Full-wave Electromagnetic Simulation Research Articles

Optimization of Microwave Wireless Power Transmission With Specific Absorption Rate Constraint for Human Safety

In this communication, we propose a new convex optimization algorithm for exciting the transmitting antennas of a microwave wireless power transmission (MPT) system that transfers maximum power under a specific absorption rate (SAR) constraint for human safety. The transformation of the initial NP-hard problem to a convex optimization problem is described in detail. We applied the proposed algorithm to several MPT scenarios with multiple transmitting antennas around and one receiver placed near a box-shaped phantom model. The channel response and the electric field response for the receiver and phantom required for the optimization process are obtained using full-wave electromagnetic simulations. The received power and the power transfer efficiency (PTE) of the proposed optimization (OPT) technique are compared with those of the time-reversal (TR) technique at 0.9 GHz. The results show that the OPT technique can transfer more power than the TR technique with a lower PTE within the SAR limit and that the proposed technique can be applied to various MPT scenarios.

Effects of Filling Material Roughness on Scattering from a Rectangular Groove

This study investigated the effects of filling material roughness on the H-polarized scattering signatures of a two-dimensional (2D) rectangular groove embedded on an infinite ground plane. Under the assumption of a weakly rough surface on the groove, simplifying suppositions can be used to estimate Green’s functions inside and outside the groove. Enforcing continuity in the tangential magnetic field on the rough surface enables the construction and resolution of a Fredholm’s integral equation with logarithmic singularity. The results were verified via two full-wave electromagnetic field simulations carried out using the finite element method and the method of moments. The findings showed that even a weakly rough surface, such as a polished exterior with weak residual roughness, can considerably affect and change 2D scattering patterns.

Modeling the Self-Capacitance of an Irregular Solenoid

Irregular solenoids, i.e., sparse solenoids, varied-pitch solenoids, and noncylindrical solenoids, has started to appear recently for specific applications, and an accurate and fast modeling of their self-capacitance is necessary to accelerate the design and optimization process of such coils. Conventional capacitance modeling methods only work for coils, which are standard tightly wound cylindrical solenoids, but fail for irregular solenoids. Full-wave electromagnetic (EM) simulators provide good accuracy, but the computation time is usually long. In this article, a fast and accurate model to calculate the self-capacitance of an irregular solenoid is proposed. A modified turn-to-turn capacitance model, which considers the capacitance in the insulation layer of a wire as well as the air capacitance between the nearest neighbouring loops and the second nearest neighbouring loops is adopted. It works for solenoids of different types (e.g., sparse, varied-pitched, and noncylindrical ones). The accuracy of the proposed model is validated by comparing the calculation results to both those obtained by using commercial full-wave simulators and experimental results. For sparse solenoids, the proposed model is always accurate for coils with either small or large pitches. For varied-pitch solenoid and noncylindrical solenoid, the error rate of the proposed model is around 10%. Computational time wise, the computation time of the proposed model is reduced by over $10^4$ times compared to that of a commercial EM simulator whereas the accuracy is comparable.

High power characterisation of the ECRH transmission lines and power deposition calculations in the TJ-II stellarator

Abstract In the TJ-II stellarator, the last mirror of both ECRH transmission lines is an in-vessel steerable phase correcting mirror that distorts the power distribution of the reflected beam whenever the mirror orientation angles deviate from the design position. Beam distortions cannot be measured due to space restrictions when accessing the inner part of the device, particularly if we have in mind the wide range of orientation angles needed to carry out ECRH and ECCD experiments, for which such experimental characterisation should be performed. Instead, a set of full wave electromagnetic simulations allows us to estimate the power distribution of the beam coupled to the plasma and how this distribution changes when the in-vessel mirror is steered. High power characterisation of the transmission lines is first accomplished and documented in order to determine the beam power distribution before entering the vessel. For the perpendicular injection case, the additional contribution to the beam distortion produced by its passage through the structure supporting the window is calculated. Leaving aside the power losses due to beam trimming (7% and 10% for the first and second line respectively), the added distortion produced by the window housing is of minor importance compared to the effect of the internal mirror. The impact of beam distortion on the width of the power deposition profiles for several launching directions is addressed using the ray/beam tracing code TRUBA. Results show that the beam waist in the plasma appears smaller and ahead of the position predicted by quasi optical theory, leading to beam widening in the magnetic axis where the resonant layer is located. Despite the evident distortion, power deposition profiles calculated with beam tracing, refined with a Gabor expansion of the wave field distribution in case of strongly oblique incidence, do not differ much from the predictions of the standard ray tracing calculation.

