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A synthesized strategy for achieving polarization-insensitive metalens is proposed by imposing identical parabolic phase patterns individually on two decoupled left-handed (spin up, σ+) and right-handed (spin down, σ-) circular polarization (CP) channels of an anisotropic scheme. This is enabled by synergizing dynamic phases and local geometric phases within 0° and 90° rotations. As a consequence, similar focusing behavior is expected when it is illuminated by a plane wave of arbitrary polarization since any incoming wave is a superposition of σ+ and σ- wave. Such an intriguing feature can be utilized to engineer a polarization-insensitive reflectarray/transmitarray in a shared aperture by placing a feeding source of different polarizations at its focal spot without altering the array aperture. Therein, the polarization of the meta-array is determined by its feeding which is readily to be changed in practice. For demonstration and possible applications, two reflectarrays composed of 50 × 50 elements at Ku band with one in normal and the other in offset configuration were designed and experimentally characterized. Numerical and experimental results illustrate that both reflectarrays are able to work in dual-linear (LP) and dual CP states by utilizing LP and CP horns of different orientations. Moreover, the versatile polarization-agile property does not pose significant penalty on its radiation performance and both reflectarrays exhibit a measured peak aperture efficiency of 63.8 and 39.6%, respectively near 14 GHz, and a 1dB gain bandwidth more than 11.8 %.

In this article, by deliberately controlling multiple resistive electric and magnetic resonances in terms of dispersion and dissipation, a low-profile, wideband, microwave metamaterial absorber with wide-angle and polarization-independent responses is proposed. The proposed absorber comprises a planar array with resistance-loaded metallic cross patterns, and a vertical periodic crossed mesh array with resistance-loaded metallic ring patterns. The vertical periodic crossed mesh array was inserted between the planar array and the metal ground to improve the wide-angle polarization-independent absorption. The involved dispersion-engineered design strategy for angular- and polarization-insensitive responses is described with numerical evidences and electromagnetic response behaviors. A proof-of-concept absorber was fabricated and measured for verification. At quasi-normal incidence, the measured bandwidth characterized by more than 90% absorption was 2.11–3.89 GHz, i.e., a fractional bandwidth (FBW) of 59.3%. At the incident angle of 50°, the FBW of the absorption larger than 90% was 48.6%. The absorber was thin with a thickness of 13 mm, corresponding to $0.09\lambda _{0}$ at the lowest operating frequency. The numerical and experimental results demonstrated that our proposed strategy provides an effective way to achieve wide-angle and polarization-independent responses in a broadband; these responses are very promising for most strategic applications.

In this article, a 2-D Airy beam is experimentally generated at microwave frequency by using a simple and thin metasurface composed of a single-layer of square C-shaped complementary split-ring resonators (C-SRRs). It is shown that the cross-polarization transmission coefficient, including the magnitude and phase, of a single C-SRR unit can be controlled by modifying the single C-SRR unit rotation angle, and this property is herein used to reshape the impinging field in accordance with the desired 2-D Airy beam profile. The designed metasurface is then fabricated to measure the most significant features of the generated microwave Airy beam, including quasi-nondiffraction, self-bending, and self-healing properties, thus achieving good results in agreement with those predicted by theory. The generated microwave 2-D Airy beam with its properties can be widely applied for high-efficiency wireless power transfer, wireless covert communication, and directional communication.

In this brief, a novel multi-mode square ring loaded resonator (SRLR) is presented, and a tri-band balanced bandpass filter (BPF) with good performances is developed. The multi-mode SRLR consists of a ring resonator and six open stubs, and its resonant characteristics is obtained by the odd/even-mode method. Compared with conventional SRLR, the symmetrical SRLR has more degree of freedom in controlling the resonance modes. Thus, a wide upper DM stopband can be achieved by choosing proper electrical lengths of open stubs. Moreover, frequency dispersion technique is used to enhance common-mode (CM) suppression in the proposed balanced tri-band BPF. By loading a cross-shaped stub and open-loop stub to the center of the first and second order SRLR respectively, five CM transmission zeros (TZs) are produced. Therefore, a wideband CM suppression can be achieved with the DM response almost intact. To verify this design method, a tri-band balanced filter operating at 2.4, 3.5 and 5.2 GHz has been designed, simulated and fabricated. Measured results exhibit good agreement with the simulated ones.

