Mode characteristics of nested eccentric waveguides constructed by two cylindrical nanowires coated with graphene

A kind of nested eccentric waveguide constructed with two cylindrical nanowires coated with graphene was designed. The mode characteristics of this waveguide were studied using the multipole method. It was found that the three lowest modes (mode 0, mode 1 and mode 2) can be combined by the zero-order mode or/and the first-order modes of two single nanowires. Mode 0 has a higher figure of merit and the best performance among these modes within the parameter range of interest. The mode characteristics can be adjusted by changing the parameters of the waveguide. For example, the propagation length will be increased when the operating wavelength, the minimum spacing between the inner and outer cylinders, the inner cylinder radius and the Fermi energy are increased. However, when the outer cylinder radius, the dielectric constants of region I, or the dielectric constants of region III are increased, the opposite effect can be seen. These results are consistent with the results obtained using the finite element method (FEM). The waveguide structure designed in this paper is easy to fabricate and can be applied to the field of micro/nano sensing.     

Wave propagation in a rarefied plasma with finite Larmor radius

Wave propagation in a rarefied two-component plasma immersed in a uniform constant magnetic field has been discussed wherein the plasma pressure is assumed to be anisotropic owing to finite Larmor radius effect. It is shown that, for propagation along the external magnetic field, there exist two modes of wave propagation, namely, the gravitational mode and the hydromagnetic mode. The former is found to be independent of the magnetic field and hence of the Larmor radius, while the latter is appreciably influenced by the finite Larmor radius. On the other hand, for transverse propagation, there are three modes of wave propagation viz. the ion-sound mode, the electron-sound mode and the electromagnetic mode. It is shown that only the lowfrequency ion-sound mode is affected by the finite Larmor radius.     

无限流体中孔隙介质圆柱周向导波的传播特性

为研究无限大流体约束的孔隙圆柱中周向导波的传播规律,分析孔隙参数对导波传播特性的影响,建立了无限流体中孔隙介质圆柱的理论模型,利用孔隙介质弹性波动理论,建立了周向导波频散方程,通过数值模拟计算得到无限流体中孔隙介质圆柱的频散曲线,探讨了圆柱半径和孔隙参数对导波传播特性的影响,并对导波的衰减特性进行了分析;通过数值计算,得到了周向导波的时域波形,讨论了孔隙参数对波形的影响.结果表明,孔隙介质圆柱半径的改变影响圆柱尺度,孔隙度的改变影响孔隙介质中体声波的波速,都对周向导波频散曲线产生一定的影响,所得到的频散曲线特征及衰减曲线与时域波形吻合.研究结果对开展无限流体中孔隙介质圆柱的超声无损评价提供了一定的理论参考.     

Analysis of wave propagation in a thin composite cylinder with periodic axial and ring stiffeners using periodic structure theory

Wave propagation characteristics of a thin composite cylinder stiffened by periodically spaced ring frames and axial stringers are investigated by an analytical method using periodic structure theory. It is used for calculating propagation constants in axial and circumferential directions of the cylindrical shell subject to a given circumferential mode or axial half-wave number. The propagation constants corresponding to several different circumferential modes and/or half-wave numbers are combined to determine the vibrational energy ratios between adjacent basic structural elements of the two-dimensional periodic structure. Vibration analyses to validate the theoretical development have been carried out on sufficiently detailed finite element model of the same dimension and configuration as the stiffened cylinder and very good agreement is obtained between the analytical and the dense finite element results. The effects of shell material properties and the length of each periodic element on the wave propagation characteristics are also examined based on the current analytical approach.     

