Experimental Observation of Topologically Protected Helical Edge Modes in Patterned Elastic Plates

M Miniaci,M Ruzzene,R K Pal,B Morvan

doi:10.1103/physrevx.8.031074

Abstract

The investigation of topologically protected waves in classical media has opened unique opportunities to achieve exotic properties like one-way phonon transport, protection from backscattering and immunity to imperfections. Contrary to acoustic and electromagnetic domains, their observation in elastic solids has so far been elusive due to the presence of both shear and longitudinal modes and their modal conversion at interfaces and free surfaces. Here we report the experimental observation of topologically protected helical edge waves in elastic media. The considered structure consists of an elastic plate patterned according to a Kagome architecture with an accidental degeneracy of two Dirac cones induced by drilling through holes. The careful breaking of symmetries couples the corresponding elastic modes which effectively emulates spin orbital coupling in the quantum spin Hall effect. The results shed light on the topological properties of the proposed plate waveguide and opens avenues for the practical realization of compact, passive and cost-effective elastic topological waveguides.

Highlights

Topological protection provides a significant potential to achieve one-way, defect immune, and scattering free wave propagation [1]
We report on the experimental investigation of topologically protected helical edge modes in elastic plates patterned with an array of triangular holes, along with circular holes that produce an accidental degeneracy of two Dirac cones
Several studies have exploited valley degrees of freedom to create spinpolarized band gaps and quantum valley Hall effect (QVHE) analogues. This is conveniently achieved by breaking spatial inversion symmetry at the unit cell level, which is an approach that has been successfully applied for flexural waves in plates [25,26] as well as in mechanical lattices with resonators [27]

Summary

INTRODUCTION

Topological protection provides a significant potential to achieve one-way, defect immune, and scattering free wave propagation [1]. This analogy is the nucleation of a double Dirac cone and the coupling of two degenerate modes corresponding to distinct irreducible representations of the reciprocal lattice symmetry group characterized by a Dirac dispersion [22] In this context, elastic plates emerge as excellent candidates due to the presence of an infinite number of modes with distinct polarizations and coupled deformation mechanisms. Several studies have exploited valley degrees of freedom to create spinpolarized band gaps and quantum valley Hall effect (QVHE) analogues This is conveniently achieved by breaking spatial inversion symmetry at the unit cell level, which is an approach that has been successfully applied for flexural waves in plates [25,26] as well as in mechanical lattices with resonators [27]. IV summarizes the main findings of the paper and describes future investigations as well as potential applications of the proposed approach to the identification of QSHE continuous mechanical analogues

QSHE ANALOGY THROUGH DISPERSION ENGINEERING

EXPERIMENTAL OBSERVATION OF HELICAL EDGE MODES

Findings

CONCLUSIONS