Experimental Evidence of Robust Acoustic Valley Hall Edge States in a Nonresonant Topological Elastic Waveguide

This paper presents experimental evidence for the existence of acoustic valley Hall edge (AVHE) states in topological elastic waveguides. The fundamental lattice is assembled on the basis of a nonresonant unit where space-inversion symmetry (SIS) is broken by our simply perturbing the underlying lattice geometry. This aspect is in contrast with existing elastic AVHE designs that exploit locally resonant units and require the addition of masses to break SIS. The experimental results presented in this study validate findings so far presented only at the theoretical and numerical level. In particular, it is found that edge modes can effectively propagate along domain walls between topologically dissimilar domains and that disorder-induced backscattering is substantially suppressed due to the weak coupling between oppositely-valley-polarized modes. The coupling between valley modes is further investigated and linked to an evident chiral flux of the mechanical energy. Finally, we show that the weak coupling between the valleys can be exploited to achieve selective mode injection at the domain wall, hence realizing a very effective excitation strategy for the chiral edge states.