Dielectric Objects Research Articles

Marching‐on‐in‐time solution of delayed PEEC models of conductive and dielectric objects

In this work, a marching-on-time solver for time-domain simulation of partial element equivalent circuit (PEEC) models of electromagnetic systems is presented. The PEEC method is based on the electric field integral equation and the continuity equation. It describes separate magnetic and electric field couplings in terms of partial inductances and coefficients of potential. When the propagation delay is taken into account, the enforcement of Kirchhoff current and voltage laws results in a set of delayed differential equations. Typically, in the PEEC method, currents and charges are expanded with rectangular basis functions in both time and space. In this work, higher order basis functions are used to expand currents and charges in time. The resulting TD-PEEC solver is tested by comparison with other solvers that operate in both the time- and the frequency-domain and exhibits a satisfactory accuracy and better stability properties.

On the Use of Focused Incident Near-Field Beams in Microwave Imaging

We consider the use of focused incident near-field (NF) beams to interrogate the object of interest (OI) in NF microwave imaging (MWI). To this end, we first discuss how focused NF beams can be advantageously utilized to suppress scattering effects from the neighbouring objects whose unknown dielectric properties are not of interest (i.e., undesired scatterers). We then discuss how this approach can also be helpful in reducing the required measured data points to perform imaging. Driven by the relation between the electromagnetic inverse source and inverse scattering problems, our approach emphasizes the importance of tailoring the induced contrast sources in the imaging domain through the utilized incident NF beams. To demonstrate this idea, we consider two recently-proposed NF beams, and simulate them for imaging applications. The first one is a subwavelength focused NF beam generated by a passive NF plate, and the other is a Bessel beam generated by a leaky radial waveguide. Simple imaging examples are considered to explore the potential advantages of this approach, in particular, toward mainly seeing the object of interest, and not the unknown undesired scatterers. The scope of this paper is limited to homogeneous dielectric objects for which the induced total field distributions in the interrogated objects are similar to the incident field distributions (e.g., those that satisfy the Born approximation). Simple inversion results for focused and non-focused beams are presented accompanied by discussions comparing the achieved reconstructed values.

Height-from-Polarisation with Unknown Lighting or Albedo.

We present a method for estimating surface height directly from a single polarisation image simply by solving a large, sparse system of linear equations. To do so, we show how to express polarisation constraints as equations that are linear in the unknown height. The local ambiguity in the surface normal azimuth angle is resolved globally when the optimal surface height is reconstructed. Our method is applicable to dielectric objects exhibiting diffuse and specular reflectance, though lighting and albedo must be known. We relax this requirement by showing that either spatially varying albedo or illumination can be estimated from the polarisation image alone using nonlinear methods. In the case of illumination, the estimate can only be made up to a binary ambiguity which we show is a generalised Bas-relief transformation corresponding to the convex/concave ambiguity. We believe that our method is the first passive, monocular shape-from-x technique that enables well-posed height estimation with only a single, uncalibrated illumination condition. We present results on real world data, including in uncontrolled, outdoor illumination.

Fast full‐wave calculation of electromagnetic fields based on weak‐form volume integral equation for MRI applications

High-field magnetic resonance imaging (MRI) scans have safety problems because energy is not homogeneously distributed in the human body. A prescan where energy and temperature distributions are calculated for input energy budgeting can significantly lower the risk. Therefore, fast and accurate calculations of the electromagnetic (EM) fields of an MRI system with a subject under scan are needed. Here, the authors present a fast EM solver based on a weak-form volume integral equation (VIE) for solving this problem. The proposed approach calculates the EM field inside the human body at a high speed without sacrificing accuracy. In the proposed approach, the VIE formulation for an inhomogeneous dielectric object is employed for near-field calculations. The operation on a dense matrix resulted by the VIE is accelerated by fast Fourier transform (FFT) in conjunction with the latest efficient iterative method. Numerical experiments are presented to show the speed and accuracy of the proposed EM solver.

