Acoustic Metalens With Space-coiling Structures

Acoustic metamaterials are composite materials consisting of artificial subwavelength structures that behave like a continuous material with unconventional effective properties,such as negative effective dynamic bulk modulus and negative effective dynamic mass density.The intensive researches expand acoustic materials and their application fields and provide a new route for sound wave manipulation.The metamaterials-based lenses offer the new types of imaging components and functions by controlling and manipulating the path of sound,which remains a great technical challenge in terms of experimental realization.This dissertation gives important studies on acoustic wave propagation with these artificial lenses.The dissertation is divided into following sections:In Chapter ?,the origin and applications of electromagnetic metamaterials and acoustic metamaterials are reviewed.The history of negative-parameter,nearzero-parameter and double-negative-parameter acoustic metamaterials are described.In Chapter ?,an acoustic metalens was proposed comprised of the anisotropic refractive index cavities with coiling-up space.The design significantly downsizes the thickness of the metalens compared to that based on the Fabry-Pérot resonance.The strong resonant tunneling compression effect is further demonstrated in the cavities.Imaging experiments have shown that the objects located at a considerably enlarged distance away from the metalens can be imaged with a subwavelength spatial resolution.In Chapter ?,we designed,fabricated,and experimentally demonstrated a gradient acoustic metasurface to manipulate sound radiation patterns.The gradient metasurface is constructed on the basis of a coiling-up space in a tunable interdigitated structure,which exhibits relative refractive index in a discretized classic hyperbolic secant profile.Capable of generating secondary sound sources with desired gradient phase shifts,the metasurface shows the ability of controlling sound radiation such as by cylindrical-to-plane-wave conversion,plane wave focusing,and effective tunable acoustic negative refraction.Owing to its deep-subwavelength thickness,the metasurface may reduce the size of acoustic devices and offer potential applications in imaging and scanning systems.In Chapter ?,a planar acoustic Luneburg lens in air was proposed by using space-coiling metamaterials,which was fabricated with epoxy resin by means of 3D printing.Good focusing ability has been shown from the measured field distributions of the proposed lens.The broadband performance of the lens is also experimentally confirmed for frequencies ranging from 1 kHz to 3 kHz.A wide-angle acoustic reflector has also been designed and demonstrated by the simulations.In Chapter ?,a two-dimensional acoustic Maxwell's fish-eye lens has been proposed by using the gradient-index metamaterials with space-coiling units.As predicted by ray trajectories on a virtual sphere,the proposed lens has the capability to focus the acoustic wave irradiated from a point source at the surface of the lens on the diametrically opposite side of the lens.The broadband and low loss performance is further demonstrated for the lens.The proposed acoustic fish-eye lens is expected to have the potential applications in directional acoustic coupler or coherent ultrasonic imaging.In Chapter ?,we demonstrate an ultraslow-fluid-like particle with intense artificial Mie resonances for low-frequency airborne sound.Eigenstate analysis and effective parameter retrieval show two individual negative bands in the single-size unit cell,one of which exhibits a negative bulk modulus supported by the monopolar Mie resonance,whereas the other exhibits a negative mass density induced by the dipolar Mie resonance.The unique single-negative nature is used to develop an ultra-sparse subwavelength metasurface with high reflectance for low-frequency sound.The designed Mie resonators provide diverse routes to construct novel acoustic devices with versatile applications.Finally,the main conclusions of the present study and the prospect for the future work are drawn in Chapter ?.