He-Xiu Xu

Compact dual-band circular polarizer using twisted Hilbert-shaped chiral metamaterial

We propose a kind of chiral metamaterial inspired from the fractal concept. The Hilbert fractal perturbation in the twisted split ring resonator element results in compact metamaterial and breaking mirror symmetry, which readily forms chirality over triple bands. The discrepancy between co-polarization conversion and cross-polarization conversion over multiple bands can be explored for multifunctional devices. A multiband circular polarizer is then numerically and experimentally studied in the X band based on the bilayered twisted Hilbert resonator with mutual 90° rotation. The ability of transforming linearly polarized incident waves to circularly polarized waves is unambiguously demonstrated with high conversion efficiency and large polarization extinction ratio of more than 20 dB across dual bands. Moreover, exceptionally strong optical activity and circular dichroism are also observed.

Compact Circularly Polarized Antennas Combining Meta-Surfaces and Strong Space-Filling Meta-Resonators

Reduced-size single-feed circularly-polarized (CP) patch antennas are proposed and investigated based on the strategy of combining meta-surfaces and meta-resonators owning strong space-filling capability. They all comprise a slot-loaded square patch printed over a well designed reactive impedance surface (artificial meta-surface with magnetic response at the resonant frequency) for improved antenna performances and size reduction. The complementary crossbar fractal tree (CCFT) slot and three-turn complementary spiral resonators (TCSRs) with asymmetric gap orientation are employed as meta-resonators to render the antennas to radiate CP waves in single-band or dual-band operation and to facilitate further miniaturization. Numerical and experimental results indicate that these antennas owning a maximum size of 0.262λ0 × 0.262λ0 around 3 GHz exhibit a comparable impedance and axial ratio (AR) bandwidth over 1.05% and high gain of more than 4.15 dBic, predicting promising applications in portable and handheld communication systems.

Dynamical control on helicity of electromagnetic waves by tunable metasurfaces

Manipulating the polarization states of electromagnetic (EM) waves, a fundamental issue in optics, attracted intensive attention recently. However, most of the devices realized so far are either too bulky in size, and/or are passive with only specific functionalities. Here we combine theory and experiment to demonstrate that, a tunable metasurface incorporating diodes as active elements can dynamically control the reflection phase of EM waves, and thus exhibits unprecedented capabilities to manipulate the helicity of incident circular-polarized (CP) EM wave. By controlling the bias voltages imparted on the embedded diodes, we demonstrate that the device can work in two distinct states. Whereas in the “On” state, the metasurface functions as a helicity convertor and a helicity hybridizer within two separate frequency bands, it behaves as a helicity keeper within an ultra-wide frequency band in the “Off” state. Our findings pave the way to realize functionality-switchable devices related to phase control, such as frequency-tunable subwavelength cavities, anomalous reflectors and even holograms.

High-Directivity Emissions with Flexible Beam Numbers and Beam Directions Using Gradient-Refractive-Index Fractal Metamaterial

A three-dimensional (3D) highly-directive emission system is proposed to enable beam shaping and beam steering capabilities in wideband frequencies. It is composed of an omnidirectional source antenna and several 3D gradient-refractive-index (GRIN) lenses. To engineer a broadband impedance match, the design method for these 3D lenses is established under the scenario of free-space excitation by using a planar printed monopole. For realizations and demonstrations, a kind of GRIN metamaterial is proposed, which is constructed by non-uniform fractal geometries. Due to the non-resonant and deep-subwavelength features of the fractal elements, the resulting 3D GRIN metamaterial lenses have extra wide bandwidth (3 to 7.5 GHz), and are capable of manipulating electromagnetic wavefronts accurately, advancing the state of the art of available GRIN lenses. The proposal for the versatile highly-directive emissions has been confirmed by simulations and measurements, showing that not only the number of beams can be arbitrarily tailored but also the beam directions can be steerable. The proposal opens a new way to control broadband highly-directive emissions with pre-designed directions, promising great potentials in modern wireless communication systems.

Analysis and Design of Two-Dimensional Resonant-Type Composite Right/Left-Handed Transmission Lines With Compact Gain-Enhanced Resonant Antennas

A two-dimensional (2-D) resonant-type composite right/left-handed transmission line (CRLH TL) is proposed consisting of complementary split-ring resonators (CSRRs) and series capacitive gaps. For characterization and design, the circuit parameters extraction procedure for the TL element is derived, and the unique 2-D dispersion characteristics (CRLH nature) are analytically investigated and further verified by eigenmode analysis performed in HFSS and phase analysis in ADS. A good agreement of dispersion diagram is obtained between full-wave simulation and theory. To demonstrate potential applications, two types of microstrip antennas have been designed, fabricated, and measured following the design procedures that are established. In the first design, a multifrequency antenna operating at three resonant modes of n = -1, n = 0, and n = + 1 is designed by arranging a 2×2 CRLH array in a conventional patch. The frequency ratios between these modes are 1.98 and 1.15, respectively, which can be arbitrarily modulated by controlling the proportion between the CRLH section and the patch. In the second design, zeroth-order resonant antennas are researched. These antennas exhibit simultaneously a leaky-wave characteristic and an omnidirectional radiation behavior, distinguishing them significantly from any previous designs. In both cases, the antenna gain is comparable, and the cell structure is uniplanar and electrically small.