Plasmonic or dielectric dimers: A generic way to control the reversal of near field optical binding force

Abstract Controlling the near field optical binding force is an emerging new area of optical manipulation. So far, there is no generic way to reverse the near field optical binding force for plasmonic or dielectric nano-dimers of distinct shapes (cube, cylinder, ring, sphere); and specially for homodimers. In this article, for both plasmonic and dielectric objects, we have demonstrated a general way to control the reversal of near field binding force for different shaped dimer sets applicable for both homodimers and heterodimers. The force reversal is achieved by simple breaking of symmetry, considering the nanoparticles are half or less than half immersed in an inhomogeneous dielectric background, i.e. at air–water interface. However, if the dimer set is placed over a dielectric interface or fully inside a homogeneous medium, the sign of binding force does not reverse. Such force reversals for plasmonic dimers have been explained based on Fano resonance, interference fields, Lorentz force and image charge theory. The mechanism of binding force reversal is quite different in dielectric dimers, which has been explained mainly based on electric dipole moment, magnetic dipole moment and the interaction between electric and magnetic dipole moment. Our proposed idea can be verified by simple experiment.

Control rod position measurement with helix-electrode capacitance sensor in nuclear heating reactor

Capacitance sensors are frequently applied for dielectric objects measurements. However, the study on the ungrounded/grounded control rod position measurements with capacitance sensor is limited. In this connection, a system based on the helix-electrode capacitance sensor is applied to measure the rod position in NHR. Based on the variations of equivalent electrical permittivity with different control rod positions in NHR, the helix-electrode capacitance sensor has been successfully used for the ungrounded/grounded control rod position measurement due to its advantages of non-invasion, low-cost, high reliability and non-radiation. The finite element method is used to calculate the sensor capacitance and the sensor sensitivity is also presented. Then, the ungrounded/grounded control rod position experiment is carried out to measure the response of the helix-electrode capacitance sensor, in which a novel method is proposed to calibrate the helix-electrode capacitance sensor based on the grating linear displacement measuring probe. The capacitances obtained by simulations were compared with experiment results to verify the proposed models. The subsequent quantitative analyses on the data indicate the reliability and accuracy of the apparatus for monitoring control rod position. It is demonstrated that such a system shows promising applications for measurements of the control rod position in NHR.

Surface Integral Equation-Based Characteristic Mode Formulation for Penetrable Bodies

A novel surface integral equation (SIE)-based theory of characteristic modes (TCM) formulation is proposed for homogeneous penetrable bodies. Analytical expressions for the characteristic eigenvalues are presented and good agreement between the numerical and analytical results is observed for both lossless and lossy, as well as for dielectric and magnetodielectric objects. The new formulations, without any postprocessing methods, avoid spurious modes and resonances appearing in existing SIE-based TCM formulations. For lossy objects, the obtained eigenvalues are complex. The real part of an eigenvalue is related to the ratio of the reactive and radiated power, as in the original TCM formulation for perfect electric conductor structures, and the imaginary part gives the ratio between the dissipated and radiated power.

An Iterative Modified Diffraction Tomography Method for Reconstruction of a High-Contrast Buried Object

An iterative subsurface inverse scattering algorithm has been proposed to profile high-contrast 2-D dielectric objects buried under a lossy ground. The proposed iterative modified diffraction tomography (IMDT) method is based on the combination of the traditional iterative Born method and DT technique which is well known for its simplicity and robustness. In effect, IMDT is an iterative Born algorithm that utilizes the spectral domain concept of DT for solving the inversion problem. The proposed iterative approach results in removing the Born approximation’s limitation of DT technique in dealing with high-contrast scatterers. To this end, the total field inside the reconstruction domain is renewed and then expanded into upgoing and downgoing plane waves in each iteration. Consequently, by exponential expansion of the fields and deriving a modified DT formulation, high-contrast targets are also efficiently reconstructed. To assess the proposed IMDT method, various high-contrast objects and noise conditions are studied. It is shown that the IMDT algorithm significantly outperforms the DT technique and is capable of reconstructing high-contrast objects efficiently and accurately even in noisy environments.

Formulation of Surface-Volume-Surface-EFIE for Solution of Three-Dimensional Scattering Problems on Composite Dielectric Objects

A novel formulation of the surface-volume-surface electric field integral equation (SVS-EFIE) is introduced for the solution of the scattering and radiation problems on the composite dielectric objects made of arbitrary number of piecewise homogeneous nonmagnetic dielectric regions. The regions composing the scatterer may or may not share common boundaries. Due to the fact that SVS-EFIE introduces the independent fictitious surface electric current densities on the boundary of each region, the proposed formulation allows for independent meshing of the scatterer regions according to its permittivity. As a result, the new SVS-EFIE formulation presents no complications in method-of-moments discretization at the material junctions. The proposed formulation exhibits the same CPU time and the memory complexities as traditional surface integral equation formulations when fields throughout the scatterer are to be computed. The new equation is validated through comparison of its solution against the Mie series and fields computed using the commercial software.

