This study presents a compact frequency-selective metasurface for transmissive and reflective polarization conversion over the V-band (40-65 GHz). The structure consists of two Rogers RO4003C substrates separated by a 0.2-mm air gap and interconnected by a metallic post to enhance electromagnetic coupling. We performed parametric optimization to obtain multiple polarization-conversion bands. The optimized unit cell has a periodicity of 3.9 mm and employs resonator dimensions of L₁ = 2.6 mm, L₂ = 2.1 mm, W = 1.0 mm, S = 0.3 mm, H = 0.2 mm, and D = 0.3 mm. We conducted full-wave simulations and free-space measurements using WR-19 horn antennas and a vector network analyzer for validation. The proposed structure exhibits frequency-selective polarization-conversion behavior, with the transmission conversion exceeding 90% over approximately 40-49 GHz and near-unity conversion at selected frequencies, while the reflection conversion approaches 100% at resonant frequencies of approximately 58 and 60.5 GHz. The fabricated prototype showed good agreement with simulations, with minor discrepancies attributed to fabrication and measurement uncertainties. The proposed structure is promising for millimeter-wave communications, intelligent electromagnetic surfaces, radar sensing, transmitarray antennas, and future 6G systems.

Silicon nitride (Si₃N₄) has emerged as a compelling platform for visible-light, quantum-photonic, and nonlinear-optical applications because of its broad transparency window, low propagation loss, and negligible two-photon absorption. However, the moderate refractive index of Si₃N₄ poses a challenge for efficient fiber-to-chip coupling using grating couplers. Although advantageous for wafer-scale testing and flexible layouts, Si₃N₄ grating couplers suffer from low grating strength, which limits their coupling efficiency. Here, we experimentally demonstrate high-efficiency grating couplers on a hybrid amorphous-silicon/silicon-nitride (a-Si/Si₃N₄) platform. Uniform devices are presented for TE and TM polarizations in both the O- and C-bands, while apodized-focalizing designs are demonstrated for both polarizations in the O-band and for TE polarization in the C-band. The fabricated devices achieved record coupling efficiencies of -1.6 dB and -1.2 dB in the O-band for TE and TM polarization, respectively, and -1.3 dB and -2.9 dB in the C-band for TE and TM polarization, respectively. The measured peak wavelengths deviated from their nominal values, primarily because of variations in the refractive index of the fabricated Si₃N₄ and the grating duty cycles relative to their design values. These shifts are well reproduced by simulations incorporating fabrication biases and can be compensated for by adjusting grating parameters or the fiber coupling angle. To the best of our knowledge, these are the highest coupling efficiencies demonstrated to date for silicon nitride grating couplers in the O-band for both polarizations and in the C-band for TM polarization. For TE polarization in the C-band, fabrication deviations shifted the peak coupling wavelength to 1512 nm, resulting in the highest efficiency reported to date for a silicon nitride grating coupler operating in the S-band. These results demonstrate a practical approach for achieving low-loss fiber-to-chip coupling in Si₃N₄ photonic integrated circuits, addressing a longstanding challenge in the platform and supporting the development of scalable systems for datacom, telecom, nonlinear photonics, and quantum technologies.

