In the past decades, with the increasing complexity of topological crystals, artificial electromagnetic (EM) materials, and EM environments, understanding their precise scattering behaviors and characteristis is turning more challenging. Traditional methods for modeling these properties often rely on full-wave simulations or analytical algorithms which are only applicable for regular shapes with plane wave incidences. These methods are inefficient for the design and broadband multiple scattering analysis of general 3D EM structures, as new simulations are required for each different scattering scenario and frequency, while solving a substantial number of unknown variables in each analysis. In this paper, a novel hybrid numerical scattering T-matrix extraction method applicable to scatterers of arbitrary shape and composition is developed in the context of the Foldy-Lax multiple scattering theory (F-L MST). Generalization is also made such that the F-L MST can be applied to multiple scattering problems with arbitrary incident fields. Once the T-matrix elements of individual scatterers are obtained through combining spherical wave expansion with full-wave numerical simulations of surface fields as proposed in the paper, it can be stored and reused, significantly reducing the overall computational complexity. Compared to conventional methods, this approach merely requires matrix inversions of moderate orders in a multiple scattering problem, offering notable efficiency advantages for about an order of magnitude. Meanwhile, the smooth frequency dependence of the T-matrix elements and incident field coefficients suggests the feasibility of interpolating these coefficients for broadband simulations. This proves particularly helpful in the swiftly evolving near-field techniques, and scenarios requiring extensive analysis such as broadband and Monte Carlo analysis. Numerical cases, involving multiple scatterer shapes and arrangements, are explored and compared with COMSOL full-wave simulations. The results validate the accuracy and efficiency of the proposed method, with potential to become a powerful tool for EM simulations and optimization of various wave-functional materials and in many other multiple scattering applications.

Reconfigurability is a crucial capability for electromagnetic devices to achieve high flexibility in accommodating various scenarios. In this study, we present a reconfigurable microwave rasorber with dynamically tunable helicity polarization for the passband wave using a compound unit cell composed of two-bit chiral meta-atoms. Our specific samples demonstrate low reflectivity (< –10 dB) across the entire C-band, while also offering four distinct states (two-bit) for the passband (reflection/transmission) wave in a narrow window around 6.25 GHz, including circular polarization control or complete blockage. We experimentally demonstrated the switching capability for both chirality and intensity of the passing band wave. These results are significant in expanding the application scenarios of rasorbers with more versatile polarization controllability.

Most of the analytical work on general transparent dispersive media to date has been confined to second-order dispersion within the framework of the paraxial approximation. It is the aim in this article to lift this restriction. Specifically, a detailed discussion is provided of modulated (3+1)-dimensional nonparaxial spatiotemporally localized waves in second-order transparent dispersive media. Novel infinite-energy invariant wavepackets and finite-energy almost undistorted solutions are discussed in detail. Illustrative numerical examples of the latter are given for normal dispersion in fused silica and for anomalous dispersion in a Lorentz plasma.

A gap-coupled design of elliptical shape microstrip antennas for wideband circularly polarized response is proposed. The wideband nature of the response is attributed to the gap-coupling between the orthogonal resonant modes on the fed and parasitic elliptical shape patches. With the total substrate thickness of 0.11λ_cAR, the gap-coupled antenna offers the reflection coefficient bandwidth of 784 MHz (55.68%) that includes circularly polarized bandwidth for axial ratio ≤3 dB of 542 MHz (35.82%). The antenna offers a broadside radiation pattern across the bandwidth, with a peak gain larger than 10 dBi. A design methodology to realize a similar gap-coupled antenna in a different frequency range is presented that yields similar wideband results. With the obtained antenna characteristics, proposed designs will find applications in GPS L and GSM 900 frequency bands. An experimental verification for the obtained simulated results is carried out, which provides a close agreement.

Directional and highly-efficient excitation of guided waves is closely related to the on-chip information processing and is of fundamental importance to plasmonics, nanophotonics, and chiral quantum optics. However, during the directional coupling between propagating waves and guided waves, there is a loss of information about the incident polarization state. It remains elusive and challenging to preserve the incident polarization information in the near-field directionality. Here we experimentally demonstrate polarization-maintaining and polarization-dependent near-field directionality at a microwave frequency of 9.5 GHz by exploiting a reflection-free, anisotropic, and gradient metasurface. The s- and p-polarized guided waves are excited only by the s- and p-polarized components of incident waves, respectively, and they propagate predominantly to opposite designated directions. Remarkably, the measured coupling efficiency between propagating waves and guided waves exceeds 85% for arbitrary incident polarization states. Our work thus reveals a promising route to directly and efficiently convert the polarization-encoded photon qubits to polarization-encoded guided waves, a process that is highly sought after in the context of optical network and plasmonic circuitry.

