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Photoplethysmography is a widely used optical technique to extract physiological information non-invasively. Despite its large use and adoption, multiple factors influence the signal shape and quality, including the instrumentation used. This work analyzes the variability of the DC component of the PPG signal at three source–detector distances (6 mm, 9 mm, and 12 mm) using green, red, and infrared light and four photodiodes per distance. The coefficient of variation (CV) is proposed as a new signal quality index (SQI) to evaluate signal variabilities. This study first characterizes the PPG system, which is then used to acquire PPG signals in the chest of 14 healthy participants. Results show a great DC variability at 6 mm, homogenizing at 9 and 12 mm. This suggests that PPG systems are also sensitive to the near- and far-field effects commonly reported and studied in optics, which can impact the accuracy of physiological parameters dependent on the DC component, such as oxygen saturation (SpO2). [ABSTRACT FROM AUTHOR]

This paper presents a time-domain (TD) approach based on hybrid artificial intelligence (AI) to speed up convergence of radiating sources characterization in power electronics. To obtain a representative equivalent model of device under test, a dedicated optimization framework has been developed in TD using a particle swarm optimization (PSO) toolbox. In addition, for elementary feature extraction, a pseudo-Zernike moment invariant (PZMI) descriptor has been defined. Finally, with the aim of identifying remaining dipole parameters and classification problems, artificial neural networks (ANN) have been implemented. A coupling of TD electromagnetic (EM) inverse method based on a PSO algorithm along with PZMI and ANN application has been investigated and applied to a real test case. Experimental measurements have been conducted using the near-field scanning technique above an alternating current (AC)/direct current (DC) converter. Obtained results are discussed based on a comparison between measured and estimated EM field distributions using both the hybrid AI method and a conventional TD inverse method based on genetic algorithms (GA) only. This study confirms that, compared with those given by nonhybrid method, the proposed algorithm further improves the convergence speed while maintaining high accuracy. Hence, the present work offers an impressive perspective for radiated emissions characterization using hybrid AI algorithms. [ABSTRACT FROM AUTHOR]

The paper employs statistical methods to model low-frequency electromagnetic field fluctuations over printed circuit boards (PCBs) using near-field measurements within the frequency domain along the Coplanar Broadband Capacitance Wave (CBCPW) transmission line. The study characterizes electromagnetic noise profiles up to 15.2 MHz, assesses the impact of transmission lines (TL) on PCB environments, and explores the effects of signal interference. Advanced statistical methods, including the Hurst exponent and Ljung-Box tests, are integrated with traditional near-field measurements to quantify electromagnetic noise persistence and independence across frequencies. Additionally, 2-D autocovariance analysis provides a precise evaluation of electromagnetic field characteristics over the PCB surface. This research further develops a model for electromagnetic field fluctuations by incorporating theoretical frameworks aligned with stochastic Maxwell’s equations and addressing challenges such as measurement uncertainty and mechanical stepping of the probe head over the PCB surface. The autocovariance function enhances data analysis robustness, which is advantageous for the sparse data typically associated with PCBs and integrated circuits. Measurements extend up to 328.27 MHz, with a focus on significant fluctuations up to 15.2MHz due to their dominance over the measured noise. This study proposes a comprehensive statistical and modelling approach tailored for applications involving CBCPW transmission lines, especially relevant to microscale detectors and sensitive electronic devices.

Abstract Characteristics of electrograms depend on the electrode design and distance to the electric source. Our aim was to assess the impact of electrode design and distance from the myocardial electric source on the unipolar and bipolar electrograms to deduce a far-field cut-off. We retrospectively analyzed left atrial electroanatomical maps of 25 patients acquired using an ablation catheter with a 4.5 mm tip-, mini- and 2 mm ring electrodes. The unipolar and bipolar electrograms were characterized based on peak-to-peak amplitude, signal duration, maximal slope, and relative power of the high frequency spectrum above 50 Hz (HF_rel). The unipolar electrograms of ring electrodes showed an increased amplitude (140%), slope (150%) and HF_rel (16% vs. 11%) compared to the tip- and mini-electrodes. The median amplitude, slope, and HF_rel for the ring electrodes followed a power-law decay with distance with a steep decline up to 4 mm. This near-field cut-off can be identified based on a HF_rel above 10% in unipolar electrograms. In conclusion, we observed a higher unipolar amplitude for small ring-electrodes compared to larger tip-electrodes. The rapid decay of the amplitude, slope, and HF_rel up to a distance of 4 mm is suggestive for near-field cut-off identified based on HF_rel above 50 Hz. Clinical Trial Registration: NCT04095559.

