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Title: Manipulating acoustic wave propagation with meta-structured surfaces
Authors: Liu, Tuo
Degree: Ph.D.
Issue Date: 2018
Abstract: Metamaterials are a class of artificial materials constructed on subwavelength scale to provide exotic properties absent in nature, enabling many counter-intuitive effects and innovative applications. Recently, the concept of metasurfaces was pushed forward as a promising evolution of metamaterials, where the modulation of wave behaviors is through the specific boundary conditions instead of the constitutive parameters. Among numerous emerging topics, this thesis focuses on metamaterials and metasurfaces for airborne sound either confined within waveguides or guided by structured surfaces open to the environment. Beyond the scope of seeking extraordinary properties along the real axis, the thesis also shows how judiciously tailored losses can play important roles in controlling sound propagation. The thesis starts with a study on a type of gradient holey-structured metasurfaces, along which the dispersive group velocity of the structure-induced surface acoustic waves (SSAWs) slowly drops from that of air to zero. Broadband incident waves parallel and close to the metasurfaces can thus be effectively converted into the SSAWs, with various frequency components being decelerated until trapped at different positions, leading to spatial-spectrally modulated and highly compressed sound field, namely, the so-called acoustic rainbow trapping effect. The thesis further considers the inherent visco-thermal losses inside the holes, a non-negligible factor in practice. Unlike the lossless case, the gradually diminished group velocity becomes anomalous rather than zero at the trapping positions, suggesting that the system's attenuation reaches maximum. Consequently, the unavoidable strong backscattering in the absence of losses, due to the facts that the trapping is temporarily achieved and the local oscillation eventually radiates backward, is almost fully absorbed.
In the following chapter, a design approach of gradient-index (GRIN) holey-structured metasurfaces is presented to manipulate airborne sound in the subwavelength regime via the SSAWs with large wave-vector values. Based on an explicit mapping relation between the effective index of the SSAWs and the hole depth of the unit cell, arbitrary GRIN profile of index values higher than that of air can be directly implemented by adjusting the depth distribution. As a representative example, subwavelength focusing is experimentally realized along a well-designed GRIN metasurface, in which the focal spot size is less than 1/7 of the wavelength in air. The thesis further demonstrates that two-dimensional (2D) subwavelength imaging is available when a scanning of the object plane is conducted, enabled by the near-field coupling between the evanescent waves and the slow SSAWs. The meta-structured surfaces are capable of manipulating not only the SSAWs but also sound waves within waveguides. By decorating the rigid inner surfaces of acoustic waveguides with micro-structures, the resultant metamaterials can modulate the refractive index in a complex plane, which offers an intriguing opportunity to the study of parity-time (PT) symmetry in passive acoustic system. The exploration of non-Hermitian Hamiltonians possessing PT symmetry has tremendously advanced in experiment with optical system through the quantum-classical analogue, yet still a challenge for acoustics due to the lack of natural gain medium. This thesis reports an all passive acoustic PT-symmetric metamaterials crystal constructed through interleaving groove-structured and holey-structured acoustic metamaterials. It provides intrinsic passive PT-symmetric potential available in 2D space, which allows a flexible manipulation of unpaired wave vectors. At the transition point from unbroken to broken PT symmetry phase, unidirectional sound focusing effect is experimentally observed.
Subjects: Hong Kong Polytechnic University -- Dissertations
Acoustical engineering
Pages: xi, 115 pages : color illustrations
Appears in Collections:Thesis

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