Download or read book Nonlocality and Willis Coupling in Acoustic Metamaterials and Metasurfaces written by Li Quan (Ph. D.) and published by . This book was released on 2020 with total page 290 pages. Available in PDF, EPUB and Kindle. Book excerpt: Acoustic metamaterials and metasurfaces are artificial materials and surfaces composited with meta-atoms and inclusions at the subwavelength scale but behave homogeneously in the macroscopic scale with unusual properties in the manipulation of sound waves. Most of the research in this field has focused so far on the study of the local response of the materials and surface properties but neglected the nonlocality. This dissertation focuses on the study of nonlocal phenomena in acoustics, and explore their unusual abilities in the control and manipulation of sound waves. The dissertation starts with the exploration of pressure-velocity coupling phenomena, also known as Willis coupling or acoustic bianisotropy, in a single subwavelength inclusion and derived general bounds on the Willis response of acoustic scatterers, indicating these bounds can be reached through proper design. A systematic design approach of reciprocal Willis inclusion has been outlined for the realistic implementation of acoustic inclusions with Willis response from zero, mild to maximum. My study shows that reciprocal Willis responses can be mainly attributed to geometrical asymmetries. By breaking the system’s time-reversal symmetry through bias flow, Willis coupling has also been observed in geometry-symmetrical systems. Then, I changed my viewpoint from single inclusion to the collective response of metamaterials at the macroscopic level. By combining moving media with zero-index acoustic propagation, giant bianisotropy and extreme nonreciprocity were achieved in the metamaterials with modest mechanical motion. This special acoustic response was utilized to induce non-reciprocal positive-to-negative sound refraction and the non-reciprocal metamaterial lens which could focus object when the excitation is from a specific side. Finally, the study was extended to metasurface, and explore their unusual properties by considering the strong coupling between neighboring elements. The general impedance relation with the consideration of coupling between neighboring elements was proposed and the mechanism was applied to the design of nonlocal passive metasurfaces which could overcome the limitations of local designs, and achieve unitary efficiency for extreme beam steering. The nonlocal metasurface has also been applied to the design of acoustic hyperbolic metasurface requiring extreme anisotropic impedance which was considered impossible in the local metasurface