The Global Navigation Satellite System Reflected (GNSS-R) Signal adopts a heterogeneous observation mode and utilizes the globally shared GNSS constellation as a multisource microwave signal transmission source, providing the opportunity signals for radar measurements. As a basis for GNSS-R bistatic remote sensing simulations, this paper analyzes wave spectrum model of sea surfaces, GNSS signal scattering model, and GNSS signal scattering power model. The modified Zavorotny and Voronovich (Z-V) model combined with two-scale method (TSM) for sea surface scattering is utilized to simulate delay Doppler map (DDM), with emphasis on the analysis of the effects of wave polarizations, delay Doppler interval, and sea states on DDM of GNSS signal scattered from sea surfaces. The correlated power model of GNSS scattering signal is validated by comparison with measured Cyclone Global Navigation Satellite System (CYGNSS) DDM data in L1 level 2.1 version. The DDM waveforms obtained from Z-V model combined with TSM are basically consistent with the CYGNSS actual data, in which strong scattering spots can be observed clearly from both simulated and measured DDMs. The modeling and analysis of DDM for spaceborne GNSS-R signal from sea surface is of great value in ocean remote sensing applications, particularly for the interpolation and utilization of various spaceborne GNSS measured data.

A hybrid continuous extended-mode inverse class-F power amplifier is designed with band-pass filtered matching networks to match transistor inputs. This design methodology increases the impedance space by incorporating free factors into the current equation of the traditional inverse class-F power amplifier (PA). The suggested matching network in this article is a reliable alternative to the commonly used low-pass structured matching network, and this synthesis method simplifies the deployment of the distributed network compared to the LC low-pass network. High efficiency is guaranteed by the constructed output band-pass matching network. To verify the validity and superiority of this design method, a broadband power amplifier operating at 2.6-4.0 GHz was designed and fabricated. Largesignal measurement results indicate that the drain efficiency (DE) ranges from 60% to 81%, 40-42.3 dBm output power, and 10.5-11.5 dB power gain across this frequency range.

Multi-input multi-output (MIMO) antennas have garnered significant attention for addressing the demands of high channel capacity, reliable and uninterrupted signal transmission, and high data rates, especially with recent advancements in 5G low Earth orbit (LEO) satellite communications. In addition to these features, automotive applications require antennas with minimal mutual coupling, high gain, multiple resonant frequencies, and compact size for user equipment. To meet these requirements, a 1×2 defected ground structure (DGS)-based fractal MIMO antenna array is proposed, covering various frequencies in the sub-6 GHz bands, including 0.7 GHz, 2.6 GHz, 3.1 GHz, and 3.5 GHz. The proposed antenna provides sufficient channel bandwidths and achieves a gain of 12.9 dBi in the n78 frequency band. The design has been fabricated, and the measured results show good agreement with the simulated ones. Moreover, the proposed antenna design can be integrated into the plastic parts of a car body, offering various automotive applications. It achieves a realistic data rate of approximately 10-12 Mbit/s, as verified through link budget calculations that consider the key parameters of LEO satellite systems.

Magneto-Acousto-Electrical Tomography (MAET), as one of the electrical characterization imaging methods, is used to image the electrical conductivity of biological tissues, which can be used for noninvasive, radiation-free imaging of biological tissues. Currently, most of the studies on MAET are simulations and experimental validations of simple structural models, and there is no sufficient validation of models with complex structures, and most of the results cannot comprehensively invert complex structural models with multi-gradient conductivity distributions. To address this problem, this paper proposes a MAET method for conductivity reconstruction of complex structural models which is applicable to 2D problems and may be extendable to 3D problems. Based on this method, the conductivity distribution of normal and diseased tissues in the simulation model of complex structures was reconstructed, and the consistency between experimental and simulated signals was verified. The results show that the MAET method for conductivity reconstruction of complex structural models proposed in this paper is conducive to improving the image resolution as well as the structural similarity, enhancing the conductivity distribution information of complex structural targets with inhomogeneous shapes and multi-gradient conductivity distributions.

Photonic crystals, often referred to as the ``semiconductors of light,'' have entered a new phase enabling exotic properties once exclusive to topological quantum matter such as topological insulators. While the development of the first three-dimensional (3D) photonic crystal marked the establishment of photonic crystals as an independent field, initial studies in topological photonic crystals focused mainly on one and two dimensions. Though a true photonic crystal counterpart of a 3D strong topological insulator remains elusive, significant progress has been made toward achieving 3D topological photonic crystals. Compared with their lower-dimensional counterparts, 3D topological photonic crystals reveal a richer variety of topological phases and surface manifestation, which enables more degrees of freedom for light manipulation. In this review, concentrating on the novel boundary states unique in 3D systems, we provide a brief survey of the 3D topological photonic crystals and recent advances in this field. We categorize and discuss various topological phases and associated phenomena observed in 3D photonic crystals, including both gapped and gapless phases. Additionally, we delve into some recent developments in this rapidly evolving area, including the realization of 3D topological phases through synthetic dimensions.

