A dual-gradient underwater meta-auricle for broadband sound signal enhancement

Perceiving the long-distance or weaker sounds is an important technical indicator for the development of acoustic detection equipment such as sonar. Especially, acoustic signal enhancement of free-field and long-distance application condition close to that in the lake or ocean environment is still challenging. To this end, we propose a dual-gradient artificial meta-auricle, which comprehensively utilizes the wavefront reflection phase modulation and geometrical concave reflection mechanisms, and could greatly enhance the underwater sound signal in a broadband frequency range. As the reflection phase of each metasurface unit from the center to the outermost side is gradient distributed, the reflected sound waves would converge on the central axis; while the height of each unit is incremental distributed from the center to the outside and forms a concave surface, so that the reflected sound waves could converge at the geometric center. In this way, in the geometric center region, the two focusing mechanisms are superimposed with each other synergistically. Therefore, the focusing amplitude could be effectively enhanced, and the working frequency band would be greatly broadened. This reflective acoustic metalens, similar to the auricle of an animal, enables signal enhancement over a wide frequency band, both in waveguide and in free-field conditions.