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.
Highlights
Spin angular momentum enables fundamental insights for topological matters, and practical implications for information devices
Because conventionally it is enough to represent the acoustic wave by scalar pressure fields only, and classical field theory concludes that scalar fields possess zero SAM27
According to the physical meanings possessed in angular momenta of acoustic waves, the spin angular momenta (SAM) density can be described as: s where ρ is the mass density, ω is the frequency, c is acoustic velocity, and v is acoustic velocity field
Summary
Nonzero acoustic spin in waveguide with symmetry breaking. According to the physical meanings possessed in angular momenta of acoustic waves, the SAM density can be described as (refs. 5–8): s. Is the unit vector in y-direction, k is the longitudinal momentum qcomffiffiffiffipffiffioffiffiffinffiffiffieffiffinffiffitffiffiffiwffiffi ith k = kez the wave vector along z-direction, κ 1⁄4 ω2=c2 À k2 is the transverse wave vector, c is the sound speed in the air, R is the radius of the waveguide, and p0 is the pressure field amplitude Metasurface waveguide modes with k > 0 at different frequencies will result in the top-viewed anticlockwise elliptically polarized velocity fields corresponding to sy > 0, which make opposite propagating modes of opposite polarizations become approximately orthogonal to each other This momentum-locked SAM profile of bulk modes within the waveguide will apparently reduce couplings between forward propagating and reflected scattering modes. The nonsymmetric transmission T(θ) ≠ T(−θ) can be found for the metasurface waveguides with opposite bending angles, which indicates that the spinful waveguide mode will pass more
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