局域共振超材料对固体中弹性波偏振的操控

Elastic deformation in solid can be both compressional and shear. Consequently, solid materials can simultaneously sustain both transverse waves and longitudinal waves. In addition, more special polarizations can exist in solids with a certain shape, such as torsional waves in a rod or a beam. Polarization conversion between transverse and longitudinal elastic waves can occur in a scattering event. Such conversion is generally an uncontrollable process. In this project, we extend local resonance for the design of elastic metamaterials for the control of elastic wave polarization. Local resonance is concept that originates in acoustic metamaterials that utilizes resonances that is localized within a unit cell for novel control of sound propagation. Through judicious design, we aim using such a class of metamaterials for solid-borne elastic waves, and to achieve controllable rotation of polarization direction of transverse elastic waves, and to achieve controllable conversion between transverse and longitudinal waves. Moreover, we will explore the possibility for polarization control in specific geometric shapes, such as torsional to flexural waves conversion in a rod. These effects will be demonstrated with experiments. Theoretical and numerical investigations will also be conducted to reveal the mechanism for these functionalities.

固体中的弹性波的偏振具有多样性和复杂性。固体中同时允许压缩和剪切两种弹性形变。故固体弹性波可以是横波或纵波。一定几何形状的固体中可以存在更特殊的偏振模式，如杆、梁等结构中的扭转波。弹性波的不同偏振可以在散射过程中相互转化。而这种偏振的转化一般不可控。本课题拟通过局域共振超材料，实现对弹性波偏振的操控。局域共振机制源于声波超材料，其原理是利用原胞内部可调共振来实现声波的新颖操控。本课题中将把局域共振机制发展到固体弹性波，并对偏振产生前所未有的可控调节。我们希望通过设计超材料，在实验上实现横波偏振方向的可控偏转；实现横波和纵波之间的转化；以及探讨针对特殊几何形状的偏振控制，如杆中扭转波和折弯波的转化。同时，我们将进行理论和数值仿真，揭示超材料偏振操控的具体物理机制。

弹性波和声波的传播的不仅具有广泛的应用价值，而且是一个重要的基本科学问题。虽然弹性波和声波皆为机械振动在时空中的传播，由于传播介质的区别，两者展现诸多出截然不同的性质。本课题以弹性波偏振调控为切入，结合近年国际学科发展新形式和国家对基础科学与日俱增的实际需求，对研究方向做出了一定调整，进行了大量探索性研究并取得多项开创性成果。其中主要成果三项。1. 通过对弹性波偏振态振型的深入研究，结合声波超材料实现各向异性等效参数的可能性，首次在声波中观察到类似弹性波的横波振动。基于此设计并制备了微极型超材料，实现了声横波的自旋轨道耦合。 2. 基于薄板折弯振动设计弹性波超材料晶格系统， 并利用合成维度，利用弹性波体系首次实现了全新的三维混合型陈绝缘体。3. 设计了由横波模和纵波模构造而成的非厄米系统弹性波耦合波导，通过对波导参数的绝热演化，利用弹性波在波导的传播实现对参数空间中奇异点的绕行，并由此实现了弹性波偏振模态的转换。