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In this paper, we numerically and experimentally demonstrate a substrate integrated plasmonic waveguide (SIPW) concept and its application in microwave bandpass filters. This SIPW consists of double arrays of slots etched on the top and bottom metal layers of a substrate integrated waveguide (SIW) to support spoof surface plasmon polariton (SSPP) modes with low and high cutoff frequencies. The simulated results show that by tuning the parameters of the SIPW's unit cell, the dispersion characteristics can be engineered at will. Then, we propose a sharp roll-off microwave bandpass filter based on this SIPW. This filter has a passband from 7.5 to 13.0 GHz with high return loss and low insertion loss. Furthermore, to demonstrate the independent tuning of the passband of the filter, we also design two microwave bandpass filters with passbands of 9.2-13.0 GHz and 7.5-10.5 GHz by decreasing the distance between two rows of via holes and increasing the slot length, respectively. Finally, to experimentally validate the filter designs, we fabricate and measure three prototypes and find that the experimental results are in excellent agreement with the simulations. This SIPW concept may have extensive potential applications in the development of various plasmonic integrated functional devices and circuits.

In this paper, a novel planar waveguide based on spoof surface plasmon polaritons (SSPPs) using fish-bone corrugated slot structure is first proposed in the microwave region. Low-dispersion band can be realized by such structure with tight field confinement of SSPPs, resulting in size miniaturization of the proposed waveguide. The high frequency stopband of the proposed ultra-wideband bandpass filter (BPF) is created by using this proposed waveguide, while the low frequency stopband is properly designed through introducing the microstrip-to-slotline transition. The 2-D E-fields distribution, surface current flow, and energy flow patterns are all calculated and illustrated to demonstrate the electromagnetic (EM) characteristics of the proposed ultra-wideband BPF. The BPF tuning characteristics is explored to provide a guideline for facilitating the design process. To validate the predicted performance, the proposed filter is finally designed, fabricated, and measured. Measured results illustrate high performance of the filter, in which the reflection coefficient is better than -10 dB from 2.1 to 8 GHz with the smallest insertion loss of 0.37 dB at 4.9 GHz, showing good agreement with numerical simulations. The proposed surface plasmon polariton waveguides are believed to be significantly promising for further developing plasmonic functional devices and integrated 2-D circuits with enhanced confinement of SSPPs in microwave and even terahertz bands.

A dual-layer patch antenna using multiple parasitic patches for the bandwidth enhancement is presented. Two additional resonances can be obtained with the help of the parasitic patches of the antenna. Consequently, a wide impedance bandwidth of 17% (|S11|

A tunable dual-band bandstop filter (BSF) in the far-infrared region is proposed based on a graphene surface plasmon. The BSF consists of a graphene waveguide and two graphene-ribbon resonators on the SiO2/Si substrate, supporting waveguide mode and edge mode, respectively. The influence of graphene structure parameters on transmission spectra is numerically investigated, which shows that the resonant properties are sensitive to the lengths of the graphene-ribbon resonators, and each stopband of dual-band BSF can be individually designed and tuned without affecting the other stopband. Moreover, the center frequencies of the two stopbands can be actively tuned independently by respective chemical potential of graphene-ribbon resonators. The proposed filter structure is promising to be utilized in future photonic integrated circuit applications as an ultracompact dual-band BSF.

A compact triplexer based on E-stub-loaded composite right-/left-handed (ESL-CRLH) resonators with quasi-lumped impedance matching network is presented in this paper. The equivalent circuit model of the ESL-CRLH resonator is presented first and its left-/right-handed capacitance/inductance elements are fully derived. Then, a quasi-lumped impedance matching circuit is designed to connect the three ESL-CRLH resonator based filter channels for the triplexer construction. Finally, the designed triplexer obtains high isolations among the ports and low in-band insertion losses of the three filter channels centered at 1.86, 2.41, and 3.25 GHz, of which a miniaturized layout has been realized. Good agreement between the simulated and measured results can be observed to validate the design idea.

Two novel microstrip patch antennas with multiple parasitic patches and shorting vias have been presented for the bandwidth enhancement. Based on the conventional triangular patch antenna, two more resonances can be obtained with the introduction of multiple parasitic patches, and consequently, the antenna bandwidth can be broadened. Parametric analysis of the patches has been studied for the verification of bandwidth enhancement. An example of the proposed antenna with multiple parasitic patches is designed, fabricated, and tested. The measured bandwidth with $\vert S_{11}\vert < -10$ dB ranges from 5.46 to 6.27 GHz (13.8%), and good far-field radiation patterns can be obtained within the frequency band. In addition, two shorting vias are inserted into the above proposed antenna to decrease the input impedance, resulting in further bandwidth enhancement of the antenna. This antenna is fabricated and tested as well, which achieves a measured 10-dB impedance bandwidth of 17.4% from 5.5 to 6.55 GHz.