Dyakonov-like waveguide modes in an interfacial strip waveguide

We study Dyakonov surface waveguide modes in a waveguide represented by an interface of two anisotropic media confined between two air half-spaces. We analyze such modes in terms of perturbation theory in the approximation of weak anisotropy. We show that in contrast to conventional Dyakonov surface waves that decay monotonically with distance from the interface, Dyakonov waveguide modes can have local maxima of the field intensity away from the interface. We confirm our analytical results by comparing them with full-wave electromagnetic simulations. We believe that this work can bring new ideas in the research of Dyakonov surface waves.

An efficient transformer modeling approach for mm-wave circuit design

In this paper, a Gaussian-process surrogate modeling methodology is used to accurately and efficiently model transformers, which are still a bottleneck in radio-frequency and millimeter-wave circuit design. The proposed model is useful for a wide range of frequencies from DC up to the millimeter-wave range (over 100 GHz). The technique is statistically validated against full-wave electromagnetic simulations. The efficient model evaluation enables its exploitation in iterative user-driven design approaches, as well as automated design exploration involving thousands of simulations. As experimental results, the model is used in several scenarios, such as the design of an inter-stage amplifier operating at 60 GHz, where the model assisted in the simulation of the transformers and baluns used, and the design of individual transformers and a matching network.

Investigation of a Radiation Pattern Frequency Dependence of a Subarrayed Slotted Waveguide Antenna Using CAD Ansys HFSS

Introduction. A resonant slotted waveguide antenna allows providing broadside radiation, which coincides with the normal line to the longitudinal axis of the array. Such an antenna can be well-matched in a very narrow frequency band. Even a slight deviation from the operating frequency leads to a significant distortion of the radiation pattern. In this regard, the distortion of the radiation pattern is becoming an urgent task. Aim. The main objective of this work is to preserve the radiation pattern of the resonant slotted waveguide antenna with longitudinal slots in the broad face in operating bandwidth. Materials and methods. Subarraying was performed to preserve the undistorted radiation pattern of the slotted waveguide antenna array in the operating bandwidth. The antenna model and its directional properties were analyzed in CAD Ansys HFSS using Visual Basic Scripting Edition macros. Results. Three slotted waveguide antennas’ models, consisting of two, four, and eight subarrays were developed in Ansys HFSS CAD. The effect of subarraying on the radiation pattern at the center frequency and the upper and the lower operating frequencies is studied. It is shown that the growing number of subarrays leads to a more stable radiation pattern in the operating bandwidth. Conclusions. Full-wave electromagnetic simulations have shown that subarraying of a resonant slotted waveguide antenna is a useful measure that preserves undistorted radiation pattern in the operating frequency band. Usage of Visual Basic Scripting Edition macros allows to minimizing the time spent creating a model in CAD Ansys HFSS.

Nested Kriging with Variable Domain Thickness for Rapid Surrogate Modeling and Design Optimization of Antennas

Design of modern antennas faces numerous difficulties, partially rooted in stringent specifications imposed on both electrical and field characteristics, demands concerning various functionalities (circular polarization, pattern diversity, band-notch operation), but also constraints imposed upon the physical size of the radiators. Conducting the design process at the level of full-wave electromagnetic (EM) simulations, otherwise dictated by reliability, entails considerable computational expenses, which is another and a serious challenge. It is especially pronounced for the procedures involving repetitive EM analyses, e.g., parametric optimization. Utilization of fast surrogate models as a way of mitigating this issue has been fostered in the recent literature. Unfortunately, construction of reliable surrogates for antenna structures is hindered by their highly nonlinear responses and even more by the utility requirements: design-ready models are to be valid over wide ranges of operating conditions and geometry parameters. Recently proposed performance-driven modeling, especially the nested kriging framework, addresses these difficulties by confining the surrogate model domain to a region that encapsulates the designs being optimum with respect to the relevant figures of interest. The result is a dramatic reduction of the number of training samples needed to render a usable model. This paper introduces a variable-thickness domain, which is an important advancement over the basic nested kriging. The major benefit demonstrated using two antenna examples is a further and significant (up to seventy percent) reduction of the training data acquisition cost. It is achieved while ensuring that the model domain covers the regions containing optimum designs for various sets of performance specifications.