Efficiently scanning for space signals and accurately detecting them from noisy environment is essential in space communication. Various unwanted interferences also present in space that may hamper the perfect detection process. This paper proposes a novel near-field circular beamformer (NCB) that will perfectly detect the desired source signal from any direction and position in space. To improve the robustness of NCB against Direction of Arrival (DOA) error, distance error, unwanted interferences and noises, this work also offers robust NCBs (RNCB) using robust Optimal Diagonal Loading (ODL) and Variable Diagonal Loading (VDL) techniques. While searching for wanted signal, the beamformer provides a primary lobe at the look direction and shows some secondary unwanted side lobes for noise and interference. Sometimes these undesired side lobe levels (SLL) become so severe that it may create conflict in locating the precise position of the desired source. To reduce these SLL, Genetic Algorithm (GA), Particle Swarm Optimization (PSO) and Grey Wolf Optimization (GWO) techniques have been applied to RNCB. The simulation results show that the optimized RNCB significantly diminishes the objectionable SLL of non-optimized RNCB by choosing appropriate weight vector of antenna array without affecting the other antenna parameters. Artificial Neural Network (ANN) have also been used to predict the weight vector for minimum SLL. [ABSTRACT FROM AUTHOR]

Synthetic aperture interferometry (SAI) is a signal processing technique that mixes the signals collected by pairs of elementary antennas to obtain high-resolution images with the aid of a computer. This note aims at studying the effects of the distance between the synthetic aperture interferometer and an observed scene with respect to the size of the antenna array onto the imaging capabilities of the instrument. Far-field conditions and near-field ones are compared from an algebraic perspective with the aid of simulations conducted at microwave frequencies with the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS) onboard the Soil Moisture and Ocean Salinity (SMOS) mission. Although in both cases the signals kept by pairs of elementary antennas are cross-correlated to obtain complex visibilities, there are several differences that deserve attention at the modeling level, as well as at the imaging one. These particularities are clearly identified, and they are all taken into account in this study: near-field imaging is investigated with spherical waves, without neglecting any terms, whereas far-field imaging approximation is considered with plane waves according to the Van–Citter Zernike theorem. From an algebraic point of view, although the corresponding modeling matrices are both rank-deficient, we explain why the singular value distributions of these matrices are different. It is also shown how the angular synthesized point-spread function of the antenna array, whose shape varies with the distance to the instrument, can be helpful for estimating the boundary between the Fresnel region and the Fraunhofer one. Finally, whatever the region concerned by the aperture synthesis operation, it is shown that the imaging capabilities and the performances in the near-field and far-field regions are almost the same, provided the appropriate modeling matrix is taken into account. [ABSTRACT FROM AUTHOR]

The demand for faster and more reliable wireless communication has led to the emergence of 6G technology. One of the key features of 6G is the utilization of terahertz (THz) frequencies for data transmission, which can provide significantly higher data rates compared to previous generations. By extending the near-field sensing range, the communication distance can be increased, leading to improved coverage and performance in 6G systems. The proposed solution is achieved through the integration of metamaterials, which are artificially designed structures with unique electromagnetic properties. By incorporating metamaterials into the design of THz antennas, we can manipulate the near-field region and enhance its sensing capabilities. Near field enhancement can also be achieved through the use of reflectors, non-uniform spacing, and dielectric lenses. Plasmonic structures and chiral metamaterials are also effective. It is achieved by tailoring the electric and magnetic response of the metamaterials, which can effectively concentrate the THz radiation within the near-field region of the antenna. The proposed model reached 98.23% resolution, 92.51% sensitivity, 93.78% range, 88.58% frequency response, 94.02% directivity, 94.62% cross talk reduction. The enhanced near-field sensing range for THz antennas will have a significant impact on the performance of 6G communication systems. It will not only extend the communication distance but also improve signal quality and reduce power consumption. It will pave the way for the realization of ultra-high-speed and reliable 6G wireless communication, making it a potential game-changer in the future of telecommunications. [ABSTRACT FROM AUTHOR]

Bio-implanted medical devices with electronic components play a crucial role due to their effectiveness in monitoring and diagnosing diseases, enhancing patient comfort, and ensuring safety. Recently, significant efforts have been conducted to develop implantable and wireless telemetric biomedical systems. Topics such as appropriate near-field wireless communication design, power use, monitoring devices, high power transfer efficiency from external to internal parts (implanted), high communication rates, and the need for low energy consumption all significantly influence the advancement of implantable systems. In this survey, a comprehensive examination is undertaken on diverse subjects associated with near-field wireless power transfer (WPT)-based biomedical applications. The scope of this study encompasses various aspects, including WPT types, a comparative analysis of WPT types and techniques for medical devices, data transmission methods employing WPT-based modulation approaches, and the integration of WPT into biomedical implantable systems. Furthermore, the study investigates the extraction of research concerning WPT topologies and corresponding mathematical models, such as power transfer, transfer efficiency, mutual inductance, quality factor, and coupling coefficient, sourced from existing literature. The article also delves into the impact of the specific absorption rate on patient tissue. It sheds light on WPT's challenges in biomedical implants while offering potential solutions.