一维固-固结构圆柱声子晶体中弹性波的传输特性

利用一维固-固结构圆柱声子晶体中弹性波横向受限的条件,推导出弹性波在其中各个模式满足的关系式,利用它研究了弹性波各模式的特性.并利用转移矩阵研究了弹性波的传输特性随模式量子数和圆柱半径的变化规律.得出了一些一维固-固结构圆柱声子晶体的新特征,即弹性波的传输特性由模式量子数和圆柱半径决定. 关键词： 圆柱声子晶体 弹性波 受限 模式     

金属线栅笼代替空心导体圆柱对辐射波模拟器电磁特性的影响

主要关注空心导体圆柱的等效替代品-圆柱线栅笼的电磁特性,分别基于保角变换和等效电磁场理论进行了等效半径的推导,在此基础上运用电磁仿真软件CST仿真分析了金属线栅的疏密和粗细程度对测试区电场波形的上升沿、脉宽和峰值的影响,并且对比了圆柱线栅笼和空心导体圆柱两种结构的电场分布。分析结果显示,双锥笼型结构完全能够等效代替空心导体圆柱,且进一步验证了等效半径理论的正确性。     

An improved perfectly matched layer for the eigenmode expansion technique

When performing optical simulations for rotationally symmetric geometries using the eigenmode expansion technique, it is necessary to place the geometry under investigation inside a cylinder with perfectly conducting walls. The parasitic reflections at the boundary of the computational domain can be suppressed by introducing a perfectly matched layer (PML) using e.g. complex coordinate stretching of the cylinder radius. However, the traditional PML suffers from an artificial field divergence limiting its usefulness. We show that the choice of a constant cylinder radius leads to mode profiles with exponentially increasing field amplitudes resulting in numerical instability. As a remedy we propose an improved PML based on a mode-dependent cylinder radius and mode profiles with stable field amplitudes. The new PML formulation eliminates the artificial field divergence and ensures numerical stability.     

Propagation in a warm collisional magnetoplasma enclosed in aconducting cylinder

A study is presented of the propagation of electron plasma waves in a warm collisional plasma filling a conducting cylinder and magnetized strongly in the axial direction, the plasma parameters being taken to be such that the electron-ion collision is the dominant damping process. The attenuation and phase constants are derived in suitably normalized form by using the electrodynamic method. The dispersion and attenuation characteristics for propagation in the lowest order mode in a hydrogen plasma are obtained for various values of the normalized plasma radius αω_pe/c, the electron temperature and electron density being held constant, and the characteristics are compared with those given by the usual quasi-static approximation, which is found to be valid only if αω_pe /c≲1, where α is the plasma radius, ω _pe is the plasma frequency, and c is the velocity of light in free space. The effect of the plasma frequency on the characteristics is investigated     

Wave diffraction by a concave statistically rough surface

We consider a statistically rough impedance surface that is concave on average in contrast to a plane. Backscattering from such a surface is considered based on the small perturbation theory method. The diffraction problem is divided into two parts which are considered separately: the problem of scattering by small roughness (assumed to be local) and the propagation of incident and scattered fields over a smooth large-scale concave surface. In contrast to the 'two-scale' scattering model, the zero-order unperturbed wavefield is not assumed to be specularly reflected from the local tangent plane to the smooth surface, but it is a solution of a corresponding diffraction problem. Two particular cases of smooth surfaces are considered: first, the inner surface of a concave cylinder with a constant radius and finite angular pattern, and second, a compound surface that consists of a coupled half-plane and the cylindrical surface mentioned above. In a geometrical optics limit and with propagation at low grazing angles, the analytical results for a zero-order (unperturbed) field are obtained for these two cases in the form of a series over multiple specular reflected fields. It is shown that these non-local processes lead to the essential increase in the backscattering cross section in comparison with the two-scale model and tangent-plane approach.     

Electron on a cylinder with topological defects in a homogeneous magnetic field

In this work we study the effects of the geometry and topology of a cylinder on the energy levels of an electron moving in a homogeneous magnetic field. We consider the existence of topological defects as a screw dislocation and a disclination. When we take the region of movement as the full cylindrical surface, we find that, by increasing the strength of the screw dislocation, the dispersion on the electronic energy levels is affected and monotonically increasing. For an electron moving in an almost flat region we show that the dispersion on the Landau levels decrease monotonically as we increase the strength of the screw dislocation. The lowest Landau level can reach a zero value, leaving the energy of the system solely given by the geometry of the cylinder, which does not depend on the magnetic field. In both situations, as we change the deficit angle of the disclination, we observe that the energy levels are shifted and the magnitude of such shift depends on the magnetic field. The Landau levels for a flat sample are recovered in the limit of an infinite cylinder radius.     

