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A detailed Volume Integral Equation (VIE) formulation for planar plasmonic nano structures with finite thickness in flat multi-layers medium is presented. The boundary condition along the localized metallic objects is expressed in terms of the unknown polarization current flowing through these objects in the form of an integral equation, which is solved using the Method of Moments (MoM). The Green's functions associated with a layered medium of practical importance are expressed in the spectral domain. The corresponding spatial domain Green's functions are obtained using the Discrete Complex Images Method (DCIM). Special treatment for the spectral function's asymptote at high spectral values is performed. The presented formulation is applied on different plasmonic structures immersed inside layered media. The structures include nano-rod and nano-patch excited by an incident plane wave. In addition, a simple band-stop filter based on quarter-wavelength stubs is considered. This filter is fed with a couple of plasmonic transmission lines. The obtained current distributions and S-parameters are compared with those obtained using a commercial full-wave electromagnetic simulator, namely CST Microwave Studio. A very good agreement is observed. The proposed integral equation formulation enjoys high degree of stability, numerical efficiency, and accuracy.

The identification of unknown antennas and analysis of electromagnetic wave vulnerability are important for effective attacks and/or protection in high-power electromagnetics (HPEM). Using electromagnetic clues, antenna modeling becomes possible, and its electromagnetic properties can be analyzed by predicting area-wide radiation patterns. To model an antenna, an infinitesimal dipole modeling (IDM) technique, utilizing clues from sparse far-field patterns, was used in the present study. The IDM approach obtained the equivalent infinitesimal dipole (ID) for the antenna configuration current using Green’s function. The sparse far-field pattern was determined for a movement trajectory, using air reconnaissance. Random far-field data acquisition paths were considered cumulatively in the 1–10 range in this case. The approach also used external exposure information about the antenna, such as the operating frequency and size. For simulations and measurements, a ridged-horn antenna operating at 2.4 GHz was used. The overall radiation pattern reconstructed using the IDM approach was correlated at 0.78 when one path was considered in the simulations. As the considered paths were accumulated, the correlation tended to increase, with the correlation for ten paths exceeding 0.98. The estimated radiation pattern characteristics of antennas can be utilized for communication reception characteristics and/or electromagnetic wave vulnerability analysis. [ABSTRACT FROM AUTHOR]

Recently, many successes have been witnessed in the field of inverse scattering problems (ISPs) with deep learning schemes (DLSs). However, most of the studies focus on the spatial characteristics of interested targets, and little attentions are paid to the information from multiple incidences that can be treated as temporal information measured in a time series. In this work, a multiple-incidence framework (MIF) is introduced by incorporating the information from multiple incidences into an extra dimension of data space, helping to enrich physical knowledge in DLSs. A 3-D convolutional neural network (CNN) structure is further introduced to explore cross-frame information of the data space, which enables simultaneous utilizations of multiple-incidence information. The proposed MIF can be easily adapted into previous DLSs in the literatures, where two widely used DLSs including back-propagation scheme (BPS) and dominant current scheme (DCS) are used to demonstrate the advantages of the proposed framework. It is shown by extensive numerical and experimental examples that MIF enables consistent improvements for BPS and DCS. It is expected that the proposed MIF will also find its applications in real-time dynamic microwave imaging and multiple-frequency imaging by effectively using physical knowledge from temporal and frequency measurements, respectively. [ABSTRACT FROM AUTHOR]

Dynamic manipulation of the surface plasmon polariton (SPP) and wave steering is important in plasmonic applications. In this work, we excite a curved SPP in topological continua by applying a radial magnetic bias. We believe that it is a new technique to create a unidirectional SPP traveling along a curved trajectory. We also derive a Green’s function (GF) model for radially biased plasma, applicable to curved SPPs. We compare the properties of unidirectional curved SPPs with the usual case when an axial bias is applied. [ABSTRACT FROM AUTHOR]

A symmetry relation of the integral operators in a layered medium proved by the reciprocal theory can be applied to improve the accuracy of the magnetic-field integral equation (MFIE). Based on the symmetric layered medium Green’s function, the normal vector can be transferred into the inner integral to keep the field integral as the inner integral of the elements in the impedance matrix. When discretized by the divergence-conforming function, this field-extracted scheme shows better performance than the classic expression in accuracy. Meanwhile, this form of expression can be applied in the combined-field integral equation (CFIE). Several numerical results are provided to validate the formulation of the symmetry relations from the layered medium Green’s function to the impedance matrices and to reveal its advantages in enhancing the accuracy of the field-extracted form of MFIE. [ABSTRACT FROM AUTHOR]

In this work, we present a novel analytical model for the magnetic field in through-the-Earth (TTE) communications. The modeling approach is based on dyadic Green’s functions to compute impressed and scattered fields in a multilayered structure. Its performance is assessed through comparisons with previous models, measurements, and simulations of magnetic field. The great advantages of the proposed approach in relation to the previous ones are its flexibility in the positioning and direction of antennas and the unlimited number of layers of soil. Moreover, this approach can be more accurate than classical ones in scenarios where the soil has a high conductivity below the antenna in an uplink scenario. [ABSTRACT FROM AUTHOR]

Ultrasonic rotational 3-D shear wave elasticity imaging (SWEI) has been used to induce and evaluate multiple shear wave modes, including both the shear horizontal (SH) and shear vertical (SV) modes in in vivo muscle. Observations of both the SH and SV modes allow the muscle to be characterized as an elastic, incompressible, transversely isotropic (ITI) material with three parameters: the longitudinal shear modulus $\mu _{L}$ , the transverse shear modulus $\mu _{T}$ , and the tensile anisotropy $\chi _{E}$. Measurement of the SV wave is necessary to characterize $\chi _{E}$ , but the factors that influence SV mode generation and characterization with ultrasonic SWEI are complicated. This work uses Green’s function (GF) simulations to perform a parametric analysis to determine the optimal interrogation parameters to facilitate visualization and quantification of SV mode shear waves in muscle. We evaluate the impact of five factors: $\mu _{L}$ , $\mu _{T}$ , $\chi _{E}$ , fiber tilt angle $\theta _{\text {tilt}}$ , and F-number of the push geometry on SV mode speed, amplitude, and rotational distribution. These analyses demonstrate that the following hold: 1) as $\mu _{L}$ increases, SV waves decrease in amplitude so are more difficult to measure in SWEI imaging; 2) as $\mu _{T}$ increases, the SV wave speeds increase; 3) as $\chi _{E}$ increases, the SV waves increase in speed and separate from the SH waves; 4) as fiber tilt angle $\theta _{\text {tilt}}$ increases, the measurable SV waves remain approximately the same speed, but change in strength and in rotational distribution; and 5) as the push beam geometry changes with F-number, the measurable SV waves remain approximately the same speed, but change in strength and rotational distribution. While specific SV mode speeds depend on the combinations of all parameters considered, measurable SV waves can be generated and characterized across the range of parameters considered. To maximize measurable SV waves separate from the SH waves, it is recommended to use an ${F}/{1}$ push geometry and $\theta _{\text {tilt}} = {15}^{\circ} $. [ABSTRACT FROM AUTHOR]