Disordered and correlated disordered photonic materials have emerged in the past few decades and have been rapidly proposed as a complementary alternative to ordered photonics. These materials have thrived in the field of photonics, revealing the considerable impact of disorder with and without structural correlations on the scattering, transport, and localization of light in matter. Scanning near-field optical microscopy (SNOM) has proven to be a fundamental tool for the study of the interaction between light and matter at the nanoscale in such systems, allowing for the investigation of optical properties and local electromagnetic fields with extremely high spatial resolution, surpassing the diffraction limit of conventional optical microscopy. In this review, the most important and recent advances obtained for disordered and correlated disordered luminescent structures by means of the aperture SNOM technique are addressed, showing how it allows the tailoring of local density of states (LDOS), as well as providing access to statistical analysis for multi-resonance disordered and hyperuniform disordered structures at telecom wavelengths. [ABSTRACT FROM AUTHOR]

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Topological insulators generally have dielectric bulk and conductive surface states. Consequently, some of these materials are shown to support polaritonic modes at visible and THz frequencies. At the same time, the optical properties of topological insulators in the mid‐infrared (mid‐IR) remain poorly investigated. Here, near‐field imaging is employed to probe the mid‐IR response from the exfoliated flakes of bismuth (Bi)/selenide (Se)/telluride (Te)/antimony (Sb) crystals with varying stoichiometry – Bi2Se3, Bi2Te2Se, and Bi1.5Sb0.5Te1.7Se1.3 – in pristine form as well as covered by thin flakes of hexagonal boron nitride (hBN) is employed. Contrary to theoretical expectations, all three materials exhibit a dielectric response with a high refractive index and with a loss below the experimental detection limit. Particularly, the near‐field mapping of propagating phonon‐polaritons in hBN demonstrates that all three van der Waals crystals with different stoichiometry act as a practically lossless dielectric substrate with an ultra‐high refractive index of up to 7.5 in Bi2Te2Se. Such a unique dielectric crystal will be of great advantage for numerous nanophotonic applications in the mid‐IR. [ABSTRACT FROM AUTHOR]

Extremely large-scale antenna array (ELAA) at millimeter wave (mmWave) and Terahertz (THz) band has been considered a key technology for combating high attenuation in high-frequency bands in future 6G communications. Uniform circular arrays (UCAs) have attracted much attention because of their ability to provide flat beamforming gain at all angles. To realize efficient beamforming, beam training is widely used to acquire channel state information. However, with a large antenna number, the beam training overhead in ELAA systems becomes overwhelming. Moreover, with a large bandwidth, the beam defocus effect severely degrades beam training accuracy. To address these issues, this paper proposes a frequency-dependent focusing (FDF)-based beam training scheme to realize effective beam training in near-field wideband ELAA systems with UCA. Specifically, we first analyze the FDF property of UCA, where signals at different subcarriers can simultaneously focus on different distances. Then, by exploiting the FDF property to search different distances using different subcarriers simultaneously, we design a hierarchical codebook and propose an FDF-based beam training scheme. To reveal the effectiveness of the proposed scheme, we compare its necessary beam training overhead with that of existing schemes. Finally, the simulation results demonstrate that the proposed scheme can achieve accurate beam training in near-field wideband UCA systems with a low beam training overhead. [ABSTRACT FROM AUTHOR]

Scattering-type scanning near-field optical microscopy (s-SNOM) is becoming a premier method for the nanoscale optical investigation of materials well beyond the diffraction limit. A number of popular numerical methods exist to predict the near-field contrast for axisymmetric configurations of scatterers on a surface in the quasi-electrostatic approximation. Here, a fully electrodynamic approach is given for the calculation of near-field contrast of several scatterers in arbitrary configuration, based on the generalized Mie scattering method. Examples for the potential of this new approach are given by showing the coupling of hyperbolic phonon polaritons in hexagonal boron nitride (hBN) layers and showing enhanced scattering in core-shell systems. In general, this method enables the numerical calculation of the near-field contrast in a variety of strongly resonant scatterers and is able to accurately recreate spatial near-field maps. [ABSTRACT FROM AUTHOR]

Landslide-generated impulsive waves (LGWs) in reservoir areas can seriously threaten waterway safety as well as human life and properties around the two side slopes. The risk reduction and mitigation of such a hazard require the accurate prediction of near-field wave characteristics, such as wave amplitude and run-up. However, near-field LGW involves complicated fluid-solid interactions. Furthermore, the wave characteristics are closely related to various aspects, including the geometry and physical features of the slide, river, and body of water. However, the empirical or analytical methods used for rough estimation cannot derive accurate results, especially for deformable landslides, due to their significant geometry changes during the sliding process. In this study, the near-field waves generated by deformable landslides were simulated by smoothed particle hydrodynamics (SPH) based on multiphase flow. The deformable landslides were generalized as a kind of viscous flow by adopting the Herschel-Bulkley-Papanastasiou (HBP)-based nonNewtonian rheology model. The HBP model is capable of producing deformable landslide dynamics even though the high-speed sliding process is involved. In this study, an idealized experiment case originating from Lituya LGW and a practical case of Gongjiafang LGW were reproduced for verification and demonstration. The simulation results of both cases show satisfactory consistency with the experiment/investigation data in terms of landslide movement and near-field impulsive wave characteristics, thus indicating the applicability and accuracy of the proposed method. Finally, the effects of the HBP model's rheological parameters on the landslide dynamics and near-field wave characteristics are discussed, providing a parameter calibration method along with suggestions for further applications. [ABSTRACT FROM AUTHOR]