Double dipoles on a single-layer substrate are utilized to construct a triple-mode printed quasi-Yagi antenna for the multi-band and broadband antenna applications. A stub-loaded dipole generating two resonant modes (i.e., lower dual-mode dipole) is allocated on the underside of a simple dipole (i.e., upper single-mode dipole) introducing the third resonant mode. Using these three resonant modes, three compact printed quasi-Yagi antennas, i.e., tri-band, dual-band, and broadband printed quasi-Yagi antennas, are designed with the same antenna prototype but different parameter values. Seen from the measured results, all of these three antennas have good unidirectional radiations, high radiation efficiencies, and low cross-polarization levels at the operating frequencies within the impedance bandwidths.

On the basis of a single multimode resonator (MMR), a miniaturised microstrip sext‐band bandpass filter (BPF) with high band‐to‐band isolations has been presented. The proposed MMR is an stepped impedance resonator (SIR) with one short‐circuited stub loaded in the middle and two identical open‐circuited stubs located at two sides. Transmission poles (TPs) and transmission zeros (TZs) can be excited when the electrical lengths of two open stubs increase. The TZs introduced by open stubs separate the TPs into multiple groups; therefore, the design of a sext‐band BPF becomes available. Moreover, two input/output tapped branches and radical stub‐loaded short‐circuited lines are treated as perturbation structures in order to increase design freedoms for improving filter performance. Good agreement between simulated and measured results validates the design method.

Abstract A low‐cost W‐band leaky‐wave antenna with two‐dimensional scanning capability is presented. By using the low‐loss transversal long‐slot array structure with a single‐layer parabolic reflector, the antenna realizes a passband from 85 to 102.8 GHz with Voltage Standing Wave Ratio

Metamaterials, an artificial electromagnetic (EM) material, have exhibited unique characteristics, enabling innovations in terahertz (THz) wavefront shaping, polarization modulation, surface wave manipulation, and spectrum modulation. In this review, we focus on the advancements in THz metamaterials for spectrum modulation, emphasizing their structural design, special materials, and applications. The review discusses various structural designs, including metal-dielectric composite structures, all-metal structures, and all-dielectric structures, each offering distinct advantages for THz applications. Key special materials such as graphene, vanadium dioxide (VO2), molybdenum disulfide (MoS2), and flexible materials are highlighted for their significant contributions to enhancing the performance and functionality of THz metamaterials. The applications of THz metamaterials in EM stealth and sensing are thoroughly introduced, with a particular focus on biosensing, pesticide detection, and other sensing applications. The review emphasizes the potential of THz metamaterials in developing highly sensitive and selective sensors, as well as efficient THz absorbers and filters. Future perspectives include the continuous development of novel materials, advanced fabrication technologies, and the integration of multifunctional capabilities to further expand the applications and efficiency of THz metamaterials. These advancements are expected to drive significant progress in THz technology, impacting fields such as medical diagnostics, environmental monitoring, food safety, wireless communication, and security.

Due to their inherent defects, circular polarization discrimination methods involving amplitude-phase or axial ratio are obstacles of the computer software-based optimal design of broadband linear-to-circular (LTC) polarization conversion metasurfaces. Therefore, based on the proposed circular polarization discrimination method relating to ellipticity angle β, this study exploits Python-programmed elitist preserving genetic algorithm to optimally design a reflective-type broadband LTC polarization conversion metasurface. The simulation results demonstrate that the final metasurface can realize the wideband LTC polarization conversion; the measured results are in excellent agreement with the simulations, thus validating the proposed design method. It provides an efficient alternative for broadband LTC polarization conversion metasurface designs, which drives the development of the design of LTC polarization conversion metasurfaces from manual methods towards automation. And this work can be applied in antenna designs, optical devices, and remote sensing.

A high-gain circularly polarized (CP) magnetoelectric (ME) dipole antenna with metallic radiating structure is presented. The proposed CP antenna element consists of two pairs of metallic posts with a supporting metallic plate, a metallic air-filled elliptical cavity, and a feeding slot on the bottom substrate integrated waveguide (SIW). The two pairs of modified rectangular metallic posts combined with the gaps between them act as radiating ME-dipoles which are surrounded by a metallic air-filled elliptical cavity. The above-mentioned radiating structures are fed by a modified cross-shaped coupling slot etched on the bottom feeding SIW to generate CP radiation, where loading the elliptical metallic cavity significantly improve the CP performance of the antenna element. Then, a 4 × 4 antenna array with full-corporate feeding network is designed, fabricated, and measured, which has an impedance bandwidth of 18.9 % from 37.4 to 45.2 GHz and a 3-dB axial ratio (AR) bandwidth of 14.2 % from 36 to 41.5 GHz. Thanks to the all-metal radiating structures, a maximum measured gain of 20.4 dBic has been achieved. The measured and simulated results are in good agreement.

Abstract A compact 60‐GHz on‐chip bandpass filter (BPF) is presented using gallium arsenide (GaAs) technology. To demonstrate the working mechanism of the proposed BPF, an inductor‐capacitor (LC) equivalent circuit model is conceived and analysed for further investigation of the transmission poles (TPs) and zeros. Finally, a prototype of the BPF is fabricated and tested to validate the proposed idea, whose simulated and measured results are in good agreement. The measurements show that it has a centre frequency of 60.2 GHz with a bandwidth of 14%, and the minimum insertion loss (IL) within the passband is 2.57 dB. The chip size, excluding the feedings, is about 0.265 mm × 0.184 mm.

