Helical Phononic Modes Induced by a Screw Dislocation

Abstract

A 1D unidirectional phononic waveguide embedded within a 3D hexagonal close-packed (HCP) phononic crystal is investigated, formed through the introduction of a screw dislocation. This approach does not rely on the non-trivial topological characteristics of the 3D crystal. The findings reveal that the screw dislocation induces a pair of helical modes with orthogonal displacements in the x- and y-directions, which are 90 degrees out of phase, resulting in distinctive rotational motion. These helical modes exhibit directional propagation, tightly linked to the helicity of the screw dislocation. The observed directionality and robustness are attributed to the interplay between the structural helicity and the existence of a bulk bandgap. This work provides new insights into the influence of dislocation-induced symmetry on wave propagation in phononic systems. It also offers a pathway for designing directionally selective waveguides without relying on topological properties.