Wide-stopband plasmonic filters are essential components in compact mid-infrared (MIR) photonic systems. This work proposes a geometrically tunable wide-stopband plasmonic filter based on a metal-insulator-metal (MIM) waveguide with dual resonator cavities. The optical response is numerically investigated using the two-dimensional finite-difference time-domain (2D FDTD) method. The influence of the resonator height H₂ and the inter-cavity distance D on the stopband characteristics is analyzed. The symmetric dual-cavity configuration enables effective control of the stopband bandwidth and central wavelength. The design achieves a significantly broadened stopband while maintaining compactness and high transmission selectivity, making it a promising candidate for integration into mid-infrared photonic and sensing systems.

This letter presents a compact, low profile singe substrate transmissive linear-to-circular polarization (LCP) converter designed and experimentally validated for point-to-point THz communication bands. The proposed LCP converter consists of an H-shaped gold metallic pattern deposited on both sides of a 100 μm-thick fused silica substrate. The LCP converter operates within the 0.225-0.307 THz frequency band, achieving a simulated 3-dB axial ratio bandwidth of 30.8% in simulation. Owing to its wide axial ratio bandwidth, the proposed design is a promising candidate for point-to-point THz communication applications. The performance of the proposed converter is verified through surface current distribution, which explains the occurrence of Huygens response and equivalent circuit model. The proposed converter exhibits a measured 3-dB axial-ratio bandwidth of 27.8% in the frequency band 0.229-0.303 THz. The simple geometry and single-substrate implementation, with a thin profile and wide 3-dB axial ratio bandwidth, make the proposed design suitable for practical deployment scenarios.

A circularly polarized (CP) millimeter-wave phased array antenna (PAA) is proposed for wide-angle scanning applications. The antenna is composed of radiating patches, coupling patches, and a ground plane. A single element consists of a centrally fed microstrip CP antenna with double arc-shaped slots, with a parasitic patch loaded on its top. A sequentially fed 2 × 2 subarray is constructed by arranging single elements in a specific orientation, and the central disc-ring structure is combined with the square ring patch structure based on the beam complementarity principle to broaden the beamwidth. Both simulations and measurements are performed on a 4 × 4 prototype array. The proposed antenna operates over a frequency band of 27.6-30.4 GHz, 3 dB AR bandwidth covers working bandwidth. When the beam scans to ±60°, the gain degradation relative to the boresight direction is only 1.1 dB, with the AR at the beam pointing angle maintained ≤3.5 dB. The proposed antenna boasts a compact size, facile fabrication process, and excellent wide-angle scanning capability, and it provides a novel design paradigm and practical solution for CP millimeter-wave wide-angle scanning PAA systems.

A novel integration method of a polarization conversion metasurface (PCM) and an array antenna for radar cross-section (RCS) reduction is presented. This method combines the PCM with a slot array antenna operating at 11.5 GHz for reducing RCS. The metasurface is composed of polarization conversion units arranged in a checkerboard pattern, and each PCM unit cell is made up of two symmetrical fork-shaped structures. The polarization conversion units can achieve a polarization conversion rate of over 90% in the frequency band of 10.12-19.93 GHz (65%). The measurements demonstrate that the antenna attains over 10 dB RCS reduction in the frequency range of 9.9-20.7 GHz (71%). Meanwhile, the radiation performance of the antenna is effectively preserved.

This study proposes a broadband, wide-angle metasurface for bistatic radar cross-section (RCS) reduction by integrating a low-profile bent-line unit design with an Adaptive Binary Particle Swarm Optimization algorithm enhanced by Array Pattern Synthesis (ABPSO-APS). The optimized metasurface achieves over 10 dB of bistatic RCS reduction across 8.4-21 GHz (86.7% fractional bandwidth), with a peak reduction of 22 dB, outperforming conventional checkerboard, genetic algorithm, and particle swarm optimization layouts by 22.82%, 15.27%, and 7.91%, respectively. The design also exhibits angular stability up to 30° and polarization insensitivity under both TE and TM incidences, while maintaining an ultrathin profile of only 0.1λ (where λ is the wavelength at the center frequency). These results demonstrate its strong potential as a compact and efficient solution for advanced electromagnetic stealth and radar signature control applications.

Supplying battery-free power to wireless sensor systems (WSS) mounted on rotating shafts remains a major challenge due to limited installation space, low rotational speed, and the requirement for long-term autonomous operation. This paper presents a compact dual-rotor energy harvester (EH) based on multilayer printed circuit board (PCB) sheets, designed for powering WSSs installed on ship propulsion shafts. Stacked multilayer PCB coils forming a three-dimensional structure are arranged on both the inner and outer rotors to enhance magnetic flux linkage and power density. The experimental results show that the EH generates power levels up to 959 mW at a shaft speed of 300 rpm. The output power improved nonlinearly with increasing rotational speed, demonstrating its suitability for real-time monitoring applications. The proposed EH offers a promising solution for powering WSS in autonomous driving technologies, with the potential for further optimization and integration into various mobility systems.