A privacy-compliant indoor localization approach utilizing a 3-D near-field (NF) passive radar imaging technique is presented. This technique leverages ubiquitously radiated electromagnetic fields for imaging, with passive tags introduced to enhance the strength of scattering fields, thereby enabling precise localization at the imaging level. The method also supports localization in non-ideal imaging scenarios, such as for limited bandwidth or in highly-reflective environments. Based on their geometrical properties the simple and low-cost passive tags enable intuitive differentiation between individuals or objects. Associated privacy protection mechanisms are discussed, where the frequency-varying properties of the passive tags provide additional flexibility and potential applications under privacy and ethical considerations. Several forms of passive tags are presented, where both simulation and experimental results validate the effectiveness of the proposed passive tag designs.

Early detection is critical for effective skin cancer treatment. Micro-/millimeter-wave spectroscopy has emerged as a promising non-invasive and cost-effective detection technique. Tissue models are essential in early numerical studies, which typically represent the first step in detector's feasibility assessment. This paper focuses on quantifying implications of numerical model complexity on computational studies of skin spectroscopy. In our comparative numerical studies, we constructed one finger model that follows anatomical structures, as well as its three simplified versions, subjected to simulated measurements with a slim dielectric probe in the 0.5-50 GHz range. Using the finite-element method (FEM) for simulation, we analyzed mesh count to estimate computational cost and return loss variation to assess model reliability. As a result, we reach recommendations for models that optimize computational resources and can yield meaningful information from the standpoint of skin cancer screening. Simplified models are adequate for lower microwave frequencies (< 10 GHz), but at higher frequencies, models with at least three tissue layers (skin, fat, and ligament) are necessary. Modeling smaller tumors requires greater tissue complexity than larger tumors to achieve comparable reliability. Additionally, squamous cell carcinoma (SCC) scenarios demand higher model complexity than basal cell carcinoma (BCC) and melanoma to achieve similar reliability.

A very compact MIMO antenna for broad-band applications which covers the complete spectrum of X-band applications is represented here. The proposed element in the represented antenna covers a total volume of around 320 mm³. The isolation among radiating elements is improved by placing individual elements orthogonally thereby improving isolation better than 20 dB. This enhanced isolation helps to provide substantial MIMO parameters including ECC, TARC, channel capacity loss, and multiplexing efficiency. The devised antenna is compact (0.066λ × 0.066λ × 0.0024λ mm³) printed over an FR4 substrate which is widely available concerning 6.2 GHz to 11.2 GHz wireless applications. Fabrication of the above-mentioned proposed antenna is done and all the desired calculations are made desirably. Furthermore, the practically measured results conclude that the antenna-measured patterns well correspond to the simulated results.

We present a self-synchronized dual-color cross-phase-modulation mode-locked (XPM ML) fiber laser with excellent wavelength tunability and signal-to-noise ratio for coherent anti-Stokes Raman scattering (CARS) detection. Cross-phase-modulation gives rise to self-synchronization between the two-color lasers, which enables rapid wavelengths scanning as time delay of the master laser cavity is electrically adjusted. The synchronized cavity without any mode-locking elements helps to improve the mode-locking stability and resistance to environmental interference. The pump (780 nm, 18.5 ps) and Stokes (881.1-899.4 nm, 1.5 ps) pulses obtained by second harmonic generation (SHG) are then sent to a focusing lens for CARS detection for scanning Raman shift of 1470-1701 cm^-1). As an example of analyte, rhodium-bisphosphine complex catalyst samples are detected. This highly stable and fast-tunable two-color XPM synchronized mode-locked laser architecture has the potential for arbitrary waveband extension would greatly improve the possibility of coherent Raman scattering imaging technology from the laboratory to practical applications in e.g. biomedical detection.

In this study, we introduce a dual-modal fluorescence hyperspectral micro-CT system developed for e.g. bioimaging applications. The system integrates an X-ray computed tomography (CT) module with a custom-designed hyperspectral fluorescence imaging module, achieving high-resolution structural imaging with detailed molecular-level insights. With a spectral resolution of 10 nm across the wavelength range of 450–750 nm, the hyperspectral fluorescence imaging module enables a fine compositional analysis. Using surface-modified nanoparticles, we demonstrate the system's capability to capture fluorescence under both X-ray and UV excitation. Imaging experiments on a mouse model further highlight the system's ability to generate comprehensive Four-dimensional (4D) datasets that integrate spatial, spectral, and structural information. To the best of our knowledge, no such a dual-modal system or the like has even been reported before. This dual-modal approach enhances the visualization and analysis of biological tissues, offering promising applications in e.g. disease diagnosis, surgical guidance, and preclinical research.

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.

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.

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.

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.