Reflection-type broadband coding metasurfaces for acoustic focusing and splitting

In this paper, we propose a kind of reflection-type broadband acoustic coding metasurfaces (BACMs), which are composed of two square helical channels and the connected air cavity at the end of the channels. This helical-cavity coupled structure is selected as a logical unit "1,"the pure air hole is set as a logical unit "0,"and the reflective phase difference of the two units is approximately equal to πin a broad frequency range. More importantly, we reveal a somewhat unconventional mechanism of the coupling resonance between the helical channel and the air cavity for the broadband characteristic, which can be hardly realized by the traditional space-coiling or Helmholtz-resonator metasurfaces. We prove that the 0/1 encoding form can be reconstructed simply by inserting the spiral structure or not. By encoding the sequence of the logical units in the BACMs, the broadband acoustic focusing lens and acoustic splitter within the frequency range of [2.4, 5.6 kHz] are demonstrated numerically and experimentally. Our study may find applications in the fields of acoustic wave devices.