Extreme renormalisations of dimer eigenmodes by strong light–matter coupling

We explore by theoretical means an extreme renormalisation of the eigenmodes of a dimer of dipolar meta-atoms due to strong light–matter interactions. Firstly, by tuning the height of an enclosing photonic cavity, we can lower the energy level of the symmetric ‘bright’ mode underneath that of the anti-symmetric ‘dark’ mode. This is possible due to the polaritonic nature of the symmetric mode, that shares simultaneously its excitation with the cavity and the dimer. For a heterogeneous dimer, we show that the polariton modes can be smoothly tuned from symmetric to anti-symmetric, resulting in a variable mode localisation from extended throughout the cavity to concentrated around the vicinity of the dimer. In addition, we reveal a critical point where one of the meta-atoms becomes ‘shrouded’, with no response to a driving electric field, and thus the field re-radiated by the dimer is only that of the other meta-atom. We provide an exact analytical description of the system from first principles, as well as full-wave electromagnetic simulations that show a strong quantitative agreement with the analytical model. Our description is relevant for any physical dimer where dipolar interactions are the dominant mechanism.

Editorial for the special issue on advances in forward and inverse surrogate modeling for high‐frequency design

Editorial for the special issue on advances in forward and inverse surrogate modeling for <scp>high‐frequency</scp> design

Intelligent Time-Varying Metasurface Transceiver for Index Modulation in 6G Wireless Networks

Index modulation (IM) is one of the candidate technologies for the upcoming sixth generation (6G) wireless communications networks. In this paper, we propose a space-time-modulated reconfigurable intelligent metasurface (RI-MTS) that is configured to implement various frequency-domain IM techniques in a multiple-input multiple-output (MIMO) array configuration. Unlike prior works which mostly analyze signal theory of general RI-MTS IM, we present novel electromagnetics-compliant designs of specific IMs such as sub-carrier index modulation (SIM) and MIMO orthogonal frequency-domain modulation IM (MIMO-OFDM-IM). Our full-wave electromagnetic simulations and analytical computations establish the programmable ability of these transceivers to vary the reflection phase and generate frequency harmonics for IM. Our experiments for bit error rate show that RI-MTS-based SIM and MIMO-OFDM-IM are lower than conventional MIMO-OFDM.

Low‐cost data‐driven modelling of microwave components using domain confinement and PCA‐based dimensionality reduction

Fast data-driven surrogate models can be employed as replacements of computationally demanding full-wave electromagnetic simulations to facilitate the microwave design procedures. Unfortunately, practical application of surrogate modelling is often hindered by the curse of dimensionality and/or considerable nonlinearity of the component characteristics. This paper proposes a simple yet reliable approach to cost-efficient modelling of miniaturized microwave components which adopts two fundamental mechanisms to improve the computational efficiency of setting up the surrogate. Firstly, the model domain is confined-using a set of pre-optimized reference designs-to the region of the parameter space containing high-quality designs with respect to the relevant performance figures. Secondly, the domain is spanned by the selected principal components of the reference set for dimensionality reduction. Consequently, the surrogate model, covering wide ranges of the device parameters and operating conditions, can be established using a fraction of training data samples required by conventional approaches, without compromising its predictive power. The proposed technique is illustrated using two miniaturized microstrip components: a rat-race coupler (RRC) and an impedance matching transformer. The following accuracies of the PCA-based surrogates have been obtained: 0.9% for RRC and 2.1% for the transformer (for 800 training data samples).

