An underwater planar lens for broadband acoustic concentrator

In this Letter, a single-cavity underwater acoustic metasurface unit with sub-wavelength thickness was designed for simultaneously realizing the high refractive index in the water environment. By arraying several metasurface units with a gradient index distribution in the radial direction, a gradient-index three-dimensional planar underwater acoustic lens was constructed. Moreover, a lens with fewer units was designed to reduce the radial size by nearly a half to the original one while maintaining the excellent broadband focusing capability unchanged. In addition, by adding a back cavity on the focusing side of the lens, a lens-based acoustic concentrator was proposed which could greatly enhance the sound energy gain in a wide frequency range. Finally, samples of the underwater acoustic planar lens and concentrator were fabricated and measured in an anechoic tank, which verified that the designed underwater acoustic concentrator has excellent broadband sound energy enhancement performance. The underwater acoustic planar lens and concentrator proposed in this work have potential application values including but not limited to underwater target detection and acoustic energy harvesting.& nbsp;Published under an exclusive license by AIP Publishing.