Study On Acoustic Artificial Microstructure Coding And Topological Properties

Artificial microstructures are composed by sub-wavelength ‘atoms’,whose physical properties are determined by the unit structure design and the spacial arrangement.Compared to traditional materials,artificial microstructures have provided extra degree of freedom in wave manipulation,and have demonstrated lots of promising applications in optic,acoustic and elastic waves.Coding artificial microstructure with flexible design manner and topological artificial microstructures with rich physical properties have aroused a tremendous interest recently.In this thesis,we focus on the coding and the topological properties of artificial microstructures in acoustics,including1.We designed coding acoustic metasurfaces by using the coding sequences of designed Boolean elements and showed that acoustic waves can be manipulated by the coding metasurfaces with great flexibility.Radiation pattern control,focusing and wave branching effects are realized with a certain bandwidth.With the concept of the acoustic coding metamaterials or metasurfaces,innovative acoustic devices may be developed and used in acoustic wave processing,digital lenses,diffusion cloaking and wave field modulations.2.We realized multiband asymmetric transmission of sound by coded metasurface which is composed of two Boolean elements of the same transmission rate and opposite phase.Utilizing the interference of the coupled coding units,angle-and frequency-dependent asymmetric propagation of sound is clearly observed.Compared to acoustic diodes proposed with sonic crystals,our ultrathin system offers advantages such as broadband operation,high contrast ratio,and suitability for directional operation.This concept is also expected to inspire other ultrathin unidirectional device designs in both acoustics and photonics.3.We study the acoustic topological insulator based on Kekulé Lattice.The effective Hamiltonian obtained by the tight-binding model,was used to analyzed the effect of Kekulé distortion in acoustic crystal.Pseudospin polarized or valley polarized topological states,emerge at the domain walls constructed by mirror and translation operations.Through numerical simulation and experiment,we analyzed the topological transport and topological refraction.This work provides a new way towards the design of waveguides and directional antennas for sound.4.We report on the experimental realization of acoustic type-II WPs in a stacked graphenelike chiral phononic crystal.The experimental results clearly exhibit the characteristics of type-II WPs by observing conical dispersion and the Lifshitz transition at Fermi frequencies.We also show the existence of acoustic Fermi arcs,which connect the bulk bands that enclose WPs of opposite charges.All observations are in good agreement with the theoretical analyses.As the analog counterpart in classical waves,the phononic crystal brings a platform for the research of type-II WPs in macroscopic systems.5.We report on the realization of acoustic three-dimensional Dirac phononic crystal.Utilizing the Hamiltonian derived from tight-binding model,we analyzed the band behaviors and topological properties,and obtain the pseudospin and topological invariant.We predict the transition of three-dimensional Dirac points towards Weyl points in chiral phononic crystal.We experimentally observed the Fermi arcs connecting the Dirac points,and investigate the interface effect to the existence of Fermi arcs.This work provides new insights towards three-dimensional topological matters.