Symmetry selective directionality in near-field acoustics

Abstract

Understanding unidirectional and topological wave phenomena requires the unveiling of intrinsic geometry and symmetry for wave dynamics. This is essential yet challenging for the flexible control of near-field evanescent waves, highly desirable in broad practical scenarios ranging from information communication to energy radiation. However, exploitations of near-field waves are limited by a lack of fundamental understanding about inherent near-field symmetry and directional coupling at sub-wavelengths, especially for longitudinal waves. Here, based on the acoustic wave platform, we show the efficient selective couplings enabled by near-field symmetry properties. Based on the inherent symmetry properties of three geometrically orthogonal vectors in near-field acoustics, we successfully realize acoustic Janus, Huygens, spin sources and quadrupole hybrid sources, respectively. Moreover, we experimentally demonstrate fertile symmetry selective directionality of those evanescent modes, supported by two opposite meta-surfaces. The symmetry properties of the near-field acoustic spin angular momenta are revealed by directly measuring local vectorial fields. Our findings advance the understanding of symmetries in near-field physics, supply feasible approaches for directional couplings, and pave the way for promising acoustic devices in the future.

Realization of acoustic spin transport in metasurface waveguides

“…According to the physical meanings possessed in angular momenta of acoustic waves, the SAM density can be described as (refs. 5 – 8 ): where ρ is the mass density, ω is the frequency, c is acoustic velocity, and v is acoustic velocity field. According to the definition in Eq.…”

“…According to the physical meanings possessed in angular momenta of acoustic waves, the SAM density can be described as (refs. [5][6][7][8] ):…”

“…Therefore, people widely believed that the curl-free longitudinal wave cannot shape into circularly polarized from as transverse optical waves so that the acoustic waves cannot possess SAM. Just recently, after fully treating the acoustics as vectorial velocity fields, the fact that the acoustics can process the SAM intrinsically has been unveiled theoretically and observed experimentally 5 – 8 . The acoustic SAM can thus be naturally associated with the circularly (elliptically, in general) polarized profile of acoustic velocity fields (nonzero v * × v ), which does not conflict with the zero vorticity ∇ × v = 0 of the curl-free acoustic waves.…”

“…he spin angular momenta (SAM) of classical waves, ranging from elastic to optical waves, are essentially associated with their local chiral polarized profiles. Many well-known waveforms, such as circularly polarized plane wave, two-wave interference, spatial confined Gaussian beam, and surface evanescent wave, have been shown to possess nonzero SAM [1][2][3][4][5][6][7][8] . The SAM exhibits a family of spin-related effects with remarkable properties for waves, i.e., spin-Hall effect (SHE) 2,9 , quantum spin Hall effect (QSHE) 10,11 and spin-locked scattering in bianisotropic media 12 .…”

Dirac cones and chiral selection of elastic waves in a soft strip

“…4B, such a chiral source excites the S1 mode which propagates toward x1 > 0; however, it cannot produce its time-reverse partner S * 1 propagating in the opposite direction. By controlling the source's chirality, we performed selective feeding and one-way wave transport, a feature which has recently been exploited in different contexts (42)(43)(44).…”