An updated strong-motion database of the Iranian earthquakes has been used to propose empirical equations for the prediction of peak ground velocity (PGV), peak ground acceleration (PGA), and 5%-damped Spectral Accelerations (SA) up to0.0 s for geometric mean of horizontal components. Some records from the NGA-West2 are added to the database to enrich it at near source distances for large magnitude events. Lack of data in the near source distances causes less accuracy in previous Iranian Ground Motion Models (GMMs) in comparison with the current study. In this work, the regression analyses have been performed on a truncated database which causes to obtain unbiased results. We used 3015 acceleration time series from 594 earthquakes after truncation of data to develop a new GMM. The provided model is valid for Joyner-Boore distances ranging from 0 km to80 km and magnitudes ranging from 4 to 7.9 and Vs30 ranging from50 m/s to500 m/s. [ABSTRACT FROM AUTHOR]

Plasmonic nanorings are promising building blocks for a variety of applications, including optical and biological sensing, and energy because the open inner spaces of the nanorings exhibit a high surface‐to‐area ratio and possess unique optical properties that are different from solid nanostructures. However, the simple architecture of mono‐rim based structures leads to low electromagnetic near‐field confinement, which requires a more complex structure to facilitate effective interaction with light. Herein, we report on recent progress of synthetic strategies for fabricating plasmonic nanorings using both top‐down and bottom‐up approaches. First, we introduce the conventional methods for achieving classical ring architectures. Then, we discuss rationally designed synthesis methods for creating advanced and structurally unique nanostructures to increase near‐field enhancement. This process involves multi‐step chemical toolkits that enable control over the shape and the introduction of repeated units in a single entity. Then, we explore the potential applications of complex nanoring architectures. [ABSTRACT FROM AUTHOR]

The work focuses on detailing the distribution of the near evanescent field depending on the geometry of the aperture-forming region, the electrical properties of the objects under study, and assessing methods for scanning the properties of objects in depth. The results of a study of the electrodynamic properties of resonator probes with a coaxial aperture, developed for local microwave diagnostics of various objects, are presented. In particular, the influence of the size and shape of the tip and the tip-sample distance on the field distribution of a classical quarter-wave resonator measuring transducer is analyzed. Quantitative dependencies are presented that comprehensively characterize the probe-object system in terms of locality and sensitivity. Various options for changing the geometry of the probe aperture assembly are discussed to optimize the conversion characteristics of microwave sensors. The results of a study of the field distribution of a probe with tunable sensitivity by changing the position of the tip tip relative to the aperture plane are presented. The dependence of the field distribution on the tip displacement in such a probe has been established. Various variations of the operating modes of a probe with tunable sensitivity were studied: immersion of the tip into the probe aperture, extension of the tip into the interior of the object, extension of the probe tip from the aperture to the surface of the object. [ABSTRACT FROM AUTHOR]

One manifestation of light-Weyl fermion interaction is the emergence of chiral magnetic effects under magnetic fields. Probing real space magnetic responses at terahertz (THz) scales is challenging but highly desired, as the local responses are less affected by the topologically trivial inhomogeneity that is ubiquitous in spatially averaged measurements. Here, we implement a cryogenic THz microscopy instrument under a magnetic field environment—a task only recently achieved. We explore the technical approach of this system and characterize the magnetic field's influence on our AFM operation by statistical noise analysis. We find evidence for local near-field spatial variations in the topological semimetal ZrTe5 up to a 5-Tesla magnetic field and obtain near-field THz spectra to discuss their implications for future studies on the chiral magnetic effect. [ABSTRACT FROM AUTHOR]

We present a scheme for line-of-sight user localization at a uniform linear array (ULA) applicable to both near-field and far-field. The proposed scheme leads to closed-form expressions for the estimates of the user location, distance and angle. The proposed scheme is based only on the phase differences between pilot signals received on neighboring antennas at the ULA. As a result, the proposed scheme can be implemented by a ULA with analog hardware and a single-tone pilot. One such analog hardware ULA is proposed. We provide numerical results with which we confirm our theoretical results and compare the proposed scheme with an existing localization scheme in the literature and to the Cramer-Reo bound.

Extremely large antenna arrays (ELAAs) are being proposed for sixth-generation (6G) systems as an evolution of massive multiple-input, multiple-output (mMIMO). Besides having a large number of antennas, ELAAs can be physically large, which makes communication in the radiative near-field very likely. This means the plane wave approximation does not hold and channel models that accurately capture the propagation effects in the near-field are required. Recent works have highlighted the beam-focusing effect available in the near-field region, where antenna arrays can generate beams not only limited in width but also limited in depth. This work leverages a precise channel model for ELAAs intending to explore the inherent physical layer security (PLS) features in the radiative near-field. For that purpose, it is presented a study on the power ratio between a legitimate and a malicious user, considering the effects of the field region, relative distance, and beamforming direction. By taking advantage of that analysis, it is shown that both the jamming rejection and the secrecy rate can be substantially improved when communicating in the near field, demonstrating that beam-focusing constitutes an interesting PLS technique for upcoming 6G communications.

