Permittivity Profile Research Articles

Magnetoelectric coupling at microwave frequencies observed in bismuth ferrite-based multiferroics at room temperature

• Room-temperature single phase multiferroic materials was obtained • Magnetoelectric coupling was observed at microwave (GHz) frequency • The coupling suggests a dynamic interaction between magnetic moments and ferroelectric polarization 0.63Bi 0.99 La 0.01 Fe 1- x Mn x O 3 -0.33BaTiO 3 -0.04Na 0.5 Bi 0.5 TiO 3 ( x = 0.10, 0.15, and 0.20) ceramics have been prepared by solid state reaction, and their structural, ferroelectric, ferromagnetic, piezoelectric, and magnetoelectric properties are studied. Except for the x = 0.2 composition, the X-ray diffraction and SEM-EDX analysis on the sintered ceramics show that the samples are single-phase, rhombohedral perovskites at room temperature. The introduction of Mn ions leads to a small deterioration in the ferroelectric and piezoelectric properties, whilst the room-temperature magnetization increases upon doping. The optimal multiferroic properties are obtained for the x = 0.15 composition. The magnetoelectric coupling in this sample was evidenced by the anomalies occurring simultaneously in the dielectric permittivity and magnetic permeability spectral profiles in the microwave region. Such a coupled phenomenon can be attributed to a dynamic interaction between magnetic moments and ferroelectric polarization.

Additive Manufacturing of Linear Continuous Permittivity Profiles and Their Application to Cylindrical Dielectric Resonator Antennas

The utilization of additive manufacturing (AM) to engineer the permittivity profile of dielectric resonator antennas (DRAs) is considered. For the first time, the capabilities of AM are exploited to create continuously swept permittivity profiles and applied to cylindrical DRAs. The spatial variant lattices (SVL) synthesis algorithm is implemented to create the desired permittivity profiles from a single material, and resulting geometries are manufactured using a high-permittivity material in a fused deposition modeling AM process. Three individual antennas for global navigation satellite system bands are designed and manufactured, two inhomogeneous DRAs with continuous permittivity profiles along the radial and vertical axis, and one homogeneous DRA for comparison. The manufactured antennas are characterized by impedance, realized gain, and axial ratio. Experimental results agree well with simulations and show increased impedance-, gain-, and axial-ratio bandwidths for both inhomogeneous antennas compared to the homogeneous one.

A Graded Index All-Dielectric Lens for Field Decorrelation and Decoupling of a Wideband MIMO Antenna

In this paper a broadband graded index all-dielectric (GID) lens that enables field decorrelation and decoupling of closely spaced wideband multiple-input-multiple-output (MIMO) antenna elements, is presented. Reduction in correlation coefficient ρ for field decorrelation and port decoupling is realized by tilting the individual antenna element radiation patterns away from each other and by suppressing the surface wave propagation inside the antenna substrate through judicious control of the permittivity profile of the GID lens using the concept of transformation electromagnetics (TE). The permittivity profile of the GID lens is designed using air-hole technology and is fabricated by additive manufacturing. To validate the proposed concept, six different examples of lens loaded wideband patch-based MIMO antenna elements arranged in two different orientations are investigated. Simulated and measured results verify the effectiveness of the proposed technique, which constitutes the reduction in ρ for both isotropic and non-isotropic propagation environments along with port-isolation enhancement without reducing the fractional bandwidth (FBW) in all of the six tested cases. Further results indicate a wide FBW having a minimum of 8.5% and a maximum of 33%, while radiation patterns with no extra backward radiation are obtained.

Physics Assisted Deep Learning for Indoor Imaging Using Phaseless Wi-Fi Measurements

A physics assisted deep learning framework to perform accurate indoor imaging using phaseless Wi-Fi measurements is proposed. It is able to image objects that are large (compared to wavelength) and have high permittivity values, that existing radio frequency (RF) inverse scattering techniques find very challenging, making it suitable for indoor RF imaging. The technique utilizes a Rytov based inverse scattering model with a deep learning framework. The inverse scattering model is based on an extended Rytov approximation (xRA) that pre-reconstructs the RF measurements. Under strong scattering conditions, this pre-reconstruction is related to the actual permittivity profile by a non-linear function, which is learned by a modified U-Net model to obtain the permittivity profile of the object. Thus, our proposed approach not only reconstructs the shape of objects, but also estimates their permittivity values accurately. We demonstrate its imaging performance using simulations as well as experimental results in an actual indoor environment using 2.4 GHz Wi-Fi phaseless measurements. For incident wavelength $\lambda_0$, the proposed framework can reconstruct objects with relative permittivity as high as 77 and electrical size as large as $40 \lambda$, where $\lambda =\lambda_0/\sqrt{77}$. This is in contrast to existing phaseless imaging techniques which cannot reconstruct permittivity values beyond 3 or 4. Thus, our proposed method is the first inverse scattering-based deep learning framework which can image large scatterers with high permittivity and achieve accurate indoor RF imaging using phaseless Wi-Fi measurements.