We present a new approach to the study of the telegraph equation, which is based on analyzing the equation in phase space. We consider the phase space of time and frequency, as well as of position and spatial frequency (or wavenumber). We show that considerable simplification, both conceptual and mathematical, results from this approach. The basic reason why a phase space approach is fruitful is that when there is dispersion, the group velocity is not constant but changes with frequency, and therefore the wave changes as it propagates over space and time. Simple but effective and insightful phase space approximations to the solution of the telegraph equation are presented, with examples. [ABSTRACT FROM AUTHOR]

A surface field analytical solution of the impedance boundary condition circular cylinder canonical problem is contributed in the work at hand. The accurate representation of the primary ray contribution of the tangential surface magnetic field, of a tangential magnetic current excited relative electromagnetic structure, is given via series expansion in powers of the normalized surface impedance. Furthermore, each term in the series is represented as series of the inverse of the cylinder radius. This is achieved by approximating the Hankel functions by a uniform asymptotic expansion within the spectral integral representation of the relevant Green’s function. All the resulted integrals are evaluated analytically in an exact fashion, and thus, the final process entails only summations and differentiations. The attained formulas are valid both within and outside the paraxial region. Validity of the contributed analytical solution is provided by successful comparison with published results. [ABSTRACT FROM AUTHOR]

The late-time evaluation of electromagnetic (EM) field quantities yielded by convolution integrals that combine Green’s functions available at discrete time samples and strictly causal excitations is critically revisited. A typical situation is used for tracing the causes of the divergent late-time behavior that is often experienced. A framework combining a suitable integral partitioning with a polynomial approximation is shown to effectively guarantee the integrals’ convergence. The formulation is validated via numerical experiments evidencing its accuracy and computational efficacy. The method is amenable to be used in a wide range of problems requiring the late-time evaluation of convolution integrals of the indicated type. [ABSTRACT FROM AUTHOR]

An accelerated integral equations (IE) field solver for determining scattered fields from electrically large electromagnetic metasurfaces using fast multipole method (FMM) is proposed and demonstrated in 2-D. In the proposed method, practical general metasurfaces are expressed using an equivalent zero thickness sheet model described using surface susceptibilities, and where the total fields around it satisfy the generalized sheet transition conditions (GSTCs). While the standard IE-GSTC offers fast field computation compared with other numerical methods, it is still computationally demanding when solving electrically large problems, with a large number of unknowns. Here, we accelerate the IE-GSTC method using the FMM technique by dividing the current elements on the metasurface into near- and far-groups, where either the rigorous or approximated Green’s function is used, respectively, to reduce the computation time without losing solution accuracy. Using numerical examples, the speed improvement of the FMM IE-GSTC method { $O(N^{3/2})$ } over the standard IE-GSTC { $O(N^{3})$ } method is confirmed. Finally, the usefulness of FMM IE-GSTC is demonstrated by applying it to solve electromagnetic propagation inside an electrically large radio environment with strategically placed metasurfaces to improve signal coverage in blind areas, where a standard IE-GSTC solver would require prohibitively large computational resources and long simulation times. [ABSTRACT FROM AUTHOR]

For wireless communications using linear large-scale antenna arrays, we define a receiving coordinate system and parameterization strategy to facilitate the study of the impact of three-dimensional position and rotation of the arrays on the achievable spatial degrees of freedom (DoF) in line-of-sight (LOS) channels. An analytical framework based on spatial bandwidth analysis is developed, under which three elementary problems corresponding to three basic orthogonal receiving directions are investigated. For each of them, accurate, simple, and interpretable closed-form approximations for the achievable spatial DoF are derived, and the spatial region where a sufficient amount of spatial DoF is expected available is determined. The expressions can easily be integrated into large-scale system-level simulations. Some interesting and surprising observations are made from simulation studies based on the analytical results. For instance, the spatial bandwidth is shown to be approximately constant in almost the entire spatial multiplexing region. Moreover, in significant parts of this region, the optimal receive array orientation is not parallel with the transmitting array. [ABSTRACT FROM AUTHOR]

We investigated how to construct low-order subspace basis sets to accurately represent electromagnetic (EM) fields generated within inhomogeneous arbitrary objects by radio frequency sources external to Huygen’s surface. The basis generation relies on the singular value decomposition of Green’s functions integrodifferential operators, which makes it feasible to derive a reduced-order yet stable model. We present a detailed study of the theoretical and numerical requisites for generating such basis and show how it can be used to calculate performance limits in magnetic resonance imaging applications. Finally, we propose a novel numerical framework for the computation of characteristic modes of arbitrary inhomogeneous objects. We validated accuracy and convergence properties of the numerical basis against a complete analytical basis in the case of a uniform spherical object. We showed that the discretization of Huygens’s surface has a minimal effect on the accuracy of the calculations, which mainly depends on the EM solver resolution and order of approximation. [ABSTRACT FROM AUTHOR]

