  Metasurfaces are a family of novel wavefront shaping devices with planar
profile and subwavelength thickness. Acoustic metasurfaces with ultralow
profile yet extraordinary wave manipulating properties would be highly
desirable for improving the performance of many acoustic wave-based
applications. However, designing acoustic metasurfaces with similar
functionality as their electromagnetic counterparts remains challenging with
traditional metamaterial design approaches. Here we present a design and
realization of an acoustic metasurface based on tapered labyrinthine
metamaterials. The demonstrated metasurface can not only can steer an acoustic
beam as expected from the generalized Snell s law, but also exhibits various
unique properties including surface wave conversion, extraordinary
beam-steering and apparent negative refraction through higher-order
diffraction. Such designer acoustic metasurfaces provide a new design
methodology for acoustic signal modulation devices and may be useful for
applications such as beam-steering, surface wave manipulation, high efficiency
sound absorption, acoustic imaging and ultrasound lens design.