Tunable microwave metasurfaces for high-performance operations: dispersion compensation and dynamical switch

Controlling the phase distributions on metasurfaces leads to fascinating effects such as anomalous light refraction/reflection, flat-lens focusing, and optics-vortex generation. However, metasurfaces realized so far largely reply on passive resonant meta-atoms, whose intrinsic dispersions limit such passive meta-devices’ performances at frequencies other than the target one. Here, based on tunable meta-atoms with varactor diodes involved, we establish a scheme to resolve these issues for microwave metasurfaces, in which the dispersive response of each meta-atom is precisely controlled by an external voltage imparted on the diode. We experimentally demonstrate two effects utilizing our scheme. First, we show that a tunable gradient metasurface exhibits single-mode high-efficiency operation within a wide frequency band, while its passive counterpart only works at a single frequency but exhibits deteriorated performances at other frequencies. Second, we demonstrate that the functionality of our metasurface can be dynamically switched from a specular reflector to a surface-wave convertor. Our approach paves the road to achieve dispersion-corrected and switchable manipulations of electromagnetic waves.

Multifunctional Microstrip Array Combining a Linear Polarizer and Focusing Metasurface

Although microstrip reflectarrays/transmitarrays have been extensively studied in the past decades, most previous designs were confined to monofunctional operations based on either transmission or reflection. In this communication, we propose a scheme to design multifunctional arrays that can simultaneously exhibit the functionalities of a reflectarray and a transmitarray on the basis of the appealing feature of a polarizer we discovered (i.e., constant phase difference between its cross-polarization transmission and copolarization reflection within a broadband). To demonstrate the proposed scheme, we designed and fabricated a multifunctional device comprising a 15 × 15 array of twisted complementary dual-split ring resonators, each carefully designed to exhibit the desired transmission phase satisfying a parabolic distribution. Feeding the device by a Vivaldi antenna at its focus, we numerically and experimentally demonstrated that our system functioned as a directive emitter working in a transmission/reflection mode for cross-polarization/copolarization radiation at low/high frequencies, and it can radiate directively in both directions with different polarizations at intermediate frequencies. The half-power beamwidth of the array antenna was ~15°, which is 40° narrower than that of a bare Vivaldi antenna. Moreover, the gain was higher than 13 dB in all cases studied, which is at least 7 dB higher than that of the Vivaldi antenna.

Compact Microstrip Antenna With Enhanced Bandwidth by Loading Magneto-Electro-Dielectric Planar Waveguided Metamaterials

A new concept of planar magneto-electro-dielectric waveguided metamaterials (MED-WG-MTM) is proposed to manipulate the effective permeability μeff and the effective permittivity εeff. The MEDWG-MTM cell consists of an electric complementary spiral ring resonator (CSR) in the upper metallic plane and a magnetic embedded Hilbert-line (EHL) in the ground plane. The characterizations and working mechanisms are investigated in depth through eletromagnetic (EM) simulation, circuit model calculation and effective material parameters analysis. Numerical results show that the MED-WG-MTM can be manipulated with a larger refractive index for miniaturization and a larger wave impedance for bandwidth (BW) enhancement. For demonstration and potential applications, a microstrip patch antenna working at 3.5 GHz and occupying an area of only 0.20λ0 × 0.20λ0 is designed by using the derived flexible three-step frequency tuning method. A good agreement of results between the simulations and measurements suggests that the designed antenna advances in many aspects such as compact dimensions with a 42.53% miniaturization, broad operation band with a 207% impedance BW enhancement, and comparable radiation performances relative to its conventional counterparts.

An Octave-Bandwidth Half Maxwell Fish-Eye Lens Antenna Using Three-Dimensional Gradient-Index Fractal Metamaterials

The design and performance of a novel octave-bandwidth highly-directive half Maxwell fish-eye (HMFE) lens antenna are presented in superextended C band. The three-dimensional (3D) HMFE lens is implemented by gradient-refractive-index (GRIN) metamaterials and launched by an omnidirectional planar microstrip trapezoid printed monopole from the perspective of high integration, light weight and low profile. A new approach is proposed to design the GRIN metamaterial element in terms of a deep subwavelength feature by incorporating fractal geometry. Numerical and experimental results coincide well, showing that the lens enables a considerable gain enhancement of the monopole near 10 dB across a frequency range of 3 to 7.5 GHz while without significantly affecting the cross-polarization patterns and impedance matching. The near-field free-space measurement is also performed in an octave to afford a physical insight into the high gain, which is attributable to the accurate conversion of quasi-spherical waves to plane waves. Moreover, the truncation and homogenization effects of the lens on the antenna directivity are investigated to illustrate the fundamental mechanisms and afford the design guidelines.

Miniaturization of 3-D Anistropic Zero-Refractive-Index Metamaterials With Application to Directive Emissions

Several strategies are explored for the first time toward the miniaturization of three-dimensional (3-D) anistropic zero-refractive-index metamaterials (AZIM). By incorporating the fractal, spiral, and meandered resonant metallic inclusions within a host dielectric medium, several AZIM elements are engineered in subwavelength at the plasma frequency. The influences of geometrical shape, arrangement, and dimensions of inclusions on electromagnetic features of the 3-D elements are also investigated to obtain the design guideline. In this frame, we have designed a set of AZIM elements with near-zero permittivity and near-zero permeability occurring at the same frequency. To demonstrate potential applications, a lens horn antenna by loading the 3-D AZIM lens has been designed, fabricated, and measured. Numerical and experimental results agree well and illustrate that the beamwidth of the E-plane pattern has reduced about 4.9° while that of the H plane has reduced about 6.7°. Moreover, the gain of the lens horn antenna has improved 1.6 dB relative to its conventional counterpart with identical aperture. The proposed avenue in 3-D AZIM design advances a step toward the compactness and homogenization.