Tribo-electric charging of dielectric solids of identical composition

Despite its long history and importance in many areas of science and technology, there is no agreement on the mechanisms responsible for tribo-electric charging, including especially the tribo-charging of chemically identical dielectric solids. Modeling of the excitation, diffusional transport, and de-excitation of electrons from hot spots shows that a difference in local surface roughness of otherwise identical solid dielectric objects leads to different transient excited electron concentrations during tribo-processes. The model predicts that excited electron concentrations are lower and concentration gradients higher in solids with rougher rather than smoother surfaces. Consequently, during contact, the flux of charge carriers (electrons or holes) from hot spots will be greater into the rougher solid than into the smoother solid. These predictions are in agreement with current and historical observations of tribo-electric charge transfer between solids of the same composition. This effect can take place in parallel with other processes and may also play a role in the charging of solids of different composition.

A butterfly‐based direct solver using hierarchical LU factorization for Poggio‐Miller‐Chang‐Harrington‐Wu‐Tsai equations

AbstractA butterfly‐based hierarchical LU factorization scheme for solving the PMCHWT equations for analyzing scattering from homogenous dielectric objects is presented. The proposed solver judiciously re‐orders the discretized integral operator and butterfly‐compresses blocks in the operator and its LU factors. The observed memory and CPU complexities scale as O(N log2 N) and O(N1.5 log N), respectively. The proposed solver is applied to the analyses of scattering several large‐scale dielectric objects.

Non-destructive testing of arbitrarily shaped refractive objects with millimetre-wave synthetic aperture radar imaging

Abstract. Millimetre-wave (mmW) imaging is an emerging technique for non-destructive testing. Since many polymers are transparent in this frequency range, mmW imaging is an attractive means in the testing of polymer devices, and images of relatively high resolution are possible. This contribution presents an algorithm for the precise imaging of arbitrarily shaped dielectric objects. The reconstruction algorithm is capable of automatically detecting the object's contour, followed by a material-sensitive reconstruction of the object's interior. As an example we examined a polyethylene device with simulated material defects, which could be depicted precisely.

A 3D spatial spectral integral equation method for electromagnetic scattering from finite objects in a layered medium

The generalization of a two-dimensional spatial spectral volume integral equation to a three-dimensional spatial spectral integral equation formulation for electromagnetic scattering from dielectric objects in a stratified dielectric medium is explained. In the spectral domain, the Green function, contrast current density, and scattered electric field are represented on a complex integration manifold that evades the poles and branch cuts that are present in the Green function. In the spatial domain, the field-material interactions are reformulated by a normal-vector field approach, which obeys the Li factorization rules. Numerical evidence is shown that the computation time of this method scales as O(N log N) on the number of unknowns. The accuracy of the method for three numerical examples is compared to a finite element method reference.

Electromagnetic behavior of dielectric objects on metallic periodically nanostructured substrates.

In this research, we investigate the electromagnetic behavior of a metallic thin-film with a periodic array of subwavelength apertures when dielectric objects are located on it. The influence of size, geometry and optical properties of the objects on the transmission spectra is numerically analyzed. We study the sensitivity of this system to changes in the refractive index of the illuminated volume induced by the presence of objects with sizes from hundreds of nanometers (submicron-sized objects) to a few microns (micron-sized objects). Parameters such as the object volume within the penetration depth of the surface plasmon in the buffer medium or the contact surface between the object and the nanostructured substrate strongly affect the sensitivity. The proposed system models the presence of objects and their detection through the spectral shifts undergone by the transmission spectra. Also, we demonstrate that these can be used for obtaining information about the refractive index of a micron-sized object immersed in a buffer and located on the nanostructured sensitive surface. We believe that results found in this study can help biomedical researchers and experimentalists in the process of detecting and monitoring biological organisms of large sizes (notably, cells).

