Study on tunable artificial acoustic materials based on topological effect

随着凝聚态物理中拓扑效应的发现，在经典的声学领域中开拓了一个新的研究方向&mdash;&mdash;拓扑声学材料。它可以引导声波在任意路径上鲁棒性传输，且不受材料本身缺陷的影响，这对于声通信和新型声学器件的研发都具有十分重大的意义。虽然拓扑声学人工材料的提出为声波的传输与控制提供了新的方法和理论，但其本身也存在着一些局限性。拓扑声学材料的构造多基于被动声学材料，造成系统的工作频带窄且固定。另外，实现拓扑声学材料的拓扑效应大多依赖于特定的晶格对称性和几何结构参数，这较大程度的限制了拓扑声学材料的设计。对于大多数高阶拓扑声学材料，其仅能实现单一的角态模式分布，难以实现多种类型角度的角态分布，使得拓扑声学材料的应用场景受到限制。因此，围绕以上拓扑声学材料存在的问题，本文开展了以下研究： 1、实现了宽频灵活操控的拓扑声学系统。基于六边形蜂窝状穿孔结构，引入面外方向高度自由度，使得系统的几何构型可以被重构。通过改变孔洞中水柱的高度改变结构几何构型，实现对拓扑声学材料色散曲线的调节。基于量子谷霍尔效应实现拓扑相变，在拓扑带隙处产生边界态的色散曲线。且边界态色散曲线也随着液柱高度变化而变化，使得拓扑边界态可以在较宽的频率范围内进行变化，实现了宽频灵活操控的拓扑声学系统。 2、设计了不依赖于狄拉克锥破缺机制的非对称手性螺线结构。采用非对称的阿基米德螺线结构，构造了手性谷拓扑声学材料。通过探究螺线相关几何结构参数与谷陈数之间的变化关系，获得了影响拓扑相变的重要几何参数。基于此，构造了具有相反陈数的谷霍尔相左旋右旋螺旋结构，在拓扑边界处实现了鲁棒传输的边界态。另外基于转角与拓扑相变间的关系，构造了一种声学开关的结构，为声信号的传输与控制提供了新的方法。 3、实现了多种类型的角态分布模式。基于手性风车结构构造了拓扑四极子单元，在拓扑带隙处不仅产生了有带隙的边界态，还产生了带隙内角态的能量分布模式。调节结构单元的几何参数，研究了系统体、边、角态随几何参数变化的演化情况。另外在直角梯形声学超晶格结构中实现了多种类型的角态模式分布，丰富了声能量传输模式。 4、设计了编码超表面并实现了焦点能量可调的效果。根据编码的设计方法构造了能量可调的聚焦声学超表面。将双开口的赫姆霍兹共振单元与无色散的声通道单元结合，构成0-&pi;相位差，组成&ldquo;0&rdquo;、&ldquo;1&rdquo;编码单元，然后根据聚焦超表面的相位分布函数进行排列组合。实现了对平面声波的宽频聚焦功能，且通过移动共振腔间的隔板可实现对焦点能量调节功能。 论文的创新点为： 1、提出了一种主动调控拓扑声学系统色散曲线的方法。通过连续的改变拓扑系统中的水量，可以使得拓扑边界态也发生连续的变化，也极大的拓宽了拓扑系统的工作频率范围，对于拓扑声学系统的实际应用具有重要意义。 2、提出了一种不依赖于狄拉克锥破缺机制的手性拓扑声学结构。考虑到基于狄拉克锥破缺机制的拓扑系统对结构单元严格的对称性要求，采用具有低对称性的手性螺线结构实现了拓扑传输特性，简化了拓扑系统的结构单元设计。 3、提出了一种可实现多种类型角态能量分布的高阶拓扑结构。采用手性风车结构构造了具有多种拐角形状的直角梯形超晶格，实现了多种类型角态能量的分布，丰富了声能量的传输模式。</p

Underwater acoustic metamaterial based on double Dirac cone characteristics in rectangular phononic crystals

