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° to 40° and strongly attenuated for negative incidence (NI) for incident angles from −10° to −40°. 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.