Structured interfaces with subwavelength features enable modulations of phase, amplitude, and polarization on demand, leading to a plethora of flat-profile devices and metasurfaces. Plasmonic and dielectric metasurfaces have been intensively explored, building up the frameworks of flat optics for ultrathin and integrated nanophotonics. The in situ controllability and tunability of aforementioned family of metasurfaces, however, has been a grand challenge, due to the intrinsic limitations of the materials. Semiconductors with diversified catalogs of material candidates thus demonstrate promising potentials, owing to the mature and versatile technologies developed nowadays. The fuse of semiconductors and nanostructured metasurfaces has been witnessed more recently, paving a distinct avenue toward active, tunable, reconfigurable light manipulation for next-generation optical nanodevices. Judicious selection of the active materials for metasurfaces empowers the active functionality of the designer applications. This paper presents a review of this merging semiconductor paradigm for active metasurfaces across a wide range of spectrum and shows unprecedented potentials in the future interface-based optoelectronics, quantum optics, nano-optics, and surface engineering with full compatibility of semiconductor foundry.

A hybrid design method for broadband radar cross section (RCS) reduction based on reflection diffusion is proposed and successfully demonstrated. To this end, we first analyze the scattering behavior of full parabolic-phased metasurface and find out its weaknesses in achieving perfect diffusive patterns. Then, we alleviate the issue of full parabolic-phased method through hybridizing two strategies, i.e., concave/convex chessboard arrangement and random focal lengths. The proposed hybrid design makes the phase profile aperiodic without any mirror symmetry, which is the key for achieving perfect diffusive scattering behavior within a wide operation band. The numerical and experimental results show that our proposed design features broadband, polarization insensitivity, and wide incidence angle and can efficiently decrease the RCS more than 10 dB within 7.8–23.2 GHz. Our approach comprehensively solves the issues of narrow band, high bistatic RCS value, time-consuming optimization process, and sensitivity to different polarization and incident angles, promising great potential in stealth applications.

This paper presents a novel broadband phase-shifting surface (PSS) element with frequency-selective surface (FSS) backing, which could independently control the phase of transmitted/reflected waves at high/low frequencies. Using the appealing features of basic building block, a high gain planar array antenna is designed and implemented by employing a set of these spatially varied phase-shifting elements. Fed by a Vivaldi antenna, the fabricated array is composed of $9 \times 9$ elements, each carefully designed to exhibit the desired transmission/refection phase at high/low frequencies. Numerical and experimental results indicate that the fabricated array is capable of showing the functionalities of transmitarray and reflectarray at different frequencies. The peak gain of either transmission or reflection beam was found to be 18.8 dB, which was at least 10 dB higher than that of bare feed source.

© 1963-2012 IEEE. Non-Foster technology facilitates the ability to surpass the Chu bandwidth limit associated with electrically small antennas (ESAs). Nonetheless, in addition to challenging stability issues, the enhanced performance can come at the cost of increased noise and resistance losses generated by the active circuit. Consequently, low total efficiency and degraded signal-to-noise ratio (SNR) values can arise. Stability and SNR have dominated most reports to date; little has been discussed with regard to the underlying innovative physics of non-Foster augmented radiators. In this communication, we propose a broad bandwidth non-Foster ESA, emphasizing those aspects. By embedding a non-Foster element into the near-field resonant parasitic element of a metamaterial-inspired antenna, its electrically small size is maintained. On the other hand, a 5-times enhancement of its -10 dB fractional bandwidth (15 times its -3 dB bandwidth) is measured, significantly surpassing its passive Chu limit. Under a good matching, the measurements demonstrate that this non-Foster ESA achieves a 1.05 dBi peak gain and realizes average 5.0 dB SNR and 17 dB gain improvements over its passive counterpart.

A stopband-extended sixth-order balanced bandpass filter with switchable passband function and stopband response is proposed in this paper. Based on the use of asymmetric coupled stepped-impedance resonators, the upper differential-mode stopband can be extended, and high common-mode suppression can be performed within passband. By means of high-order structure and cross coupling, which introduce transmission zeros closely around the passband, high selectivity is attained. The configuration utilizing switchable cross coupling is eventually employed in the balanced filter to perform either Elliptic function with steep passband edge or Chebyshev function with flat and deep stopband. The circuit is fabricated in high-temperature superconducting technique on an MgO substrate with YBCO thin films, which can result in relatively low insertion losses. Experiment at the temperature of 77 K was carried out to validate the design concept. The results attain good agreement with the predicted ones in simulation.

Metasurface-based vortex beam generators are very promising and applicable to enhance transmission data capacity in wireless communication system. However, most designs to date are ceased to predict their far-field characteristic, which is actually in demand when applied to wireless communication. Here, we find for the first time a deterministic and robust strategy to fastly estimate the exact far-field patterns and gain limit of a vortex beam based on the theoretical analysis of aperture field with spiral phase profile. For verification, a broadband metasurface-based vortex-beam generator based on geometric phase is designed, numerically calculated and experimentally measured at microwave regime. Excellent agreements are observed among far-field results obtained based on the proposed method, numerical simulations and experimental measurements, firmly demonstrating the validity and correctness of the proposed strategy. Such a deterministic and robust strategy of predicting far-field characteristics of a vortex beam may pave the way for its applications in many engineering scenarios, such as conical beam design, wireless communication, et al.