We present a surface integral equation (SIE) solution for a special class of anisotropic media, whose permittivity and permeability are both symmetric full tensors with the same or multiple values. This kind of media is generated from the invisible cloaking or absorbing material of a perfectly matched layer (PML) design. A new derivation based on the potential functions is presented to obtain the closed-form Green’s function. Detailed and rigorous proofs of some vector identities are given for the first time. Special treatments are detailed to address the singularity of the anisotropic Green’s function for discretizing the SIE. Numerical experiments validate the accuracy, stability, and high efficiency of the proposed solution compared with the analytical solution or the finite element-boundary integral (FE-BI) method. [ABSTRACT FROM AUTHOR]

A plane-wave spectrum method is presented for the fast computation of radiation integrals for source and observation plates, which can be arbitrarily tilted and separated by an intermediate-to-far-field distance. The method expedites the direct and inverse transforms between spatial and spectral domains using unequally spaced fast Laplace transforms. These fast algorithms allow one to combine fast Fourier transforms (FFTs) and contour deformation in the complex plane with controllable accuracy and computation times close to that of a single 2-D FFT. Hence, scattering interactions between plates of sizes up to thousands of wavelengths can now be computed within seconds using current desktop computers. The fast method is validated here for the case of radio propagation in a canyon street modeled with electrically large facets using the physical optics approximation. [ABSTRACT FROM AUTHOR]

In this work, a full-wave partial element equivalent circuit (PEEC) model for 2-D periodic metallic structures is proposed. The mixed potential integral equations (MPIEs) with periodic Green’s function (GF) are interpreted as loop equations following Kirchhoff’s voltage law (KVL) in a circuit domain. The impact of gauge freedom in the MPIE on the corresponding PEEC model is discussed. A tailored gauge function is introduced to form a generalized PEEC model for periodic structures, which demonstrates superior passivity and frequency stability. With the proposed PEEC model, lumped and active elements can be easily integrated with conducting bodies for field-circuit co-simulation. Through the distributed circuit including coupling elements, the PEEC model provides a clear physical insight into the electromagnetic behavior of the physical models for design and optimization. Three numerical examples, including frequency-selective surface (FSS), array antenna, and pixelwise meta-atom, are given to illustrate the accuracy and efficiency of the proposed PEEC model. [ABSTRACT FROM AUTHOR]

Modeling the electromagnetic response of carbon nanotube (CNT) reinforced composites is inherently a 3-D multiscale problem that is challenging to solve in real time for nondestructive evaluation (NDE) applications. This article presents a fast and accurate full-wave electromagnetic solver based on a multilayer dyadic Green’s function approach. In this approach, we account for the effects of the dielectric slab, where the CNTs are embedded, without explicitly discretizing its interfaces. Due to their large aspect ratios, the CNTs are modeled as arbitrary thin wires (ATWs), and the method-of-moment (MoM) formulation with distributed line impedance is used to solve for their coupled currents. The accuracy of the in-house solver is validated against the commercial MoM and the finite element method (FEM) solvers over a broad range of frequencies (from 1 GHz to 10 THz) and for a wide range of dielectric slab properties. Examples of 100 nm-long vertical and horizontal CNTs embedded in a 1 $\mu \text{m}$ -thick lossy dielectric substrate are presented. The in-house solver provides more than $50\times $ speed up while solving the vertical CNT and more than $570\times $ speed up while solving the horizontal CNT than a commercial MoM solver over the GHz-to-THz frequency range. [ABSTRACT FROM AUTHOR]

In this communication, a semianalytical solution for the computation of the 2-D Green’s function for a cylindrical perfect electric conducting (PEC) enclosure containing an arbitrary number of confocal elliptic dielectric layers is provided. The approach relies on an expansion of the Green’s function into Mathieu functions, whose coefficients are found in an efficient way through a recursive procedure. The effectiveness of the technique is evaluated by comparisons with analytical formulas and against the results provided by independent numerical solvers. Moreover, the applicability of the solution to forward scattering problems using integral formulations is also assessed. In all the considered cases, the developed approach has been found to provide accurate results. [ABSTRACT FROM AUTHOR]

The volume integral equation (VIE) that describes the forward scattering problem is generally solved by iterative methods, such as the conjugate gradient (CG) method. In this work, we unfold the CG method into an iterative deep neural network to accelerate solving the VIE. After the dielectric scatterer’s relative permittivity and the incident field are input into the network, the total field is trained to converge to the ground truth iteratively. In the neural network, Green’s function is taken as an explicit operator to describe wave physics, and the fast Fourier transform (FFT) is applied to accelerate the computation of volume integrations. The global influence of all points in the space is compressed into a layer by the volume integration. In numerical tests, we validate the accuracy, efficiency, and generalization ability of the proposed neural network, and investigate the feasibility of changing the input size and the frequency in the prediction. Results show that the network is scale-independent and adaptable to predict fields in a narrow frequency band. This work provides us a new perspective of incorporating both learned parameters and physics into numerical algorithms for fast computation, and has the potential of being applied in deep-learning-based inverse scattering problems. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) and nonequilibrium Green’s function (NEGF) framework are used to explore the structural, spin-polarized electronic, and spin-based transport properties of edge-hydrogenated zigzag aluminum nitride nanoribbons (ZAlNNRs). The proposed ZAlNNR is observed to be structurally stable and exhibits half-metallic nature in the magnetic state. The quantum transport property of the proposed two-terminal device model of 1H-AlN-1H demonstrates the bipolar spin-filter characteristics along with giant magnetoresistance (GMR), spin-based peak to valley current ratio (spin-PVCR), and spin-based rectification ratio (spin-RR) of the order of 1015, 1012, and 108, respectively. The calculated GMR and spin-RR of the 1H-AlN-1H device are 107 and 102 times higher than zigzag silicene nanoribbon (ZSiNR) and doped-zigzag graphene nanoribbon (doped ZGNR), respectively. The observed GMR, spin-PVCR, and spin-rectifying behavior of the reported ZAlNNR device could be deployed for multifunctional spintronic device applications. [ABSTRACT FROM AUTHOR]

