Hybrid porous Helmholtz resonator metasurfaces with decoupled amplitude-phase modulations for finely tunable frequency-multiplexed acoustic holograms

We propose a metasurface design filled with porous medium inside Helmholtz resonators, which can project multiple predesigned images at different frequencies. It is easier to fabricate and more practical than the conventional multiplexed acoustic holographic metasurface of collateral Helmholtz resonators. Moreover, the amplitude and phase of the reflected acoustic wave it modulates have decoupled characteristics, which means there is no need to search for amplitude-phase space by algorithms. We theoretically, numerically, and experimentally demonstrate the meta-unit can achieve arbitrary amplitude-phase combinations at four frequencies. Then, the fourfold acoustic holographic images are numerically generated by one metasurface at 1180, 1490, 2340, and 3480 Hz, respectively. These proposed strategies significantly increase the degree of freedom of the classical metasurfaces, which may open up additional avenues for multifunctional compact acoustic devices.