Effect of A-site covalency on ferroelectric phase transition in Ba doped morphotropic phase boundary PbTiO3–BiFeO3 solid solutions

This work investigates the role of A-site chemical bonding in morphotropic phase boundary (MPB) compositions of PbTiO3–BiFeO3 solid solutions exhibiting high tetragonality. Ba substitution significantly reduces the ferroelectric critical temperature, which correlates well with the structural parameters and manifests marked change in the free energy profile of structural phases within the MPB regime. A reduction in the grain size with Ba substitution has been observed which shows the reduction in the grain growth with Ba incorporation. The presence of pinned defect dipoles has been confirmed from the observed elliptical hysteresis loops at room temperature, making high-temperature permittivity studies unique to unraveling the finer details of this system. Inter and Intra grain contributions in the electrical response are quantified from the equivalent circuit analysis and compared with the complex permittivity profile. The frequency and temperature-dependent ac conductivity responses were analyzed, and observed conductivity was related to charge carriers hopping between vacant sites.

Frequency-Dependent Dielectric Permittivity in Poisson-Nernst-Planck Model.

The approaches developed for studying the polarization of molecules and the dynamics of ions in dielectric materials are usually considered separately. The two effects are often believed to take place in different frequency ranges. The low frequency response is usually dominated by ionic migration, whereas the high frequency response is played by molecular polarization. The goal here is to clarify the interplay between free and bound charge densities and their influences on permittivity and impedance profiles by proposing a version of the Poisson-Nernst-Planck (PNP) model that allows to include the effect of a frequency-dependent (and thus not instantaneous) polarizability.

Single-slice microwave imaging of breast cancer by reverse time migration.

Microwave imaging of breast cancer is considered and a new microwave imaging prototype including the imaging algorithm, the antenna array, and the measurement configuration is presented. The prototype aims to project the geometrical features of the anomalies inside the breast to a single-slice image at the coronal plane depending on the complex dielectric permittivity variation among the tissues to aid the diagnosis. The imaging prototype uses a solid cylindrical dielectric platform, where a total of 24 optimized Vivaldi antennas are embedded inside to form a uniform circular antenna array. The center of the platform is carved to create a hollow part for placement of the breast and the multistatic, microwave scattering parameters are collected with the antenna array around the hollow center. The dielectric platform further enhances the microwave impedance matching against the breast fat tissue and preserves the vertical polarization during the measurements. In the imaging phase, a computationally efficient inverse electromagnetic scattering method-reverse time migration (RTM)-is considered and adapted in terms of scattering parameters to comply with the actualmeasurements. The prototype system is experimentally tested against tissue-mimicking breast phantoms with realistic dielectric permittivity profiles. The reconstructed single-slice images accurately determined the locations and the geometrical extents of the tumor phantoms. These experiments not only verified the microwave imaging prototype but also provided the first experimental results of the imagingalgorithm. The presented prototype system implementing the RTM method is capable of reconstructing single-slice, nonanatomical images, where the hotspots correspond to the geometrical projections of the anomalies inside the breast.

Sub-surface stratification and dielectric permittivity distribution at the Chang’E-4 landing site revealed by the lunar penetrating radar

Context.In 2019, China’s Chang’E-4 (CE-4) probe landed on the far side of the Moon: a first in lunar exploration. The Lunar Penetrating Radar (LPR) mounted on the Yutu-2 rover allows the mapping of the near-surface structure and the dielectric permittivity of the landing area. The dielectric properties of the lunar soil affect the propagation of the LPR signals, which can be used to infer the depth of sub-surface boundaries and derive the composition of the component materials.Aims.Our objectives are to estimate the fine-resolution spatial distribution of relative permittivity and to improve the interpretation of the geological processes combined with the radargram of the CE-4 landing area.Methods.We used a modified method that combines the F-K migration and the minimum entropy of the ground penetrating radar (GPR) signals to estimate the velocity and permittivity values; this has the advantage of obtaining the appropriate velocity and permittivity, even with the incomplete or unnoticeable hyperbolic curves in the radar imageResults.The sub-surface stratification of the CE-4 landing area is seen in the first 31 lunar days of the LPR data. A fine-resolution dielectric permittivity profile ranging from ~2.3 to ~6.3 is obtained with our method, and the actual depths of the observed prominent sub-surface interfaces are determined, giving a maximum average depth of ~38 m. The thickness of the regolith layer is in the range of ~5.7–15.6 m, with an average of 11.8 m. The permittivity of the near-surface regolith (<30 cm) is ~2.78 ± 0.01, the bulk density is 1.57 ± 0.01 g cm−3, which is close to the results of ~1.61 g cm−3at the Apollo 15 landing area. The permittivity map is consistent with the radargram; the regolith and the paleo-regolith layer have relatively low permittivity and low echo strengths, while the rock debris has high permittivity and shows strong echos in the radargram. Two buried craters of different diameters beneath the navigation sites 4–11 and 16–31 are revealed in the radar profile. The permittivity distribution map can show detailed variations of material properties both inside and outside craters.

