Asymmetric phase modulation of acoustic waves through unidirectional metasurfaces

An acoustic asymmetric phase modulation metasurface (APMM) has been proposed theoretically and demonstrated numerically. We find that the proposed APMM could realize the asymmetric transmission of the acoustic wave and the conversion of the propagating wave into the surface wave. The underlying mechanism is based on an asymmetric phase modulation by coupling an acoustic gradient index metasurface (AGIM) and a near-zero-index medium (ZIM). The AGIM and ZIM are implemented by only employing the coiling-up-space structures. Numerical simulations also show that the APMM could provide the high transmission contrast for the asymmetric transmission. The subwavelength thickness and flat geometry of the APMM may be useful for the further actual applications in the acoustic artificial device.