copolarized and cross-polarized transmitted beams, we spatially modulate this phase by constructing a spatially nonuniform metasurface of self-complementary unit cells. With this approach, we experimentally demonstrate a focusing circular-polarization beam splitter operating near 0.345 THz.

Characteristic Mode Analysis of a Nonuniform Metasurface Antenna for Wearable Applications

A novel nonuniform metasurface antenna is designed based on the characteristic mode analysis for wearable applications. A metasurface layer consists of a 3 × 3 unit cell array, in which the center and corner cells are rotated counterclockwise, whereas the other four unit cells are rotated in the opposite direction with a same rotation angle. The structure of a nonuniform metasurface is optimized according to the modal significance curves and surface currents, which resulted that mode 1 and mode 2 are combined to widen the bandwidth, which can cover the 5 GHz WLAN band. The volume of the proposed antenna is equal to a cylinder with a diameter of 55 mm and a thickness of 4 mm. The measured efficiency is greater than 80% and the average gain can achieve 7.63 dBi in the working band. The proposed antenna is harmless to human body, which can be evidenced by the simulated 1 and 10 g averaged specific absorption rate values.

Miniature Wideband Non-Uniform Metasurface Antenna Using Equivalent Circuit Model

A miniaturized wideband non-uniform metasurface antenna is proposed and analyzed using an equivalent circuit model. The nonuniform metasurface consists of an array of rectangular metallic patches separated by varying gaps. The effects of the varying gap widths on the resonant frequencies of the dual-mode non-uniform metasurface antenna are studied using the proposed equivalent circuit model. The resonant frequency of the first TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode decreases with narrowing of either the center or side gaps; the resonant frequency of the antiphase TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> mode is hardly affected by the center gap width but decreases while reducing the side gap width. The miniaturization of the metasurface antenna is therefore achieved by narrowing the side gaps, and the wide bandwidth with good impedance matching is realized by widening the center gap. A proof-of-concept prototype demonstrates a peak gain of 7.2 dBi over a -10 dB reflection coefficient bandwidth of 21.8%, with a miniaturized non-uniform metasurface of 0.46λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.49λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.06λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the center operating wavelength in free space) which achieves 54% size reduction compared with the earlier developed wideband antenna with a uniform metasurface of 0.7λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.7λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.06λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> .

Scattering Field Solutions of Metasurfaces Based on the Boundary Element Method for Interconnected Regions in 2-D

A methodology for determining the scattered Electromagnetic (EM) fields present for interconnected regions with common metasurface boundaries is presented. The method uses a Boundary Element Method (BEM) formulation of the frequency domain version of Maxwell's equations - which expresses the fields present in a region due to surface currents on the boundaries. Metasurface boundaries are represented in terms of surface susceptibilities which when integrated with the Generalized Sheet Transition Conditions (GSTCs), gave rise to an equivalent configuration in terms of electric and magnetic currents. Such a representation is then naturally incorporated into the BEM methodology. Two examples are presented for EM scattering of a Gaussian beam to illustrate the proposed method. In the first example, metasurface is excited with a diverging Gaussian beam, and the scattered fields are validated using a semi-analytical method. Second example concerned with a non-uniform metasurface modeling a diffraction grating, whose results were confirmed with conventional Finite Difference Frequency Domain (FDFD) method.

A nonuniform metasurface loaded circularly polarized differentially-fed antenna

ABSTRACTThis paper reports a novel differentially driven circularly polarized metasurface-based antenna with symmetrical and unidirectional radiation pattern. The introduction of metasurface could generate an additional resonance. The location of lower resonant frequency could be tuned independently by varying the length of cross gap in the ground plane while the higher resonant frequency is insensitive to this change. In addition, it is also proved that the circularly polarized performance could be better using the nonuniform metasurface than the uniform one. Furthermore, an antenna prototype has been fabricated to experimentally validate the principle and design approach. The measurements agree reasonably with the simulated ones.

A low‐profile high‐gain filtering antenna for fifth generation systems based on nonuniform metasurface

AbstractThis article presents a low‐profile filtering antenna based on nonuniform metasurface for fifth generation (5G) systems at 3.5 GHz. A crossed stub, slotted ground, shorting via, and the nonuniform metasurface is used to achieve high gain and filtering response in lower and upper stopbands. The antenna and the metasurface are printed on Rogers RO4003C (εr = 3.38, tan δ = 0.0027) and Taconic TLX‐9 (εr = 2.5, tan δ = 0.0019), respectively. Measured results show that the antenna has an impedance bandwidth of 3.24 to 3.8 GHz (15.91%) below −10 dB with a stable gain having a maximum value of 10.43 dBi and good radiation patterns. Moreover, this antenna passband satisfies the typical bandwidth required for the global 5G spectrum at the 3.5 GHz band. The gain drops significantly and the S11 goes up to 0 dB outside the passband. Due to the excellent filtering characteristics, the proposed antenna is a good candidate for 5G applications.