In this article, we demonstrate a reliable physics-based simulation approach to accurately model high-speed In0.53Ga0.47As/AlAs double-barrier resonant tunneling diodes (RTDs). It relies on the nonequilibrium Green’s function (NEGF) formalism implemented in SILVACO Atlas TCAD quantum simulation package to closely mimic the actual device physics, together with the judicious choice of the material parameters, models, and suitable discretization of the associated epitaxial layer structure. The validity of the approach was proved by comparing simulated data with experimental measurements resulting from fabricated micrometer-sized RTD devices featuring two different epitaxially grown layer stacks. Our results show that the simulation software can correctly compute the peak current density ${J} _{p}$ , peak voltage ${V} _{p}$ , and the valley-to-peak voltage difference $\Delta {V}$ = ${V} _{v} - {V} _{p}$ associated with the negative differential resistance (NDR) region of the RTD heterostructure static current density–voltage (${J}$ – ${V}$) characteristic at room temperature (RT), all of which are key parameters in the design of these devices for use in oscillator circuits. We believe that this work will now help in optimizing the RTD epitaxial structure to maximize its radio-frequency (RF) power performance, accelerating developments in the rapidly evolving RTD technology for emerging applications, including next-generation ultra-broadband short-range wireless communication links and high-resolution imaging systems. [ABSTRACT FROM AUTHOR]

Least-squares reverse-time migration (LSRTM) is linearized inversion based on the Born approximation, which commonly seeks to high-quality migration result by the least-squares sense. Contrary to time-domain LSRTM which performs the wavefield simulation as much as several times of the number of shots, frequency-domain LSRTM (F-LSRTM) has the advantage to efficiently deal with multiple shot records. Furthermore, if Green’s function can be saved on memory storage, the full wavefield simulation is implemented only once at the first iteration during entire LSRTM iterations. However, huge memory storage may be required to save the Green’s function for large dataset and model size. To alleviate this computational issue, we propose an efficient F-LSRTM scheme using singular value decomposition (SVD). In our scheme, Green’s function can be saved efficiently as two unitary matrices and one singular value vector with a few number of dominant singular values. Because the number of dominant singular values, called the optimal rank, is much smaller than the minimum value in each dimension size of Green’s function, the proposed method can make it possible to save the Green’s function into the computing memory with keeping the accuracy. After demonstrating the feasibility of reducing the rank of Green’s function, we examine our proposed F-LSRTM scheme and comparative F-LSRTM schemes (F-LSRTM using an adjoint-state method and saved full Green’s function, respectively) using the simple layered and modified marmousi-2 model. Numerical tests indicate that our proposed F-LSRTM scheme can generate migration results as accurate as comparative F-LSRTM schemes with less memory usage and computational cost. [ABSTRACT FROM AUTHOR]

In this work, to characterize the radiated emission from electronic circuits in shielding enclosure, an improved electric dipole-based source reconstruction method (SRM) is developed. Moreover, by resorting to this reconstructed equivalent source, the estimation of electromagnetic interference (EMI) between different circuit modules in the enclosure can be conveniently and accurately evaluated. Different from the free-space SRM, the equivalent dipoles of the proposed SRM are directly placed over the shielding box enclosed circuit board, and the numerical Green’s function (NGF) is developed to bridge the connection between the equivalent dipoles and the planar scanned magnetic near-field. With the NGF strategy, the effects of surrounding environments (including the presence of substrate, ground plane, and shielding enclosure) are inclusively considered, which makes the proposed SRM valid for any complex EM environments in theory. Since the proposed SRM also has the capability of reproducing the radiated emission inside the shielding enclosure, it is critically helpful to evaluate the EMI between different circuit modules inside the shielding enclosure. To reach this aim, the Rayleigh-Carson Reciprocity theorem is referred, in which the original EM coupling problem is decomposed into the “forward problem” and “reverse problem.” A Huygens box enclosing the victim circuit module is built and the EM fields on the box are obtained using the reconstructed dipoles in both problems. Then the EMI can be calculated accordingly. Therefore, the novelty and merits of the proposed approach are threefold: 1) the physically-based equivalent dipole model considers the interactions between the radiation sources and the surrounding objects, leading to the accurate prediction of radiated emissions both outside and inside the enclosure; 2) due to the utilization of the NGF, there is no approximation involved in the solution of the dipole’s radiation, thus improving the calculation precision; and 3) the near-field coupling between multiple electronic circuit modules inside the shielding enclosure can be identified precisely, which is useful for diagnostics of radiation sources. The effectiveness of this algorithm is verified by representative numerical examples. [ABSTRACT FROM AUTHOR]

Carbon nanotube field effect transistors (CNFETs) have potential applications in future logic technology as they display good electrostatic control and excellent transport properties. However, contact resistance and leakage currents could limit scaling of CNFETs. Non-equilibrium Green’s function (NEGF) simulation investigates that coupling between contact metal and CNT impacts both contact resistance and leakage current. The physical mechanisms underlying the effects are analyzed. A model with calibrated metal coupling strength from experimental data projects ION-IOFF design space to understand the trade-off between shrinking contact and extension lengths. For CNT with diameter of 1 nm, both contact and extension lengths greater than 8 nm are a good compromise between ION and IOFF for digital logic in advanced technology nodes. [ABSTRACT FROM AUTHOR]

A method of moments (MoM)-based procedure is developed to calculate the scattering from homogeneous dielectric bodies. The technique uses quadrilaterals for the approximation of geometry and piecewise sinusoidal (PS) basis functions for the approximation of surface currents. The notable advantage of the PS basis is its radiated fields can be expressed in closed-form. The proposed approach therefore circumvents the need to use Green’s function in formulating the integral equations. Combined with a simple testing procedure, the matrix filling time is reduced and the concern of singularity is removed. To further reduce the computational complexity and enhance the performance, a characteristic basis function method (CBFM) is implemented and embellished with a new characteristic basis function (CBF) generation strategy. Specifically, a cubical surface bounding the entire target is first specified and discretized. A set of auxiliary sources are then defined in a manner similar to that done for a scattering body. These auxiliary sources are defined once and shared by different blocks. In comparison to the traditional plane wave spectrum (PWS) method, the CBFs generated by the new approach are valid not only for plane wave excitations but also for localized excitations. Numerical results are provided to validate the efficacy of the proposed algorithm. [ABSTRACT FROM AUTHOR]

