Shaping elastic wave mode conversion with a piezoelectric-based programmable meta-boundary

Pressure-shear wave conversions on free boundaries or interfaces of solids are peculiar and unavoidable wave phenomena in two-dimensional (2D) elasticity, compared with electromagnetic and acoustic wave systems. However, flexibly tailoring their conversions in a reversible and programmable manner to meet a predefined conversion efficiency and spatial distributions of converted wave fields has never been touched before. In this letter, we introduce a programmable meta-boundary with deep subwavelength thickness that is composed of an array of piezoelectric sensing-and-actuating units controlled by electrical circuits such that pressure to shear wave conversions are able to be electrically reconfigured. Through numerical simulations, we show the meta-boundary can nearly totally convert an incident pressure wave to a reflected shear wave even for normal incidences. Thanks to the programmability of electrical control circuits, incident waves can span a large range of angles, from negative to positive values, and reflected shear waves converted from pressure waves are also able to be steered to different directions. The design could find potential applications in protection of underwater structures, acoustic cloaks, ultrasonic imaging and new types of shear-wave-transducer devices. (C) 2020 Elsevier Ltd. All rights reserved.