Elastic Wave Energy Entrapment for Reflectionless Metasurface

This paper presents an elastic metasurface that can entrap the elastic wave energy of both longitudinal and shear wave modes in a lossy region, resulting in almost no reflection environment, which we call a reflectionless metasurface. Unlike previous attempts in acoustics, elastic waves are governed by different physics, such as mode conversion, and different behavior of shear waves. Accordingly, achieving a reflectionless elastic metasurface requires a different approach to that of previous studies on acoustic metasurfaces for sound absorption. Here, physics for entrapping the elastic wave energy is proposed by combining two phenomena: the local resonance of metasurface unit cells and the surface wave conversion. As a result, the elastic wave energy originating from an incident longitudinal wave can be highly concentrated in the metasurface, and energy dissipation can be significantly increased if any damping is implemented in the metasurface region. To realize the idea, a reflectionless metasurface, consisting of rod-shaped unit cells with various lengths, is designed. Numerical and experimental investigations show that the proposed reflectionless metasurface, with a small amount of loss, can effectively reduce both the reflected longitudinal and shear waves under longitudinal wave incidence with broad angles. We expect that the proposed idea, based on a thin artificial elastic layer, will provide an alternative route to design ultrasonic absorbers.