In this paper, a feature extraction technique based on the electromagnetic (EM) representation of radar signals is presented. In particular, we focus on ground-penetrating radar (GPR) imaging, where we model the backscatter from varying 2-D geometric shapes with arbitrary local coordinate rotations. Due to the electrically small nature of buried targets and the bending of the radar signal at the air–soil interface, we focus on exact methods to model the surface current density induced on scattering surfaces. Overcomplete basis sets are derived from the EM descriptions to represent the scene sparsely. From this proposed modeling framework, we devise a novel methodology to exploit the prediction of scattering behavior to extract features for classification from radar scenes when multiple buried scattering surfaces are present. We see that our method can identify and reconstruct buried scattering geometries in the presence of false targets that are brought about by the nonlinear nature of the exact EM modeling methods. A noniterative algorithm based on the conjugate of Green’s function is developed to solve for the surface current in an unknown domain using multifrequency, multiaperture data. Our modeling and feature extraction algorithms are numerically validated for different target shapes buried in lossy soil profiles. [ABSTRACT FROM AUTHOR]

The Intensity Vector Autonomous Recorder (IVAR) measures acoustic particle velocity and pressure simultaneously. IVAR was deployed on the seabed during the 2017 Seabed Characterization Experiment (SBCEX) with the primary objective to study sound propagation within underwater waveguides for which the seabed consists of fine-grained, muddy sediments. In this study, a Bayesian framework is applied to underwater noise recorded by IVAR from a cargo ship traversing the central region of the SBCEX2017 area for the purpose of inversion to characterize sediment properties. The vector acoustic data are in the form of a bounded, nondimensional form known as circularity, a quantity that is independent of the ship noise-source spectrum and that can be interpreted as the normalized curl of active intensity. The inversion model space for the seabed consists of a low-compressional speed layer and underlying basement half-space, with each having compressional and shear components. The interpretative model for producing a replica of the data is based on the plane wave reflection coefficient for a layered, elastic seabed in conjunction with the depth-dependent Green’s function that is integrated in the complex wave number plane to obtain pressure and particle velocity fields. The small change in water depth between the location of the ship source and IVAR is addressed using adiabatic mode theory. The inversion results exhibit slow variation over the 20-min observation period, representing approximately 5 km of travel by the ship source. [ABSTRACT FROM AUTHOR]

The emerging field of plasmonics may lead to enhanced light–matter interactions at extremely nanoscale regions. Plasmonic (metallic) devices promise to efficiently control classical and quantum properties of light. Plasmonic waveguides are usually employed to excite confined electromagnetic modes at nanoscale that can strongly interact with matter. Analysis shows that nanowaveguides share similarities with their low-frequency microwave counterparts. In this article, we review ways to study plasmonic nanostructures coupled to quantum optical emitters from a classical electromagnetic perspective. Quantum emitters are mainly used to generate single-photon quantum light that can be employed as a quantum bit, or “qubit,” in envisioned quantum information technologies. We demonstrate different ways to enhance a diverse range of quantum electrodynamic phenomena based on plasmonic configurations by using the Green’s function formalism, a classical dyadic tensor. More specifically, spontaneous emission and superradiance are analyzed through Green’s function-based field quantization. The exciting new field of quantum plasmonics could lead to a plethora of novel optical devices for communications and computing applications in the quantum realm, such as efficient single-photon sources, quantum sensors, and compact on-chip nanophotonic circuits. [ABSTRACT FROM AUTHOR]

This work presents a full-wave Green’s function approach for the rigorous investigation of a probe-coupled dielectric resonator antenna (DRA) configuration with an imperfect inner conductor. The impedance boundary condition is applied over the imperfect conductor in the formulation to derive the fields inside and outside the dielectric region, considering all higher-order modes. The technique enhances the applicability of the full-wave Green’s function in the evaluation of input impedance and radiation characteristics for an antenna structure with imperfectly conducting spherical surface, which to the best of our knowledge has not been reported before in the DRA literature. The role of the surface impedance on the antenna resonance and quality factor is thoroughly characterized. A suitable choice of the surface reactance depending on the radius of the imperfect conductor and dielectric-coating thickness enables the simultaneous reduction in the resonant frequency and quality factor for the antenna configuration. The effect of the surface reactance with varying coating permittivities is also investigated. The parametric variations in the input impedance, reflection coefficient, and antenna bandwidth with change in the surface impedance and coating permittivity for the antenna configuration are analyzed. The radiation characteristics for the antenna configuration are also examined. [ABSTRACT FROM AUTHOR]

This article addresses the problem of efficiently and accurately evaluating the singular or near-singular double volume (6-D) reaction integrals fundamental to the solution of volume integral equations (VIEs) using the method of moments (MoM). VIE techniques based on the MoM are used to obtain accurate solutions for electromagnetic (EM) problems of all types, and the accurate computation of the singular and near-singular reaction integrals that occur in the resulting system matrices is crucial for accurate EM modeling. Here we propose a scheme to treat, as a whole, the 6-D reaction integrals appearing in the MoM. The divergence theorem is directly applied twice in the physical space domain, not a transformed domain, thus eliminating restrictions to well-shaped elements. With appropriate integration reordering, the 6-D volume integrals are expressed as two radial integrals followed by two surface integrals over source and observation domain boundaries. We further smooth the integral by removing the static form of the kernel and evaluating it separately using a semi-analytical approach. The method is numerically validated for static and dynamic kernels arising in the electric field VIEs (i.e., for kernels having $1/R$ singularities) and for linear basis functions. [ABSTRACT FROM AUTHOR]

Nearfield (NF) 3-D imaging provides an effective solution of objects’ radar cross section (RCS) within a compact range. This article proposes a precise RCS extrapolation via NF 3-D imaging with adaptive parameter optimization Bayesian learning (APOBL), i.e., first, in the process of NF 3-D imaging, objects’ scattering centers may vary with the observation angle, while it is hard for the existing Bayesian learning via presetting parameters to reach an optimal estimation. For this issue, we present a parameter self-adaption solution, improving precision, and stability. Second, we also apply a block-based optimization idea in Bayesian-learning-based 3-D imaging, ensuring NF 3-D imaging quality. Third, in the process of RCS extrapolation, we apply a weighted Green’s function operator into the 3-D imaging-based NF-to-far-field (NF–FF) compensation, further ensuring high precision. The simulation and experiment results verify that the proposed method has an advantage in precision over the existing 3-D imaging-based RCS extrapolation methods. [ABSTRACT FROM AUTHOR]