In this paper, a novel dual-band compact printed monopole antenna is presented. The antenna is composed of three spatially distributed layers: a modified folded-line stepped impedance resonator (SIR) on the upper layer, a printed monopole antenna on the middle layer, and another folded-line SIR on the lower layer. A pair of resonant frequencies can be introduced by loading two different modified folded-line SIRs as near-field resonant parasitic elements (NFRPs) in the near field of a printed monopole antenna. Simultaneously, the resonant frequencies of the two are lower than the fundamental mode frequency of a traditional printed monopole antenna, thereby enabling the realization of a compact antenna. Furthermore, the resonant frequency can be adjusted through the wide edge strong coupling between the folded-line SIR and the printed monopole antenna. After elaborating on the design principles, a sample antenna is fabricated and underwent measurements to validate the anticipated performance of the proposed antenna. The measured outcomes closely align with the predicted results, providing compelling evidence that the design methodology expounded in this paper is indeed precise and efficacious.

With the successful deployment of fifth-generation (5G) systems, both industry and academia anticipate the evolution of 6G systems to meet future data-intensive needs. 6G aims to enhance coverage, capacity, and spectral efficiency, with extremely large-input multiple-output (XL-MIMO) base stations (BS) being central to this vision. In addition, the mid-band spectrum (7-15 GHz) becomes a strategic choice, providing sufficient bandwidth and reasonable path loss characteristics. However, the implementation of XL-MIMO in the mid-band will bring fundamental changes to the electromagnetic signal propagation characteristics, especially due to the increase in Fraunhofer distance. Therefore, it is necessary to model the waves as spherical for near-field scenarios. Furthermore, the efficient operation of communication systems depends on accurate knowledge of channel state information (CSI). However, existing CSI feedback schemes (Type-1 and Type-2) were originally designed based on far-field assumptions and exhibit obvious limitations when applied to XL-MIMO systems operating within the mid-band spectrum. To address this issue, this paper proposes two innovative CSI feedback frameworks: Type-3 for single-user MIMO (SU-MIMO) scenarios and Type-4 for multi-user MIMO (MU-MIMO), providing comparable uplink overhead compared to Type-1 and Type-2 respectively. Furthermore, the performance of the proposed Type-3 and Type-4 feedback schemes in terms of spectral efficiency and overhead is also evaluated and compared with existing Type-1 and Type-2 feedback schemes. Results demonstrate the superiority of the proposed CSI feedback schemes over existing feedback schemes, remaining resilient to performance degradation with an increase in the oversampling factor while maintaining similar uplink overhead. Notably, Type-4 demonstrates significant performance improvements compared to existing Type-2 feedback, and the gain amplifies as the number of UEs increases in MU-MIMO scenarios. The results show that the proposed CSI feedback scheme alleviates the limitations of existing feedback schemes and significantly enhances the system performance in near-field XL-MIMO environments.

Terahertz (THz) communication is a cutting-edge technology for the sixth-generation (6G) networks, offering vast bandwidths and data rates up to terabits per second, significantly advancing vehicular connectivity and services. However, THz signals are impacted by attenuation, path loss, and beam misalignment. Furthermore, the requisite high Nyquist sampling rates for THz systems increase the computational and system complexity of the receiver. A promising solution to navigate these obstacles involves the use of intelligent reflecting surfaces (IRS)-enhanced multiple-input multiple-output (MIMO) technology, which steers THz wave propagation. However, the substantial dimensions associated with IRS and MIMO extend the near-field, particularly at THz frequencies, as indicated by the Rayleigh distance and suffer from beam squint. To reduce system complexity and reduce sampling to sub-Nyquist rate, we propose a novel receiver design for an IRS-assisted near-field MIMO THz system that employs low-complexity compressed sensing. This method introduces an IRS signal-matched (IRSSM) measurement matrix with beam squint for capturing the transmitted signal at a sub-Nyquist rate, taking advantage of the sparsity in the signal and THz channels, and signal recovery using the deep learning (DL) model. Simulation results for symbol error rate (SER) and normalized mean square error (NMSE) performance indicate that the proposed DL-based receiver outperforms conventional recovery algorithms based on orthogonal matching pursuit (OMP) CS-recovery and dictionary-shrinkage estimation (DSE).

