Harnessing curvature for helical-wave generation in spiral-based metamaterial structures

Linearly polarized elastic waves propagating in a linear path have been extensively explored in numerous applications from biomedical imaging to structural health monitoring. However, elastic waves propagating in a circularly polarized helical path are less explored because of the challenges in their generation and control. In this paper, we harness conventional actuation methods combined with sheets decorated by Archimedean spirals to generate elastic helical waves. We show that our metamaterials can support the propagation of such waves along a curved path without backscattering in a topologically protected manner. Moreover, we also show the creation and propagation of helical waves in our metamaterials without the need to wave guiding or domain interfaces, all with a single mode excitation source. We establish our methodology for flat plates and show the wave evolution as the metamaterials transition from slightly curved plates to fully curved cylinders. We observe the preservation of topologically protected edge states and helical wave beaming at different frequencies without the need to domain interferences. Furthermore, we leverage the mode to tune the helicity of the propagating waves. Our methodology can open new avenues for the generation and control of elastic helical waves using single source actuation that can be used in numerous applications.