Topology optimization design of broadband elastic wave coding metasurfaces

In this paper, a kind of broadband elastic wave coding metasurfaces is proposed. The metastructure unit designed by topology optimization is used as logical unit “1,” and the pure plate is set as logical unit “0.” The phase difference of the transmitted longitudinal wave between two units is introduced as the objective function in optimization. The transmittance of the designed unit is kept above 60%, and the transmitted phase difference between the two coding units is approximately π in the broadband frequency range. In addition, a set of achromatic focusing coding units is designed topologically to achieve customized dispersion. By coding the logical units, the designed metasurface can control the elastic wave between frequencies of 20.4 and 23.6 kHz, and wave performances, such as beam focusing and splitting, are achieved. The experimental results of the designed metasurface are in good agreement with numerical simulations. Therefore, our design has potential applications in the fields of engineering such as nondestructive testing.