HEM11模磁绝缘线振荡器的高频分析

作为小型化和紧凑型的高功率微波源,磁绝缘线振荡器(MILO)在过去十几年里得到了广泛的研究和发展.在大多数研究中,最低的对称模一直被当作器件的主模.然而,由于结构的对称性或者电子发射均匀度不理想等原因,很容易激励起非对称模式.计算了MILO同轴结构中同时包含对称模和非对称模的本征方程.在此基础上,通过对结构的优化设计,提出了一种HEM₁₁模工作的MILO,并开展了原理性实验.在二极管的电压为480kV,电流为39kA条件下得到了功率为1.2GW,脉冲宽度为40ns的微波输出,功率转换 关键词： 磁绝缘线振荡器 高频特性 11模')" href="#">HEM₁₁模 开放腔     

Higher-order core mode resonances in a mechanically induced long-period holey fiber grating

We present the spectral analysis of higher-order core mode resonances in a long period holey fiber grating induced mechanically in an asymmetric holey fiber. Calculations based on a fast-Fourier transform mode solver shows that the mode resonances obtained experimentally correspond to the odd- and even-LP_1,1 core modes. Additionally, we analyze the twist and polarization response of these mode resonances in the long period holey fiber grating. The results obtained in this work are of great importance in the design of new all-fiber optical devices that involve couplings of higher order core modes in asymmetric holey fibers.     

非对称型Kerr类介质膜漏波导中的非线性导波

本文研究了特定边界条件下非对称型Kerr类介质膜漏波导中TE₀光波的传播特性。计算结果表明,介质膜厚存在一临界值。临界值之上没有导波存在,临界值之下能同时存在强场TE₀模和弱场TE₀模,它们具有不同的传播波数。这两个模的膜内传播功率、场强峰值位置随膜厚和光场频率的变化各不相同,色散关系也有极大差别。在对称极限之下,弱场TE₀模的色散为零。 关键词：     

Effects of tunneling coupling on plasmon modes in asymmetric double-quantum-well structures

The collective charge density excitations in asymmetric double-quantum-well (DQW) structures with different tunneling strengths are systematically studied. In particular, the damping properties of the plasmon modes in various tunneling strengths are investigated in detail. It is shown that plasmon modes in asymmetric DQW structures are quite different from those in symmetric DQW systems. In weak tunneling regime, an intra-subband mode ω _- with an acoustic-like dispersion relation which is damped in symmetric DQW structures arises and coexists with the optical-like mode ω ₊ while the inter-subband mode ω ₁₀ is highly damped. With the tunneling strength being increased, the ω ₁₀ branch gradually becomes undamped and emerges out of the (1-0) single-particle continuum, whereas the ω _- branch gradually approaches the (0-0) single-particle continuum. In intermediate coupling regime, these three branches of modes coexist undamped. In strong tunneling regime, ω _- enters the (0-0) single-particle continuum and becomes damped. Consequently, only the ω ₊ and ω ₁₀ modes exist in this regime. Received 10 July 2001 and Received in final form 17 September 2001     

Guided modes in asymmetric graphene waveguides

An asymmetric quantum well in graphene can act as a slab waveguide for electron waves in a manner analogous to the electromagnetic waves in dielectrics. Guided modes and the probability current density are analyzed in the graphene electron waveguide induced by asymmetric electrostatic potential. The modes in an asymmetric graphene waveguide include guided modes, “cover modes”, “substrate modes” and “radiation modes”. The conditions for a guided mode are quantified. It is found that the fundamental mode is absent when both the Klein tunneling and classical motion are present. The confinement of electrons for lower order mode is stronger than for higher order mode. We hope that these characteristics in asymmetric graphene waveguide can provide potential applications in graphene-based waveguide devices.     

