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In this letter, a dual-band polarization-independent terahertz (THz) metamaterial absorber (MA) based on metal-insulator-metal (MIM) configuration is proposed and measured. The flexible material polyimide is chosen as the substrate. The periodic metal array cell on the top surface is fabricated utilizing four F-shaped metal strips, and due to the high symmetry of the structure, the designed absorber is insensitive to polarization. Furthermore, the impact of structural parameters on the absorption characteristics is numerically investigated, and the results indicate that the absorption peak can be flexibly adjusted by changing the length of the top metal strip. The proposed MA achieves near-perfect absorption at 0.555 THz and 0.797 THz under normal TE and TM polarized incidence, with Q-factors of 23.56 and 45.28, respectively. This work provides a new design strategy for the realization of multi-band THz MA.

A reflective polarization converter (PC) with independently controllable passband is proposed using two dipole antennas and a bandpass filter. The vertically allocated dipole antennas and the horizontally placed bandpass filter are combined with each other to form an antenna-filter-antenna structure, which can realize polarization conversion and filtering performance simultaneously. Single- and dual-band PCs with controllable filtering responses and high selectivity are designed, where single- and dual-band filtering characteristics can be achieved as long as the microstrip filter structure is changed. To validate the design concept, a dual-band PC is fabricated and measured, where the measurement and simulation results are in good agreement. The measured lower and upper band-edge roll-off rates of the dual-band PC are 234 dB/GHz and 95 dB/GHz, and 93 dB/GHz and 69 dB/GHz, respectively, featuring high selectivity. The proposed PC will have significant potential applications in wireless communication systems.

In this article, a new design method of waveguide bandpass filters (BPFs) using spoof surface plasmon polaritons (SSPPs) is proposed based on the double-layer grounded (DLG) technology. Two-layer metallic strips of the SSPP unit cells are coated on dielectric substrate and grounded to the rectangular waveguide (RWG) wall, where the substrate is inserted into the $E$ -plane of RWG. Due to the DLG structure, the capacitance coupling between two layers is introduced, and consequently the bandpass filtering responses are realized by the SSPP itself. For validation, two different types of Ka-band DLG-SSPP waveguide BPFs are presented and discussed. Their corresponding unit cells’ equivalent circuit models are extracted to clarify the filtering characteristics and transmission zero (TZ) generation. The electromagnetic (EM) simulation results show that the center frequency can be flexibly controlled by adjusting the groove length. Furthermore, in order to control the upper cutoff frequency independently, an improved structure of DLG-SSPP unit cell with central patch is also proposed. To demonstrate and verify the design feasibility, three examples of waveguide BPFs are fabricated. Good agreement between measured and simulated results show that the proposed idea will be a good candidate for the waveguide BPF design with compact size and flexibly tuning ability.

A novel method for estimation and optimizing structural parameters of coupled-line bandpass filters (BPFs) using transformer-based estimator (TBE) and multilayer perceptron (MLP) is proposed. Once trained, the TBE can quickly obtain the predicted values of the BPF structural parameters from desired S-parameters, while the trained MLP can replace the time-consuming electromagnetic (EM) simulation process, and establish the mapping from structural parameters to S-parameters. In order to obtain the optimal structural parameters, the trained MLP is combined with genetic algorithm (GA) for fast optimization. To demonstrate the effectiveness of the proposed method, a BPF using three pairs of coupled-line microstrip structure is designed and fabricated. The experimental results demonstrate that the proposed method can quickly and accurately obtain the optimal structural parameters from the desired frequency response of the coupled-line BPF.

In this article, a wideband diplexer is proposed based on spoof surface plasmon polaritons (SSPPs). The dispersion properties of the SSPP unit cell are analyzed, which shows that the upper/lower cutoff frequency and the bandwidth of the channel filter can be flexibly controlled. Based on the proposed SSPP-based bandpass filter (BPF), a WR-4 band wideband diplexer is further developed by the space-mapping technique, which is used to achieve high computational efficiency and design accuracy. For demonstration, two SSPP-based channel BPFs and the proposed diplexer are fabricated by high-precision computer numerical control (CNC) milling technology. The measured results show that the average insertion losses (ILs) of two BPFs are 1.6 and 2.4 dB, respectively, and the return losses are both better than 17 dB. Meanwhile, the measured 3-dB bandwidth of the proposed diplexer is 174–204 GHz and 225–255 GHz for two channels. The average ILs are 1.8 and 2.7 dB within the passbands of two channels, and the corresponding return losses are both better than 16 dB, which are all in good agreement with the simulated ones. Furthermore, a WR-4 band dual-carrier wireless link is built based on the proposed diplexer for future 6G terahertz (THz) communications. The corresponding real-time date rate is 41.6 Gbps for the proposed wireless link and the transmission distance is up to 3 km.

Topological waveguides and traditional microwave transmission lines often face impedance and mode mismatch issues, which limit its application. We designed a new topological waveguide in a microstrip system that addresses these challenges by the interface of different valley Chern numbers. Through simulation and experimental validation, we have demonstrated effective direct feeding via microstrip lines. Furthermore, we have shown that topological state maintains transmission stability, even in the presence of corners and defects, due to their topologically protected characteristics. It will serve as a novel platform for practical use of topological circuit and system.

In this brief, a voltage-controlled tunable filtering attenuator using PIN diodes is proposed. A coupled-line structure loaded by PIN diodes is utilized to realize the integration of filtering and attenuation. The ideal circuit of the proposed tunable filtering attenuator is derived and analyzed by odd- and even-mode method. For demonstration, a filtering attenuator prototype with inductors and capacitors for the DC bias circuit is fabricated, where the measured results agree well with the corresponding simulations. The measured results show that the filtering attenuator can realize the attenuation of $2.76 \sim 16.99$ dB within a bandwidth from 1.34 GHz to 2.65 GHz when the DC bias voltage is changed from 0 V to 0.62 V.