Full-wave Numerical Technique Research Articles

Numerical techniques of the estimation the amount of waveguides surface roughness for the THz regime

The article is devoted to the problem of the efficient design of THz waveguides devices. Extreme small sizes of THz devices such as subharmonic pumped mixers for radiometer witch waveguides are integrated in copper block covered with 3 μm gold are presented. The three numerical techniques and direct measurements in the THz regime have been used to predict the effective conductivity reduction due to protrusions. Efficient design of THz systems requires the ability to predict effective conductivity, allowing engineers to determine the amount of surface roughness for acceptable reflection losses prior to fabrication. Full-wave numerical technique to predict the field enhancement factors for both the rf electric field and the rf magnetic field on the protrusion above smooth sample in the THz regime has been mentioned. The electric field and magnetic field enhancement factors on the hemispherical protrusion should be excluded in case the protrusions size are smaller than δ/50 and δ/1.5 correspondingly (δ is skin depth). Mie-Scattering-based approach approximates a unit cell of the grating as a circular cylinder protrusion the additional loss due to the protrusion compared with a perfectly flat surface of the same material. Hammerstad–Bekkadal Model demonstrates an analytical function which depends of sheet resistance for a sample with surface roughness on the surface roughness and skin depth of the metal. Numerical calculations using this technique predict 10% power absorption due to surface features 35 nm at 400 GHz. A layout of the apparatus for direct measurements of the effective conductivity of samples with roughness features using an open quasi-optical resonator is given. The results of theoretical calculation and empirical evaluation have been compared with computer simulation software taking into consideration of the effective conductivity only due to the change in the surface geometry. The comparison empirical results and numerical calculations full-wave numerical technique are more accurate for samples that are smooth relative to the skin depth. For surface features greater than the skin depth, we found the Hammerstad and Bekkadal model to be a better. The observed methods can be used as a rapid means to estimate waveguides surface roughness in terms of power loss in the THz regime.

Approach to the analysis and synthesis of cylindrical metasurfaces with noncircular cross sections based on conformal transformations

We present methods for analyzing and designing cylindrical electromagnetic metasurfaces with non-circular cross sections based on conformal transformations. It can be difficult to treat surfaces with non-canonical geometries since they generally do not admit straightforward solutions to the Helmholtz wave equation subject to the appropriate boundary conditions. This leads to the reliance on full wave numerical techniques which are only suitable for the analysis, but not the synthesis, of these surfaces. We address this issue by employing conformal transformations to map the physical space into a computational space in which the surface coincides with a circular cylinder. The electromagnetic boundary conditions on the surface remain intact under the transformations due to their angle-preserving nature. However, they are much more easily enforced. As a result, analytical modal solutions for the scattered fields are readily obtainable, which facilitate closed-form analysis and synthesis equations for general non-circular cylindrical metasurfaces. One important utility enabled by the proposed framework is the efficient identification of electromagnetic field distributions that satisfy local power conservation. This leads to passive and lossless surface designs, which are highly desirable in practice as they do not require active and/or lossy components.

Design of symmetrical wide-angle graphene-based mid-infrared broadband perfect absorber based on circuit model

A mid-infrared, polarization insensitive, broadband, and wide-angle graphene-based absorber exploiting arrays of nano-disks is proposed. The interaction between arrays of graphene disks provides broadband absorption over 90% from 21 to 40 THz. Circuit model and impedance matching concept are used to estimate the dimensions of the proposed absorber while the results are verified using Full-wave numerical techniques. A well-defined design procedure is proposed which is promising method to be expanded for the design of similar multi-layer absorbers. Compared with previously reported broadband absorbers, a broadband absorption of 63.3% with respect to the central frequency is achieved using a simple symmetrical structure suggested. Compared with Full-wave numerical techniques, the proposed approach not only dramatically decreases the simulation time, but it also immensely reduces the requirement of memory for simulations. The proposed structure is a multi-layer one, thus, it can be manufactured by chemical vapor deposition (CVD).

Characterization of Nematic Liquid Crystal at Microwave Frequencies Using Split-Cylinder Resonator Method

Liquid crystal (LC) is an anisotropic liquid material, which flows like a liquid, but at the same time, its molecules have an orientational order like in the solid state. Thus, LC is a promising dielectric material for designing reconfigurable devices at microwave frequencies. In order to optimize the design of reconfigurable microwave devices, accurate values of the dielectric permittivity and the loss tangent of LCs are needed. However, new LCs are not well characterized at these frequencies because of its recent use for microwave applications. Therefore, the characterization in this frequency range is required for its practical use within microwave components and devices. In this paper, a split-cylinder resonator method is used for the characterization of four different nematic LCs at two frequency points, i.e., 5 and 11 GHz. This characterization includes the extraction of their complex dielectric permittivity values at these frequencies. The employed method allows to obtain the two extreme permittivity values without applying any external electric or magnetic field to polarize the LC molecules. Two different approaches, a modal analysis method and a full-wave numerical technique, have been used for determining the LC parameters obtaining similar results in both cases.