Asymmetric propagation of the first order antisymmetric lamb wave in a tapered plate based on time domain analysis

The asymmetric propagation of the first order antisymmetric (A₁) Lamb wave in a tapered plate respectively carved with sharp bottom corner and round bottom corner is theoretically investigated. Through numerical simulation of A₁ Lamb wave in time domain, we find that when the thickness of the waveguide abruptly decreases to below the cut-off thickness, about half of the A₁ mode is converted into the fundamental symmetrical S₀ and antisymmetrical A₀ modes to pass through the defected region. Furthermore, the transmitted modes A₀ and S₀ are completely apart from each other and can be quantitatively evaluated. Conversely, when the thickness change is very smooth, most of the energy of A₁ Lamb wave is reflected back. It is the unique mode conversion behavior that leads to great transmission difference value of A₁ Lamb wave along the opposite directions. Finally, the influence of geometrical parameters on the transmission coefficient is also studied. The higher efficiency and proper working frequency range can be realized by adjusting the slope angle θ, height h ₁ and h ₂. The simple asymmetric systems will be potentially significant in applications of ultrasound diagnosis and therapy.     

Optical characteristic analysis and TE0 mode selection for asymmetric metal-clad waveguides

The optical characteristics of the asymmetric metal-clad waveguide are analysed, more accurate formulae are derived for calculating the propagation constants and loss coefficients of TE and TM modes. The computed results for Al/GaAs/AlGaAs waveguides are in good agreement with the numerical results, and are much more accurate than the results in some other papers. The method for selecting the TE₀ mode is presented and discussed.     

A Study of the Phase Transition in the System [Ag1−x(Na)x]NO3 by Infrared Spectroscopy

A complete infrared study of the mixed crystal system of AgNO₃ and NaNO₃ is carried out in the region 400–4000 cm^?1. The study includes internal fundamental normal vibrations of the NO^?₃ ion in the ordered and disordered phases of AgNO₃ at different values of the sodium concentration, the I.R. spectra, spectral band shape intensities, and frequencies of the internal modes as functions of the sodium ion concentration. Special attention is paid to bending mode, combination mode, asymmetric stretching mode, and over-tone. The rotational energy barrier is determined at different concentrations of the sodium ions in the system.     

The effect of the number of nodal diameters on non-linear interactions in two asymmetric vibration modes of a circular plate

In order to investigate the effect of the number of nodal diameters on non-linear interactions in asymmetric vibrations of a circular plate, a primary resonance of the plate is considered. The plate is assumed to have an internal resonance in which the ratio of the natural frequencies of two asymmetric modes is three to one. The response of the plate is expressed as an expansion in terms of the linear, free oscillation modes, and its amplitude is considered to be small but finite, and the method of multiple scales is used. In view of the corrected solvability conditions for the responses, it has been found that in order for the modes to interact, the ratio of the numbers of nodal diameters of two modes must be either three to one or one to one. In this study the one-to-one case, in which the modes have the same number of nodal diameters, is examined. The non-linear governing equations are reduced to a system of autonomous ordinary differential equations for amplitude and phase variables by means of the corrected solvability conditions. The steady state responses and their stability are determined by using this system. The result shows very complicated interactions between two modes by telling existence of non-vanishing amplitudes of the mode not directly excited.     

Mass asymmetry dependence of angular distribution in237Np(α29 and 44 MeV,f): single particle effect

Angular distributions of fission products have been measured as a function of mass asymmetry in the odd-Z ²³⁷Np(α_{29 and 44 MeV},f) system using a recoil-catcher technique and off-line gamma spectrometry. Higher angular anisotropies were observed for the asymmetric mode products compared to the symmetric mode products at both energies. Average anisotropies for individual modes are lower than those for neighbouring even-even fissioning nucleus²⁴²Pu due to odd-nucleon spin effect. Present data have been analysed according to the transition state model assuming two modes of fission with characteristic saddle-shapes, barriers and multichance fission probabilities. It is seen that angular distributions for the symmetric and asymmetric modes are decided at and well past the corresponding saddle points respectively. Odd-nucleon spin contribution (〈k ²〉) to the tilting mode variance have been deduced. For (²⁴¹)Am fission, 〈k ²〉 values for the asymmetric and symmetric modes are ≤ 14 and > 14 ?² respectively. The 〈k ²〉 value averaged over several nuclei from preactinide (²⁰¹Ti) to actinide (²⁴⁸Cf) is 11.5 ± 4.2 ?². Average 〈k ²〉 value is in close agreement with the theoretical estimate.     