A Broadband Dual-Polarized Antenna With Low Profile Using Nonuniform Metasurface

A novel dual-polarized antenna with very low profile using nonuniform metasurface is presented in this letter. The proposed antenna is composed of two dipoles placed crosswise, of which the radiators are formed by four square metal plates. The antenna is designed to work at the lower frequency band (0.69–0.96 GHz) of base station. By adding the metasurface atop the dipole antennas, the overall height of the antenna together with metasurface can be reduced to 0.1 λ0 (wavelength of the center frequency), which is much lower compared to the normal height of a dipole antenna (0.25 λ0). Simulated and measured results show that the height reduction is achieved without degrading the performance of the antenna, making the proposed antenna a competitive candidate for base-station application.

Realizing Broadband Transparency via Manipulating the Hybrid Coupling Modes in Metasurfaces for High‐Efficiency Metalens

AbstractMetasurfaces with locally controlled phase discontinuity are promising for novel manipulations of the optical fields. However, the high insert loss at off‐resonance frequencies and narrow operation band of the resonant metasurface lead to the common low efficiency, obstructing it from practical applications. Here, it is shown that a broadband transparent metasurface, made of three‐layer complementary square ring resonator (CSRR) arrays, can be employed for high‐efficiency and broadband beam manipulation. The nonuniform metasurface can be optimized with high transmittance (above 90%) while maintaining full range (from 0 to 2π) phase modulation by adjusting the geometric parameters of the CSRR on the middle layer. The physical insight can be understood with a coupled oscillator model, it is found that the interlayer coupling constant is the key factor to realize phase modulation with high efficiency. A transmittance metalens with long focal depth in broadband is experimentally demonstrated as an example, the measured transmission efficiency is higher than 80%. The developed broadband wavefront engineering elements can be employed for high‐performance, miniaturized and superior environmental suitability sensing, imaging, and communication system.

The gain enhancement of a graphene loaded reconfigurable antenna with non-uniform metasurface in terahertz band

In this article, two uniform and non-uniform metasurfaces with near zero permeability are designed and simulated. Retrieval method is used to extract the effective parameters of the designed metasurfaces. Both of designed metasurfaces are placed as the superstrate on the top of a planar terahertz antenna. Therefore, the significant 8 dB gain improvement and 12% efficiency enhancement are achieved for the total structure in the desired frequency of 1 THz. Moreover, using the non-uniform metasurface improves the front to back ratio of the antenna from 13.8 dB to 17.4 dB. The effect of number of unit cells and the distance between antenna and metasurface on the radiation parameters of the antenna are investigated. Finally, three parasitic graphene based strips with optimized lengths are placed near the patch antenna, which can be used to adjust or tune the response of the designed high gain antenna.

Modeling and design of metasurfaces for beam scanning

The aim of the present contribution is to show that a judicious phase engineering in metasurfaces can be efficiently used in the design of low-profile beam-steerable antennas. We present the design, simulation and experimental validation of the proposed low-profile antennas. The phase modulation on the metasurface is derived from the ray optics analysis. Such a non-uniform metasurface is utilized as a partially reflective surface in Fabry–Perot cavity antenna. Beam scanning is obtained, and depending on the phase modulation applied, the scan angle can be controlled. Furthermore, an active metasurface incorporating electronic components is fabricated and tested in an electronically steerable antenna.

Planar bifunctional Luneburg-fisheye lens made of an anisotropic metasurface

Luneburg lens and Maxwell-fisheye lens are well-known microwave and optical devices with distinct focusing properties. Here, a planar bifunctional Luneburg-fisheye lens made of an anisotropic metasurface is presented, which features as a Luneburg along the horizontal optical axis, while as a fisheye along the vertical optical axis. A method to control the inhomogeneous indices of refraction along the two optical axes independently is proposed by designing an anisotropic and nonuniform metasurface, which can provide the required distributions of refractive indices approximately for Luneburg and fisheye lenses viewing from the two optical axes. Experiments in the microwave frequency range demonstrate very good performance of the planar bifunctional Luneburg-fisheye lens. The proposed method opens up an avenue to design other kinds of bifunctional devices using metasurfaces in the microwave, terahertz, and even optical ranges.