Tunable acoustic metasurface based on tunable piezoelectric composite structure.

Due to the potential engineering needs, the passive tunable metasurfaces with a high performance equivalent to the active phased array is worthy of research. Here, a passive ultrathin metasurface unit composed of a piezoelectric composite structure (PCS) connected to an external capacitor, which can modulate the phase of the transmitted acoustic waves at a deep subwavelength scale only by controlling the external capacitor but without changing the structure, is proposed. Then, a tunable acoustic metasurface composed of 20 identical PCSs is introduced to realize three acoustic functions, beam steering, beam focusing, and tweezer-like beam generating, just by changing the external capacitors. The phase-control abilities of the PCS unit and three functions of the designed metasurface are proved both numerically and experimentally. This study provides the possibility to design ultrathin tunable acoustic metasurfaces with the ability of precise control and passive materials.