Metasurfaces offer remarkable capabilities for manipulating electromagnetic waves and by incorporating multiplexing techniques can significantly increase the versatility of design possibilities. Here, we designed and experimentally demonstrated a series of dual non-diffractive beam generators for terahertz radiation based on all-dielectric metasurfaces. These generators could produce switchable Bessel beams and abruptly autofocusing beams depending on the spin and frequency of the incident terahertz waves. In addition, by further applying appropriate phase gradients in the design, these non-diffractive beams could be deflected in specified directions. It is also possible to simultaneously generate multiple non-diffractive beams with different properties. The generated non-diffractive beams were measured with near-field scanning terahertz microscopy, and the results agreed well with simulations. We believe that these metasurface-based beam generators hold tremendous potential in terahertz imaging, communications, non-destructive evaluation, and many other applications.

This study introduces a machine learning (ML)-based method for point-of-care (POC) colorimetric testing of total antioxidant concentration (TAC) in saliva, an important biomarker for health monitoring. The approach leverages ML to accurately classify color intensity in the POC test. Saliva samples were collected and imaged at specific intervals during the colorimetric reaction, generating a dataset representative of various antioxidant levels. Four classifiers (Convolutional Neural Network, Support Vector Machine, K-Nearest Neighbors, and Single-layer Feed-Forward Neural Network) were evaluated on distinct datasets, with Convolutional Neural Network (CNN) consistently achieving superior performance. To enhance classification accuracy, stacking-based ensemble learning was applied, combining CNN predictions with a Support Vector Machine (SVM) meta-classifier, achieving up to 92% accuracy. Additionally, YOLOv4-tiny was utilized for object detection to isolate regions of interest in the images, creating a refined dataset that a CNN model is then classified with ca. 98% accuracy. This YOLOv4-CNN approach not only improved accuracy but also simplified the model architecture. The integrated object detection and CNN models were deployed on an Android application, enabling real-time TAC analysis on a smartphone with 98% accuracy and a fast readout time of 2 minutes. This method offers a robust, efficient, and accessible solution for POC antioxidant testing.

In this paper, a novel H-slot resonator of size 30 mm x 20 mm x 1.56 mm for fracture detection, backed with a perfect electric conductor (PEC), designed at 2.54 GHz is presented, and its performance is evaluated. Concurrently, an equivalent circuit model of the resonator is developed, and its performance coherently agrees with the CST model. The design is tested using a hybrid tissue phantom based on a second-order Debye dielectric tissue model. The detection of fractures of several thicknesses with a minimum width of 2 mm and a maximum of 10 mm was compared. Overall, the proposed design improves the detection of fractured regions in a bone with a 2 mm crack width as the smallest detectable crack size.

Polaritons, part-light−part-matter waves, enable the control of light at the subwavelength scale. Interfacial behaviors play a critical role in polariton manipulation, with negative refraction showing promise for high-resolution focusing. However, reflections pose a substantial challenge, especially in applications where backscattering is unwanted. To address this issue, we propose a structure composed of planar junctions of metasurfaces, each supporting in-plane hyperbolic polaritons with misaligned optical axes. We demonstrate that when the asymptote of the incident hyperbolic isofrequency contours (IFCs) aligns with the interface normal, the reflected waves transform into highly lossy one-dimensional ghost polaritons (HL-1DGPs), channeling energy near the interface. The refracted waves also convert into HL-1DGPs when the outgoing IFC asymptote aligns with the interface normal. Leveraging these phenomena, we design polaritonic lenses and absorbers with greatly reduced reflection. These insights into the interfacial behaviors of hyperbolic polaritons under symmetry breaking have implications for creating polaritonic elements beyond the diffraction limit.

To increase the detection dynamic range of slope-assisted Brillouin optical time-domain analysis (SA-BOTDA) system, we propose a configuration using unbalanced frequency-shifted stokes and anti-stokes sidebands as continuous probe light simultaneously to expand the region of effective stimulated Brillouin scattering (SBS) spectrum existed in frequency domain. The proposed scheme fully utilizes unbalanced double sidebands' gain, loss and corresponding phase spectra, constructing linear regions by specific data processing methods and has pump power-independence characteristic. Besides, scheme of dual-frequency agile change is employed to broaden the linear region at the cost of detection speed. The dynamic detection range of the proposed system can be increased to over 180 MHz, with a spatial resolution of 3.5 m and 500 MHz sampling rate for vibration detection.