This manuscript presents a simple, novel design of a dual broadband absorber with merged double bands and double-split concentric ring resonators in each quadrant-based metamaterial absorber (DBBMDBMMA). The proposed structure exhibits two experimentally validated absorptance broadbands; one extending from 7.73 GHz-9.54 GHz and the other extending from 10.65 GHz-13.27 GHz. The two broadbands cover X-band and Ku-band, respectively. The DBBMDBMMA is fabricated on a double-sided FR4 copper plate with electrical dimensions of 0.5λ_o × 0.5λ_o × 0.05λ_o (λ_o at 8.5 GHz). The unit cell of the metamaterial absorber consists of concentric double split-ring resonators in each quadrant, with splits oriented vertically opposite and separated by a 90° inclination difference. The novelty lies in the response and design of the proposed structure that exhibits dual broadbands extending in two microwave bands without using parasitic elements, a multilayered structure, or an extended substrate width. The response of the DBBMDBMMA structure is stable with variation in incidence and polarization angle. The metamaterial behaviour is studied in terms of effective values of normalized matched impedance, permeability and permittivity. The proposed DBBMDBMMA exhibits reflectance of -21.62 dB at 8.59 GHz, -26.18 dB at 8.88 GHz, -15.11 dB at 11.24 GHz, and -18.63 dB at 12.86 GHz. The first absorptance peak is contributed by outer split-ring resonators in quadrants II and IV; the second absorptance peak is contributed by outer split-ring resonators in quadrants I and III; the third absorptance peak is contributed by inner split-ring resonators in quadrant II and IV; and the fourth absorptance peak is contributed by inner split-ring resonators in quadrants I and III. The proposed DBBMDBMMA structure exhibits polarization and wide incident angle insensitive response that finds applications in stealth technology and RCS reduction.

This paper proposes a compact harmonic communication rectifier for a simultaneous wireless information and power transfer (SWIPT) receiver front-end employing quadrature frequency-shift keying (QFSK) modulation. First, we theoretically analyzed the feasibility of communication using the second harmonic of QFSK signals. Second, a compact harmonic communication rectifier is proposed by combining a frequency-selective duplexer and an impedance-matching network. The duplexer with two output branches partially extracts the low-power second harmonic for communication and fully transmits the fundamental wave for wireless power transfer. The matching network with π-type can transmit the fundamental wave to the diode for rectification and adjust the energy-information power ratio for practical requirements. Finally, the proposed rectifier is fabricated and measured with good agreement between simulation and measurement results. A rectifying efficiency of 76.7% is achieved for QFSK-modulated waves with a carrier frequency of 2.5 GHz and a frequency offset of 20 MHz at an input power of 14 dBm. The extracted second-harmonic power is -5.3 dBm with an energy-to-information power ratio of 64:1. The signal-to-noise ratio (SNR) is 30 dB and the bit error rate (BER) is less than 10^-12.

Time-modulated metasurfaces (TMMs) enable low-observable and reconfigurable radar deception, but existing approaches remain limited in synthesizing complex range-velocity (R-V) false-target clusters because of forward-designed modulation schemes and single-platform operation. This paper presents a task-oriented framework for multidimensional false-target synthesis against frequency-modulated continuous-wave (FMCW) radars. An analytical model explicitly connects TMM temporal modulation with the FMCW R-V estimation chain. Intra-frame modulation reshapes the echo spectrum to generate programmable range offsets, while inter-frame control modifies the slow-time response to generate programmable velocity offsets. The modulation-sequence design is then formulated as a combinatorial inverse-design problem and solved using Physics-Informed Initialization-Assisted Particle Swarm Optimization (PIIA-PSO), which exploits analytical R-V relations to construct a physically informative initial population, thereby reducing ineffective early-stage exploration and improving convergence efficiency toward high-quality realizable solutions. The framework is further extended to distributed multi-platform cooperation to increase spatial degrees of freedom and enable coherent energy aggregation. Results show range and velocity errors below 10% for single-platform synthesis and improve the overall SSIM by 7-10 percentage points under distributed configurations, demonstrating enhanced synthesis fidelity, flexibility, and scalability.