In this paper, a wide-stopband high-temperature superconducting (HTS) bandpass filter (BPF) is presented using slitted stepped-impedance resonators (slitted-SIRs). The use of SIR is essential in widening the filter stopband. Besides, it is found that introduced slits in SIRs provide additional freedoms to control resonance modes when compared with a conventional SIR. Hence, four types of slitted-SIRs are arranged at same fundamental mode f 0 and different higher order modes by adjusting the slits. The coupling of higher order modes is reduced and wide stopband is achieved. In addition, a composite spurline structure is employed to generate two independently controllable transmission zeros, which further improves the stopband performance. Finally, a cascade quadruplet HTS BPF is designed and fabricated on an MgO wafer with double-sided YBCO films. Measurement shows the passband of the BPF is operated at 2.35 GHz and its minimum insertion loss is less than 0.15 dB. The rejection level is better than 21.9 dB in the stopband up to 13.26 GHz (about 5.64 f 0 ). The designed HTS filter tends to the potential application for the desired wide stopband and the high selectivity performance.

Vortex beams have been extensively realized using different approaches. Typically, the efficiency and bandwidth of a vortex beam are limited by impure copolarized components and the intrinsic dispersion of passive resonant structures. Here, we propose a strategy to generate wideband vortex beams by using a Pancharatnam–Berry metasurface in which two orthogonal reflections exhibit a broadband out-of-phase difference. To achieve this, a broadband strategy based on multimode operation and dispersion engineering methods was established. A dual-layer meta-atom is proposed; each layer comprises of five metallic dipoles, and the geometrical parameters are carefully adjusted to tune the resonant frequencies. Because the dipole orientations in each layer are orthogonal, the reflection responses under the two orthogonal polarizations can be independently engineered. Both numerical and experimental results indicate that our method not only enables a high-efficiency spiral beam conversion over a broad range of 6.95–18 GHz (>82%) but also causes a polarization-insensitive effect; thus, it can be adapted for any linear or circular polarization.

A quad-channel high-temperature superconducting (HTS) diplexer with high isolation using spiral stub-loaded resonators (SSLRs) is presented in this paper. The quad-channel diplexer contains two paths with four SSLRs. Each path consists of two passbands that each passband shares two common SSLRs for size reduction. The SSLR structure of this study offers several design freedoms, such as the passband frequency can be adjusted by properly tuning its geometrical parameters and its bandwidth can be controlled through changing the coupling coefficients between the adjacent SSLRs. Moreover, a T-junction matching network is designed to minimize the undesired interactions between the two paths of the diplexer. To verify the designed methods, an HTS quad-channel diplexer operating at 1.91/2.157/2.36/2.59 GHz is successfully designed and fabricated by using YBCO thin film on a 0.5-mm-thick MgO substrate and it shows a compact size of $0.16\,\lambda _{g}\times 0.23\,\lambda _{g}$ . The maximum insertion loss of four passbands is 0.28 dB and the isolation between channels at the four passbands higher than 50 dB is obtained by adopting T-junction matching network.

A novel triple-layer dual-mode meta-atom wherein an H-shaped structure is combined with a pair of symmetric patches is proposed. The composite structure substantially lowered the operation frequency, and balanced phase agility with transmission magnitude bandwidth. The transmission phase limit of the proposed composite structure approaches the theoretical limit. Because this structure has appealing features, we employed a set of these ultrathin meta-atoms to implement the phase distribution of an optimized array using the alternating projection method. An X-band single-feed quad-beam transmitarray consisting of $25 \times 25$ elements, each carefully designed to exhibit the desired transmission phase, was designed, simulated, physically implemented, and measured. Feeding the meta-array using a horn at its focus, four-beam radiation patterns with satisfactory sidelobe levels and gain were numerically and experimentally demonstrated. The peak gain was found to be 18.8 dB at 9.6 GHz, and the aperture efficiency was calculated as 38.3%. Moreover, the half-power beamwidth of the array antenna was approximately 7°, which was 50° narrower than that of a bare feed horn. Moreover, the gain of each beam was higher than 17 dB in all studied cases, which was at least 7 dB higher than that of a bare feed horn.

