Asymmetric transmission via lossy gradient-index metasurfaces: Role of diffraction

Abstract

The development of acoustic metasurfaces has enabled numerous wave control abilities. The effect of losses in acoustic metasurfaces for sound transmission manipulation, however, is largely unexplored. Here, we show that robust asymmetric transmission can be achieved by harnessing judiciously tailored losses. Theoretical investigations show that the asymmetric behavior stems from loss-induced suppression of high order diffractions. Multiple reflections occur inside the individual slits for the negative incident direction, which lead to different orders of diffraction and asymmetric responses for opposite directions. Numerical simulations based on effective medium are performed and are in good agreement with theoretical analysis. Real structures based on unit cells with designed internal viscous loss are fabricated and measured in a 2D waveguide. The peak energy contrast is about 10 times in a certain range of incident angles and frequencies. This study may open up new possibilities in lossy acoustic metamaterials and metasurfaces. The theory can also be readily extended to electromagnetic waves.