In this letter, a frequency and linear polarization(LP) reconfigurable antenna is proposed. The antenna consists of two pairs of printed dipoles as the primary radiating patches. By independently controlling the direction of flowing current using loaded PIN diodes, the dynamic reconfiguration of both frequency and linear polarization (LP) can be realized. In addition, a dual-band artificial magnetic conductor (AMC) reflector is added under the radiator, which can effectively reduce the antenna profile to 0.1λ₀ (12.4 mm, where λ₀ is the wavelength at low operating frequency). Both simulated and experimental results show that the proposed antenna can operate in four modes: 0° LP low frequency (2.36-2.77 GHz) state, 0° LP high frequency (3.25-3.68 GHz) state, 90° LP low frequency state, and 90° LP high frequency state. The antenna exhibits stable radiation patterns, with gain values of 7.56 dBi in the low frequency state and 8.03 dBi in the high frequency state. This antenna is suitable for ISM band applications, such as Wi-Fi (2.4-2.48 GHz) and Bluetooth (2.4-2.48 GHz), as well as TDD Band 42, meeting the requirements of modern wireless communication systems.

This study proposes an independently controlled polarization rotator with transmissive and reflective capabilities operating in two different frequency bands. The proposed independently controlled transmissive and reflective polarization rotator (ICTR-PR) unit cell consists of four metal layers separated by three substrates. The transmissive polarization rotator mode is realized by two strips (receiving strips) on the top layer, which are connected with two vias through circular holes inside the ground plane to two 90° rotated strips (transmitting strips) on the bottom layer. The reflective polarization rotator mode was produced by connecting another pair of strips on the top layer to a microstrip line located in the middle layer. Properly adjusting the length of each strip allows both transmissive and reflective features to be independently controlled. The proposed rotator exhibits dual-frequency band resonances at 7.7 and 9.48 GHz for reflection and transmission responses, respectively. Furthermore, a high polarization conversion ratio (PCR) of more than 80% was achieved for both modes. A prototype was fabricated and measured to validate the simulation results. A good agreement between the experimental and simulated results was obtained.

We present the design, fabrication, and experimental characterization of two 150 mm Luneburg lenses for X-band (10 GHz), produced by FFF using standard PLA. The printed PLA permittivity was measured with a 7 mm coaxial cell and EpsiMu, yielding ε_{r, PLA} ≈ 2.5 at 100% infill; a volume-weighted mixing model with perimeter correction was used to set discrete radial infill fractions. Two infill patterns (grid and gyroid) were tested, and waveguide mounts were integrated for reproducible alignment. Insertion-loss tests give 1.5 dB (grid) and 1.1 dB (gyroid) at 10 GHz. Far-field measurements (R = 1.5 m) and Friis-based estimates yield peak gains of 20.5 dBi (grid) and 19.4 dBi (gyroid) (simulation: 20.8 dBi); the waveguide reference gain is 4.9 dBi. Near-field tests at R = 0.15 m show an on-axis S₂₁ improvement of +2.33 dB, which corresponds to a low apparent near-field aperture efficiency (1.8-2.3%) while far-field efficiencies inferred from the measured gains are substantially higher (35-45%). These results confirm that discrete infill grading in low-cost FFF-printed PLA can realize effective Luneburg lenses at X-band, with quantifiable trade-offs among insertion loss, infill geometry, and realized aperture efficiency.

This study proposes a two-element Vivaldi antenna array that achieves broadband mutual coupling suppression and gain enhancement. First, by etching multiple spoof surface plasmon polariton (SSPP) slots on the ground plane to suppress surface-wave coupling, the inter-element isolation has increased from 20-31 dB to 20-45 dB, with an improvement of 5-10 dB (a peak of 20 dB) within the operating band of 1.8-4.5 GHz. Then, a quasi-transparent metasurface (MS) is placed above the aperture to enable phase compensation, converting spherical wavefronts to quasi-planar ones and thereby improving the gain of 0.5-2 dBi across the operating band. Finally, the designed Vivaldi antenna array is fabricated and measured, which exhibits S₁₁ < -10 dB (1.3-4.5 GHz), enhanced isolation, and stable gain performance.

Radar-based vital sensing methods have received significant attention due to their potential to provide continuous, non-contact measurements for heartbeat and respiration monitoring. Our original two-wave model extracts respiration and heartbeat data by formulating the estimation process as a minimization problem. Although the original method examines temporal changes in respiration and heartbeat signals in a different manner from existing methods, it remains sensitive to the slight body movements that often occur in laboratory experiments. In this study, we propose a modified two-wave model with improved robustness against such movements. Using experimental data collected with a millimeter-wave Multi-Input Multi-Output (MIMO) frequency-modulated continuous-wave (FM-CW) radar system, we demonstrate that the improved model can successfully measure both respiration and heartbeat signals even in cases where the original method fails, thereby improving the capability for non-contact vital signal detection.