This article studies the sub-linearity of the output characteristics measured in Schottky-barrier metal-oxide-semiconductor field-effect transistors with simulations and experiments. It is shown that the sub-linearity is not due to the forward-biased Schottky diode at the drain contact interface but due to the drain bias impact on the source-side Schottky-barrier, resulting in an increased carrier injection with increasing drain–source voltage. The simulation results are confirmed with the measurements of fabricated dual-gate Schottky-barrier transistors. [ABSTRACT FROM AUTHOR]

A Green’s function in an acoustic medium can be retrieved from reflection data by solving a multidimensional Marchenko equation. This procedure requires a priori knowledge of the initial focusing function, which can be interpreted as the inverse of a transmitted wavefield as it would propagate through the medium, excluding (multiply) reflected waveforms. In practice, the initial focusing function is often replaced by a time-reversed direct wave, which is computed with help of a macro velocity model. Green’s functions that are retrieved under this (direct-wave) approximation typically lack forward-scattered waveforms and their associated multiple reflections. We examine whether this problem can be mitigated by incorporating transmission data. Based on these transmission data, we derive an auxiliary equation for the forward-scattered components of the initial focusing function. We demonstrate that this equation can be solved in an acoustic medium with mass density contrast and constant propagation velocity. By solving the auxiliary and Marchenko equation successively, we can include forward-scattered waveforms in our Green’s function estimates, as we demonstrate with a numerical example. [ABSTRACT FROM AUTHOR]

Marine seismic data with towered streamers have played an important role in marine exploration. However, the distance between adjacent sources and the distance between adjacent receivers/channels are inconsistent (i.e., like regularly missing shots) and near-offset information is unrecorded, which can decrease the performances of surface-related multiple elimination (SRME) and seismic migration. Traditional algorithms to provide prestack seismic data with consistent trace interval and to recover near-offset data have some drawbacks, including low efficiency of computation and super-parameter selection by trial and error. Thus, we propose a novel self-supervised deep learning (DL) algorithm to reconstruct regularly missing shots and recover near-offset information with an improved U-net by combining U-net and residual learning of ResNet. Via the spatial reciprocity of Green’s function, common shot gathers (CSGs) have similar features as common receiver gathers (CRGs). The reconstruction performances of regularly missing shots in CRGs can be guaranteed by using the network that is trained and validated by adaptively extracted CSGs. To reconstruct near-offset information of CSGs, we first construct pseudo-seismic data with the dip approaching 0 at near-offset parts by a rotation-truncation strategy. Pseudo-seismic data can be regarded as seismic data with approximate near-offset information to train and validate the designed network, which is later used to reconstruct near-offset information for CSGs. Finally, field marine seismic data with towered streamers is used to demonstrate the validity and effectiveness of the proposed self-supervised algorithm in reconstructing regularly missing shots and recovering near-offset information, which are beneficial for subsequent processing of seismic data. [ABSTRACT FROM AUTHOR]

We present a multifilament current method (MFCM) to accurately and efficiently determine the cutoff wavenumbers (cutoff frequencies or eigenfrequencies) of homogeneous anisotropic material-filled metallic waveguides with arbitrary shapes. A set of $z$ -directed filamentary line sources placed outside of a waveguide are responsible for generating the simulated fields within the anisotropic material-filled waveguide in light of the derived 2-D anisotropic dyadic Green’s functions. Instead of solving an excitation-free eigenvalue problem, a substantial line source presented as an excitation is incorporated into the MFCM to formulate an eigenmode analyses technique without spurious eigenmodes. The internal field intensity response versus free space wavenumber are subsequently simulated, and a high magnitude response can be observed when an eigenmode is excited. In this case, the cutoff wavenumbers of physical eigenmodes are revealed by maximum locations of simulated field intensity response. Several numerical examples with different configurations on a waveguide are investigated. The simulated results are compared with those obtained from commercial software packages and an excellent agreement is achieved. In addition, the study of the oscillating phenomenon on magnitudes of filamentary sources and the ill-conditioning issue of constructed impedance matrix with respect to the placement of filamentary sources are also discussed. [ABSTRACT FROM AUTHOR]

A novel, robust, and computationally efficient approach is proposed for the evaluation of the Michalski–Zheng’s mixed-potential Green’s functions of general shielded layered media. A high-order variant of the spectral differential equation approximation method (SDEAM) is used to cast the spectra of Green’s functions’ components into pole-residue forms. The latter allows for closed-form evaluation of the Sommerfeld integrals (SIs) defining spatial components of the mixed-potential Green’s functions required by Method-of-Moment (MoM) discretization of different types of integral equations. The method produces Green’s functions for all elevations of interest at a fixed cost, making it substantially faster than popular alternative approaches. Numerical results validate the developed high-order SDEAM scheme and demonstrate its extended range of validity compared to the discrete complex image method (DCIM). [ABSTRACT FROM AUTHOR]

Precharacterizing the transition cubes containing stratified dielectrics is inevitable for the floating random walk (FRW)-based capacitance extraction. Each multilayer-dielectric transition cube is characterized by a pair of Green’s function table (GFT) and weight value table (WVT), and all these GFTs and WVTs usually have large volume and constitute the major memory cost of the FRW algorithm. In this work, we explore the geometric symmetry of the multilayer-dielectric transition cube to enable volume reduction and fast generation of the GFT and WVT. For a general transition cube with stratified dielectrics and the one with four equal-thickness dielectrics, two schemes are proposed to reduce the volume of GFT and WVT by $8\times $ and over $10\times $ , respectively. Accordingly, an approach for fast generation of the reduced GFT/WVT is proposed, which is proved to produce the same result as the original GFT/WVT values. And, an improved FRW algorithm is proposed to utilize the reduced GFTs/WVTs without the sacrifice of runtime or accuracy. Both theoretical analysis and numerical experiments are conducted to demonstrate the remarkable volume reduction of precharacterization data (GFTs/WVTs). The fast GFT/WVT generation approach and the improved FRW algorithm are also validated with numerical experiments, showing over $10\times $ speedup of the precharacterization process, and accurate and memory-efficient capacitance extraction as well. [ABSTRACT FROM AUTHOR]