The second near-infrared region (NIR-II, 900-1880 nm) spectral window has garnered significant attention in bioimaging due to its moderate light absorption, diminished photon scattering and reduced autofluorescence. Exploiting NIR-II fluorescence, confocal microscopy has achieved deep in vivo imaging. In this study, we have identified that the fluorescence with wavelength beyond 1400 nm offers superior imaging quality for NIR-II confocal microscopy, irrespective of the laser excitation source being continuous-wave or pulsed. Furthermore, leveraging the multiphoton excitation capabilities of femtosecond laser, we have successfully integrated multiphoton excited visible fluorescence channels into the NIR-II fluorescence confocal microscopic system. We have successfully employed this novel system to acquire up to six distinct fluorescence microscopic imaging channels with negligible cross-channel interference, as well as multi-channel and large-depth in vivo observation of mouse brain and kidney.

In this article, body scanning and imaging using a triangular microstrip antenna (MSA) with microstrip line feeding is presented. The resonant frequency of this antenna is 2.383 GHz having 20 dB bandwidth 3 MHz. The results comply with the 2.36 to 2.39 GHz band that the Federal Communication Commission (FCC) has designated for medical related applications. This antenna is used in scanning the human body model to detect the presence of tumor. The scan results are used to generate a 2-D color contour plot, which shows the location of tumor. Parametric analysis is carried out to fix the slot dimension to get optimum antenna performance. After successful simulation, the antenna structure is fabricated, and testing is carried out using Power Network Analyzer.

Constructing and utilizing toroidal modes in clusters of dielectric particles opens pathways to creating more efficient, compact, and functional devices across various fields, from sensing and telecommunications to energy and defense applications. Toroidal modes contribute to unusual material properties related to artificial magnetism, which is essential for designing innovative metamaterials. In this paper, we establish a relationship between eigenoscillations (modes) and scattering characteristics of a toroidal dielectric particle (torus) and clusters of particles composed of different numbers of dielectric disks arranged in a circular configuration (rings) in terms of the manifestation of their toroidal response. In particular, we examine the multipole contributions to the scattering cross-sections obtained in the exact form and long-wavelength approximation. A toroidal mode is introduced as a mode of the system for which the second-order term related to the exact electric dipole in the multipole decomposition is much greater than the first-order term. We show that the individual modes of the torus and hybrid modes of the ring consisting of an electromagnetically coupled ensemble of particles can be uniquely related, including the lowest-frequency toroidal dipole mode. Unlike the torus, the toroidal dipole mode in the ring can be separated in frequency from other multipole contributions, allowing excitation of the pure toroidal dipole resonance when providing corresponding irradiation conditions for external electromagnetic waves. This study provides an opportunity to better understand the physics of toroidal resonances in structures containing ensembles of dielectric particles and the peculiarities of their application in advanced microwave and photonic systems.

By using the time-dependent three-dimensional coupled-wave theory (3D-CWT), the transient analysis of photonic-crystal surface-emitting lasers (PCSELs) with double-lattice photonic crystals is performed. By optimizing the size of the PCSELs and the shape of the double-lattice photonic crystals, the resonance frequency is increased, and the damping (photon lifetime) is decreased, which enables over 40 GHz intrinsic 3 dB modulation bandwidth of the PCSELs. 100 Gb/s open eye under non-return-to-zero (NRZ) modulation is demonstrated by using such PCSELs. The large bandwidth enables single-lane 200 G optical transmission under four-level pulse-amplitude modulation (PAM-4). This study shows the design principles of large-bandwidth PCSELs and promises PCSELs to be an ideal candidate for the application of high-speed, high-power, free-space optical communication.

This research paper presents a novel dual-band slot array cavity-backed metamaterial antenna designed using advanced substrate integrated waveguide (SIW) technology. The antenna is specifically optimized for operation in the Ka-band frequencies. The incorporation of slots in the proposed design yields substantial advantages, such as enhanced impedance matching, exceptional directivity, and the capability for dual-band operation. Furthermore, the designed antenna showcases a gain of 6.40 dBi at 27.42 GHz and 4.77 dBi at 28.70 GHz, which is a result of the innovative incorporation of metamaterials within the SIW cavity. This demonstrates the antenna's ability to provide efficient signal reception and transmission, particularly in the specified frequency bands. The antenna's capability to operate in dual bands and its exceptional performance have been confirmed through rigorous simulation and experimental validation. These results substantiate its reliability and suitability for demanding millimeter-wave applications. The compact design and exceptional performance metrics of the proposed antenna underscore its potential for seamless integration into contemporary wireless communication systems, thereby laying the groundwork for continued progress in SIW and metamaterial-based antenna technology.