We theoretically construct a rectangular phononic crystal (PC) structure surrounded by water with C-2v symmetry, and then place a steel rectangular scatterer at each quarter position inside each cell. The final complex crystal has two forms: the vertical type, in which the distance s between the center of the scatterer and its right-angle point is greater than 0.5a, and the transverse type, in which s is smaller than 0.5a (where a is the crystal constant in the x direction). Each rectangular scatterer has three variables: length L, width D, and rotation angle theta around its centroid. We find that, when L and D change and theta is kept at zero, there is always a linear quadruply degenerate state at the corner of the irreducible Brillouin zone. Then, we vary theta and find that the quadruply degenerate point splits into two doubly-degenerate states with odd and even parities. At the same time, the band structure reverses and undergoes a phase change from topologically non-trivial to topologically trivial. Then we construct an acoustic system consisting of a trivial and a non-trivial PC with equal numbers of layers, and calculate the projected band structure. A helical one-way transmission edge state is found in the frequency range of the body band gap. Then, we use the finite-element software Comsol to simulate the unidirectional transmission of this edge state and the backscattering suppression of right-angle, disorder, and cavity defects. This acoustic wave system with rectangular phononic crystal form broadens the scope of acoustic wave topology and provides a platform for easy acoustic operation

Subwavelength Chiral Spiral Acoustic Metamaterials for a Robust Topological Acoustic Insulator

Topological acoustic insulators enable sound waves to transmit along the surface without backscattering, which builds a new pathway towards sound wave control. However, a large share of topological acoustic insulators are realized based on special point group symmetry and Bragg scattering mechanism. This method not only exerts a restriction on the unit cell design but also requires the lattice constant to be comparable with the wavelength. In this paper, the chiral spiral acoustic metamaterials are constructed based on an Archimedean spiral structure. This structure enjoys subwavelength characteristics and is easy to construct. Taking advantage of the chirality of the spiral structure topological phases with opposite energy flow direction can be constructed. The edge state is formed at the interface composed of the spiral units sharing different chirality, which does not depend on point group symmetry. The topological transportation on the interfaces shows strong robustness despite sharp corners verified by straight and zigzag waveguides. The topological acoustic insulator with a chiral spiral structure provides a 11el strategy for small acoustic devices with robust sound transmission

Labyrinthine Structure with Subwavelength and Broadband Sound Insulation

In this text, the combination of spiral structure and zigzag channels is introduced to design labyrinthine structures, in which sound waves can propagate alternately in the clockwise and counterclockwise directions. Finite element method and S-parameter retrieval method are used to calculate band structures, effective parameters, and transmission properties of the structures. The influences of different structural parameters on their acoustic properties are also studied. These results show labyrinthine structures have multiple bandgaps in the range of 0 Hz-1000 Hz, and the proportion of bandgaps exceeds 33%, which indicates labyrinthine structures have good broadband properties. The normalized frequency of the lowest bandgaps is far smaller than 1, which indicates the structures take good control of sound waves on subwavelength scale. Combining units with different structural parameters can achieve better sound insulation. This research provides a new kind of space-coiling structure for low-frequency and broadband sound waves control, which have excellent application prospects

Three-dimensional fractal structure with double negative and density-near-zero properties on a subwavelength scale

We constructed a three-dimensional fractal acoustic metamaterial using the combination of zigzag channels and Menger fractal structures that had high structural symmetry and could extend into 3D space easily. Reflection and transmission coefficients were numerically calculated and experimentally measured, and the results matched well with each other. Using the finite element method and the S-parameter retrieval method, the band structures, the equivalent frequency surface and the effective parameters of the acoustic metamaterial were calculated. The results showed that the 3D acoustic metamaterial had double negative property in the normalized frequency range of 0.205-0.269 and density-near-zero property in the normalized frequency vicinity of 0.356. A plate lens model and a quarter-bending model were constructed to demonstrate the double negative property and the density-near-zero property of the acoustic metamaterial, respectively. These results showed that the 3D acoustic metamaterial could achieve excellent acoustic properties and would be promising to construct the 3D double negative structure and control acoustic waves on a subwavelength scale within a single unit. (C) 2020 The Authors. Published by Elsevier Ltd

