Polarization-independent full mode-converting elastic metasurfaces

Abstract

This investigation proposes a novel elastic metasurface tailored to convert a longitudinal wave into a transverse wave and vice versa at an arbitrary refraction angle with full (100 %) conversion efficiency. The previous mode-converting elastic metasurface based on Snell's critical angle can convert a longitudinal wave into a transverse wave, but the refraction angle of the transverse wave cannot be smaller than the critical angle. In addition, the previous metasurface cannot convert a transverse wave into a longitudinal wave without generating a parasitic transverse wave. Here, we propose an elastic metasurface that enables full mode-converting transmission at an arbitrary refraction angle, regardless of whether the incident wave's polarization is longitudinal or transverse. To this end, we present a novel strategy to design unit cells that yield total mode-conversion efficiency as well as the desired phase shifts covering the entire 2π range. Shape optimization and geometry-flipping strategies were employed to design the unit cells constituting the metasurface. The concept of the unit cell's effective length was useful for the unit cell design. The designed metasurfaces were then used for various applications, such as longitudinal-to-transverse and transverse-to-longitudinal wave steering, splitting, and focusing. The performance of our metasurface was verified by ultrasonic experiments performed on an aluminum plate with the designed metasurface embedded. The realized full mode-converting transmission applicable for an arbitrary angle is expected to be critically valuable for nondestructive testing and medical ultrasounds requiring a specific elastic wave mode of the desired arbitrary beam pattern.