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

Although focusing sound waves beyond the diffraction limit offers possibilities for many applications, such as acoustic detection, sensing, and imaging, experimental realization remains a challenge. This study offers a straightforward way of superfocusing sound, through modulating spoof surface acoustic waves along a gradient-index metasurface. The same system also allows subwavelength acoustic imaging. Intriguingly, the enhanced and confined sound field is measurable even away from the metasurface, allowing ``visualization'' of the entire sound-field evolution process at subwavelength scales. This capability suggests an enhanced platform for observing many phenomena in wave physics.