This paper explores the potential near-field beamforming (NFBF) in integrated sensing and communication (ISAC) systems with extremely large-scale arrays (XL-arrays). The large-scale antenna arrays increase the possibility of having communication users and targets of interest in the near field of the base station (BS). The paper first establishes the models of near-field spherical waves and far-field plane waves. With the models, we analyze the near-field beam focusing ability and the far-field beam steering ability by finding the gain-loss mathematical expression caused by the far-field steering vector mismatch in the near-field case. Subsequently, we analyzed the performance degradation caused by traditional far-field beamforming in the near field for both communication and sensing. We formulate the transceiver NFBF design problem as maximizing the sensing signal-to-interference-plus-noise ratio (SINR) while ensuring the required communication quality-of-service (QoS) and total power constraint. We decompose it into two subproblems and solve them using the generalized Rayleigh entropy theory and the Semi-Definite Relaxation (SDR) technique. Additionally, we prove the attainability of the optimal solution for SDR. Additionally, a low-complexity design scheme is proposed as an alternative to the SDR approach for obtaining transmit beamforming. The simulation results validate the effectiveness of the proposed NFBF scheme, demonstrating its capability to manage co-angle interference and enhance both communication and sensing performance.

Optical nanoantennas demonstrate the ability to confine and enhance electromagnetic fields. This ability makes nanoantennas essential tools for highresolution microscopy. The nanoantenna resonance and response can be tuned by changing their size, shape, and material as well as adjusting the probing conditions, e.g. excitation wavelength. In this paper we simulated the propagation and interaction of visible light with computer generated models of butterfly nanoantenna arrays using the finite-difference time-domain (FDTD) method. The simulations were used to understand and predict the experimental results obtained with scanning nearfield microscopy (SNOM) on commercially available samples. Simulation parameters are chosen carefully to reflect the measurement conditions. [ABSTRACT FROM AUTHOR]

We model the effects of the photosensitive parameters of aerosols on their optical properties to provide a solid framework for further experimental and theoretical studies. A spherical dust particle is used to study the effects of the ambient medium, size, surface roughness, wavelength, and imaginary part of the complex refractive index. Five Gaussian random spheres with different aspect ratios are simulated to study the dependence of aerosol light scattering properties on particle shape distribution. To investigate the influence of composition, we model two typical kaolinite-like particles (pure and composite) collected from Southwest Sahara, with 0 and 2% hematite at different mixing states. Using the method of discrete-dipole approximation in DDSCAT, a comparative study is performed with the Mueller matrix elements, scattering, absorption, extinction efficiencies, single scattering albedo, and linear depolarization ratio as indicators. For single, microscopic dust particles, near-field calculations are carried out. The results show that the intensity of backscattering and the scattering efficiency decreases in water compared to dry air. Light in the visible range is more efficient for aerosol scattering experiments. A small number of impurities in the sample will increase its absorbing properties, but, in general, the scattering efficiencies strongly depend on the single-particle mixing state. Smaller particles with a diameter comparable to the wavelength of incident light show higher scattering efficiencies but lower backscattering intensities than larger particles, while surface roughness is shown to strongly alter the polarizability of the particle but has a negligible effect on its single-scattering albedo. Moreover, different shapes have a strong effect on the degree of linear polarization, but, in general, using the spherical over elliptic shape model can underestimate the scattering efficiencies by up to 4%. Finally, variation in the imaginary part of the complex RI can underestimate the single scattering albedo by up to 35.8%.

Disordered and correlated disordered photonic materials have emerged in the past few decades and have been rapidly proposed as a complementary alternative to ordered photonics. These materials have thrived in the field of photonics, revealing the considerable impact of disorder with and without structural correlations on the scattering, transport, and localization of light in matter. Scanning near-field optical microscopy (SNOM) has proven to be a fundamental tool for the study of the interaction between light and matter at the nanoscale in such systems, allowing for the investigation of optical properties and local electromagnetic fields with extremely high spatial resolution, surpassing the diffraction limit of conventional optical microscopy. In this review, the most important and recent advances obtained for disordered and correlated disordered luminescent structures by means of the aperture SNOM technique are addressed, showing how it allows the tailoring of local density of states (LDOS), as well as providing access to statistical analysis for multi-resonance disordered and hyperuniform disordered structures at telecom wavelengths.

Metamaterial-based quarter-wave plates (QWPs) have emerged as promising candidates for advanced polarization control in a variety of optical applications, owing to their unique properties, such as ultra-thin profiles and tailored spectral responses. We design an ultra-thin, high-efficiency, and broadband QWP in transmission mode based on a TiO 2 /Au grating structure. We show that multiple reflections and the near-field effects associated with the integration of these devices pose challenges that must be considered when combining multiple metamaterials. We present insights that facilitate improved design methodology and the optimization of integrated metamaterial QWPs and other metadevices. Our results contribute to the development of miniaturized and high-density advanced lightwave and polarization control devices in optical systems. [ABSTRACT FROM AUTHOR]

In this paper, a novel algorithm is presented for estimating three-dimensional (3-D) parameters of near-field (NF) sources with a centro-symmetric cross array, where the sub-array x and the sub-array y are uniform linear arrays equipped with the same number of array elements. By exploiting the time-delay-based spatial correlation of the received signal, the proper array elements are selected, respectively, from the two sub-arrays to define two sets of variables, and then two time-delay-based vectors are constructed. Further, a series of delay lags are uniformly sampled to form pseudo snapshots so as to obtain two corresponding time delay matrices, which facilitates the formation of the parallel factor (PARAFAC) model in time domain. Finally, trilinear alternating least squares decomposition is utilized to jointly estimate the two-dimensional (2-D) direction-of arrival and range parameters of NF sources from the PARAFAC model. In the case of low signal-to-noise ratio and small snapshots conditions, the estimation performance in the 3-D parameters of the proposed algorithm is superior to that of the subspace-based algorithm. In addition, the proposed algorithm only involves simple parameter pairing, which requires no eigenvalue decomposition and spectral peak searching. [ABSTRACT FROM AUTHOR]

