声子晶体的负折射

通过对声子晶体负折射的实验和理论进行梳理,提出可以通过从对称性角度理解声波在周期性结构中的散射,来理解声学负折射现象的物理机制.在此启发下,本文继而运用有限元模拟,通过改变背景介质的声学常数,成功展示了声波在声子晶体中的负折射现象.     

新型功能材料——声子晶体

声子晶体是20世纪90年代初提出的一种新型声学功能材料，这种周期性弹性结构具有许多重要性质，如声波带隙特性，即处于禁带频率范围内的振动或声波将被禁止在晶体中传播。通过求解声波在晶体中的运动方程可以设计一定的声子禁带和允带，而声子禁带与声波异质结构中声子的安德森局域化问题密切相关。文章重点阐述了声子晶体的主要特征、理论研究方法、潜在应用及前景展望。     

混合物介质中声波尾波成因的随机过程分析

该文基于声波在混合物介质中传播时反射及散射的随机特性，把混合物介质抽象为三维各向同性的马尔科夫链，把声波在混合物介质中传播过程抽象为声波在三维马尔科夫链中以声速进行“随机游走”的随机过程。用空间内某点接收到声波的概率类比该点接收波振幅，以声波到达该点所走过的步数类比接收波时域曲线的时间。此理论模型可较好解释声波在混合物介质中传播时“峰波延后”及“尾波”等现象。     

二维周期性复合介质中声波带隙结构及其缺陷态

用平面波展开方法和超元胞方法,研究了钢柱体在水中按周期性排列时声波的能带结构.讨论了钢柱体的横截面形状(圆形和正方形)、柱体的体积填充率及缺陷等因素对声波带隙结构的影响. 关键词： 声波 带隙 缺陷态     

重复脉冲泵浦管状固体激光器的介质温升

 对重复脉冲泵浦的管状YAG激光器的3维温度分布进行了深入研究，计算模拟了管状介质在不同时刻的3维动态温升分布。计算结果表明，泵浦初始阶段，介质中各点的瞬态温升均随泵浦脉冲个数呈锯齿形增长，最大温升点的位置由介质内壁随时间逐渐向介质内部移动；经过大约490个脉冲泵浦后，介质内的瞬态温升最终随时间呈重复周期性变化，最大温升点将不再移动。当温升分布呈重复周期性变化后，激光管中心部分的温升较大，内外壁的温升较小，且外壁温升低于内壁温升。     

黄土层内的声波传播衰减

通过对声波在黄土层内传播的实验研究，本文分析了黄土介质中声波传播的强烈衰耗特性，得出了声波的衰减，这可为浅层地下声探设备的研制提供依据，黄土的声学特性依赖于其结构形状等多种因素而有所不同，而我们的实验环境和所获数据有限，因而我们的研究结果只是该问题的一个特例。     

周期性管柱信道的声波通信技术研究综述

井下声波通信技术利用周期性管柱信道实时传输声波数据，相比于其他通信方式，具有适应性好、复杂度低、传输效率高等优点，在随钻地质导向、油水井监测等方面有广阔的应用前景，是发展智能油田的关键技术之一。本文主要综述上世纪90年代以来关于信道物理模型、信道容量、声信号调制与接收方法等关键问题的研究历程，并对通信系统的设计要点与国内外相关产品的现场应用进行梳理，为后续的技术改进提供依据。     

关于声波教学中的几个关系

在声波的教学中,由于学生知识结构的关系,对于声波性质的了解有一定的欠缺,为了让学生充分认识声波的特性,在实际教学过程中,通过对如下几个关系的认识,学生对知识的了解将会更加清晰、明了.1 声波的传播与波速的关系 在教材中,列出了声波在各种不同介质中的传播速度,以及它们随温度变化的情况,     

