Researches On Abnormal Manipulation Of Acoustic Wavefront Through Metasurfaces

Recent years have witnessed a tremendous progress in the design of metasurfaces, which greatly reduce the thicknesses of optical elements, and result in various unconventional phenomena and functionalities, such as anomalous reflection/refraction, optical vortex plates, wave plates, and ultra-thin focusing lenses. The central idea of metasurfaces is to introduce the desired phase profile by patterning subwavelength structures at the interface between two natural materials. The rationally designed phase offers an additional, yet a very important degree of freedom to fully control the wave propagation. On the other hand, the practical applications of conventional acoustic elements are frequently hampered by two critical factors. One is the acoustic opaqueness of natural solids for airborne sound due to the extreme impedance contrast with respect to air. Another one is the issue of device thickness, especially for airborne sound in the audible regime that is closely related to our daily life. Based on the Huygens Fresnel’s Principle, in this thesis, inspired by the development of optical metasurfaces, we will give a demonstration of several unconventional acoustic devices, including planar lens made of zigzag slits, acoustic metasurfaces consisting of space-coiling units and multi-scaled blazed gratings. The main works are listed below:1 Focusing and directional beaming effects of airborne sound through a planar lens with zigzag slitsBased on the Huygens-Fresnel principle, we design a planar lens to efficiently realize the interconversion between the point-like sound source and Gaussian beam in ambient air. The lens is constructed by a planar plate perforated elaborately with a nonuniform array of zigzag slits, where the slit exits act as subwavelength-sized secondary sources carrying desired sound responses. The experiments operated at audible regime agree well with the theoretical predictions. This compact device could be useful in daily life applications, such as for medical and detection purposes.2 Anomalous refraction of airborne sound through ultrathin metasurfacesSimilar to their optic counterparts, acoustic components are anticipated to flexibly tailor the propagation of sound. However, the practical applications, e.g. for audible sound with large wavelengths, are frequently hampered by the issue of device thickness. Here we present an effective design of metasurface structures that can deflect the transmitted airborne sound in an anomalous way. This flat lens, made of spatially varied coiling-slit subunits, has a thickness of deep subwavelength. By elaborately optimizing its microstructures, the proposed lens exhibits high performance in steering sound wavefronts. Good agreement has been demonstrated experimentally by a sample around the frequency 2.55 kHz, incident with a Gaussian beam at normal or oblique incidence. This study may open new avenues for numerous daily life applications, such as controlling indoor sound effects by decorating rooms with light metasurface walls.3 Making acoustic half-Bessel beams with metasurfacesUnlike the well-known Airy beams that deform beyond paraxial angles, the Half-Bessel (HB) Beams can bend to steeper angles, which greatly benefits to practical applications. Here we propose an effective metasurface design to construct two-dimensional acoustic HB beams. The design is based on an array of spatially varied coiling-slit units, each of them mimics the wave responses derived from the analytical expression of the acoustic HB beam. A tradeoff method is utilized here to simplify the variable amplitude responses, for the sake of reducing the design complexity. The full-wave simulations and experimental measurements consistently manifest the effectiveness of this design process. Potential applications can be anticipated from the HB beam constructed by metasurface, such as large-angle bending transport of particles.4 Highly efficient blazed gratings based on gradient-comb-like unitsHere we propose a multi-scaled reflective grating with excellent blazed performance (nearly perfect blazed effect at the well-predicted frequency and orientation). The blazed grating consists of a periodical array of metallic super-cells, each made of several equal-distant subwavelength slits with linearly reduced depth. A simple model based on Huygens-Fresnel principle is established to forecast the microwave response for the incidence of different polarizations:for transverse-electric polarization, the structure provides only the ordinary total reflection (i.e., without orientation deflected); for transverse-magnetic (TM) polarization, the waves are deflected to specific orientation due to the linear phase delay of the slit exits. Similar design route can be extended to acoustic systems, considering the mathematic similarity between the acoustic wave and the electromagnetic wave of TM-polarization.