Subwavelength Sound Focusing and Imaging Via Gradient Metasurface-Enabled Spoof Surface Acoustic Wave Modulation

Manipulating acoustic waves beyond the diffraction limit offers alternative possibilities to many applications in wave physics. Here, we show that a gradient-index acoustic metasurface is able to achieve this purpose by modulating airborne sound at subwavelength scales via the local-resonance-controlled spoof surface acoustic waves. From the dispersion relation of such a surface mode, a design approach of explicitly correlating the effective refractive index with the geometrical unit-cell parameters can be obtained. With one carefully designed gradient-index acoustic metasurface constructed using locally resonant elements, we demonstrate the experimental realizations of both subwavelength plane-wave sound focusing and super-resolution acoustic imaging. This work provides a feasible pathway to the subwavelength manipulation of airborne sound. Moreover, it offers an ideal experimental platform to directly observe how acoustic energy flow is governed by refractive-index modulation. The proposed design approach can be easily applied to various spoof surface acoustic wave devices that demand gradient index distribution.