EPR Uniform Field Signal Enhancement by Dielectric Tubes in Cavities

The dielectric tube resonator (DTR) for electron paramagnetic resonance spectroscopy is introduced. It is defined as a metallic cylindrical TE₀₁₁ microwave cavity that contains a dielectric tube centered on the axis of the cylinder. Contour plots of dimensions of the metallic cylinder to achieve resonance at 9.5 GHz are shown for quartz, sapphire, and rutile tubes as a function of wall thickness and average radius. These contour plots were developed using analytical equations and confirmed by finite-element modeling. They can be used in two ways: design of the metallic cylinder for use at 9.5 GHz that incorporates a readily available tube such as a sapphire tube intended for NMR and design of a custom procured tube for optimized performance for specific sample-size constraints. The charts extend to the limiting condition where the dielectric fills the tube. However, the structure at this limit is not a dielectric resonator due to the metal wall and does not radiate. In addition, the uniform field (UF) DTR is introduced. Development of the UF resonator starting with a DTR is shown. The diameter of the tube remains constant along the cavity axis, and the diameter of the cylindrical metallic enclosure increases at the ends of the cavity to satisfy the uniform field condition. This structure has advantages over the previously developed UF TE₀₁₁ resonators: higher resonator efficiency parameter Λ, convenient overall size when using sapphire tubes, and higher quality data for small samples. The DTR and UF DTR structures fill the gap between free space and dielectric resonator limits in a continuous manner.     

Wave diffraction by a concave statistically rough surface

Abstract

We consider a statistically rough impedance surface that is concave on average in contrast to a plane. Backscattering from such a surface is considered based on the small perturbation theory method. The diffraction problem is divided into two parts which are considered separately: the problem of scattering by small roughness (assumed to be local) and the propagation of incident and scattered fields over a smooth large-scale concave surface. In contrast to the ‘two-scale’ scattering model, the zero-order unperturbed wavefield is not assumed to be specularly reflected from the local tangent plane to the smooth surface, but it is a solution of a corresponding diffraction problem. Two particular cases of smooth surfaces are considered: first, the inner surface of a concave cylinder with a constant radius and finite angular pattern, and second, a compound surface that consists of a coupled half-plane and the cylindrical surface mentioned above. In a geometrical optics limit and with propagation at low grazing angles, the analytical results for a zero-order (unperturbed) field are obtained for these two cases in the form of a series over multiple specular reflected fields. It is shown that these non-local processes lead to the essential increase in the backscattering cross section in comparison with the two-scale model and tangent-plane approach.     

EM Wave Propagation in Ferrite-Filled Transversely Magnetized Circular Waveguide

This paper presents a rigorous method of solving Maxwell's equations for ferrite-filled transversely magnetized circular waveguide. Solutions for cylindrical coordinate system are derived by using power series expansion of the field's potentials. Complex propagation constants are obtained by applying boundary conditions and representing fields in partial cylindrical modes. Plane wave representation of propagation is also considered. Numerical calculations of the propagation constants as well as the dominant waveguide mode reflection and transmission from a ferrite-filled waveguide section are presented.     

THz波段介质涂敷空芯金属圆波导的传输特性

针对THz波段介质涂敷空芯金属圆波导传输特性的精确分析问题,基于波导中场方程及边界条件建立关于传播常数的特征方程,并且采用改进的Muller法求解特征方程得到涂敷圆波导主模HE11模的传播常数。计算中对THz波段的非理想导体电导率采用经典弛豫效应模型。仿真结果表明:内径为1.8 mm的银波导,当聚苯乙烯涂敷层厚度为17 m时,HE11模在1.5~3.0 THz的衰减常数在1 dB/m以下,且具有较好的色散特性;内径为2.2 mm的银波导,在2.5 THz时其衰减常数随涂敷层厚度的增加先增大后减小,且存在最佳介质涂敷层厚度,可实现THz波低损耗传输。     

Guided wave propagation and mode differentiation in hollow cylinders with viscoelastic coatings