Microwave imaging with noise waveforms using FDTD time reversal method

AbstractThis article presents a microwave imaging method based on the time‐reversed finite‐difference time‐domain method (FDTD‐TR) that enhances immunity to undesired interference and jamming signals. The method uses random noise as the source waveform that illuminates the imaging area. Signals scattered by objects in the imaging area are collected on a planar grid, followed by processing through a correlation operation, and then used in the FDTD‐TR algorithm to construct images on any desired plane in the computational domain. The method capability in suppressing interference noise is demonstrated through successful reconstruction of images for a test case consisting of a dielectric object behind a wall. The method is compared to conventional FDTD‐TR methods that fail the same image reconstruction test.

High-Order Sparse Shape Imaging of PEC and Dielectric Targets Using TE Polarized Fields

In this paper, we present a novel method for microwave imaging using sensors with transverse electric (TE) polarization. The method is applicable to metallic and dielectric objects and exploits the sparseness of the sought solution, by imposing a measurement model based on multipole sources. In order to verify the achievable performance, the method has been numerically tested against different scenarios, including targets with complex concave cross sections as well as multiple objects. The investigation has shown that, opposite to the transverse magnetic (TM) polarization, the standard (zero order) sparse processing yields no or very little information about the targets. In contrast, using higher order multipole sources to build the sparse imaging dictionaries allows retrieving complicated target shapes, even in presence of noise. In addition, we have compared the reconstruction results obtained using the TE and TM polarization.

A Discontinuous Galerkin Surface Integral Equation Method for Scattering From Multiscale Homogeneous Objects

A discontinuous Galerkin (DG) surface integral equation approach is proposed for scattering from homogeneous dielectric objects. The formulation of DG for homogeneous bodies is derived from the combined tangential field integral equation. The differences of DG for penetrable and nonpenetrable objects are presented by numerical experiments to demonstrate the numerical mechanism of DG. Numerical experiments demonstrate the great advantages of our presented formulation of DG for homogeneous objects in efficiency, flexibility, and scalability. A series numerical results are presented to show the capability of the presented DG solution for homogeneous bodies, especially for multiscale homogeneous bodies.

Frequency Domain Inverse Profiling of Buried Dielectric Elliptical-Cylindrical Objects Using Evolutionary Programming

An efficient method based on the evolutionary programming (EP) technique is proposed for inverse profiling of 2-D buried dielectric objects with elliptical cross sections. In particular, EP with Cauchy mutation operator (EP-CMO), as its first reported implementation to inverse problems, is utilized as a stochastic optimization tool for quantitatively reconstructing buried objects. Moreover, the method of moments technique in conjunction with conjugate gradient-fast Fourier transform method is used, as a fast and simple frequency domain forward solver, in each iteration of the proposed method. Numerical results for different case studies are presented and analyzed. To assess the proposed EP-CMO method, the results are also compared statistically with that of three other well-known optimization techniques, namely, EP with Gaussian mutation, particle swarm optimization, and genetic algorithms. The results reveal that EP-CMO is a significantly more robust and efficient optimization tool in reconstruction of this class of buried objects.

Analytical Approach for Making Multilayer and Inhomogeneous Structures Invisible Based on Mantle Cloak

Abstract We investigate an analytical approach based on transmission line model to design mantle cloak for multilayer and/or inhomogeneous dielectric objects. Our proposed model is verified with different numerical results. The results matched well with each other. Also, the required impedance for making an inhomogeneous material invisible is calculated analytically. Finally, some complex structures are implemented using CST simulator and suitable performance is reached. So, it is proved that mantle cloak is applicable for making complex inhomogeneous materials invisible.

Scattering center models of backscattering waves by dielectric spheroid objects.

Scattering center models provide a simple and effective way of describing the complex electromagnetic scattering phenomena of targets and have been successfully applied in radar applications. However, the existing models are limited to conducting objects. Numerical results show that scattering centers of dielectric objects are far more complex than conducting objects and most of them are distributed beyond the object. For the lossless and low-loss media, the major scattering contributions to total fields are surface waves and multiple internal reflections rather than the direct reflection. Concise scattering center models for backscattering from dielectric spheroid objects are proposed in this work, which can characterize the backscattered waves by scattering centers with sparse and physical parameters. Good agreement has been demonstrated between the high resolution range profiles simulated by this model with those obtained by Mie series and the full wave numerical method.