In a heat-assisted magnetic recording, the thermal effect of the head/disk interface has an important influence on the stability of the recording data. In this paper, we will discuss the thermal radiation from the disk, more specifically, the magnetic recording layer, which is at high temperature after laser heating, to the magnetic head, which is at room temperature. The radiative heat flux can be represented by the Poynting vector. In the near-field band, an effective way to obtain the electromagnetic fields is to solve the Maxwell's equations combined with fluctuational electrodynamics. The near-field back radiation between specific head and disk material is calculated by the fluctuation-volume-current method. The radiative heat energy will induce the thermal deformation of the magnetic head, which will be discussed by the simulation, laying the foundation for adjusting and controlling the flying status. [ABSTRACT FROM AUTHOR]

Modern scattering-type scanning near-field optical microscopy (s-SNOM) has become an indispensable tool in material research. However, as the s-SNOM technique marches into the far-infrared (IR) and terahertz (THz) regimes, emerging experiments sometimes produce puzzling results. For example, "anomalies" in the near-field optical contrast have been widely reported. In this Letter, we systematically investigate a series of extreme subwavelength metallic nanostructures via s-SNOM near-field imaging in the GHz to THz frequency range. We find that the near-field material contrast is greatly impacted by the lateral size of the nanostructure, while the spatial resolution is practically independent of it. The contrast is also strongly affected by the connectivity of the metallic structures to a larger metallic "ground plane". The observed effect can be largely explained by a quasi-electrostatic analysis. We also compare the THz s-SNOM results to those of the mid-IR regime, where the size-dependence becomes significant only for smaller structures. Our results reveal that the quantitative analysis of the near-field optical material contrasts in the long-wavelength regime requires a careful assessment of the size and configuration of metallic (optically conductive) structures.

Abstract Direction of arrival (DOA) estimation of mixed near‐field (NF) and far‐field (FF) signals is one of the key issues in the study of array signal processing. For more precise localisation performance, sparse arrays have drawn the attention of an increasing number of academics. Excitingly, sparse arrays enable larger array apertures and higher degrees of freedom (DOFs) than traditionally used uniform linear arrays. Consequently, this study proposes a transformed symmetric nested array (TSNA) for the passive localisation application regarding mixed NF and FF. There are three processes that go into building the proposed array. In the first step, we swap the positions of the uniform linear arrays and the sparse linear arrays in the nested array. In the second step, we place the corresponding array sensors extracted from the uniform linear array behind the uniform linear array to form a new subarray. In the last step, the TSNA is constructed by folding the formed array in half. Furthermore, we give the formula of the largest consecutive lags of the TSNA and its corresponding constructor. Finally, through theoretical analysis and computer performance simulation, this study illustrates the proposed array's numerous advantages over currently used symmetric sparse arrays.

For the demonstration of ultra-wideband bandwidth and pencil-beamforming, the terahertz (THz)-band has been envisioned as one of the key enabling technologies for the sixth generation networks. However, the acquisition of the THz channel entails several unique challenges such as severe path loss and beam-split. Prior works usually employ ultra-massive arrays and additional hardware components comprised of time-delayers to compensate for these loses. In order to provide a cost-effective solution, this paper introduces a sparse-Bayesian-learning (SBL) technique for joint channel and beam-split estimation. Specifically, we first model the beam-split as an array perturbation inspired from array signal processing. Next, a low-complexity approach is developed by exploiting the line-of-sight-dominant feature of THz channel to reduce the computational complexity involved in the proposed SBL technique for channel estimation (SBCE). Additionally, based on federated-learning, we implement a model-free technique to the proposed model-based SBCE solution. Further to that, we examine the near-field considerations of THz channel, and introduce the range-dependent near-field beam-split. The theoretical performance bounds, i.e., Cramér-Rao lower bounds, are derived both for near- and far-field parameters, e.g., user directions, beam-split and ranges. Numerical simulations demonstrate that SBCE outperforms the existing approaches and exhibits lower hardware cost.