This paper presents a compact single-feed, rectangular slotted-patched antenna (SPA) for UWB applications. The proposed design adds a triangular part of the tail of the rectangular patch, cuts the edge of the patch, etches a rectangular slot in the ground plane, and then tunes the basic parameters of the design to achieve the UWB passband. The proposed antenna including slots on the patch for compact functionality is readily identifiable. The bandwidth and realized gain of the UWB antenna can be extremely improved to show the ability of a slot loading technique. The new conception of the rectangular patch antenna is considered. A feed mechanism using an inset patch feedline is implemented and analyzed. The parameters of the antenna are demonstrated, and the antenna is fabricated with an inexpensive FR4 substrate and validated experimentally. The antenna occupies frequency band (2.56-12) GHz. Making slots in the modified patch results in a significant gain improvement of 4.8 dBi as well as extending the UWB passband. The measured values of the reflection coefficient, VSWR, realized gain, and power pattern are in good agreement with the simulated results.

A band-pass filter utilizing a dual-mode Substrate Integrated Waveguide (SIW) cavity, enhanced by a novel Defected Ground Structure (DGS) is proposed in this paper. The SIW cavity operates in TE₁₁₀ and TE₁₂₀ modes, and the electric field of TE₁₁₀ is modified by introducing a series of metallized disturbance holes at the center of SIW cavity to increase the resonant frequency of TE₁₁₀ mode to that of TE₁₂₀ mode, thereby forming a passband with two transmission poles. A DGS that combines a dumbbell structure with a Complementary Split Ring Resonator (CSRR) is employed on the ground plane of the filter to improve the stopband rejection and suppress the parasitic passband. EM simulation and measurement results suggest that the center frequency of the filter is 4.8 GHz. It achieves a 3 dB-bandwidth of 300 MHz, with its insertion loss in the passband up to 0.5 dB and return loss greater than 20 dB. The designed DGS introduces a transmission zero near 7.2 GHz to suppress the parasitic passband and enhance the selectivity of the filter, while maintaining the original insertion loss and return loss within the passband. Its overall layout is simple and innovative. The designed filter is specifically engineered for application in the receiver of Nonlinear Junction Detection (NLJD) systems, aiming to suppress interference signals and allow only the second harmonic to pass through, which holds certain practical significance in RF engineering.

A single-band, linearly polarized Substrate Integrated Waveguide (SIW) antenna is designed specifically for WLAN 802.11a applications. The SIW design consists of four rectangular slots adjacent to each other through the SIW wall, with appropriate rectangular patch elements inserted in the two vertical slots for bandwidth enhancement. The structure is optimized to radiate at a frequency of 5.22 GHz, resulting in linear polarization caused by the excitation of the TE110 mode. The simulated design offers a gain of 7.275 dBi and a bandwidth of 47 MHz. The radiation pattern of the proposed fabricated antenna is measured in test environments where it is found to be unidirectional. The proposed design is compact and minimal in complexity, offering a higher gain.

This paper presents a dual-band microstrip filter with left-handed characteristics, featuring high selectivity and miniaturization. The design achieves negative permittivity and permeability by integrating H-shaped complementary split-ring resonators (CSRRs) within a substrate integrated waveguide (SIW). To enhance out-of-band rejection performance, a defected ground structure (DGS) is introduced. By applying the Half Mode SIW (HMSIW) principle, the equivalent magnetic walls of the SIW are cut, resulting in a 50% size reduction. Dual-frequency characteristics are realized using a symmetrical H-shaped CSRR, with the filter operating in the Sub-6G frequency band. Experimental results demonstrate that the filter exhibits good selectivity and low insertion loss at 3.5 GHz and 4.8 GHz. Tuning of the second frequency band is achieved by adjusting the coupling distance between the CSRR and metal via. This work has significant application potential in the fields of wireless communication and RF technology. The study provides theoretical support and technical insights for the design of future compact multi-band filters.

This paper investigates the effects of winding configurations on force density and fault tolerance in linear primary permanent magnet vernier (LPPMV) machines. Firstly, the LPPMV machines with integral slot distributed windings (ISDWs) and fractional slot concentrated windings (FSCWs) are discussed. Due to the high modulation ratio of ISDW machine, it has the potential to achieve higher thrust force capabilities. Then, the operation principle of the LPPMV machines is analyzed from the perspective of air-gap magnetic modulation. Furthermore, it should be noted that the winding configurations of ISDW machine has larger spans, resulting in insufficient fault-tolerance. To solve this limitation, a new modular ISDW LPPMV machine was proposed and optimized. In the modular ISDW LPPMV machine, a 3×3-phase winding configuration is employed. It is demonstrated that modular ISDW LPPMV machines exhibit superior characteristics in both thrust density and fault tolerance. Finally, the experiments are carried out in a linear test bench, verifying the theoretical analysis.