Sound-Source Localization in Range-Dependent Shallow-Water Environments Using a Four-Layer Model

Sound-source localization in shallow water is a difficult task due to the complicated environment, e.g., complex sound-speed profile and irregular water bottom reflections. Full-wave numerical techniques are currently able to accurately simulate the propagation of sound waves in such complex environments. However, the source localization problem, which generally involves a large number of sound propagation calculations, still requires a fast computation of the wave equation, and thus a simplified model is well advised. In this paper, a four-layer model is considered, which is able to approximate a wide range of shallow-water environments, particularly those in summer conditions. More specifically, the medium is assumed to be horizontally stratified and vertically divided into four layers, and the sound speed in each layer is assumed to be constant or varying linearly. Under this assumption, the wave propagation can be rapidly computed via a classical wave number integration method. The main contribution of this paper is to show the suitability of the four-layer model in terms of source localization in a complex (range-dependent) environment. The sound-speed profile is assumed to be vertically irregular and horizontally slowly varying and the bottom is nonflat. In the forward problem, sound propagation in complex underwater environments is simulated via a time-domain full-wave simulation approach called the spectral-element method. The source localization error due to model imprecision is analyzed.

An UTD-Analysis of Electromagnetic Scattering From Periodic Array Structures With a Straight Truncation Boundary

The uniform geometrical theory of diffraction (UTD)-type analysis of electromagnetic (EM) scattering from an array of periodic structures is presented, whose solutions were developed by an extension from the analogous 2-D EM scattering problem. The array is semi-infinite along one of its two dimensions. The scattering fields are interpreted by the diffraction mechanisms of the UTD framework including the reflection, transmission, and truncation diffractions. This technique is particularly useful to analyze the EM properties of frequency-selective surfaces and metamaterials composed of periodic structures with identical elements by an integration of full-wave numerical techniques to analyze the array elements. Thus, it may treat a relatively flexible elemental structure and makes the solution very useful in practical applications. Numerical examples are presented to demonstrate the applicability of these solutions.

Global Modeling of Active Terahertz Plasmonic Devices

In this study, a full wave numerical technique is employed to characterize the propagation properties of 2-D plasmons along two-dimensional electron gas (2DEG) layers of biased hetero-structures at terahertz frequencies. This method is based on a coupled solution of Maxwell and hydrodynamic transport equations. In this manner, a complete description of carrier-wave interactions inside the 2DEG layer is obtained. Particularly, this simulator is employed to investigate the 2-D plasmon variations initiated by the application of an external bias along the hetero-structure. Substantial changes in the plasmon characteristics such as wavelength and decay length are reported. It is also revealed that two symmetrical plasmonic modes along the unbiased 2DEG layer split into new asymmetrical ones after applying the bias voltage. The simulation has been performed in different structures to examine the effects of various electron densities and the presence of periodic metallic gratings on the plasmon properties. Moreover, the 2-D plasmon reflections from boundaries terminated by ohmic contacts are separately studied. This research demonstrates the potentials of the 2-D conductors in the design of novel active terahertz plasmonic devices as modulators and amplifiers while proposing a new approach for their modeling. The results of this simulation are verified independently with an analytical model.

MICROWAVE-INDUCED THERMO-ACOUSTIC TOMOGRAPHY SYSTEM USING TRM-PSTD TECHNIQUE

Time reversal imaging method based on full wave numerical technique for likely breast tumors biological tissue in the Microwave-Induced Thermo-Acoustic Tomography (MITAT) system is discussed. In this paper, the mechanism of microwave-induced thermo-acoustic is strictly described based on thermodynamics and thermo-difiusion principles; the equivalent relationship between the absorbed microwave energy distribution of the biological tissue and the induced thermo-acoustic source distribution is used as the basis of the imaging algorithm. Due to its unique noise suppression feature and the stability of the algorithm, the Time Reversal Method (TRM) based on the Pseudospectral Time-Domain (PSTD) technique is applied to image heterogeneous phantom tissues from low Signal-to-Noise-Ratio (SNR) thermo-acoustic signals. Thereafter, an integrated MITAT prototype system is presented to obtain the thermo-acoustic signals from some biologic tissue with millimeter scale. The proposed TRM method is based on PSTD technique produced two-dimensional images, presented to study the performances of the MITAT in terms of contrast and resolution. These images prove predominant advantages in both contrast and resolution compared with conventional microwave and ultrasound imaging systems for malignant tumor detection. Based on the current results, our TRM-PSTD MITAT system provides evidence to predict breast tumor in an early stage and millimeter scale.

Conformal cubical 3D transformation-based metamaterial invisibility cloak

A conformal cubical transformation-based metamaterial invisibility cloak is presented and verified, in the near and the far field, by a rigorous full-wave numerical technique based on a higher-order, large-domain finite element method, employing large anisotropic, continuously inhomogeneous generalized hexahedral finite elements, with no need for discretization of the permittivity and permeability profiles of the cloak. The analysis requires about 30 times fewer unknowns than with commercial software. To our knowledge, this is the first conformal cubical cloak and the first full-wave computational characterization of such a structure with sharp edges. The presented methodology can also be used in development of conformal, transformation-based perfectly matched layers.

