Asymmetric acoustic transmission with a lossy gradient-index metasurface

We theoretically and experimentally present the asymmetric acoustic transmission phenomenon in a lossy gradient-index metasurface (GIM). The lossy GIM design is a reflectionless planar layer for acoustic waves freely transmitted for positive incidence (PI) for incident angles from 10 degrees to 40 degrees and strongly attenuated for negative incidence (NI) for incident angles from -10 degrees to -40 degrees. The underlying mechanism is ascribed to the loss-induced suppression of the high diffraction order of the period grating. The diffraction order associated with the period grating takes a value of m = 0 (without multiple reflections) for PI and m = 2 (with multiple reflections) for NI, and hence, the asymmetric acoustic transmission occurs. We exemplify the lossy GIM by the coating unit cells consisting of three-layers of acoustic labyrinthine metamaterials, which provide a full phase control and around 26% transmission loss. The experimental results agree well with the theoretical analysis and numerical simulations. Our design may offer applications in noise control and acoustic sensors. Published by AIP Publishing.