In this letter, an efficient approach for broadband radar cross-section (RCS) reduction is proposed. By coding eight types of linear-phase gradients in a spiral pattern, the reflected wave was uniformly diffused, and thus, the backscatter RCS reduction was achieved within an ultrabroadband. For verification, a spiral-coded metasurface composed of $8\times 8$ subarrays is designed, fabricated, and measured. Numerical and experimental results both show that the thin metasurface enables a 10 dB RCS reduction over an ultrabroadband ranging from 12.2 to 23.4 GHz. Moreover, the good merit of our approach is further demonstrated by comparing the scattering behavior to the steering beam of a regular sequential layout pattern. Our approach features broad band, polarization insensitivity, wide incident angles, and easy design without any optimization, promising great potential in real-world low-scattering stealth applications.

A balanced tri-band bandpass filter (BPF) is proposed based on octo-section stepped-impedance ring resonator (SIRR). The relation and design graphs between the differential-mode (DM) and common-mode (CM) characteristics with different physical parameters are analyzed. It reveals that the proposed SIRR has more degrees of freedom in controlling DM and CM characteristics for tri-band design with excellent out-of-band performances and high CM suppression. Moreover, the SIRR is loaded by open stubs with different lengths to further improve the CM suppression. Finally, a balanced tri-band BPF prototype is fabricated and measured. Good agreement between simulated and experimental results verifies this design method.

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 $15 \times 15$ 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 $\sim 15^{\circ }$ , 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.

We propose a system of antenna array and metamaterial lens in the millimeter-wave frequency, in which the linear antenna array controls the radiation beam in the horizontal direction, while the metamaterial lens controls the radiation beam in the vertical direction. Numerical simulation and experiment results agree well with each other. It is shown that the antenna system with gain of 28.7 dB exhibits a desirable beamwidth of 9.9 ° in the yz-plane and beamwidth of 3.5 ° in the xz-plane at the central frequency 37.5 GHz.

A new 2-D ultra-thin transmissive phase gradient metasurface (TPGM) with polarization-independent property is proposed based on the generalized Snell’s law of refraction. The super unit cell of the tri-layer TPGM consists of $6 \times 6$ suitably selected elements with $- {60}^\circ $ phase gradient at $x $ -direction and 60° phase gradient at $y $ -direction, respectively. The characterizations and working mechanisms of the TPGM are investigated in depth through theoretical calculation, comparative analysis, and eletromagnetic (EM) simulation. Four types of TPGMs with different functionalities are proposed based on the derived four-step design method. Numerical results show that the TPGMs are capable of manipulating the differently polarized wave independently. For demonstration and potential applications, an ultra-thin polarization beam splitter (PBS) working at X-band is implemented by a specially designed 2-D TPGM and is launched by a wideband horn antenna from the perspective of high integration, simple structure, and low cost. Numerical and experimental results coincide well, indicating that the PBS advances in many aspects such as separating and controlling the orthogonally polarized waves with a polarized splitting ratio better than 18 dB, obtaining a comparable bandwidth of more than 600 MHz, gaining a high transmission efficiency and also adopting a simple fabrication process based on the convenient print circuit board (PCB) technology.

We propose a high-gain transmitting lens antenna by employing layered phase-gradient metasurface (MS). The MS is engineered to focus the propagating plane wave to a point with high efficiency. An X-band patch antenna is placed at the focal point of the MS as a feed source, and then the quasi-spherical wave emitted by the source is transformed to plane wave. Due to the successful conversion of quasi-spherical wave to plane wave, the beam width of the patch antenna has been decreased 66° and the gain has been enhanced 11.6 dB. The proposed lens antenna not only opens up a new route for the applications of phase-gradient MS in microwave band, but also affords an alternative for high-gain antenna.

A new concept of planar magneto-electro-dielectric waveguided metamaterials (MED-WG-MTM) is proposed to manipulate the effective permeability ${\mu_{\mathrm {eff}}}$ and the effective permittivity ${\varepsilon_{\mathrm {eff}}}$ . The MED-WG-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 ${0.20 \lambda _0 \times 0.20 \lambda _0}$ 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.

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.

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 $^{\circ}$ while that of the H plane has reduced about 6.7 $^{\circ}$ . 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.

A set of two-dimensional (2-D) left handed (LH) transmission line (TL) structures are proposed based on the single-layered and double-layered fractal-perturbed mushrooms. The effects of the Koch Island on the LH characteristics have been investigated physically by examining the dispersion diagram of the LH structure with and without fractal perturbations, showing that the parasitic transverse electric mode has been considerably suppressed without any deteriorative penalty on the dominant transverse magnetic mode. Therefore, the interaction of the degenerated parasitic mode with the operation modes has been considerably weakened. The concept has been employed to suppress the cross-polarization (XP) radiation of a microstrip antenna which incorporates a 2 × 2 array of the LH structure into a conventional microstrip patch. Numerical and experimental results indicate that the XP radiation (