Design and implementation of miniaturized wideband microstrip patch antenna for high-speed terahertz applications

In the twenty-first century, graphene is widely used in wireless communication, especially in terahertz applications because of its amazing electrical, mechanical, and optical properties. This paper presents a graphene-based microstrip patch antenna at 0.72 THz resonant frequency for wireless communication. The proposed antenna shows 37.50% impedance bandwidth ranging from 0.53 to 0.84 THz at the center frequency of 0.72 THz. The result is demonstrated in terms of return loss (s11 < − 10 dB), voltage standing wave ratio (VSWR), input impedance, E-plane, and H-plane radiation pattern. The designing and simulation are performed using a full-wave electromagnetic simulator CST microwave studio based on the finite difference time domain method. The simulation output shows a minimal return loss − 59.97 dB, VSWR 1.007, and good radiation pattern at 0.72 THz resonant frequency, which would be an excellent candidate for future wireless communication as well as medical imaging, homeland defense system, explosive detection, and material characterization application.

Prismatic discontinuous Galerkin time domain method with an integrated generalized dispersion model for efficient optical metasurface analysis

Planar photonics technology is expected to facilitate new physics and enhanced functionality for a new generation of disruptive optical devices. To analyze such planar optical metasurfaces efficiently, we propose a prismatic discontinuous Galerkin time domain (DGTD) method with a generalized dispersive material (GDM) model to conduct the full-wave electromagnetic simulation of planar photonic nanostructures. Prism-based DGTD allows for triangular prismatic space discretization, which is optimal for planar geometries. In order to achieve an accurate universal model for arbitrary dispersive materials, the GDM model is integrated within the prism-based DGTD. As an advantage of prismatic spatial discretization, the prism-based DGTD with GDM has fewer elements than conventional tetrahedral methods, which in turn brings higher computational efficiency. Finally, the accuracy, convergence behavior, and efficiency improvements of the proposed algorithm is validated by several numerical examples. A simulation toolkit with the proposed algorithm has been released online, enabling users to efficiently analyze metasurfaces with customized pixel patterns.

Plasmonic Metamaterial-Based Label-Free Microfluidic Microwave Sensor for Aqueous Biological Applications

This paper reports a label-free, highly sensitive plasmonic metamaterial inspired multi-band planar microwave sensor for aqueous biological samples. The proposed sensor consists of a spoof surface whispering gallery mode (SS-WGM) resonator connected to a spoof surface plasmons polariton (SSPP) transmission line in a special arrangement. The SS-WGM resonator of the proposed sensor is capable of localizing the electromagnetic (EM) field into a specific region due to its slow-wave propagation characteristics. This feature enhances the interaction time of the sample under test (SUT) with EM wave and offers higher sensitivity. The EM wave localization enables the proposed design to sense the small volume of the bio-samples. This is required in the case of the aqueous samples because a large volume of SUT absorbs radio frequency (RF) signals and reduces the ${Q}$ -factor of the sensors. The microfluidic approach has been adopted as it supports a small sample size. The proposed sensor is numerically analyzed and optimized for multi-band capability using full-wave EM simulation. To ensure maximum sensitivity, the microfluidic channel is kept appropriately above the hot-spot of the sensor. Glucose aqueous solution is being used here as a biological sample. Experimental validation of the proposed approach has been done using different concentrations of the SUT. The proposed sensor offers a maximum measured sensitivity of 77.3e−02 MHz/mgml−1, which shows a fair improvement in the sensitivity as compared to the state-of-the-art. It is anticipated that the proposed microfluidic planar microwave sensor is paving the path for the development of the microwave-based modern lab-on-chip system arrangement.

Cascade domino lithography for extreme photon squeezing

Squeezing photons into deep sub-wavelength volumes and few-nanometer gaps has led to the investigation of interesting phenomena, including strong coupling, quantum plasmonics, nonlinearity enhancement, nonlocality, and molecular junctions. The common configuration of bowtie nanoantennas has been extensively studied owing to the great enhancement of the localized electromagnetic field. The enhancement rapidly increases as the tips become sharper and the gap becomes narrower. However, reliable fabrication of nanoantennas that are extremely sharp with sub-10 nm gaps is statistically prohibited due to the fundamental limitation of the proximity effect of electron beam lithography. Here, an intuitive “fall-to-rise” scheme is proposed and experimentally validated using a new concept of cascade domino lithography. In this report, we successfully establish a controllable lithography method of making extremely sharp bowtie nanoantennas with sub-1 nm radius of curvature reaching the size of a gold nanocluster as well as single-digit-nanometer gaps between such sharp tips. In addition, a proof-of-concept application of surface enhanced Raman spectroscopy is demonstrated along with rigorous full-wave electromagnetic simulations and numerical analysis. This control of falling nanostructures opens up an unexplored gateway towards conquering the limitations of experimentally exploring the realm of plasmonics down to the sub-nanometer regime.