Wave field in a layer with a linear background profile and multiscale random irregularities

The paper addresses the problem of determining the field of a reflected wave in a multiscale inhomogeneous medium, which consists of a background deterministic large-scale irregularity and multiscale random irregularities with scales both larger and smaller than the wavelength. A layer with a linear permittivity profile is taken as the background irregularity. The small-scale irregularities are responsible for wide-angle scattering including backscattering. The first approximation of the perturbation theory is used to account for scattering from these irregularities. As the zero approximation and Green’s function in determining the field backscattered by small-scale irregularities, we utilize the integral representation of the field, obtained in our earlier work by combining the method of double weighted Fourier transform (DWFT) and the Fock proper-time method. Asymptotic methods are used to reduce the sevenfold integral representation of a field to lower-order integrals. Conditions for validity of such representations are obtained. Formulas of the frequency coherence functions of waves reflected and backscattered from the turbulent plasma layer are given. The paper presents the results of the simulation of pulse sounding of a randomly inhomogeneous reflecting plasma layer demonstrating the effect of large-scale irregularities on backscattering by small-scale irregularities.

Quasi-Newton-Based Inversion Method for Determining Complex Dielectric Permittivity of 3-D Inhomogeneous Objects

We present a new method for determining the complex dielectric permittivity profile of 3-D inhomogeneous dielectric objects from measurements of the scattered electric field vectors in the frequency domain. The method is formulated as a minimization of a cost function defined in terms of electric field integral equations known as the object and data equations. Instead of an unknown object function containing the electromagnetic parameters of the dielectrics, the contrast sources induced within the scatterers are designated as the unknowns of the inversion scheme to avoid solving the forward scattering problem at each step. Later, the minimization of the cost function is achieved via a limited-memory quasi-Newton scheme, based on the Broyden–Fletcher–Goldfarb–Shanno (BFGS) formula, which iteratively updates the Hessian matrix estimation. The numerical results with the simulated and experimental scattered electric fields demonstrate that the presented method is capable of reconstructing scatterers with complex shapes.

Microwaves as Diagnostic Tool for Pituitary Tumors: Preliminary Investigations

To date, tumors, the second cause of death worldwide, are a modern medicine plight. The development of rapid, cost-effective and reliable prevention and diagnostics tools is mandatory to support clinicians and ensure patients’ adequate intervention. Pituitary tumors are a class of neoplasm, which calls for suitable and ad hoc diagnostic tools. Recently, microwaves have gained interest as a non-ionizing, non-invasive valuable diagnostic approach for identifying pathologic tissues according to their dielectric properties. This work deals with the preliminary investigation of the feasibility of using microwaves to diagnose pituitary tumors. In particular, it focuses on benign tumors of the adenohypophysis, e.g., the pituitary adenomas. It is assumed to access the region of interest of the pituitary region by following a trans-sphenoidal approach. The problem was modeled by developing an equivalent transmission line model of the multi-layered, lossy tissues (front bone of sphenoid sinuses, air in the sinuses, posterior bone of sphenoid sinuses, the pituitary gland and the tumor). The forward problem was developed to investigate the transmission coefficient for identifying the most favorable propagation conditions. Then, it was analyzed if, by the solution of an inverse problem, it is possible to reconstruct the permittivity and electrical conductivity profiles and identify the tumor presence. The results are promising since a maximum reconstruction error of 8% is found, in the worst case, thus paving the way for the use of microwaves for the diagnosis of pituitary tumors.

