  We propose the design and measurement of an acoustic metasurface
retroreflector that works at three discrete incident angles. An impedance model
is developed such that for acoustic waves impinging at -60 degrees, the
reflected wave is defined by the surface impedance of the metasurface, which is
realized by a periodic grating. At 0 and 60 degrees, the retroreflection
condition can be fulfilled by the diffraction of the surface. The thickness of
the metasurface is about half of the operating wavelength and the
retroreflector functions without parasitic diffraction associated with
conventional gradient-index metasurfaces. Such highly efficient and compact
retroreflectors open up possibilities in metamaterial-based acoustic sensing
and communications.
