Observation of Acoustic Skyrmions

Despite a long history of studies, acoustic waves are generally regarded as spinless scalar waves, until recent research revealed their rich structures. Here, we report the experimental observation of skyrmion configurations in acoustic waves. We find that surface acoustic waves trapped by a designed hexagonal acoustic metasurface give rise to skyrmion lattice patterns in the dynamic acoustic velocity fields (i.e., the oscillating acoustic air flows). Using an acoustic velocity sensing technique, we directly visualize a Néel-type skyrmion configuration of the acoustic velocity fields. We further demonstrate, respectively, the controllability and robustness of the acoustic skyrmion lattices by tuning the phase differences between the acoustic sources and by introducing local perturbations in our setup. Our study unveils a fundamental acoustic phenomenon that may enable unprecedented manipulation of acoustic waves and may inspire future technologies including advanced acoustic tweezers for the control of small particles.

Figure 1

Formation of acoustic skyrmions. (a) An acoustic metasurface with periodically perforated holes is utilized to support spoof surface acoustic waves (SSAWs). The interference of SSAWs excited by six pairs of speakers leads to the acoustic skyrmion lattice. (b) Structure of the acoustic metasurface. Left: top-down view. Right: the unit-cell structure of the acoustic metasurface. (c) Left: The distribution of the acoustic velocity vectors of the propagating SSAWs. Right: The distribution of the acoustic pressure field in the $x\text{−}y$ plane (top) and the $x\text{−}z$ plane (down). In the latter, the white region depicts the solid region of the acoustic metasurface. (d) The experimental setup where six pairs of speakers are utilized to excite the acoustic skyrmion lattice.

Figure 2

Experimental observation of acoustic skyrmions. (a) Experimentally measured acoustic velocity field that exhibits features of a Néel-type skyrmion lattice pattern. Arrows indicate the normalized velocity field $\overrightarrow{n}=Re(\overrightarrow{v})/|Re(\overrightarrow{v})|$. (b) Distributions of the out-of-plane velocity field $v_z$ from both calculation and experiments. (c) Distributions of the amplitude of the in-plane velocity field $|v_{\Vert }|=\sqrt{v_x^2+v_y^2}$ from both calculation and experiments. (d) The skyrmion number density profile $s=\overrightarrow{n}·({\partial }_x\overrightarrow{n}\times {\partial }_y\overrightarrow{n})$ obtained from the experiments (right) together with the skyrmion number density profile obtained from the theoretical calculations (left).

Figure 3

Manipulation of acoustic skyrmion lattices by tuning the phases of the SSAWs excited by the six pairs of speakers at the boundaries [marked by 1–6 in the inset of (a)]. Two cases, denoted as configuration I and II (see details in the main text), are measured. (a) and (b), the out-of-plane velocity fields $v_z$ for the configuration I and II, respectively. (c) and (d), the in-plane velocity fields $|v_{\Vert }|$ for the configuration I and II, respectively.

Figure 4

Robustness of the acoustic skyrmion lattice. (a) A defect made of eight screws blocking the air holes is placed on the metasurface sample, which causes scattering of the acoustic waves and distort the acoustic fields. (b) Experimentally measured acoustic velocity field for the sample with the defect. The acoustic skyrmion lattice is negligibly affected. Arrows indicate the normalized velocity field $\overrightarrow{n}=Re(\overrightarrow{v})/|Re(\overrightarrow{v})|$. (c) and (d) The measured out-of-plane and in-plane velocity field distributions, showing slight deformations near the defect (the dashed parallelogram).