Bubble-type coding metasurfaces for broadband deep subwavelength underwater acoustic manipulations

We construct a reflective broadband acoustic coding metasurface using bubbles as the unit. The reflected phase difference between the cube bubble unit and the pure water unit is about π over a wide frequency range. The broadband phenomenon could be explained by the reflection theory between the soft water-air interface and the hard water-rigid interface. Its relative bandwidth reaches 187%, close to 200% of the theoretical limit. By reconstructing the 0/1 sequence, we performed numerical simulation on broadband acoustic focusing and diffusing reflection. The research in this work has potential applications in improving the performance of underwater communication and cloaking.