Investigating dielectric spectroscopy and soft sensing for nondestructive quality assessment of engineered tissues

Non-destructive, inline quality monitoring techniques that can overcome the limitations of traditional, offline assays are essential to support the scale-up production of tissue engineered medical products (TEMP). In this work, we investigate a new soft-sensing approach with non-destructive dielectric spectroscopy (DS) that synergistically utilizes inline sensor data and predictive analytics to estimate unmeasured TEMP quality profiles. First, the performance of DS during the assessment of gelatin methacrylate (GelMA) constructs containing human adipose-derived stem cells was investigated in comparison to a traditional biochemical assay. The effects of two critical biofabrication parameters (photocrosslinking duration and volume of growth media) on a key scalar metric (Δϵ) were determined over 11 days of in vitro culture, where the metric was associated with the permittivity response of cells to alternating electric fields during DS and corresponding cellular metabolic activity. To enable accurate quality prediction while minimizing direct data collection to reduce the risk of cytotoxicity from prolonged exposure to the DS sensor electrodes and electric fields, we then developed a bilinear basis mixed model (BBMM) as a soft sensor. With comprehensive consideration of different variation sources, this model was designed to estimate missing permittivity profiles of constructs based on the measured DS dataset and biofabrication parameters. Results of benchmarking showed that BBMM outperformed state-of-the-art vector-prediction methods from literature in two different missing data estimation mechanisms. The high-accuracy BBMM provides a novel DS-driven soft sensing system as an inline monitoring tool suitable for scaled-up or scaled-out TEMP production systems.

A frequency-independent inhomogeneous planar radome with high angular stability based on permittivity manipulating

In this paper, an inhomogeneous planar radome (IPR) has been thoroughly investigated from an electromagnetic perspective. The inhomogeneity is considered to be an exponential function. Closed-form expressions have been derived for the transmission and reflection coefficients. A radome with frequency-independent response and angular stability up to 53° is achieved by manipulating the dielectric Permittivity Profile Variation (PPV) function with definite curvature. Implementation has been done with the use of the equivalent material theory (EMT). The optimal radome wall is prototyped by perforating the host medium continuously (which eliminates losses due to the discretization). An ultra-wide-band (3–18 GHz) free space measurement setup is used to characterize the frequency response of this prototype. Measurements are performed for both polarizations of TE and TM and various angles of the incidence. The results are in good agreement with the unit-cell full-wave simulations. The design procedure is generally described, which can be implemented for various electromagnetic and mechanical requirements.

A Miniaturized Anechoic Chamber: Omnidirectional Impedance Matching Based on Truncated Spatial Kramers–Kronig Medium

AbstractSpatial Kramers–Kronig (KK) medium provides a new platform to achieve omnidirectionally reflectionless absorption without using any gain elements. However, impedance‐matched spatial KK media realized in previous experiments generally have flat geometries with large transverse dimensions. Realizing finite spatial KK absorbers in free space background is important for many practical applications but has not been demonstrated experimentally thus far. Here, the concept of spatial KK relation to cylindrical coordinates is extended and a double‐resonance space‐frequency KK permittivity profile is implemented in a circular geometry. Omnidirectional impedance matching to free space is achieved by truncating the spatial KK absorber at the boundary where the relative permittivity is unity. For the experimental verification, a two‐dimensional 6.6‐wavelength‐diameter doughnut‐shaped anechoic chamber is implemented using the designed spatial KK medium. The measured electric field distributions and experimental imaging confirm the omnidirectionally reflectionless property, demonstrating that this miniaturized anechoic chamber is suitable for electrically small scatterer measurements and imaging. The proposed finite spatial KK absorber and fabricated miniaturized anechoic chamber show great promises for electromagnetic shielding and compatibility.

A Combined Inverse Source and Scattering Technique for Dielectric Profile Design to Tailor Electromagnetic Fields

This article augments existing gradient-based inverse scattering algorithms to enable the design of reflectionless lossless permittivity profiles within a given design domain that can transform an input incident field into an output field of desired characteristics. These desired characteristics are often some user-defined far-field performance criteria, such as main beam directions, null directions, and half-power beamwidth (HPBW). To this end, two extra steps will be performed prior to the inverse scattering step. First, an inverse source algorithm inverts the desired far-field performance criteria to infer a set of equivalent surface currents on a boundary close to the design domain. These equivalent currents are then converted to a set of field values that constitute the required aperture fields. Second, these aperture fields are scaled such that the input incident power to the design domain is approximately equal to the output power leaving the design domain. Finally, the desired scattered fields are formed and then inverted by an inverse scattering algorithm to reconstruct a lossless reflectionless dielectric profile within the design domain. We also show that the inverse scattering algorithm can employ appropriate regularization methods, in particular a binary regularization term, to facilitate the physical implementation of reconstructed dielectric profiles.

Arbitrary-order three-point finite difference method for the modal analysis of chiral waveguides.