Slow-light photonic crystals with group indices exceeding 100 typically require complex defect engineering - line-defect waveguides, coupled resonator optical waveguides (CROWs), or carefully designed heterostructures. These approaches, while effective, introduce fabrication complexity, optical losses, and bandwidth limitations. Recent advances achieving group indices of 50-100 in engineered structures still fall short of theoretical predictions for bulk photonic crystals operating in higher-order bands. However, a critical knowledge gap persists: most higher bands (3-8) largely unexplored. Here, we demonstrate that bulk square lattice photonic crystals operating in higher-order TM bands achieve ultra-slow light (group index 226.5) and extreme anisotropic dispersion (ratio 58.36) - performance comparable to state-of-the-art defect-engineered waveguide structures, achieved in a bulk crystal without any line defects, coupled resonators, or heterostructures, thereby maintaining fabrication simplicity. Through systematic equifrequency contour (EFC) analysis of bands 1-8 using high-density k-space sampling, we establish a quantitative framework revealing that higher bands offer superior slow-light characteristics unattainable in conventionally studied lower bands. Our GaAs/organic semiconductor system achieves this performance with moderate dielectric contrast (ε_r = 6.86), compatible with standard electron-beam lithography fabrication. A systematic parametric sweep of the rod radius (R/a = 0.30-0.50) further reveals that this slow-light performance is structurally robust and tunable by geometry: the Band 4 group index varies by less than 25% across the full sweep; Band 3 sustains a high normalized delay–bandwidth product (NDBP = 0.40-0.48) over a wide fabrication-tolerant range; Band 4 reaches an NDBP optimum of 0.403 at R/a = 0.38; Band 7 uniquely combines NDBP > 0.1 with an anisotropy ratio above 200; Band 8 exhibits extreme anisotropy (ratio up to 1369).

Owing to glass's excellent electrical properties and stability, Through-Glass Via (TGV) technology is regarded as a key technology for next-generation three-dimensional integration. Research on TGV-based transmission structures and Substrate-Integrated-Waveguide (SIW) structures has been relatively extensive. However, the investigated frequency range remains limited, rarely exceeding 100 GHz, and studies focusing on the intrinsic characteristics of the vias are still lacking. To address this issue, this study designs a set of TGV calibration standards operating in the D-band to extract the S-parameters and electrical parameters of the vias and to analyze their performance, thereby providing useful insights for the subsequent design of TGV devices in the terahertz band.

This letter presents a cross-coupled SIW filter with a quasi-elliptic response and improved stopband suppression. For cross-coupled SIW filters, suppressing spurious modes often requires rearranging feeding ports, but this rearrangement tends to increase port-to-port crosstalk and limit achievable out-of-band rejection. To address this inherent issue, we keep port positions unchanged and change the port configuration from conventional microstrip lines to an opposite-face GCPW feeding structure. The input and output ports are placed on opposite metal layers, with their ground planes separated by the dielectric substrate. Simulation results show that the port-to-port crosstalk is reduced from -25 to -40 dB to below -100 dB. Measured results show that the proposed filter maintains a similar stopband width as the previous design, while the suppression of the first spurious passband is improved by about 20 dB, reaching better than -50 dB. The improvement is achieved without hybrid structures or extra fabrication steps.

In‎ this ‎paper, ‎ a low-cost, lightweight open ‎transverse electromagnetic (TEM) ‎cell with an operating bandwidth from DC up to around 1.5 GHz is‎ designed ‎and‎ fabricated‎. Initially, a fully metallic open cell is simulated, and associated TEM modes are exhibited by the spatial electric field distributions. The cell length (L_b), width (W_b), and height (H_b) are respectively 398, 200, and 103.93 mm. In the next step, a printed circuit board (PCB) is used as the septum to reduce the device's overall weight. Moreover, longitudinal slots are cut on both sides of the septum to prevent the transverse currents, which results in the excitation of higher-order modes. For further weight reduction, sub-wavelength slots are cut in the metallic sidewalls without affecting the overall performance. The metal filling factor reduction is of great importance for reaching a portable test facility with reduced cost. Finally, the device's input ports are further manipulated to reduce discontinuity by gradually tapering them. The device is realized by the combination of metal sheet, computer numerical control (CNC) water jet, machining, and PCB technologies. The measured scattering parameters confirm the acceptable performance of the fabricated device.

