Observation of localized acoustic skyrmions

Recently, acoustic skyrmions have been explored by tailoring velocity vectorial near-field distributions based on the interference of multiple spoof surface acoustic waves, providing new dimensions for advanced sound information processing, transport, and data storage. Here, we theoretically investigate and experimentally demonstrate that a deep-subwavelength spiral metastructure can also generate the acoustic skyrmion configuration. Analyzing the resonant response of the metastructure and observing the spatial profile of the velocity field, we find that the localized skyrmionic modes correspond to eigenmodes of the spiral structure. Thus, the skyrmionic modes do not require carefully tailored external excitation condition and they have multiple resonating frequencies unlike the single skyrmionic mode realized by the interference of multiple waves. We also demonstrate that the topological protected skyrmions supported by the subwavelength metastructure is robust against structure deformations and existence of structure defects. The real-space acoustic skyrmion topology may open new avenues for designing ultra-compact and robust acoustic devices, such as acoustic sensors, acoustic tweezers, and acoustic antennas.