Cummer, Steven ALi, Junfei2020-06-102020-06-102020https://hdl.handle.net/10161/21044<p>Metamaterials are artificially engineered materials or structures that exhibit exotic properties that are not found in nature. They have been serving as a primary approach to fully control the behavior of electromagnetic waves, acoustic waves and elastic waves in recent years, and is at present a highly active research area. Metasurfaces, as the 2D version of metamaterials, have opened up unprecedented possibilities for controlling waves at will, offering a solution of molding wave propagation within a thin sheet of structures. Most metasurface designs are based on the so-called generalized Snell's Law (GSL) which achieves their functionalities by engineering the local phase shift in the unit cells. However, the efficiency of phase-gradient metasurfaces is fundamentally limited by the impedance mismatch and local porer flow mismatch between incident field and reflected/transmitted field, so that part of the energy is scattered into unwanted higher-order diffracted modes, which hinders the applicability in various scenarios. In this work, we approach these issues by exploiting acoustic bianisotropy (Willis coupling for acoustics) as an additional degree of freedom to control waves. We have explored highly efficient wavefront engineering in airborne acoustics, from manipulating simple plane waves and cylindrical harmonics to more complicated fields and finally, arbitrary wavefronts. Then we extended the application of bianisotropic metasurfaces to general impedance matching problems and demonstrated wavefront engineering in underwater acoustics with two examples: an aberration-layer penetration metasurface and a 3D acoustic tweezer. </p><p>This dissertation provides a summary of the work undertaken to achieve highly efficient and functional wavefront engineering devices, and briefly outlines some objectives for future work. Firstly, we designed an acoustic bianisotropic unit cell with full control over its scattering properties and demonstrated bianisotropic metasurfaces that overcome the fundamental limits of phase-gradient based metasurfaces. Second, we mapped the approach from Cartesian coordinates into cylindrical coordinates and demonstrated the generation of a pure field with high angular momentum. Third, we introduced surface waves to help power redistribution along the metasurface and achieved highly-efficient beam splitting and reflection. Forth, we further introduced the power-flow conformal metasurface to meet the power balance requirements for an arbitrary perfect wavefront transformation. Then we extended the application of bianisotropic metasurfaces and proposed a general impedance matching strategy, and demonstrated the idea with a case of aberration-layer penetration in water. Last but not least, by shaping the wavefront of underwater ultrasound, a 3D acoustic tweezer is demonstrated for manipulating a wide range of particles in a contact-less manner.</p>EngineeringAcousticsElectromagneticsBianisotropicHighly-efficientMetasurfaceTweezerDissertation