Accurate characterization of magnetodielectric materials is required for the development of microwave sensors and high-frequency communication systems. This study proposes a compact planar microwave sensor that uses a single dual-Split-Ring Resonator (SRR) to simultaneously measure permittivity and permeability. The core innovation lies in the spatial segregation of electric-field-dominant and magnetic-field-dominant sensing regions within a unified resonator framework, which effectively suppresses the mutual interference between the dielectric and magnetic sensing channels. Designed to resonate at 4.15 GHz, the sensor's performance was evaluated through full-wave electromagnetic simulations and experimental validation using six different materials under test (MUT) (including Magtrex555 and various Rogers's substrates). Sixty independent measurements were conducted to ensure reliability. The experimental results demonstrate high sensitivity, with 144.2 MHz/ε_r for permittivity and 45.4 MHz/μ_r for permeability. The proposed sensor enables characterization across a measured permittivity range of 2.2-6.5 and a permeability range of 1-6, demonstrating its capability to evaluate both dielectric and magnetic properties within the specified measurement domain. The proposed sensor achieved average characterization accuracies of 96.93% and 98.81% for permittivity and permeability, respectively, with minimum errors as low as 0.03%. These results confirm that dual-SRR architecture provides a robust, repeatable, and non-destructive solution for magnetodielectric material characterization with a highly compact form factor.

A flexible multifunctional passive metasurface reflector is proposed, which integrates broadband absorption, linear-to-linear polarization conversion, and dual-band linear-to-circular polarization conversion on a single passive platform. The proposed unit cell consists of a circular split-ring resonator and X-shaped metallic patches loaded with lumped resistors. The metallic patterns were fabricated on a flexible polyimide (PI) substrate, followed by a polymethacrylimide (PMI) spacer layer and a metallic ground plane. Without employing any active components, the metasurface achieves an absorptivity higher than 90% over the 4.45-8.46 GHz band. In the 12.1-15.2 GHz frequency range, the polarization conversion ratio exceeds 90%, enabling efficient linear-to-linear polarization conversion. Furthermore, within the frequency bands of 16.5-17.3 GHz and 17.7-18.2 GHz, the proposed metasurface converts incident linearly polarized waves into right-handed circularly polarized (RHCP) and left-handed circularly polarized (LHCP) waves, respectively, while the output handedness can be interchanged by rotating the incident polarization by 90°. Conformal measurements demonstrate that the proposed metasurface maintains stable absorption performance under moderate bending, with only slight degradation in polarization conversion performance and a small frequency shift in the circular polarization conversion bands. Experimental measurements show good agreement with the simulated results, verifying the effectiveness of the proposed design. Owing to its simple configuration, low cost, multifunctional integration, and conformal capability, the proposed metasurface provides a promising solution for multifunctional electromagnetic manipulation in conformal integrated electromagnetic systems.

Reconfigurable Intelligent Surfaces (RISs) have emerged as a disruptive technology with the potential to fundamentally transform 5G and future 6G wireless communication systems. By intelligently controlling electromagnetic (EM) wave propagation through large arrays of reconfigurable meta-atoms, RIS enables dynamic beam shaping, interference mitigation, coverage enhancement, and energy-efficient communication. This paper presents a comprehensive review of RIS technology, focusing on its fundamental principles, classification, architectural design considerations, and diverse reconfiguration mechanisms. Key aspects, including unit cell structures, material advancements, operating modes, spatial resolution, and control strategies, are examined in detail to provide a holistic understanding of RIS functionality. The review further explores RIS performance modelling, electromagnetic simulation techniques, prototyping methods, and measurement approaches used in evaluating real-world deployments. Emerging application scenarios, including mmWave/THz communications, indoor localization, wireless power transfer, UAV-assisted networks, and smart radio environments, are also highlighted to demonstrate RIS's versatility in next-generation networks. Finally, the paper identifies existing challenges, potential limitations, and future research directions that must be addressed to realize RIS-enabled intelligent wireless ecosystems fully. The insights presented herein aim to guide researchers and industry practitioners in advancing RIS technology as a key enabler for the evolution of future 6G communication systems.