High resolution imaging techniques will play an essential role in the future smart imaging applications to provide potential solutions for accurate detection, monitoring and classification of the target. The potential of synthetic aperture radar (SAR) systems to acquire high resolution images has opened new frontiers and led to its exploration in new applications. The conventional microwave radar imaging suffers limited range resolution in the sub-millimeter wave range and optical imaging methods are constrained by the diffraction limit. In the terahertz (THz) regime of the spectrum, the SAR systems could be operated in all the weather, all the time and are capable of penetrating through clouds, smoke, dust etc. to achieve high resolution images beyond the diffraction limit. The curved SAR presents a SAR system created by circularly encircling the target scene. The Circular SAR and Spiral SAR which are based on a circular and cylindrical spiral scanning trajectory, respectively are the scanning methods used for curved SAR. In this paper, we have explored a new toolbox that enables the rapid development of near-field THz-SAR imaging systems and generation of large near-field THz imaging scenarios with the goal to support and facilitate data driven research such as deep learning in the THz community through synthetically generated SAR images as well as its potential for THz based high resolution and 2D/3D near-field imaging application. Initially, the potential advantages of THz-SAR over Microwave SAR and optical imaging methods are presented. Further, a frequency modulated continuous wave (FMCW) radar testbed is simulated in multi-input multi-output (MIMO) configuration for the target frequency band. Moreover, the circular and cylindrical SAR scanning methods are explored for near-field imaging of point targets to obtain 2D and 3D THz-SAR images, respectively. Also, the radar images are reconstructed using the Back Propagation and Polar Formatting Algorithms, focusing on short range applications like indoor environments. Finally, the SAR performance evaluation metrics are reported and a roadmap for future work is presented.

Terahertz (THz) band is expected to be one of the key enabling technologies of the sixth generation (6G) wireless networks because of its abundant available bandwidth and very narrow beamwidth. Due to high frequency operations, electrically small array apertures are employed, and the signal wavefront becomes spherical in the near-field. Therefore, near-field signal model should be considered for channel acquisition in THz systems. Unlike prior works which mostly ignore the impact of near-field beam-squint (NB) and consider either narrowband scenario or far-field models, this paper introduces both a model-based and a model-free techniques for wideband THz channel estimation in the presence of NB. The model-based approach is based on orthogonal matching pursuit (OMP) algorithm, for which we design an NB-aware dictionary. The key idea is to exploit the angular and range deviations due to the NB. We then employ the OMP algorithm, which accounts for the deviations thereby ipso facto mitigating the effect of NB. We further introduce a federated learning (FL)-based approach as a model-free solution for channel estimation in a multi-user scenario to achieve reduced complexity and training overhead. Through numerical simulations, we demonstrate the effectiveness of the proposed channel estimation techniques for wideband THz systems in comparison with the existing state-of-the-art techniques.

Active millimeter-wave beamforming arrays and focal plane array antennas require extensive calibration to compensate for impairments, such as phase and gain variations between elements, as well as unintended coupling between changes in gain and phase. When far-field calibration methods are used for calibrating focal plane array antennas, the required size of the anechoic chamber is exceedingly large. For this reason, a near-field calibration method for millimeter-wave analog beamforming array antennas and focal plane array antennas is proposed. Nonidealities of the beamformer integrated circuits with vector modulators are taken into consideration, and a way to reduce the measurement set for vector modulators with high resolution is proposed. This method is both practical and achieves a good calibration as evidenced by measurements of the radiation patterns, and it is suitable for use in an automated factory calibration setup. The important trade-off between the radiated power and side-lobe level is highlighted.

A novel decoupling method combining magnetic coupling and electric coupling is proposed in this paper to design a miniaturized two-element antenna array. Different from the traditional planar decoupling structure, a three-dimensional quasi-split resonant ring decoupling structure which can realize electric coupling is introduced. The quasi-split resonant ring is composed of two meandered antenna elements printed on the top and bottom of the substrate. The other decoupling structure is composed of two parasitic patches, which realize magnetic coupling with the open end of the dipole element in the opposite plane. By properly controlling the strength of magnetic coupling and electric coupling, the induced currents can cancel each other out, thereby enhancing the isolation between adjacent ports. Compared with the antenna array employing the planar decoupling method, the size of the antenna array in this paper is significantly reduced by half. In terms of $S$ -parameter and near-field radiation, the simulated results of the full-wave analysis are compared with the measured results. Essentially identical results successfully verify that the antenna achieves an isolation up to 27.5 dB and omnidirectional radiation mode in the resonant bandwidth of 1.76-1.80 GHz.

In this paper, a novel design of a near-field dipole antenna sensor array for breast tumor detection is presented. The proposed sensor consists of four electrically small dipole antennas fed by a single port. Due to the proven fact that breast tumors have higher dielectric properties than the surrounding normal tissues, the proposed sensor is utilized for detecting breast tumors by evaluating the variations of the sensor’s response of two cases, normal and abnormal, of the breast tissues. A simulation study is performed using both normal and abnormal numerical breast phantoms with different sizes of tumors inserted at different locations. Simulation results show the proposed sensor detected the inserted tumors at various locations inside the normal breast phantom due to an increased area of sensitivity of the sensor by using multiple sensors array. An experimental study is conducted on breast chicken meat that mimics a healthy breast and two cases of tumors including tumors made of oil and gelatin mixture and conductive spheres with different sizes inserted at different locations inside the chicken meat. Experimental results show that the proposed sensor has higher sensitivity for detecting different sizes of breast tumors placed at multiple locations.