Longitudinal wave steering using beam-type elastic metagratings

Abstract

Metagratings have recently received much attention for effective realization of anomalous wave steering. Governed by the diffraction grating theory and the concept of metamaterials, metagratings allow to suppress superfluous propagations of high-order scattering modes in a selective manner such that wavefronts are accurately steered as designated. While numerous metagratings have been reported in electromagnetic wave research field, little has been explored in the elastic wave regime because of difficulties caused by complicatedly coupled motions among various types of elastic waves. In this circumstance, we newly propose a novel elastic metagrating model to realize anomalous steering of longitudinal waves in elastic continuum medium. Modeled by a periodic arrangement of a single or array of so-called beam-type elastic members, the present elastic metagratings resolve the difficulties in steering a specific scattered mode of elastic longitudinal waves by suppressing undesired modes in the scattered wave field. As the metagratings are attached to a boundary of elastic continuum, both longitudinal and flexural motions in the beam-type members are so coupled with elasticity wave solutions in the elastic medium as to thoroughly manipulate the reflected longitudinal waves from the interface. A set of appropriate geometric parameters of the beam-type members are searched for a direct control of the scattered modes quantitatively. As practical applications, the proposed elastic metagratings are applied to realize anomalous reflections and asymmetric splitting of longitudinal elastic waves. As well as experimental verifications, performance of the designed elastic metagratings are physically interpreted in the perspective of phase modulation.