TRANSMISSION PHASE CHARACTERIZATIONS OF METAMATERIAL COVERS FOR ANTENNA APPLICATION

Metamaterial covers exhibit inimitable electromagnetic properties which make them popular in antenna engineering. A traditional metamaterial cover has an identical transmission phase for a normally incident plane wave regardless of its polarization state. The purpose of this research is to show the possibility of using a polarization dependent metamaterial cover to change the polarization state of the incident plane wave. Novel polarization- dependent metamaterial (PDMTM) covers, whose transmission phases for two principal polarizations are difierent, are presented. A full-wave FDTD numerical technique developed by the authors is adopted for the simulations.

Numerical study of photonic crystals with a split band edge: Polarization dependence and sensitivity analysis

We examine photonic crystals (PhC) made of periodic stacks of anisotropic dielectric layers with a split band edge (SBE) on the band diagram. Just below the band edge frequency, the dispersion relation $\ensuremath{\omega}(k)$ of SBE PhCs can be approximated as a linear combination of a quadratic term and a quartic term. This is in contrast to regular (conventional) band edge PhCs, which produce a quadratic dispersion relation, and to degenerate band edge (DBE) PhCs, which produce a quartic dispersion relation. Finite-size DBE PhCs and SBE PhCs are of interest because they both can support slow-wave Fabry-Perot resonances with very good transmittance. One of the most significant differences between DBE and SBE is that the transmittance of the former depends on the incident wave polarization whereas in the latter it does not. In this work, we investigate the transmittance behavior of SBE PhCs and perform a sensitivity analysis of their responses against geometrical and/or material perturbations. Dielectric losses, thickness perturbations, and misalignment angle perturbations are considered. The analysis uses a full-wave numerical technique for transient Maxwell's equations in inhomogeneous anisotropic media based on a complex-envelope alternating-direction-implicit finite-difference time-domain. A comparison is also made between the sensitivity of the SBE PhCs response versus that of DBE PhCs.

Broadband Rational Macromodeling Based on the Adaptive Frequency Sampling Algorithm and the Partial Element Equivalent Circuit Method

The increasing operating frequencies in modern designs call for broadband macromodeling techniques. The problem of computing high-accuracy simulation models for high-speed interconnects is of great importance in the modeling arena. Nowadays, many full-wave numerical techniques are available that provide high accuracy, often at a significant cost in terms of memory storage and computing time. Furthermore, designers are usually only interested in a few electrical quantities such as port voltages and currents. So, model order reduction techniques are commonly used to achieve accurate results in a reasonable time. This paper presents a new technique, based on the partial element equivalent circuit method, which allows to generate reduced-order models by adaptively selecting the complexity (order) of the macromodel and suitable frequency samples. Thus, the proposed algorithm allows to limit the computing time while preserving the accuracy. Validation examples are given.

Design Optimization of $Z$-Cut Lithium Niobate Electrooptic Modulator With Profiled Metal Electrodes and Waveguides

Analysis and optimization of an ultra-high-speed Z-cut lithium niobate (LN) electrooptic modulator, operating at a high-frequency region, by using a full-wave finite-element numerical technique, has been demonstrated. Investigation of the effects of adjusting the buffer layer thickness, the electrode height, the electrode trapezoidal profile, and the waveguide trapezoidal profile on the microwave effective index N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> , the characteristic impedance Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , the microwave losses alpha <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , and the half-wave voltage-length product V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pi</sub> L has been reported. Optimization of these parameters yield to a novel design of the LN modulator, with a low V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pi</sub> L and a high bandwidth, operating at a frequency range between 1 and 100 GHz. The frequency-dependent dispersion of the key device parameters, with the aim of determining the device suitability for high-speed operation, has been demonstrated.

Effects of superstrates on the resonant frequency of rectangular microstrip antennas

AbstractThis article presents the extended expression to calculate the environmental effect on resonant frequency of multilayer rectangular microstrip antennas. The expression has been investigated for effective dielectric constant εeff and resonant frequency fr. The effective dielectric constant εeff of Present Method has been compared with full wave numerical technique with and without loading. These are in good agreement within 1% accuracy. It has been observed that % change in fr due to cover and substrate thickness is around 3.2% and 20%, respectively, as their thickness is varying 19 times of its initial value. This variation is drastic for h3/h2 = 0.01. The maximum % variation in fr for one substrate and two superstrate is around 5% as h4 varies from 1 mm to 5 mm. In case of unwanted air (h3), the % variation in fr of present method in comparison to experimental value is less than 1.0% for small air‐gap and 0.4% for large air‐gap for various spaced dielectric (εr4), which reveals that the effect of superstrate in fr is small than substrate above the ground plane. The resonant frequency in suspended substrate microstrip antenna with and without superstrate is 21.8% and 28.15%, respectively, as h1/h2 varies from 0.065 to 1.94. The resonant frequency of all the possible configuration is in good agreement with the experimental results. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2946–2950, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22954