Broadband and High-Transmission Metasurface for Converting Underwater Cylindrical Waves to Plane Waves

Underwater acoustic plane waves are usually generated using transducer arrays with sophisticated control and synchronizing systems. Here, a subwavelength metasurface is proposed to convert incident cylindrical waves into plane waves by rectifying the local transmission phase through pentamode material design. The metasurface allows a high transmission because of the matched impedance between the pentamode material and water. It is also broadband since only the static material property is used in the design. A metasurface with graded unit cells is fabricated and tested in a two-dimensional water waveguide using transient cylindrical wave excitation. The experiment shows that the metasurface can convert cylindrical waves to plane waves over a broad frequency band of 15-23 kHz with an energy converting ratio as high as 69%. The working frequency band and converting efficiency can be further enhanced with smaller cells and a larger lateral size of the metasurface. This research may lead to new applications for controlling low-frequency waterborne sound, such as acoustic surveying, locating, as well as plane-wave generation for underwater acoustic-testing systems.