An annulus acoustic metasurface (AAM) composed of composite labyrinthine structure (CLS) subunits has been well designed to generate fractional acoustic vortices (FAVs) in air. The FAVs with different topological charges (TCs) are realized by modulating the transmitted phase shifts through the CLS subunits. The evolution of the pressure field and phase distributions of the FAV is investigated numerically using the finite element method and demonstrated theoretically. As TC increases from 1 to 2, the central phase singularity first splits into two phase singularities and then gradually merges into a higher-order phase singularity. Meanwhile, the corresponding pressure field distribution first evolves from the annular intensity pattern to two discontinuous parts and then gradually recovers to the annular ring distribution with larger radius. We further find that the FAV generated by the AAM could extend to a relatively long distance. Finally, experiments are performed to verify the FAV by the AAM and demonstrate its long-distance propagation. The airborne FAVs by the AAMs may find potential applications in micro-particle manipulation, acoustic communication, and edge-detection imaging.

A 3D acoustic metasurface carpet cloak (AMCC) based on groove structure units is proposed and investigated. The key idea behind this acoustic invisibility is the phase compensation via local phase modulation, and thus a cloak with a thickness of only of a half wavelength could be designed for objects with arbitrary geometric shapes and sizes. We first demonstrate the cloaking effect of hiding a conical object numerically, and find that the working bandwidth of the designed AMCC is from 6200-7500 Hz. Further experimental results are in good agreement with the numerical simulations, verifying that the AMCC works well for the normal incident wave and the small-angled incident waves. As another example, we also confirm that the proposed AMCC can hide a semispherical object. The present AMCC is thin, has a simple geometrical structure, and is easy to implement, which benefits future realistic applications.

We propose an acoustic waveguide consisting of periodic parity-time-symmetric zeroindex metamaterials separated by layers of passive medium. Consistent analytical derivations and numerical simulations show that a rich variety of extraordinary scattering effects can be achieved. For example, unidirectional transparency is found to occur at an exceptional point. At singular points, the system behaves as an acoustic coherent perfect absorber (CPA) laser. Bidirectional transparency, induced by the Fabry-Pérot resonance or balanced dissipation and amplification, is also observed. In contrast to the previous scheme without structural periodicity, the proposed periodic structure allows a greater number of conditions under which the CPA laser behavior and bidirectional transparency can be achieved, which may facilitate the experimental realization of such extraordinary effects.

Acoustic zero-index metamaterials (ZIMs) with extremely large phase velocity can be used to manipulate the acoustic transmission by introducing various kinds of defects. However, previous works are based on ideal effective zero-index materials and are restricted to cylindrical defects to predigest the model complexity, which may hamper the practical applications. Here, we theoretically and numerically investigate the acoustic transmission through a ZIM waveguide structure embedded with a rectangular defect. The consistent results demonstrate that the total reflection, total transmission, and cloaking effect can be achieved by introducing suitable rectangular defect into the ideal ZIM. Moreover, the labyrinthine metamaterial, whose effective mass density and reciprocal modulus are simultaneously near zero in a certain frequency region, is further employed to implement a practical ZIM. Numerical simulations show that the transmission amplitude of the labyrinthine ZIM waveguide can cover an entire range of [0, 1] by tailoring the acoustic parameters of the rectangular defect, resulting in the similar intriguing transmission properties obtained with the ideal ZIM. This work provides a systematical study in manipulating acoustic wave propagation through labyrinthine ZIM with rectangular defect in addition to the widely studied cylindrical defects.

Abstract. The light scattering characteristics of sulfate, one of the main pollutant particles in haze, are calculated by T-Matrix method at a target wavelength of 550 nm. The variation between shape factors (such as effective radius and aspect ratio) and scattering phase functions with different types and shapes are analysed in small scale range. The influence of shape factors on scattering cross section and depolarization ratio of particles are also discussed. Results show that the shape of particles has great effects on the spatial distribution of scattering energy, and the scattering properties of particles are sensitive to aspect ratio. The depolarization of spherical particles is close to zero, while the difference between ellipsoidal and cylindrical particles reaches several orders of magnitude. When the equivalent radius is larger than 1.0 μm, the mean depolarization ratio of the non-spherical particles is greater than 0.2. The mean depolarization ratio and scattering cross section of non-spherical particle change continuously with a certain aspect ratio and particle size range, and the shape of some particles can be therefore distinguished under certain conditions.