Sound vortex diffraction via topological charge in phase gradient metagratings

Abstract

Wave fields with orbital angular momentum (OAM) have been widely investigated in metasurfaces. By engineering acoustic metasurfaces with phase gradient elements, phase twisting is commonly used to obtain acoustic OAM. However, it has limited ability to manipulate sound vortices, and a more powerful mechanism for sound vortex manipulation is strongly desired. Here, we propose the diffraction mechanism to manipulate sound vortices in a cylindrical waveguide with phase gradient metagratings (PGMs). A sound vortex diffraction law is theoretically revealed based on the generalized conservation principle of topological charge. This diffraction law can explain and predict the complicated diffraction phenomena of sound vortices, as confirmed by numerical simulations. To exemplify our findings, we designed and experimentally verified a PGM based on Helmholtz resonators that support asymmetric transmission of sound vortices. Our work provides previously unidentified opportunities for manipulating sound vortices, which can advance more versatile design for OAM-based devices.

Systematic Design and Experimental Demonstration of Transmission‐Type Multiplexed Acoustic Metaholograms

“…In this case, we search the value of β and set β=0.8, so that the heights of the FM, AM and PM are h0 = 9 mm, h1 = 10 mm, and h2 = 26 mm, respectively, and the FP resonance for the C2 (which corresponds to h1 + h2) occurs at the second desired working frequency, f2 = 17 kHz. It is also useful to note here that, β ≈ f1/f2, due to the fact that the resonant wavelength of the acoustic meta-materials can be approximately scaled up with the structural size [31][32][33][34][35][36]. For more elaborate analytical and descriptive details about the unit cell, the reader is referenced to Supplementary Note 1.…”

Structuring total angular momentum of light along the propagation direction with polarization-controlled meta-optics

“…For paraxial beams, SAM and OAM are additive so that the total angular momentum (TAM) per photon is expressed as ± ℏ + ℓ ℏ 1 . Notably, vortex beams have been widely exploited in classical and quantum communications 9 – 12 , micromanipulation 13 – 15 , and other applications 16 – 22 , and have been studied beyond the domain of optics, for, e.g., acoustic and electron vortex beams 23 , 24 , to name a few.…”

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

“…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. 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.…”

“…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.…”