Controlling sound waves in gradient spoof-fluid-spoof waveguides

In this work, we theoretically and experimentally demonstrate that effective trapping, guiding, and manipulation of sound waves can be realized in spoof-fluid-spoof acoustic waveguides with gradient index modulation. Empowered by the abundant mode evolution physics between propagation waves and spoof acoustic surface waves in the gradient waveguide structure, various functional sound propagation phenomena, including broadband transmission, broadband reflection, Fabry–Pérot resonances, and Fano resonances, are unveiled. The underlying principle stems from the interplay of various mechanisms composed of gradient mode conversion, high-order mode resonances, and symmetry-protected bound states in the continuum. These effects can be effectively modulated through the manipulation of the fluid gap and doped defects within the waveguide structure. Our findings can offer possibilities for manipulating sound waves in a versatile manner and holding significant potential for various acoustic applications such as sensing, filtering, insulation, and wavefront engineering.