To address the challenge of limited bandwidth in magnetic induction communication, this study proposes a magnetic induction antenna adopted a gradient-radius structure. The antenna adopts a discretized gradient-radius design, which effectively broadens the system bandwidth by increasing the antenna's resistance-to-inductance ratio. The study integrates theoretical analysis, simulation, and experimental testing to systematically investigate the effects of different taper ratio structures on resistance, inductance, and bandwidth. Experimental results demonstrated that the gradient-radius structure can significantly extend the bandwidth, and the extension effect became stronger as the taper ratio increased. Meanwhile, combined with simulation and experimental data, the attenuation effect of this structure on the near-zone magnetic field was evaluated, verifying that the bandwidth improvement came at the cost of coupling strength. Specifically, when the bandwidth was extended to nearly twice its original value, the received signal strength decreased by approximately 6.4 dBm. This study provides a simple and practical implementation approach for the broadband design of magnetic induction communication systems.

Electromagnetic wave propagation and scattering in materials with time-variant parameters is subject of an active field of research. The theory is usually restricted to transverse-electric or transverse-magnetic modes in one- or two-dimensional settings. Here we discuss the theory of three-dimensional, multi-component electromagnetic waves in a homogeneous, time-variant material. We present 3 × 3 causal and acausal Green's matrices and a 6 × 6 propagator matrix. We derive a number of fundamental properties such as conservation of momentum density for a piecewise continuous, time-variant material, symmetry properties of the propagator matrix, and a relation between the causal and acausal Green's matrices. These properties are used in derivation of a general wave-field representation (the counterpart of the Kirchhoff integral for a time-invariant material) and an expression for Green's function retrieval with space-correlations (the counterpart of Green's function retrieval with time-correlations in a time-invariant material).

Ghost surface polaritons (GSPs) in anisotropic materials exhibit an unusual bi-state nature with a complex-valued out-of-plane wavevector, providing unique opportunities for nanoscale light manipulation. However, the exploration and application of GSPs have been limited by the scarcity of natural crystals. Here, we revisit GSPs from the perspective of transformation optics. By introducing a shift parameter, we establish a mapping between material tensors and ghost modes, demonstrating the existence of GSPs at various transformed uniaxial interfaces. It is further shown that the wavefront can be modulated via shift parameters, enabling precise control of GSP propagation. Our study provides new insight into GSPs and establishes a versatile framework for their flexible manipulation.

A novel finite-gap lumped-port model is presented to improve the accuracy of the volume surface integral equation (VSIE) solver for electromagnetic (EM) radiation from composite metallic-dielectric structures. This port model is implemented by modifying the traditional method of moments (MoM) solution to use a novel divergence-conforming testing function at the gap as well as half-basis functions connected across the gap, where a small gap region in the domain of analysis is linked to the lumped-circuit voltage/current source. Then, a hybrid multilevel fast multipole algorithm with multilevel accelerated Cartesian expansion algorithm (MLFMA-MLACEA) is adopted to enhance the capability of this VSIE-based finite-gap lumped-port model for electrically large and multi-scale EM radiation problems. Numerical results are provided to demonstrate the accuracy and efficiency of this VSIE method.

Halide perovskites have demonstrated both short-term and long-term synaptic plasticity behaviors through various physical mechanisms, making them suitable neuromorphic electronics. However, most studies rely on distinct materials or device architectures optimized for one single mechanism, limiting the realization of both forms of synaptic plasticity within a unified material system. Here, we report ionic-ferroelectric halide perovskite (IFHP) artificial synapses that integrate two fundamentally different resistance switching processes within a single material. This functionality is enabled by the ferroelectric Dion-Jacobson perovskite (4-AMP)PbI₄ (4-AMP = 4-(aminomethyl)piperidinium), which exhibits ion-migration-dominated behavior below the coercive voltage (V_C ~ 3.5 V) and ferroelectric polarization switching above. Accordingly, the device exhibits volatile paired-pulse facilitation without stable non-volatile memory retention under sub-coercive stimulation, while exhibiting increasing non-volatile memory above V_C. The ferroelectric-polarization-driven long-term plasticity is further supported by extended retention over 2 × 10⁴ s, cycling operation, and device-to-device reproducibility. In addition, the IFHP artificial synapse emulates bio-inspired associative learning and nociceptive sensory functions, highlighting its potential as neuromorphic hardware. These results establish a single-material pathway for voltage-programmable mixed plasticity, addressing a key challenge in halide perovskite neuromorphic hardware.