Guided wave propagation theories have been widely explored for about one century. Earlier theories on single-layer elastic hollow cylinders have been very beneficial for practical nondestructive testing on piping and tubing systems. Guided wave flexural (nonaxisymmetric) modes in cylinders can be generated by a partial source loading or any nonaxisymmetric discontinuity. They are especially important for guided wave mode control and defect analysis. Previous investigations on guided wave propagation in multilayered hollow cylindrical structures mostly concentrate on the axisymmetric wave mode characteristics. In this paper, the problem of guided wave propagation in free hollow cylinders with viscoelastic coatings is solved by a semianalytical finite element (SAFE) method. Guided wave dispersion curves and attenuation characteristics for both axisymmetric and flexural modes are presented. Due to the fact that dispersion curve modes obtained from SAFE calculations are difficult to differentiate from each other, a mode sorting method is established to distinguish modes by their orthogonality. Theoretical proof of the orthogonality between guided wave modes in a viscoelastic coated hollow cylinder is provided. Wave structures are also calculated and discussed in view of wave mechanics in multilayered cylindrical structures containing viscoelastic materials.     

部分填充手征等离子体波导的电磁波

 把麦克斯韦方程和电磁场分解成横向和纵向分量, 导出了简明的一般性的手征等离子体填充的任意截面波导的波动方程和横向纵向场的关系式。对于部分填充手征等离子体波导, 导出了它的色散方程。这一方程的数值解给出了HE₁₁, EH₀₁和HE₂₁模式的色散图。随着手征等离子体填充面积的增大, EH₀₁模式的色散曲线向更高的频率移动。随着手征导纳的增加, 传播常数减小。     

Polaron effects and boundary conditions in cylindrical wires

In this work we show that the polaron effects in cylindrical quantum wires are function of the cylinder radius R₀ through the boundary conditions for both the ionic and the electronic motion and through the size dependence of the static and high frequency dielectric constants. We find that the dielectric constants are increasing functions of R₀. This fact and the different boundary conditions for the ions and the electrons have the final consequence that polaron self-energy can either be an increasing or a decreasing function of R₀.     

Ring microresonator as a photonic structure with complex eigenfrequency

For the analysis of light propagation in photonic guided-wave ring microresonators, leaky-mode solvers for bent channel waveguides are often used. In the analytical approach, the leaky mode field is expressed in terms of cylindrical functions of complex arguments and a complex order which plays a role of the (azimuthal) propagation constant. In this contribution we present a modified approach which takes into account the circular symmetry of the structure. We calculate the eigenmodes of a lossy microresonator as modes with complex eigenfrequencies. In this approach, the eigenmodes are described by cylindrical functions of an integer order and a complex argument. Similarities and differences of both approaches are demonstrated using simple examples of 2D (planar) structures.     

Average scattered field from a random PEC cylinder buried below a slightly rough surface

Scattering from a perfect electric conducting cylinder with random radius buried below a half space dielectric homogenous interface is studied. The cylindrical wave scattered by cylinder is expanded in terms of plane wave spectrum. Small perturbation method is used to study the interaction of each plane wave with the interface. The zeroth order term yields solution for a flat interface, whereas scattering from a rough surface is given by first-order term. Results are obtained for both TM and TE polarizations. Analytical expressions of the average scattered field are obtained and verified using numerical evaluation. Different scattering scenarios are simulated by varying the distribution of the radius. It is observed that average scattering cross section of an ensemble with normal/uniform distribution is almost equal to that of a cylinder with mean radius.     

Effect of symmetrical boundary conditions on the vibration of thick hollow cylinders

A finite element for axisymmetric elasticity is formulated directly in cylindrical coordinates. The element is used to study the vibration of hollow, isotropic finite length cylinders. The ratio of the inside radius to the outside radius is assumed as 0.2, 0.5 and 0.9 for a given cylinder length. Frequencies are computed for free-free end boundary conditions that can be compared with the existing literature. Additionally, fixed-fixed boundary conditions are studied. Five different configurations for simple supported boundary conditions are analyzed and compared. Numerical results are given in tabular form and some fundamental mode shapes are compared graphically.