空间盘绕型声学超材料的亚波长拓扑谷自旋态

声子晶体的Dirac线性色散关系,使其具有奇特的声拓扑特性,在声波控制领域具有良好的应用前景.目前,声子晶体的拓扑边缘态主要基于Bragg散射所产生的能带结构,难以实现低频声波的受拓扑保护单向边缘传输.本文引入空间盘绕结构,设计了具有C_(3v)对称性的空间盘绕型声学超材料,并研究其布里渊区高对称点(K/K'点)的亚波长Dirac锥形线性色散.接着,通过旋转打破空间盘绕型声学超材料的镜像对称性,使其Dirac简并锥裂开而产生亚波长拓扑相变和亚波长拓扑谷自旋态.最后,采用拓扑相位互逆的声学超材料构造拓扑界面,实现声拓扑谷自旋传输.空间盘绕型声学超材料的亚波长Dirac线性色散与亚波长拓扑谷自旋态突破了声子拓扑绝缘体的几何尺寸限制,为声拓扑稳健传输在低频段的应用提供理论基础.

Subwavelength topological valley-spin states in the space-coiling acoustic metamaterials

Phononic crystals possess Dirac linear dispersion bands. In the vicinity of Dirac cones, phononic crystals exhibit topological properties which have good application prospects in control of acoustic waves. Up to now, the topological edge states of phononic crystals, based on the band structures arising from the Bragg scattering, cannot realize low-frequency sound waves by the topologically protected one-way edge transmission. In this paper, by introducing the space-coiling structure, a space-coiling phononic metamaterial with C_3v symmetry is designed. At the K (K') points of the Brillouin zone, the bands linearly cross to a subwavelength Dirac degenerated cones. With a rotation of the acoustic metamaterials, the mirror symmetry will be broken and the Dirac degenerated cones will be reopened, leading to subwavelength topological phase transition and subwavelength topological valley-spin states. Lastly, along the topological interface between acoustic metamaterials with different topological valley-spin states, we successfully observe the phononic topologically valley-spin transmission. The subwavelength Dirac conical dispersion and the subwavelength topological valley-spin state breakthrough the limitation of the geometric dimension of the phononic topological insulator, and provide a theoretical basis for the application of the phononic topologically robust transmission in a subwavelength scale.