A three-point finite difference method with an arbitrary order of accuracy is proposed for the modal analysis of chiral planar waveguides. The elaborate application of a nonuniform grid, compact finite difference technique, and boundary conditions results in an efficient, easily implemented, and versatile tool for the modal analysis of chiral planar waveguides with an arbitrarily discontinuous profile of permittivity, permeability, and chirality. In particular, this method efficiently resolves the fine structures in plasmon and photonic crystal waveguides. For the test model of a chiral-metallic plasmon waveguide, stable convergence up to a sixteenth order of accuracy can be obtained, which produces a relative error on the effective index that approaches the machine precision with only eighty grid points.

Contrast Source Inversion-Based Multilayered Object Analysis for Terahertz Wave Imaging

A complex permittivity profile reconstruction for multilayered objects was presented in this study by incorporating compressed sensing (CS)-based thickness estimation with contrast source inversion (CSI) non-linear inverse scattering (IS) method using a terahertz (THz) frequency band. Several studies investigated permittivity estimation for multiple layers. However, they require a prior knowledge of the thickness of each layer. Moreover, a critical problem in this field is the simultaneous estimation of both the dielectric constant and the thickness of each layer. To address this, a super-resolution thickness estimator using a CS filter and the CSI-based dielectric profile reconstruction scheme was used. This problem was effectively solved by introducing the cost function estimated using the CSI scheme, where the number of layers is given. The finite-difference time-domain (FDTD) numerical test indicated that the proposed method provides an accurate estimation of the thickness and dielectric profile in double-layered objects.

Deep-Learning-Based Calibration in Contrast Source Inversion Based Microwave Subsurface Imaging

A deep-learning (DL)-based data calibration technique applied to quantitative microwave inverse-scattering analysis is presented. This technique aims at subsurface inspection for the buried object under a concrete road or soil. The inverse-scattering analysis provides a complex permittivity profile, which is useful for object identification such as air gap or water. Contrast source inversion (CSI) is one of the most promising inverse-scattering methods. This method is capable of avoiding the iterative use of highly computational forward solvers. However, when applied to the measured data, an appropriate calibration capable of converting measured data to simulation data is required. In this work, a DL-based calibration suitable for nonlinear inverse problems is proposed. Its efficiency is experimentally demonstrated using a concrete cylinder containing water with different salinities.

Field Decorrelation and Isolation Improvement in an MIMO Antenna Using an All-Dielectric Device Based on Transformation Electromagnetics.

This work presents a new technique for enhancing the performance of a multiple-input multiple-output (MIMO) antenna by improving its correlation coefficient ρ. A broadband dielectric structure is designed using the transformation electromagnetics (TE) concept to decorrelate the fields of closely placed radiating elements of an MIMO antenna, thereby decreasing ρ and mutual coupling. The desired properties of the broadband dielectric wave tilting structure (DWTS) are determined by using quasi-conformal transformation electromagnetics (QCTE). Next, the permittivity profile of the DWTS is realized by employing air-hole technology, which is based on the effective medium theory, and the DWTS is fabricated using the additive manufacturing (3D printing) technique. The effectiveness of the proposed technique is verified by designing two-element patch-based MIMO antenna prototypes operating at 3 GHz, 5 GHz, and 7 GHz, respectively. The proposed technique helped to reduce the correlation coefficient ρ in the range of 37% to 99% in the respective operating bandwidth of each MIMO antenna, thereby, in each case, improving the isolation between antenna elements by better than 3 dB, which is an excellent performance.

Flow monitoring by microwave imaging inside a cuboid cavity: Theory and numerical feasibility analysis

In this paper, we investigate multiphase flow monitoring inside metallic pipes by qualitative microwave imaging, where foreign objects inside the liquid flow are monitored to eliminate contamination. The motivation stems from the fact that conventional tomographic approaches of microwave imaging are inherently not suitable for real-time flow monitoring due to their computational requirements as these techniques aim to retrieve the dielectric permittivity profile inside the pipeline. In this context, we envision that the factorization method of qualitative inverse scattering theory, which is a shape retrieval algorithm from scattered field measurements, provides a better alternative for flow monitoring. To demonstrate the feasibility of such an approach, we first reformulate the factorization method by considering dyadic Green’s functions inside a cuboid cavity with perfectly conducting walls. Later, the formulation is simplified for a specific microwave measurement configuration to accelerate the reconstructions for real-time flow monitoring. Finally, flow monitoring is realized as a differential imaging procedure where foreign objects are detected by using differences of multi-static scattering parameters that are measured consecutively. The simulation studies performed for food and petroleum flows reveal the capabilities of the technique as a viable solution for different industrial flows.