Acoustic Valley-Locked Waveguides in Heterostructures of a Square Lattice

Abstract

The transport behaviors of topological guiding modes for classical waves in heterostructures with width degree of freedom have attracted much attention recently. With the width degree of freedom, topological waveguides are endowed with increased capacity in energy transport and improved flexibility in interfacing existing devices. However, to date, such topological waveguides are only realized in hexagonal lattices, by virtue of the known valleys existing at the Brillouin zone corners. Here, we report on the implementation of topological waveguides in heterostructures of a square lattice, consisting of three domains of acoustic crystals: the middle domain has Dirac points and the two side domains have valleys of opposite valley Chern numbers. A pair of counterpropagating topological waveguide modes, with amplitudes that are uniform in the central domain and attenuated sidewards in the two side domains, are supported in the heterostructure. We experimentally verify that the topological waveguide modes are valley locked and backscattering immune, and they have a high capacity for energy transport. We also demonstrate experimentally the conversion between the valley waveguide states and the usual valley edge states. More interestingly, when the central domain of the Dirac points is applied with a transverse structural gradient mirror symmetrically, a different kind of topological waveguide mode emerges. We verify that the waveguide modes possess the same topological properties.