双沃拉斯顿棱镜光强分束比精确分析

利用折射定律,介质膜两侧折射率不同时多光束干涉理论和菲涅耳公式,精确推导了双沃拉斯顿棱镜的光强分束比的具体表达式。以公式为基础,通过Matlab软件数值模拟作图分析光强分束比随入射角、入射波长和结构角的变化关系曲线。结果表明:在棱镜为介质胶合型时,光强分束比随入射角和入射波长的变化很小,光强分束比基本为1;棱镜为空气胶合型时,光强分束比随入射角,结构角和波长的变化很大。两种情况下,光强分束比随各参量的变化基本呈周期性变化。     

颗粒样品形变对声波传播影响的实验探究

为了更好地理解颗粒间接触结构的变化对通过颗粒介质中的声波的影响,本文利用单轴压缩实验,通过一系列增加的轴向压力使样品塑性应变不断增大,这在颗粒尺度上对应于颗粒间接触结构的改变.我们测量了此过程中通过颗粒样品的声波变化,结果表明颗粒体系内接触结构的变化对声波波形中的非相干波部分和频率有明显的影响,并且在样品接触结构变化的初始阶段声速是偏离有效介质理论的预测的.     

强迫布鲁塞尔振子中从准周期运动到混沌态的过渡

我们在周期外力作用下的布鲁塞尔振子中发现了从准周期行为到混沌态的过渡。这样,强迫布鲁塞尔振子成为数值实验中第一个同时看到各种通向混沌的道路并存的模型,而这是与某些流体力学实验一致的。原来光滑的周期采样图在局部起皱折迭,同时准周期的功率谱中出现越来越多的噪声背景,这种过渡方式与人们在圆周映象中讨论的准周期轨道全局失去光滑性成为对比,可能在非线性微分方程所描述的系统中更具有代表性。 关键词：     

Photonic properties of metallic-mean quasiperiodic chains

The light propagation through a stack of two media with different refractive indices, which are aligned according to different quasiperiodic sequences determined by metallic means, is studied using the transfer matrix method. The focus lies on the investigation of the influence of the underlying quasiperiodic sequence as well as the dependence of the transmission on the frequency, the incidence angle of the light wave and different ratios of the refractive indices. In contrast to a periodically aligned stack we find complete transmission for the quasiperiodic systems for a wide range of different refractive indices for small incidence angles. Additional bands of moderate transmission occur for frequencies in the range of the photonic band gaps of the periodic system. Further, for fixed indices of refraction we find a range of almost perfect transmission for angles close to the angle of total reflection, which is caused by the bending of photonic transmission bands towards higher frequencies for increasing incidence angles. Comparing with the results of a periodic stack the quasiperiodicity seems to have only an influence in the region around the midgap frequency of a periodic stack.     

SH wave propagation in magnetic–electric periodically layered plates

In this paper, dispersive behavior and band structure of SH waves in magnetic–electric (ME) periodically layered plate are investigated by the transfer matrix method. Two kinds of unit cell form, one is piezoelectric (PE)/piezomagnetic (PM)/PE, and the other is PM/PE/PM, are considered in detailed. A peculiar case of the generalized single-celled plate is first presented and then the multi-celled periodical layered plates for discussion on the propagation behaviors and the band characteristics of SH waves, respectively. The effects on dispersive curves of four kinds of ME boundary conditions at the free surface of plates are discussed. Numerical examples for phase velocities or frequencies all show that the zero-order mode is non-dispersive both for single-celled and multi-celled periodically layered plates. The high modes of dispersive curves tend to the shear wave velocity of the slower sub-layer as frequencies increase both for these two structures. The frequency pass-band and frequency band gaps appear in multi-layered ME periodically layered plates. Electrical and magnetic boundary conditions, respectively, determine the dispersive curves for SH waves in the PE/PM/PE and PM/PE/PM periodically layered plates.     