Conventional spin-transfer torque-based magnetoresistive random access memory (STT-MRAM) with CoFeB electrodes has great potential as universal memory. However, state-of-the-art STT-MRAM technology has encountered the issues such as high writing current density and low thermal stability for scaling down to $1\times $ nm. Heusler alloy has been suggested as an alternative to resolve these problems by significantly reducing the Gilbert damping constant while preserving approximately 100% spin polarization. In particular, $L2_{1}$ -ordered Co2FeAl (CFA)-based magnetic tunnel junction (MTJ) exhibits outstanding half-metallicity and perpendicular magnetorcystalline anisotropy characteristics arising from Co(Fe)-O orbital hybridization at the interface. In this work, we investigate the biaxial strain effects of CFA-based MTJ by adjusting in-plane lattice constants from −4% to +4%. Our density functional theory - nonequilibrium Green’s function (DFT-NEGF) calculations present that FeAl-O interfaced MTJ shows a converged tunneling magnetoresistance (TMR) ratio under compressive strain while Co2-O interfaced MTJ shows strain-sensitive TMR ratio under both compressive and tensile strain. The difference in the current-density trends for the two types of MTJs is mainly attributed to the additional state arising from Fe-O bonding. Our results emphasize the careful control of straintronic techniques on CFA-MTJs. [ABSTRACT FROM AUTHOR]

An efficient ping-pong algorithm is presented to deal with electromagnetic scattering from reflecting surface antennas such as the reconfigurable intelligent surfaces (RISs) of arbitrary geometric shapes. The proposed computational electromagnetics (CEM) algorithm does not require formation, storage, and inverse of the impedance matrix used by the method of moments (MoM). Instead, a rigorous formula is obtained for the computation of the surface current from the scattered electric field. Besides, the algorithm only contains convolution operations with the scalar Green’s function and second derivatives operations to iteratively update the surface currents and scattered electric field. Also, hypersingular integration arising from the singularities of the surface current and scattered electric field is properly regularized. Furthermore, the algorithm converges fast and satisfactory results are obtained after a few iterations, and it is about ten times as fast as the MoM. Besides, the first iteration gives the solution for reflecting surface antennas on substrates of perfect absorption. Numerical solutions are obtained for some typical antennas and comparisons are made to those in the literature and from the MoM. [ABSTRACT FROM AUTHOR]

This paper deals with the mathematical expressions called Sommerfeld integrals. Introduced by A. Sommerfeld in 1909, they are mathematically equivalent to inverse Hankel transforms. The original historical goal of these integrals was to provide an accurate mathematical description of the electromagnetic phenomena involved in long-distance wireless radio and telegraphy. However, their scope was quickly enlarged thanks to the so-called spectral-domain stratified theory, and now they are ubiquitous in the mathematical models associated to many electromagnetic technologies, ranging from EMC lightning modelization and ground penetrating radar to optical and plasmonic integrated devices and going through the familiar microwave and millimeter-wave planar structures using printed circuit technology. In all these areas, Sommerfeld integrals can provide direct evaluations of the involved electromagnetic fields or they can be used as Green's functions in the frame of integral equation formulations. Other disciplines involving stratified media, like seismology and geological prospection, also benefit from these integrals. After discussing the most canonical Sommerfeld integral, appearing in the so-called Sommerfeld identity, this paper reviews three classical structures, namely, the original Sommerfeld problem involving two semi-infinite media, and the microstrip and stripline geometries. It is shown that Sommerfeld integrals provide a unifying treatment of these three problems and that their mathematical features have a direct translation in terms of the physical properties exhibited by the electromagnetic fields that can exist in them.

In this work, we show that the near field of leaky-wave resonant antennas (LWAs) radiating into a dense medium can be locally represented as a spherical wave in a certain solid angle around broadside using an accurate definition of the phase center. The near field in this solid angle can be efficiently evaluated through the integration of the spectral Green’s function along the steepest descent path (SDP). Beyond this solid angle, defined as the shadow boundary angle, a residual contribution due to the leaky-wave pole must also be added to fully describe the near field. It is found that this shadow boundary angle can be used to define the phase center and geometry of a truncated lens that couples well to leaky-wave antennas, even in electrically small-to-medium sized lenses and low-contrast cases. To demonstrate the applicability of the proposed study, we combine the SDP field calculation with a Fourier optics (FO) methodology to evaluate the aperture efficiency and radiation patterns of small-to-medium sized lenses in reception. A truncated silicon lens with a diameter of only four free-space wavelengths is presented with almost 80% aperture efficiency. Excellent agreement with full-wave simulations is achieved, which demonstrates the accuracy of the proposed design and analysis methodology. [ABSTRACT FROM AUTHOR]

An analytical drain current model is developed for a short-channel, dual-gate, monolayer 2-D transition metal dichalcogenide (TMD), lateral tunnel field-effect transistor (TFET) by considering 2-D ${p}^{+}-{n}$ and ${n}-{n}^{+}$ junction at source–channel and channel–drain junction, respectively. This work includes the effect of both front- and back-gate dielectric fringing field effects and 2-D source and drain depletion charges. A unified piecewise surface potential model is developed by capturing the fringing field effects in the channel and 2-D junction behavior at the source–channel and channel–drain junction by ensuring continuity of electric field at the junction regions. A tangent-line approximation method is employed for accurate numerical evaluation of the tunneling current. The proposed model is validated with simulation results obtained using nonequilibrium Green’s function (NEGF)-based simulator for different 2-D layered materials and a close agreement is observed. The veracity of the proposed model is tested with data in reported studies for different bias conditions and excellent matching is observed. The applicability of the model is demonstrated for any 2-D layered material-based TFET from ultrashort channel to a channel length of 50 nm. The proposed model will be extremely useful for further exploration of 2-D TMD material TFET-based very large-scale integration (VLSI) circuits. [ABSTRACT FROM AUTHOR]

