Asymmetric acoustic metagrating enabled by parity-time symmetry

Passive parity-time (PT) symmetric metamaterials as a fundamental carrier to investigate non-Hermitian systems have attracted significant interest, but previous studies have rarely focused on the diffraction properties of the PT system, especially the diffraction effect in the metasurfaces with phase gradient modulation. Here, by merging two concepts of phase gradient and PT symmetry together, we design and study a non-Hermitian acoustic gradient metasurface (i.e., metagrating). We show that the PT symmetry breaks spatially the symmetry of reflected wave diffraction in the acoustic metagrating, making it an asymmetric mirror. In particular, at the PT-ensured exceptional point, for incident waves from one side, such metagrating behaves as a phase gradient metasurface, in which high-efficiency retroreflection governed generalized Snell's law occurs. For an incident wave from the opposite direction, such a metagrating is a common grating, whose diffraction in the reflection follows the common diffraction equation. Our work builds a bridge connecting the phase gradient and passive PT symmetric system, enriching the methods for wavefront manipulation and exhibiting potential applications in the design of multifunctional acoustic devices.