凝聚态物理学的新进展Ⅱ

本文综述了凝聚态物理学近年来的重要进展,并举出若干例子予以阐述。第Ⅱ部分主要关注的问题为非周期结构与低维体系的物理学。讨论的课题包括:无序体系中经典波的局域化;分形结构的动力学行为;准周期结构的能谱;量子霍尔效应;二维熔化;有机导体与超导体。     

Optical Filters Using Optical Multi-Layer Structures for Optical Communication Systems

Abstract

In this article, light reflection spectrums for different one-dimensional multi-layer structures are obtained. Optical reflectivities for periodic, superperiodic, and quasiperiodic structures have been calculated using the transfer matrix method. PBGs of each structure have been obtained using the optical reflectivity pattern. From a comparison of reflectivity and standard dense wavelength division multiplexing grids, it is demonstrated that narrow and dense band filters can be built from generalized Fibonacci quasiperiodic structures. Reflectivity of these structures contains bands that meet the ITU-T dense wavelength division multiplexing standard. Reflectivity of other multi-layer structures has poor or less potential to be used in dense wavelength division multiplexing systems.     

Scanning tunneling spectroscopy of Ag films: the effect of periodic versus quasiperiodic modulation

By using scanning tunneling spectroscopy to probe a silver thin film that contains both periodic and quasiperiodic modulation, and by using Fourier analysis, we unravel the influences of individual Fourier components of the scattering potential (periodic versus quasiperiodic) on the electronic structure of a one-dimensional quasiperiodically modulated thin Ag film. Along the periodically modulated direction, a Bragg reflection-induced energy gap is observed in k space. On the other hand, the exotic E vs k spectrum with many minigaps was observed along the quasiperiodic direction.     

Improving the ballistic AC conductivity through quantum resonance in branched nanowires

Frequency-dependent electrical conductivity is studied by means of the Kubo-Greenwood formula and a real-space renormalization plus convolution method. An analytical solution of the alternating current (AC) conductivity is found for periodic chains. In this article, we report enhancements to this ballistic AC conductivity when periodically or quasiperiodically placed Fano-Anderson impurities are introduced to an otherwise periodic chain, which is connected to two semi-infinite periodic leads at its ends. Moreover, the temperature effects on these resonant AC conducting states are remarkably different in periodic and in quasiperiodic systems. Finally, such enhancement is further analysed in branched nanowires with a small cross section.     

Plasmon analysis and homogenization in plane layered photonic crystals and hyperbolic metamaterials

Dispersion equations are obtained and analysis and homogenization are carried out in periodic and quasiperiodic plane layered structures consisting of alternating dielectric layers, metal and dielectric layers, as well as graphene sheets and dielectric (SiO₂) layers. Situations are considered when these structures acquire the properties of hyperbolic metamaterials (HMMs), i.e., materials the real parts of whose effective permittivity tensor have opposite signs. It is shown that the application of solely dielectric layers is more promising in the context of reducing losses.     

Transition to instability in a kicked bose-einstein condensate

A periodically kicked ring of a Bose-Einstein condensate is considered as a nonlinear generalization of the quantum kicked rotor. For weak interactions between atoms, periodic motion (antiresonance) becomes quasiperiodic (quantum beating) but remains stable. There exists a critical strength of interactions beyond which quasiperiodic motion becomes chaotic, resulting in an instability of the condensate manifested by exponential growth in the number of noncondensed atoms. Similar behavior is observed for dynamically localized states (essentially quasiperiodic motions), where stability remains for weak interactions but is destroyed by strong interactions.     

Linear periodic and quasiperiodic anisotropic layered media with arbitrary orientation of optic axis—A numerical study

We study numerically the linear properties of periodic and quasiperiodic anisotropic layered media. Each anisotropic slab can have arbitrary orientation of optic axis. We apply the general numerical code to recover the known results for solc filters. We propose novel periodic structures where the location and width of the gaps can be controlled easily. We also study the transmission properties of a Fibonacci sequence of anisotropic layers and show the interesting features like self-similarity and scaling.