Acoustic Topological States In Artificial Structures

The counterparts of the natural materials,artificial structural materials(i.e.,Metamaterials,metasurfaces,artificial crystals),have received extensive interest in the last few decades,due to its ability in wave manipulation.Metamaterials,one of the artificial materials,could provide many acoustic and electromagnetic properties unavailable in nature,such as,zero or negative refractive index,effective mass density with negative value,negative effective bulk modulus.By employing these artificial materials,the acoustic and electromagnetic waves can be controlled in ways that are not possible in conventional materials.Comparing with metamaterials,metasurfaces with subwavelength thickness have added value and unusual functionalities and provided a new route for wave manipulation.Recently,some interesting topology properties have been realized in artificial crystal.In this thesis,we have observed some interesting volatility behavior in artificial crystal,such as valley-selective transport,one-way edge states,and so on.Especially,here we present the first experimental demonstration of acoustic semimetal with topologically charged nodal surface.Specific information of the main works is as follows: 1.Observation of Acoustic Valley Vortex States and Valley-Chirality Locked Beam SplittingWe report an experimental observation of the classical version of valley polarized states in a two-dimensional hexagonal sonic crystal,where the inversion-symmetry breaking of scatterers induces an omnidirectional frequency gap.The acoustic valley states,which carry specific linear momenta and orbital angular momenta,were selectively excited by external Gaussian beams and conveniently confirmed by the pressure distribution outside the crystal,according to the criterion of momentum conservation.The vortex nature of such intriguing crystal states was directly characterized by scanning the phase profile inside the crystal.In addition,we observed a peculiar beam splitting phenomenon,in which the separated beams are constructed by different valleys and locked to the opposite vortex chirality.The exceptional sound transport,encoded with valley-chirality locked information,may serve as the basis of designing conceptually novel acoustic devices with unconventional functions.2.Observation of valley-selective microwave transport in photonic crystalsRecently,the discrete valley degree of freedom has attracted extensive attention in condensed matter physics.Here,we present an experimental observation of the intriguing valley transport for microwaves in photonic crystals,including the bulk valley transport and the valley-projected edge modes along the interface separating different photonic insulating phases.For both cases,valley selective excitations are realized by a point-like chiral source located at proper locations inside the samples.Our results are promising for exploring unprecedented routes to manipulate microwaves.3.Experimental Demonstration of Acoustic Semimetal with Topologically Charged Nodal SurfaceWe report the existence of topologically charged nodal surface – a twodimensional band degeneracy in momentum space that carries non-zero total Berry flux.We develop an effective Hamiltonian for such a charged nodal surface,and show that this Hamiltonian can be implemented in a tight-binding model as well as in an airborne phononic crystal.We have experimentally measured the “Femi-arcs” on the surfaces of the phononic crystal,which can serve as an evidence for the nonzero charge possessed by the nodal surface.Meanwhile,the end points of the measured “Fermi arcs” on the nodal surface depends on the position of the truncation,which distinguishes charged nodal surface semimetals from the traditional Weyl semimetals.Our result indicates that in the band theory,topologically charged objects are not restricted to zero dimension as in a Weyl point,and thus pointing to previously unexplored opportunities for the design of topological materials.4.Making sound vortices by metasurfacesBased on the Huygens-Fresnel principle,a metasurface structure is designed to generate a sound vortex beam in airborne environment.The metasurface is constructed by a thin planar plate perforated with a circular array of deep subwavelength resonators with desired phase and amplitude responses.The metasurface approach in making sound vortices is validated well by full-wave simulations and experimental measurements.Potential applications of such artificial spiral beams can be anticipated,as exemplified experimentally by the torque effect exerting on an absorbing disk.