This study explores the anisotropic electromagnetic properties of carbon nanotube (CNT)/polylactic acid (PLA) nanocomposites, fabricated in-house and shaped using traditional compression molding and advanced 3D printing techniques. By examining the effects of CNT content (ranging over 1-4 wt.% (weight percent)) and 3D printing path orientation, this research investigates how these factors influence shielding effectiveness (SE) and the corresponding nanocomposite complex dielectric permittivity tensor. Notably, a significant variation in SE was observed between the different printing path orientations, with a difference of over 20 dB at 4 wt.% CNT. Experimental measurements were used to develop an anisotropic model for the complex dielectric permittivity, with the permittivity components for samples at 4 wt.% CNT extracted to be 36.5-j44.5 along the printing direction (ε_||) and 8.3-j3.1 in the perpendicular direction (ε_⊥) over the X-band frequency range (8.2-12.4 GHz). These findings demonstrate that CNT alignment during 3D printing induces highly directional electromagnetic properties. Furthermore, we demonstrate that anisotropic simulation models provide a more accurate prediction of the electromagnetic response of 3D-printed nanocomposite structures than isotropic models. In brief, this study emphasizes the necessity of considering anisotropic properties in the design and simulation of 3D-printed nanocomposites for electromagnetic shielding and other applications.

Metamaterials are emerging as a key enabler for 6G wireless communications, attracting growing attention from industry due to their engineered electromagnetic properties that enable control over wave propagation. Metamaterials have shown promise across diverse applications; however, the desire to achieve 1 TBPS data rates in 6G communications is partially constrained by a major fundamental challenge in metamaterials: achieving gigahertz of instantaneous bandwidth (IBW) values. To address the IBW issue, we designed, fabricated, and tested a fundamental component of metamaterial, i.e., a modified split ring resonator (MSRR), achieving 90% of the targeted 1 GHz IBW and an effective permeability (μ_eff) close to 25 within the IBW. In addition, the gain-bandwidth product (GBWP) is over 1.5× greater than that of commercial 5G antennas, while maintaining the same aperture size of 1.59λ_c. We studied and reported the effect of MSRR frequency-dependent μ_eff on the IBW and GBWP and proposed an optimized MSRR design that achieves 3× the bandwidth of a conventional SRR. Finally, we integrated the proposed MSRR atop a wideband patch antenna, enhancing peak realized gain by 6 dBi.

This paper describes a flexible dual-port multiple-input multiple-output (MIMO) antenna system tailored for multi-band operation, covering 2.4 GHz Wi-Fi, 3.5 GHz 5G, as well as the 5-7 GHz band used in wireless applications such as Wi-Fi 7. The antenna is fabricated on a liquid crystal polymer (LCP) substrate, featuring an ultra-thin profile of 0.1 mm and a compact size of 60 × 35 mm², making it highly suitable for integration into modern flexible and wearable devices. To achieve high port isolation, a complementary split-ring resonator (CSRR) structure is incorporated between the two radiating elements. The measurement results indicate that the antenna achieves impedance bandwidths across 1.36-2.71 GHz, 3.07-3.655 GHz, and 4.015-8.245 GHz, which fully cover the target frequency bands of 2.4-2.483 GHz, 3.4-3.5 GHz, and 5.15-7.125 GHz. The antenna's performance is comprehensively characterized by evaluating key parameters including S-parameters, envelope correlation coefficient (ECC), diversity gain (DG), total active reflection coefficient (TARC), gain, mean effective gain (MEG), and radiation patterns, along with other relevant metrics. All measured results confirm that the antenna meets the essential requirements for MIMO and diversity systems. Furthermore, bending tests conducted at two distinct radii of 30 mm and 50 mm confirm stable antenna performance, verifying its mechanical robustness and reliability under practical bending conditions.

Based on the time-dependent Schrӧdinger equation, a finite-difference time-domain (FDTD) method is proposed to investigate electron propagation with the presence of tunneling potential distributions in metal-oxide-semiconductor field-effect transistors (MOSFETs). The channel current equation in the drift-diffusion model for classical transport is derived from the probability current formula with a plane wave assumption of the electron's state function. In both classical and quantum regimes, channel currents are numerically simulated based on quantum transport in MOSFETs using transmission functions and Fermi-Dirac distributions. The transmission function is obtained from the non-equilibrium Green's function (NEGF), indicating the probability of electrons through a channel. To determine the number of electrons at both source and drain terminals of a MOSFET, the Fermi-Dirac distributions are calculated. Numerical simulations of channel currents with various external gate-source and drain-source voltages are investigated, showing that a similar peak channel current can be generated with lower external voltages in a smaller MOSFET with a shorter gate length. Electron forward and backward propagations are obtained through FDTD simulations to demonstrate the difference of cutoff modes in classical and quantum MOSFETs.

