Conversion between longitudinal and shear waves at normal incidence using tailored meta-structures

The ability to manipulate elastic waves and achieve efficient mode conversion is important for many applications including energy harvesting, vibration mitigation and elastic wave control. In this paper, we present a novel metamaterial-based wave mode conversion device that enables the conversion of longitudinal-to-shear waves (and vice versa) at normal incidence. The devices achieve this by rotating the direction of polarisation (i.e. the motion vector) of the longitudinal waves to match the (normally orthogonal) polarisation of shear waves. Previously we have demonstrated mode conversion by adjusting the spatial–temporal distribution of the incident wave amplitude, but this approach cannot convert between modes with orthogonal motion vectors. Here we demonstrate conversion between orthogonal motion vectors without changing the spatial–temporal distribution of the field. The devices presented here reorient the direction of motion by coupling the waves into thin waveguides or “elastic wave pipes” such that a single mode is supported. The pipes are curved, and the motion vectors of the guide waves are thus rotated as the wave follows the pipe. Fabrication of these geometrically complex structures has recently been made practical through additive manufacturing allowing devices to be built that operate in the megahertz frequency range. We present the design methodology, finite element simulations and experimental demonstration of broadband mode conversion.