CO2激光写入旋转折变型长周期光纤光栅的制作及理论分析

提出了一种光纤横截面折射率变化呈旋转非对称变化的长周期光纤光栅（R-LPFG） 结构,并利用多层圆波导理论和横截面折射率离散分析方法,结合模式耦合方程组和数值求解方法理论分析了这种光栅的模式耦合特征.理论分析表明R-LPFG纤芯基模主要与一阶非对称包层模发生耦合,当光栅旋转度逐渐变大时,R-LPFG基模会与一阶非对称包层模的奇模和偶模同时发生耦合,这就会使原来单一的谐振峰逐渐分裂成双峰,这是常规光栅类型所不具有的透射谱特征.由于R-LPFG的双峰来自同一对耦合模式,它们对温度的响应很相似,因此可利用双峰间 关键词： 光纤传感 光纤光栅 2激光')" href="#">CO₂激光 旋转折变     

非对称金属包覆左手介质平板波导的微扰分析

基于微扰方法分析非对称金属包覆左手介质平板波导的特性,给出了该波导的复有效折射率的一级近似解,并对波导的传输特性和损耗特性进行了数值模拟.结果表明：非对称金属包覆左手介质平板波导没有零阶模式|TE1模传播系数随波导厚度的增加迅速减小,损耗系数随波导厚度的增加快速增加,达到最大值后又迅速减小|非对称金属包覆左手介质平板波导传输特性相对于左手介质三层平板波导传输特性发生了很大改变,TM1模,TM2和 TE2模式以及更高阶模式在截止厚度附近出现双值现象|特别在二阶及高阶模式中,TE模式相对于TM模式具有高传输低损耗的特性,而以右手介质为芯层的金属包覆波导不具有这一特性.     

涂覆石墨烯的非对称并行电介质纳米线波导的模式特性分析

采用多级展开方法,对涂覆石墨烯的非对称并行电介质纳米线波导的模式特性进行了分析.首先对这种波导中的表面等离子模式进行分类,然后对七种低阶模式的有效折射率和传播长度随工作频率、几何结构参数和石墨烯费米能的依赖关系进行详细的分析.结果表明,通过改变工作频率、几何结构参数和石墨烯的费米能,可以在较大范围内调节模式的特性.与有限元法进行的对比表明,基于多级方法的半解析结果与有限元法的数值结果非常符合.研究结果可为涂覆石墨烯的非对称并行电介质纳米线的设计和制作提供一定的理论基础.     

Extension de la representation scalaire du champ aux fibres optiques à gaine finie

Résumé Le concept de mode LP introduit dans la théorie du guidage faible est étendu aux fibres optiques à gaine finie, pour les modes de gaine dont l'indice effectif reste voisin de l'indice de la gaine. Une étude plus précise montre ensuite comment les modes LP_mn dégénérés se scindent en modes HE_{m+1, n} et EH_{m–1, n} non dégénérés. On indique comment écrire la condition aux limites sur l'interface fibre/air pour la fonction scalaire qui décrit le champ; à l'ordre 0 (modes LP), il suffit d'annuler cette fonction.

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The LP mode concept, which was introduced in the weak guiding theory is extended to optical fibers with finite cladding, for cladding modes the effective index of which remains close to the cladding refractive index. A more accurate study shows how the degenerate LP_mn modes split into nondegenerate HE_{m+1, n} and EH_{m–1, n} modes. The boundary condition to be satisfied by the scalar field on the air/fibre interface is derived; for zeroth order (LP modes), this scalar field must be zero on this interface.