This paper presents an innovative technique to synthesize a virtual antenna array (VAA) that consumes less energy than conventional antenna arrays that are used in mobile communications systems. We have shown that for a specific spectral efficiency a wireless system using the proposed virtual antenna array consumes significantly less energy per bit (∼3 dB) than a wireless system using a conventional multiple-input multiple-output (MIMO) array. This means the adoption of the proposed VAA technology in smartphones, iPad, Tablets and even base-stations should significantly reduce the carbon footprint of wireless systems. The proposed VAA is realized by employing a pair of linear antenna arrays that are placed in an orthogonal configuration relative to each other. This orthogonal arrangement ensures the radiation is circularly polarized. The size of the standard radiating elements constituting the VAA were miniaturized using the topology optimization method. The design of the VAA incorporates substrate integrated waveguide (SIW) and metasurface technologies. The function of SIW in the design was twofold, namely, to reduce energy loss in the substrate on which the VAA is implemented, and secondly to mitigate unwanted electromagnetic interactions between the neighboring radiating elements and thereby enhancing isolation which otherwise would degrade the radiation characteristics of the array. Metasurface technology served to effectively increase the effective aperture of the array with no impact on the footprint of the array. The consequence of SIW and metasurface technologies was improvement in the gain and radiation efficiency of the array. The proposed four orthogonal 4-element VAA covers the entire sub-5 GHz frequency range, and it radiates bidirectional in the azimuth plane and omni-directional in the elevation plane. Moreover, it is relatively easy to design and fabricate. The proposed VAA has dimensions of 0.96λ0 × 0.96λ0 × 0.0016λ0 at mid-band frequency of 3 GHz. VAA has a measured gain of 25 dBi and radiates with 90% efficiency. The average isolation between the linear arrays constituting the virtual array is better than 27 dB.

Plasmonic skyrmions are a subject of significant interest due to their potential applications in optics, photonics, and electromagnetic wave technology. These skyrmions are created by the interference of surface plasmon polaritons or spoof localized surface plasmons (SLSP), offering new possibilities for controlling light–matter interactions and structuring light. In this study, we have demonstrated the generation of both electric and magnetic skyrmions simultaneously using a rotational symmetric SLSP spiral meta-structure lattice, operating across a broad range from microwave to terahertz frequencies. By implementing them into a resonance configuration of the quasi-bound state in the continuum through symmetry breaking, we enhance the Q factor and fields, resulting in highly sensitive sensing performance. The SLSP metasurface enables tunable plasmonic skyrmions controlled by the incident polarization. Our findings have potential applications in highly sensitive sensing, filtering, modulation, and communication.

Abstract This work outlines the design, implementation, and experimental results of a Class‐F power amplifier (PA) with a novel output matching topology by utilizing substrate integrated waveguide (SIW). The design methodology is first proposed with one highly integrated SIW matching structure. The impedances at fundamental, second harmonic and third harmonic frequencies are analysed to explain good impedance matching for the PA as well as the isolation mechanism of the direct current (DC) and radio frequency (RF). For demonstration, an S‐band 10‐W Class‐F PA is fabricated and tested. The simulation and measurement results agree very well with each other, which verifies the proposed technology of SIW output matching. To the authors’ knowledge, this is the first time to use a highly integrated SIW matching in Class‐F PA, which simplifies the output matching for third‐order harmonics and DC/RF isolation structures.

Abstract The letter presents a triple‐mode short‐circuited circular patch resonator for bandpass filter (BPF) design. The resonator employs the TM010 mode as its dominant mode, the resonant frequency of which is lower than that of the TM110 mode. In addition, four radial slots are etched on the patch to extend the current paths of the TM110 and TM210 modes, shifting down the resonant frequencies of the two modes. The adjacent triple modes can be used to design wideband BPF. Meanwhile, the centre frequency (CF) and bandwidth (BW) can be tuned by the length of the slots. Furthermore, the short‐circuited vias are arranged in a circle providing a tuneable parameter to control the CF and BW as well. According to the symmetry of the geometry, the resonator can be bisected to realise half‐mode resonance. Two cascaded half‐mode semi‐circular patch resonators are applied to design BPF with high selectivity. BPFs using the circular patch resonator and the semi‐circular patch resonator are designed and measured for demonstration.

Abstract We numerically propose a dual-band absorber in the infrared region based on periodic elliptical graphene-black phosphorus (BP) pairs. The proposed absorber exhibits near-unity anisotropic absorption for both resonances due to the combination of graphene and BP. Each of the resonances is independently tunable via adjusting the geometric parameters. Besides, doping levels of graphene and BP can also tune resonant properties effectively. By analyzing the electric field distributions, surface plasmon resonances are observed in the graphene-BP ellipses, contributing to the strong and anisotropic plasmonic response. Moreover, the robustness for incident angles and polarization sensitivity are also illustrated.