Wireless acoustic energy harvesting through an air-water metasurface with dual coupling resonators

Extremely large acoustic impedance mismatching generates a natural acoustic barrier at the air-water interface, resulting in significantly impeding bidirectional acoustic wave propagation across the heterogeneous interface. Here, an air-water metasurface with dual coupling resonators is proposed to enhance the acoustic transmission at the air-water interface, which facilitates the implementation of wireless harvesting for acoustic energy across the heterogeneous interface. A theoretical model is established and derived to obtain the analytical expressions between acoustic energy transmission and microstructural geometric parameters. The theoretical analysis reveals that the highly efficient energy transmission mechanism depends on the impedance coupling effect of the resonant cavities for nonresonance modes. The enhanced bidirectional acoustic energy transmission at the air-water interface is investigated and verified numerically and experimentally, and the maximum enhancement of energy transmission is measured to be approximately 19 dB at the peak frequency. Finally, wireless acoustic energy harvesting across the air-water interface is implemented experimentally by integrating the designed metasurface with a contact-separation-mode triboelectric nanogenerator, and the captured energy from the waves effectively operates six LED lamps. The proposed "bottom-up"design methodology of air-water wave energy harvesting based on an acoustic-metasurface-embedded system opens promising routes for underwater wireless energy-supplying platforms and medical ultrasound therapy.