Efficient asymmetrical transmission through a metagrating for underwater acoustic waves

Abstract

Acoustic asymmetrical transmission is a theoretical and engineering challenge because of the reciprocity of the linear acoustic wave equation. It can be achieved by systems breaking reciprocity or by reciprocal systems relying solely on spatial symmetry breaking. Metagratings are planar structures relying on Bragg's diffraction to reroute wave energy toward a desired direction and are eventually able to achieve asymmetrical transmission when build from an asymmetrical pattern of multiple basic elements. The challenge for water-like media is to combine the geometrical complexity of the structure with good acoustic impedance contrast and practical feasibility. In this work, we build a reciprocal metagrating from brass cylinders arranged according to a numerically optimized pattern and obtain highly efficient asymmetrical transmission for underwater acoustic waves. Around 200 kHz, the structure transmits nearly all incident energy toward a 45° angle when insonified from one side, but act as a near perfect reflector when insonified from the other. The effect relies entirely on the simple phenomena of linear wave diffraction and interference. The generality and efficiency of this device could be of interest for applications in underwater acoustics or medical ultrasounds.