Research ArticlePHYSICS

Sound vortex diffraction via topological charge in phase gradient metagratings

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Science Advances  02 Oct 2020:
Vol. 6, no. 40, eaba9876
DOI: 10.1126/sciadv.aba9876
  • Fig. 1 Diffraction mechanism of SVs in a 3D cylindrical waveguide with PGM.

    (A) Schematic diagram of a 3D cylindrical waveguide with a PGM, which is composed of lξ fanlike supercells. (B) Topography of the fanlike supercell consisting of m groups of fanlike unit cells. These unit cells with gradient indices can produce a discrete phase modulation to cover a complete range of 2π. (C) Dispersion relationship of propagating vortex modes in a cylindrical waveguide, where there are two propagating modes of l = 0 (l = 1) taking values of v = 0 and v = 1 (v = 1 and v = 2), respectively, corresponding to the lower and upper curves, and the dashed line is R = 0.64λ. (D) Sketch map of SV diffraction in PGM.

  • Fig. 2 Numerical demonstration for SV diffraction through ideal PGMs.

    (A to D) The simulated total acoustic pressure field patterns for incident SVs with different topological charge through the PGM with lξ = 2 and m = 5. (E to H) The simulated total acoustic pressure field patterns for incident SVs with different topological charge through the PGM with lξ = 2 and m = 6. In all cases, ϑ2 = 0.9ϑ1, R = 0.64λ, h = 0.5λ, and λ = 10 cm. In addition, a cylinder with 0.05R is inserted in the center of PGMs for the convenience of simulations.

  • Fig. 3 Practical design for PGM.

    (A) Schematic of one supercell (left side) of the designed PGM (lξ = 2) with five (m = 5) different fanlike resonators at λ = 10 cm, where each resonator is realized by rotating its azimuthal section (right side) along the z axis with ϑ1 = 36°. The azimuthal section consists of four rows of four identical subresonators with height of w = R/4 = 1.6 cm, and the thickness of walls is t = 1.5 mm. Five rectangular blocks with h × R × t are placed behind each resonator. (B) Transmission and phase shift versus the height ratio (w0/w) of the 2D subresonator, in which the neck of the Helmholtz cavity is 1.5 mm. (C) and (D) are the simulated acoustic total field patterns of SV with lin = − 1 incident on the designed PGM from the left side and the right side of the waveguide, respectively.

  • Fig. 4 Experimental demonstration for asymmetric transmission of SVs.

    (A) Fabricated sample. (B) Experimental setup. (C) Simulated (left) and measured (right) phase and amplitude distributions at z = 2.6λ for SV with lin = − 1 incident from the left side of the waveguide. (D) Simulated (left) and measured (right) phase and amplitude distributions at z = − 2.6λ for SV with lin = − 1 incident from the right side of the waveguide. Photo Credit: Yangyang Fu, Nanjing University of Aeronautics and Astronautics.

  • Fig. 5 Unidirectional OAM-based communication in PGM-based waveguide system.

    (A) and (B) are the simulated total pressure field patterns for SV with lin = − 1 and SV with lin = 1 incident from the left and right sides, respectively. (C) and (D) are input and output phase and amplitude, corresponding to (A) and (B), respectively. In simulations, the viscous and thermal dissipation in Helmholtz resonators is mimicked by including losses in the air channels, given as ρ = 1.21 kg/m3 and c = 343(1 + γi) with γ = 0.015. Two designed PGMs with opposite rotation direction are placed with a distance of 20 cm, and the output phase and amplitude are evaluated at planes of z = ± 3λ.

Supplementary Materials

  • Supplementary Materials

    Sound vortex diffraction via topological charge in phase gradient metagratings

    Yangyang Fu, Chen Shen, Xiaohui Zhu, Junfei Li, Youwen Liu, Steven A. Cummer, Yadong Xu

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