Objective: This study aims to experimentally and numerically investigate the modulation of electromagnetic wave physical characteristics (transmittance, absorptance, impedance) by strongly an isotropic acoustic materialism in the 2-12 GHz frequency band, and to quantify the influence of material parameters (thickness, density, elastic modulus, an isotropic direction) on wave control performance. Methods: Three types of material units (metal composite, polymer, ceramic reinforced) with thicknesses of 1.5-3 mm and anisotropic directions along X, Y, Z axes were fabricated. Transmittance and absorbance were measured using a vector network analyzer with a WR-90 guideway system. Finite element simulations incorporating acoustic-electromagnetic coupling (strain-induced permittivity modulation) were conducted. Sensitivity analysis was performed by varying thickness (±10%), elastic modulus (±5 GPa), density (±500 kg/m³), and direction angle (±15°). Results: High-frequency absorbance reached 0.91 (metal composite at 9.5 GHz), while low-frequency transmittance remained above 0.70. Thickness and elastic modulus predominantly affected the amplitude and position of high-frequency absorption peaks, whereas density and direction angle primarily regulated low-frequency response, resulting in multi-peak broadband absorption. The deviation between experimental data and finite element simulation was less than 3%. Sensitivity analysis revealed that thickness (±10%) and elastic modulus (±5 GPa) produced the most significant changes in transmittance (0.60-0.91) and absorptance (0.70-0.91). Conclusion: Strongly an isotropic acoustic materialism offer mechanically t unable electromagnetic wave control, with thickness and elastic modulus as key design parameters. These findings provide experimental and theoretical references for high-frequency communication antennas, radar stealth coatings, and broadband absorption devices.

This study presents millimeter-wave propagation measurements at 60 GHz conducted in a reverberation chamber to emulate realistic multipath-rich indoor environments. Both line-of-sight (LOS) and non-line-of-sight (NLOS) scenarios are investigated using commercial off-the-shelf devices. The impact of stirring conditions and lossy environments is evaluated through key performance indicators, including Received Signal Strength Indicator (RSSI), Signal-to-Noise Ratio (SNR), Modulation and Coding Scheme (MCS), and throughput. Results show that mechanical stirring speed and the placement of absorbing materials significantly affect channel characteristics and system performance. In NLOS conditions, multipath propagation dominates, where dynamic beamforming demonstrates improved robustness. In contrast, LOS scenarios are highly sensitive to blockage, leading to notable performance degradation. Overall, the findings confirm that the reverberation chamber is an effective platform for over-the-air testing and characterization of 60 GHz wireless systems, offering valuable insights into system behavior under complex indoor propagation conditions relevant to future 5G and 6G deployments.

This paper presents a wideband circularly polarized (CP) antenna featuring a significantly enhanced front-to-back ratio (FBR). The proposed design employs a composite structure that integrates a dipole with a loop resonator and is fabricated using a multilayer architecture. The top layer is a dielectric substrate loaded with a cyclic zigzag patch. The middle layer contains an L-shaped radiating patch with etched rectangular grooves, and the bottom layer comprises a metal ground plane and surrounding vertically tilted-slot fences. Circular polarization is achieved by exciting orthogonal degenerate modes via strategic rectangular-groove perturbations that work in concert with the loop resonator's inherent asymmetric current path. The tilted-slot fences, in conjunction with the zigzag patch layer, function collectively to suppress backward radiation and improve the FBR. Measured results demonstrate that the antenna achieves a 3 dB axial ratio (AR) bandwidth of 54% from 2.45 to 4.3 GHz, a stable FBR of more than 27.5 dB with 2.5 dB variation, and a peak gain of 9.3 dBic. This design offers a high-performance, planar antenna solution well-suited for satellite communication and radar systems.