可重构力学超材料的设计与波动特性研究

力学超材料中的弯曲梁双稳态结构由于其主动调控性强且调控精度高等优点近年来受到广泛关注.文章利用中心受压弯曲梁的不稳定性设计了六角型双稳态结构,首先建立了等效弯曲梁模型,基于梁变形微分方程及能量最低原理探明了结构双稳态特性的产生基理,之后利用有限元数值计算研究了结构几何参数对其整体力学性能的影响,分别得到了具备自恢复及双稳态性能的结构几何参数范围,绘制了几何参数与力学性能之间的相图.同时,可重构结构的可控变形能力有助于调整整体的色散特性,利用数值仿真研究了具备双稳态特性的结构在拉伸和压缩两种构型下的色散关系,对比分析了不同结构几何参数及构型转变对结构产生的带隙位置及范围的影响,之后对由不同构型单胞组成的周期性结构进行了频响分析来验证带隙计算的准确性.通过六角型可重构结构的力学特性、色散特性研究及频响分析表明可以通过结构几何参数的设计实现对结构整体性能的主动调控,为可逆向设计的弹性波超材料结构研究分析提供了一条可靠路径

Broadband periodic and aperiodic acoustic topological insulator based on composite honeycomb structure

具有良好可重构性、良好缺陷兼容性及紧凑型的声学拓扑结构可能成为声学发展中一个有前景的方向.本文设计了一种可调谐、应用于空气声的二维宽带复合蜂窝形晶格结构,其元胞拥有两个变量:一个是中心圆的缩放参数s,另一个是&quot;花瓣&quot;图案围绕其质心的旋转角度q.研究发现当s为1.2, q为&plusmn;33&deg;时,在结构的布里渊区中心点出现四重简并态.在&plusmn;33&deg;两侧,能带会发生反转,体系经历拓扑相变;同时,结构的相对带隙宽带逐渐增加,其中q为0&deg;和60&deg;时,相对带宽分别为0.39和0.33.本研究还计算了由这两种转角的声子晶体组成的拼合结构的投影能带,发现在其体带隙中存在着边界态并验证了此拓扑边界的缺陷免疫特性.最后通过变化s,构建了一种非周期性双狄拉克锥型的声拓扑绝缘体并验证了其缺陷免疫性.本研究的体系相对带宽显著超过已知体系,将为利用声拓扑边界的声波器件微型化打下良好的基础.</p

一种基于声子晶体的水声隐身结构设计

本文提出了一种可以应用于水声隐身的声子晶体结构。首先介绍声子晶体的结构设计与组成,然后通过有限元软件计算得出结构的能带图。从图中发现在结构布里渊区的中心点存在四重简并点。接着利用此点处结构的有效折射率接近于零的特性,本文设计了一种由14×6个基本结构组成的长方形"隐身区"。通过模拟发现,这种结构对"方形"与"船形"物体具有良好的隐身效果

Acoustic edge mode in spiral-based metamaterials at subwavelength scale

Recently, the quantum effect of condensed matter physics is introduced into the acoustic field, which lays a new pathway to manipulate the acoustic wave. However, the acoustic topological insulator based on Bragg scattering requires their lattice constant to be comparable with the wavelength. In this paper, a 11el subwavelength spiral element on the basis of the Archimedean spiral is proposed. Thanks to the central resonator with a slender curved channels scheme, the subwavelength Dirac cone forms in the band structure. The eigenfrequency of the element can be changed by the spiral geometric parameters, which triggers the topological phase transition characterized by opposite valley Chern numbers. The backscattering-immune unidirectional transmission edge state exists at different topological boundaries, suggesting its great robustness even at the sharp bends. Also the topological edge modes along the Z-shaped interfaces are verified by the experiments and the calculation, which provides an effective structure to control the low frequency acoustic wave transmission

Wave characteristics of reconfigurable elastic metamaterials based on a multi stable structure

Elastic metamaterials have been considered as important tools for vibration and noise reduction because of their subwavelength bandgaps. However, acritical problem hindering their practical application is the difficulty of constructing an adjustable unit cell that facilitates bandgap tuning. In this study, reconfigurable two dimensional metamaterials based on four curved beams are developed such that they could sustain multi stable configurations. Our calculations and experiments reveal that the developed unit cell can exhibit bidirectional phase transformations to three stable configurations. The three stable configurations have different wave characteristics, allowing for elastic wave propagation to be tuned at the subwavelength scale. Based on this tuneability, a waveguide can be designed and the wave direction of travel can be controlled. This study provides a novel approach for designing reconfigurable elastic wave materials, elastic wave logic circuits, and waveguides