This paper describes a Pseudo-Noise (PN) sequence evaluation tool that analyzes potentially corrupted PN sequences and assigns a metric score indicating the quality of the received sequence. The PN tester is designed to support Spread Spectrum Time Domain Reflectometry (SSTDR) by evaluating reflected PN sequences and determining whether the received signal is valid or too corrupted for use. Signal degradation is influenced by noise levels and channel filters encountered by the sequence. To simulate real-world conditions, various types of noise and filtering effects - representing capacitive or inductive coupling - were applied to a maximum-length PN sequence. The evaluation model demonstrated a consistent decline in correlation as signal distortion increased, confirming its effectiveness in assessing signal quality.

The determination of the complex permittivity of materials is a fundamental aspect of experimental electromagnetics. This study introduces a method that estimates the complex permittivity by comparing the measured bistatic field diffracted by spherical samples in an anechoic chamber with fields computed using Mie theory. The approach is applied to a molded PMMA sphere and two 3D-printed materials (Clear Resin V4.1 and Rigid 10K) over the 2-18 GHz band. The retrieved permittivity values show excellent agreement with reference data for PMMA and enable reliable characterization of low-loss 3D-printed materials, with uncertainties quantified from both experimental and numerical contributions. These results confirm the effectiveness of microwave-diffraction-based characterization and highlight promising perspectives for future investigations on an even larger frequency band.

The control of all of light's degrees of freedom and its harnessing for applications is captured by the emergent field of structured light. The modern toolkit includes external modulation of light with devices such as metasurfaces and spatial light modulators, their intra-cavity insertion for structured light directly at the source, and their deployment to engineer quantum structured light at the single photon and entangled state regimes. Historically, this control has involved linear optical elements, with nonlinear optics only recently coming to the fore. This has opened unprecedented functionality while revealing new paradigms for nonlinear optics beyond plane waves. In this review we look at the recent progress in structured light with nonlinear optics, covering the fundamentals and the powerful applications they are facilitating in both the classical and quantum domains.

With increasing demand for renewable energy, perovskite solar cells (PSCs) have emerged as a promising alternative due to their high efficiency and solution-based manufacturing processes. However, the fabrication of PSCs in ambient conditions, as opposed to inert environments, remains challenging due to environmental factors such as moisture and oxygen that degrade perovskite materials. Developing air-processed PSCs is therefore critical for reducing fabrication cost, simplifying manufacturing infrastructure, and enabling scalable production compatible with industrial processes. Moreover, air processing represents a key step toward realistic deployment, bridging the gap between laboratory demonstrations and commercial applications. This perspective discusses the progress of air-processed PSCs, highlights the environmental challenges related to stability and performance, and outlines potential strategies for future research, including precursor chemistry, solvent and additive engineering, and interface optimization. In addition, emerging scalable deposition techniques, automated platforms, and machine learning-assisted control are expected to accelerate device optimization and reproducibility. Despite remaining challenges, commercializing air-processed PSCs is increasingly viable, promising a sustainable and efficient approach for solar energy technology.

Plasmonic designs for mid-infrared extraordinary optical transmission (EOT), a direct route to tailored filtering with broadband out-of-band rejection, have long been constrained by a fundamental trade-off between high transmission efficiency and narrow linewidths, a challenge rooted in the material properties of noble metals. Here, we theoretically propose and numerically demonstrate a versatile design paradigm that resolves this challenge by functionally decoupling the tasks of light coupling and resonant filtering. Our approach uses a dual-stacked noble metal-dielectric grating architecture to surpass the intrinsic limitations of single-layer structures. This paradigm provides the flexibility to engineer devices for ultra-high spectral selectivity and transmission efficiency. We demonstrate this with distinct designs: one at 10 μm with a quality factor (Q-factor) >2000 and >91% transmission; a high-Q design at 4 μm and >80% transmission; and a high-efficiency design at 4 μm with >92% transmission over a uniquely broad spectral-angular range. These generic designs produce solitary, narrow EOT peaks originating from a ``triple-coupling'' mechanism that mitigates reflection and absorption losses, with symmetry-broken configurations capable of exceeding Q-factors of 16,000 while maintaining a peak transmission efficiency > 60%. Crucially, these compact two-layer designs exhibit exceptional robustness against fabrication variations, offering a broadly applicable route to ultra-compact, low-cost infrared components, enabling advanced architectures such as angular sensing, spectro-polarimetric imaging, and isotope-resolved gas diagnostics.