Single-mode interface states in heterostructure waveguides with Bragg and non-Bragg gaps

Ya-Xian Fan,Ting Liu,Lan-Lan Xu,Tang-Qing Sang,Zhi-Yong Tao

doi:10.1038/srep44381

Abstract

Interface states can always arise in heterostructures that consist of two or more (artificial) materials with topologically different energy bands. The gapped band structure can be classified by the Chern number (a topological invariant) generally or the Zak phase in one-dimensional periodic systems. Recently, topological properties have been employed to investigate the interface states occurring at the connecting regions of the heterostructures of mechanical isostatic lattices and acoustical waveguides. Here, we study this heterostructure phenomenon by carefully connecting two corrugated stainless steel waveguides with Bragg and non-Bragg gaps at approximately the same frequency. These two waveguide structures can be achieved by continuously varying their geometry parameters when a topological transition exists in the forbidden bands, in which the reflection impedance changes the sign. Furthermore, a localized single high-order mode has been observed at the interface because of the transverse mode interactions, which relate to the non-Bragg gaps created by the different transverse mode resonances. Such a localized acoustic single mode with very large enhanced intensity could find its applications in sound detection, biomedical imaging, and underwater sound control, and could also enrich our means of wave front manipulations in various engineering fields.

Highlights

Heterostructures, the combinations of multiple semiconductor materials with unequal band gaps[1], have been developed to indicate any combined materials or structures with energy or frequency gaps[2,3,4,5,6,7]
The topological classification provides the equivalence states when the Bloch Hamiltonian changes with varying geometry parameters, and the bulk band structures can be identified by the geometry phase (Berry’s phase) or the Chern invariant, in terms of holonomy[13,20]
We have demonstrated that band interactions and their topological transition can be realized in a classical acoustic system

Summary

Introduction

Heterostructures, the combinations of multiple semiconductor materials with unequal band gaps[1], have been developed to indicate any combined materials or structures with energy or frequency gaps[2,3,4,5,6,7]. In quantum and classical physics, connecting several structures (or materials) with topological different band gaps has led to so many interface states with exciting physical concepts and functional materials. When the geometry parameters of a waveguide vary, the topological characteristics of the Bragg and non-Bragg gaps are invariant but their frequency ranges would shift. We can select the same frequency for the Bragg and non-Bragg gaps and elaborate the acoustic heterostructure waveguides by carefully connecting two structures. What is more intriguing is that the localized interface states can consist of a single high-order mode owing to mode coupling in non-Bragg gaps, which will be demonstrated numerically and experimentally in the following. The localized single mode could find its applications in various fields, such as trap and control of microparticals, boundary control of soft matter, and imaging

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Results

Conclusion