Metasurfaces for perfect transmission of sound from air into water

Three orders of magnitude difference in acoustic impedance between air and water has so far prevented effective sound transmission through the interface. A conventional method for an antireflection coating is a quaterwave plate. Such a coating would be too thick and bulky because acoustic waves in audible range are long. More importantly, one cannot find a suitable coating material among the naturally occurring materials, the acoustic impedance of which is required to be a geometrical mean of those of air and water. Here, we present fabrication of acoustic metasurfaces of thickness much smaller than the wavelength, which allows perfect transmission of sound through the air/water interface. The metasurface is a two dimensional array of unit cells which are consisting of membrane and cavities. Our work demonstrates usage of metamaterials for impedance matching in extreme conditions.