Realization of acoustic spin transport in metasurface waveguides

Abstract

Spin angular momentum enables fundamental insights for topological matters, and practical implications for information devices. Exploiting the spin of carriers and waves is critical to achieving more controllable degrees of freedom and robust transport processes. Yet, due to the curl-free nature of longitudinal waves distinct from transverse electromagnetic waves, spin angular momenta of acoustic waves in solids and fluids have never been unveiled only until recently. Here, we demonstrate a metasurface waveguide for sound carrying non-zero acoustic spin with tight spin-momentum coupling, which can assist the suppression of backscattering when scatters fail to flip the acoustic spin. This is achieved by imposing a soft boundary of the π reflection phase, realized by comb-like metasurfaces. With the special-boundary-defined spin texture, the acoustic spin transports are experimentally manifested, such as the suppression of acoustic corner-scattering, the spin-selected acoustic router with spin-Hall-like effect, and the phase modulator with rotated acoustic spin.

Acoustic Bessel Vortex Beam by Quasi-Three-Dimensional Reflected Metasurfaces

“…Furthermore, multifunctional vortex beams of sound can also be obtained by changing phase profiles, such as focusing vortices [38,39] and vortex beams with asymmetric propagation [40], which have potential special applications. Additionally, the finite element method based on the COMSOL Multiphysics software has been introduced to numerically design and optimize different types of acoustic vortex devices, such as unidirectional vortex beams through acoustic Weyl crystal with a topological lattice defect, and a vortex converter composed of an acoustic metagrating in a waveguide [10,37,40], and the corresponding simulated results agree well with the measured ones. However, these aforementioned vortex beams are generally observed on structure surfaces or in waveguides with a hard boundary owing to the characteristic of easy diffusion in free space.…”

“…However, their structures must satisfy the geometric characteristics of spiral distribution. On the other hand, by tailoring phase profiles of acoustic metasurfaces, sound energy can also be formed as an acoustic vortex beam based Micromachines 2021, 12, 1388 2 of 8 on phase modulation [33][34][35][36][37]. For instance, by designing phase profiles of Helmholtz resonators in a circular waveguide, acoustic vortex beams are also observed, and the sound energy of vortex beams can propagate in the waveguide with a hard boundary [34,37].…”

“…On the other hand, by tailoring phase profiles of acoustic metasurfaces, sound energy can also be formed as an acoustic vortex beam based Micromachines 2021, 12, 1388 2 of 8 on phase modulation [33][34][35][36][37]. For instance, by designing phase profiles of Helmholtz resonators in a circular waveguide, acoustic vortex beams are also observed, and the sound energy of vortex beams can propagate in the waveguide with a hard boundary [34,37]. Furthermore, multifunctional vortex beams of sound can also be obtained by changing phase profiles, such as focusing vortices [38,39] and vortex beams with asymmetric propagation [40], which have potential special applications.…”

Tunable Topological Surface States of Three-Dimensional Acoustic Crystals

“…Moreover, the interface is an anti-phase boundary formed by shifting a portion of the crystal lattice as well, which can support pseudospin-momentum locking [50,51]. The acoustic pseudospin can be defined by the rotating direction of the energy flow [52][53][54][55]. The acoustic field distributions of anti-symmetry surface mode P 1 and symmetry mode P 2 at X point, with counter-rotation of energy flow (black arrows), are presented in Figure 3G.…”

Ultrafast electron cycloids driven by the transverse spin of a surface acoustic wave

“…Consequently, these macroscopic dimensions and moderate periods do not demand advanced detection schemes. Hence, macroscopic pressure sensors can be directly used to sense the local pressure field and, thus, acoustic spin of airborne sound ( 28 , 29 ). In strong contrast, these methods cannot be applied to detect the spin of a radio frequency (rf) Rayleigh SAW at ≥100 MHz because the approximately one order of magnitude higher phase velocity shrinks the wavelength to a few micrometers.…”