Topological Effects In Artificial Acoustic Materials

The emergence of phononic crystals and acoustic metamaterials has provided us with new,powerful materials to manipulate sound,owing to their anomalous properties.There is no strict distinction between those two types of acoustic artificial materials.The phononic crystals concern with the propagation of sound waves in periodic structures,and the physical properties origin from the band structures in the momentum space;while the designs of building blocks are more important for acoustic metamaterials,and the subwavelength building blocks can be described by effective parameters,with the unusual properties originating from local resonances.In recent years,with the discovery of topological phases in condensed matter physics,acoustic topological states have drawn considerable attention,and numerous theoretical and experimental investigations demonstrated the feasibility of topological acoustics,such as acoustic quantum Hall effect,acoustic topological insulator,acoustic valley Hall effect,Floquet topological insulator,Weyl phononic crystal and acoustic higher-order topological state.Acoustic systems provide a superior platform for studying topological physics and its extensive ramifications,which in turn leads to a fascinating way to manipulate acoustic wavesThis thesis is mainly focused on the topological effects in artificial acoustic materials,including the study of the theoretical model,finite element calculations,fabrication of samples,and experimental measurements.The details are listed as follows:1.We measure the acoustic spin states and acoustic skyrmion lattice in acoustic artificial materials,by using the particle velocity sensor.Inside each unit cell of the skyrmion lattice,the orientation of the particle velocity vector gradually changes from inside to outside,and finally flips on the boundary.2.We demonstrate an acoustic Weyl phononic crystal by breaking space inversion symmetry using a combination of slanted acoustic waveguides.We directly characterize the acoustic Weyl points by performing angle-resolved transmission measurements.We also observe acoustic "Fermi arcs" by scanning the distribution of surface waves and demonstrate robust one-way acoustic transport,where the surface waves can overcome a step barrier without reflection.3.We realize the inversion of acoustic energy bands at a double Dirac cone and provide an experimental demonstration of an acoustic topological insulator.By manipulating the hopping interaction of neighboring atoms in this new topological material,we successfully demonstrate the acoustic quantum spin Hall effect,characterized by robust pseudospin-dependent one-way edge sound transport.4.We experimentally construct two-dimensional acoustic topological pseudospin-valley coupled saddle surface states,designed from glide symmetry in a three-dimensional system.Due to the hyperbolic character of the dispersion of saddle surface states,multi-directional anisotropic controllable robust sound transport with little backscattering is observed.Above all,focused on the topological effects in artificial acoustic materials,this thesis yielded some comprehensive accomplishments from fundamental designing,material preparation,its physical properties to practical applications.