This study demonstrates the saturation regime of the multipactor effect in a one-sided, partial, dielectric-loaded rectangular waveguide. The Monte Carlo algorithm is used to simulate the multipactor effect. The temporal evolution of the secondary electrons is tracked numerically, and the electrostatic field is obtained by solving the Green’s function of the dielectric’s surface charge. It is found that the positive charge on the dielectric surface produces a nonuniform electrostatic field that finally breaks the resonant motion of the secondary electrons and causes the double-sided multipactor to reach a steady state on the dielectric surface. Although the generation of electrons on the metal surface does not cease, the production ratio can be remarkably attenuated. While the inhomogeneous radio frequency (RF) field prevents the large-scale multipactor effect in the dielectric-loaded rectangular waveguide, the electrostatic field would promote the generation of secondary electrons in the high RF electric field region where the multipactor effect cannot be triggered, resulting in the expansion of the multipactor zones in the saturation stage. [ABSTRACT FROM AUTHOR]

2-D Janus materials are easy to produce strong piezoelectric properties due to the lack of mirror symmetry. In this article, we have performed the first-principles study on the piezoelectric properties of Janus BiTe ${X}$ (${X} =$ Cl, Br, I) monolayers. The electronic properties are calculated to analyze the causes of piezoelectric properties. Based on the density functional perturbation theory (DFPT), the piezoelectric tensor of Janus BiTe ${X}$ monolayer is calculated, and the piezoelectric coefficient is further calculated. The in-plane piezoelectric coefficient ${d}_{{11}}$ of BiTeI monolayer reaches 7.79 pm/V and the out-of-plane piezoelectric coefficient ${d}_{{31}}$ reaches 0.62 pm/V. Based on nonequilibrium Green’s function, the ${I}$ – ${V}$ response is calculated to analyze the piezoelectric response of the material in the device. In addition, these materials have suitable carrier mobility. Our research work shows that Janus BiTeI monolayer has potential applications in piezoelectric electronic devices, which provides a theoretical basis for further exploration of more 2-D Janus piezoelectric materials. [ABSTRACT FROM AUTHOR]

The hybrid spectral-element spectral-integral (SESI) method is developed for the electromagnetic (EM) scattering in 1-D Bloch (Floquet) periodic problems with scatterers embedded in multiple regions of 2-D layered media. The periodic layered medium Green’s function (PLMGF) is derived for the SESI method so that it can simulate EM scattering by horizontally periodic scatterers placed in an arbitrary number of layers. The SESI method is a combination of the spectral-element method (SEM) and the spectral-integral method (SIM). The SEM is applied only in regions with scatterers and has the merits of exponential convergence at a low spatial sampling density, while the SIM serves as an exact radiation boundary condition for the bottom and top boundaries of each SEM region. Therefore, there is no need to discretize the regions with pure layered media; furthermore, the inverses of these SIM matrices are given exactly and explicitly with only $O(N\log N)$ CPU time. Three numerical experiments are performed to show the accuracy and efficiency of the SESI method. [ABSTRACT FROM AUTHOR]

A $k$ -space approach for magnetostatic field computation in magnetic thin films is proposed. The convolution of truncated Green’s function and the magnetization field is computed via the fast Fourier transform, where the discretization process is done in the $k$ -space (Fourier space) instead of the ordinary space. In this respect, removal of the influence of replica in the ordinary space when the magnetostatic field is reconstructed is achieved. The accuracy and the efficiency of the proposed approach are investigated. [ABSTRACT FROM AUTHOR]

A rigorous expression for the critical length of backscattering ($\text{L}_{\textbf {Crit}}$) is reported. In the new model, $\text{L}_{\textbf {Crit}}$ is obtained from a weighted sum over the transport domain. It is shown that anisotropy of mean free path (MFP) tunes the profile of weight function. Under equilibrium condition with isotropic MFP, the weight is a uniform distribution. As the field grows, it builds up the anisotropy and confines the weight function around the injection boundary. This process quantifies the transition of $\text{L}_{\textbf {Crit}}$ from low to high field regime. We implemented this model in the non-equilibrium Green’s function (NEGF) solver and investigate the compact model form. Contrary to standard model, it is found that MFP of backscattering ($\boldsymbol {\lambda } $) elongates in high drain-source (Vds) bias. Based on these findings, an update of compact model is proposed and verified. [ABSTRACT FROM AUTHOR]

We present a study on beam coupling in radiating structures that support multiple simultaneous beams. The formation of multiple beams is relevant in modern wireless communication applications when diverse data streams can be sent from a single transmitter to users located in different directions. General expressions are provided that relate the coupling between radiated beams to the associated current distributions on the radiating aperture. When expanding the currents in terms of basis functions, the beam coupling can be also written in terms of coupling contributions due to each pair of basis functions. The analysis is applied to antenna arrays with different levels of mutual coupling between the array elements. More specifically, arrays of resonant elements, characterized by very low mutual coupling, are compared with arrays of connected elements, which are strongly coupled. We show that, even for a very high level of mutual coupling between individual array elements, it is possible to obtain orthogonal beams if the total current distributions associated with the beams are uncoupled. [ABSTRACT FROM AUTHOR]

We present an analytical approach for the calculation of direct source-to-drain tunneling (DSDT) probability of electrons in gate-all-around (GAA) silicon nanosheet (SiNS) MOSFETs. The used method is based on the wavelet transform and leads to a quasi-compact model (QCM) for the DSDT current of ultrashort-channel devices. Among them, we introduce a four-piece parabolic approximation method for the conduction band edge and present analytical expressions for the tunneling distances of electrons with different energy levels. The development of a QCM is achieved by limiting the number of interpolation points for the tunneling current density to seven specific electron energies, distributed around the energy level that makes the largest contribution to the tunneling current. A further simplification is achieved by the Gaussian approximation of the tunneling current density in transverse direction so that only the center and surface potentials ($\Phi _ {\text {C}}$ and $\Phi _ {\text {S}}$) at the barrier are of interest for the modeling. For comparison, all those approximations are also implemented in the Wentzel–Kramer–Brillouin (WKB) approximation. Furthermore, the approach is verified by nonequilibrium Green’s function (NEGF) simulation. [ABSTRACT FROM AUTHOR]