Electron energy loss of ultraviolet plasmonic modes in aluminum nanodisks.

We theoretically investigated electron energy loss spectroscopy (EELS) of ultraviolet surface plasmon modes in aluminum nanodisks. Using full-wave Maxell electromagnetic simulations, we studied the impact of the diameter on the resonant modes of the nanodisks. We found that the mode behavior can be separately classified for two distinct cases: (1) flat nanodisks where the diameter is much larger than the thickness and (2) thick nanodisks where the diameter is comparable to the thickness. While the multipolar edge modes and breathing modes of flat nanostructures have previously been interpreted using intuitive, analytical models based on surface plasmon polariton (SPP) modes of a thin-film stack, it has been found that the true dispersion relation of the multipolar edge modes deviates significantly from the SPP dispersion relation. Here, we developed a modified intuitive model that uses effective wavelength theory to accurately model this dispersion relation with significantly less computational overhead compared to full-wave Maxwell electromagnetic simulations. However, for the case of thick nanodisks, this effective wavelength theory breaks down, and such intuitive models are no longer viable. We found that this is because some modes of the thick nanodisks carry a polar (i.e., out of the substrate plane or along the electron beam direction) dependence and cannot be simply categorized as radial breathing modes or angular (azimuthal) multipolar edge modes. This polar dependence leads to radiative losses, motivating the use of simultaneous EELS and cathodoluminescence measurements when experimentally investigating the complex mode behavior of thick nanostructures.

Design of a terahertz metamaterial sensor based on split ring resonator nested square ring resonator

A terahertz (THz) sensor based on the metamaterial structure, split ring resonator with four-gaps relied on centrosymmetric nested square ring resonator, is presented. The two resonant elements of the metamaterial structure generate a corresponding resonant valley on the transmission spectrum curve in the frequency range from 0.1 to 1.9 THz respectively, and both of these resonant valleys show different redshifts when the surface permittivity of the structure changes. This feature is very suitable for THz sensing, especially the quantum interference effect between the two resonant elements, which results in the formation of an electromagnetically induced transparency (EIT)-like resonance peak on the transmission spectrum curve. The sensing performances are simulated by using commercialized full-wave electromagnetic simulation software. The results demonstrated that the proposed sensor is polarization-insensitive and has a highly boosted sensitivity, which has a promising application prospect in the fields of biomedical science and drug industry.

Mode Switching With Waveguide-Coupled Plasmonic Nanogratings

Active photonic elements enable a wide range of on-chip optical applications, including enhanced light emission, integrated optical sensing and spectroscopy, photonic signal routing, and optical computing. The ability to select between different optical channels forms the basis for many of these systems. Both spectral and spatial selectivity of optical modes are desirable and together offer flexibility in tailoring the optical response of the system. In this work, we experimentally demonstrate a platform for the active switching of hybrid plasmonic-photonic Fano resonances. Our structure consists of gold nanogratings sandwiched between two dielectric thin films. Diffractive coupling into each layer is modulated by varying the refractive index above the structure. By controlling the interaction of the substrate and superstrate modes through refractive index tuning, we experimentally demonstrate on/off switching of the narrow (FWHM = 30-40 nm) Fano resonance transparency windows with up to 85% resonance contrast, and tunability across a wide spectral range (600-850 nm). Full-wave electromagnetic simulations support these results and reveal that the tuning of these modes enables both spectral and spatial selectivity.

Impact of Graphene Thickness on EM Modelling of Antenna

This paper presents illustrative electromagnetic modelling and simulation of graphene antenna using a two-dimensional graphene sheet of zero thickness and a three-dimensional graphene slab of finite thickness. The properties of the antenna are analyzed in terms of the S11 parameter, input impedance, VSWR, radiation pattern, and frequency reconfiguration using the full-wave electromagnetic simulator. Furthermore, this work numerically studies the modelling of graphene antenna using a three-dimensional graphene thin slab and the impact of graphene slab thickness on the performance of graphene antenna.