     

ASYMMETRIC NON-LINEAR FORCED VIBRATIONS OF FREE-EDGE CIRCULAR PLATES. PART 1: THEORY

In this article, a detailed study of the forced asymmetric non-linear vibrations of circular plates with a free edge is presented. The dynamic analogue of the von Kàrmàn equations is used to establish the governing equations. The plate displacement at a given point is expanded on the linear natural modes. The forcing is harmonic, with a frequency close to the natural frequency ω_kn of one asymmetric mode of the plate. Thus, the vibration is governed by the two degenerated modes corresponding to ω_kn, which are one-to-one internally resonant. An approximate analytical solution, using the method of multiple scales, is presented. Attention is focused on the case where one configuration which is not directly excited by the load gets energy through non-linear coupling with the other configuration. Slight imperfections of the plate are taken into account. Experimental validations are presented in the second part of this paper.     

Detailed I.R. Study of the Phase Transition in the System AgNO3-Fe(NO3)3

A complete infrared study of the mixed crystal system of AgNO₃ and Fe(NO₃)₃ is carried out in the region 400–4000 cm^?1. The study includes internal fundamental normal vibrations of the NO₃^? ion in the ordered and disordered phases of AgNO₃ at different values of the ferric concentration, I.R. spectra, spectral band shape intensities, and frequencies of the internal modes as functions of the ferric ion concentration. Special attention is paid to bending mode, combination mode, asymmetric stretching mode, and over-tone. The rotational energy barrier is determined at different concentration of the ferric ions in the system of AgNO₃-Fe(NO₃)₃.     

Plasmons in asymmetric double quantum well structures

We investigate the effects of spatial asymmetry, tunneling coupling, and exchange-correlation correction on the plasmon modes in asymmetric double quantum well (DQW) structures in a time-dependent local-density approximation. Special attention is paid to the properties of the ω _- mode which is always damped in symmetric DQW systems. In addition, the results on the spectral weight of the excitations are also presented. In general, all the modes carry finite spectral weights and should be observable in resonant inelastic light scattering experiments for the specified values of the parameters. Received 2 July 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: c412-1@aphy.iphy.ac.cn     

Corrosion and erosion monitoring in plates and pipes using constant group velocity Lamb wave inspection

Recent improvements in tomographic reconstruction techniques generated a renewed interest in short-range ultrasonic guided wave inspection for real-time monitoring of internal corrosion and erosion in pipes and other plate-like structures. Emerging evidence suggests that in most cases the fundamental asymmetric A₀ mode holds a distinct advantage over the earlier market leader fundamental symmetric S₀ mode. Most existing A₀ mode inspections operate at relatively low inspection frequencies where the mode is highly dispersive therefore very sensitive to variations in wall thickness. This paper examines the potential advantages of increasing the inspection frequency to the so-called constant group velocity (CGV) point where the group velocity remains essentially constant over a wide range of wall thickness variation, but the phase velocity is still dispersive enough to allow accurate wall thickness assessment from phase angle measurements. This paper shows that in the CGV region the crucial issue of temperature correction becomes especially simple, which is particularly beneficial when higher-order helical modes are also exploited for tomography. One disadvantage of working at such relatively high inspection frequency is that, as the slower A₀ mode becomes faster and less dispersive, the competing faster S₀ mode becomes slower and more dispersive. At higher inspection frequencies these modes cannot be separated any longer based on their vibration polarization only, which is mostly tangential for the S₀ mode while mostly normal for the A₀ at low frequencies, as the two modes become more similar as the frequency increases. Therefore, we propose a novel method for suppressing the unwanted S₀ mode based on the Poisson effect of the material by optimizing the angle of inclination of the equivalent transduction force of the Electromagnetic Acoustic Transducers (EMATs) used for generation and detection purposes.     

Oscillating Guided Modes in Graphene-Based Asymmetric Waveguides

We investigate the guided modes in monolayer graphene-based waveguides with asymmetric quantum well structure induced by unequal dc voltages. The dispersion relation for the guided modes is obtained analytically, the structures of the guided modes are discussed under three distinct cases. For the cases of the classical motion and the Klein tunneling, the asymmetric structure does not influence the mode structures dramatically compared with that in the symmetric waveguide. But for the mixing case of the former two, the mode structures and the motion characteristics for the electron and the hole exhibit different behaviors at same condition. The results may be helpful for the practical application of graphene-based quantum devices.