Research On Abnormal Manipulations Of Acoustic Waves Based On Acoustic Artificial Materials

Acoustic artificial materials(including phononic crystals and acoustic metamaterials)further expand the elastic wave properties of acoustic materials in the natural world,and can achieve extraordinary and interesting physical effects,such as negative refraction,super-resolution imaging,acoustic cloaking,etc.In this paper,by constructing several types of Dirac-cone dispersion relations and transmissive metasurface in artificial materials,various abnormal manipulations of acoustic waves are realized.The first chapter mainly reviews the classification of acoustic artificial materials,related research background,the latest progress,and several types of Dirac-cone dispersion relations in phononic crystals,and summarizes the main research contents of this thesis.In Chapter 2,based on the rectangular lattice phononic crystal composed of scatterers with different material parameters,two types of acoustic semi-Dirac cones are constructed to realize anisotropic acoustic transmission.One is formed by accidental degeneracy of the monopole and dipole modes,located in the center of the first Brillouin zone.The scatterers are acoustic soft materials.By analyzing the equifrequency surface and effective parameters of the semi-Dirac cone,acoustic "switching",acoustic beam splitting,unidirectional wave-front shaping and two types of acoustic cloaking are realized.The other one is formed by the degeneracy of dipole and quadrupole modes and is located at the corner of the first Brillouin zone.The scatterers are acoustic hard materials.This semi-Dirac cone is actually formed by the degeneration of the Dirac-like cone in the square lattice.The flat band in the Dirac-like cone is split into two band structures in different directions due to the reduced symmetry of the lattice.Finally,we have experimentally verified the phenomena of acoustic "switching",sound beam expanding,and unidirectional wavefront shaping.In Chapter 3,pseudospin modes of spoof surface acoustic waves(SAW)and topological phase transition along the surface of phononic crystals are constructed by air cylindrical holes in honeycomb lattice arranged on rigid substrate.Six air cylindrical holes were selected as a supercell and a fourfold degenerate double-Dirac cone is constructed by combining the band-folding theory.The double-Dirac cone can be broken to form two degenerated states and complete band gap by only shrinking or expanding the spacing of adjacent holes in the supercell.It is found that the direction of energy flows clockwise or counterclockwise,thus the pseudospin modes of spoof SAW are constructed.The shrinkage to expansion of the compound cell leads to band inversion,and the system changes from trivial to nontrivial state,accompanied by the phase transition.According to the bulk-boundary correspondence,the unidirectional acoustic edge states can be found at the interface of trivial and nontrivial system.Then we can construct a topologically protected waveguide to realize the unidirectional transmission of surface waves without backscattering.In Chapter 4,a new design method of ultra-thin transmission acoustic cloak is proposed.The ultrathin conformal cloaking for obstacles with arbitrary shapes is achieved based on the hybrid of the phase-control metasurface(PCM)with a high transmittance and a near-zero-index metasurface(NZIM)with a near-zero density.The hybrid metasurface tightly wraps around the obstacle,in which PCM is in the outer layer and NZIM is adjacent to the obstacle.The NZIM is functionally equivalent to an equiphase area and can guide the waves around the obstacle,while the PCM can perpendicularly transfer the incident waves to the NZIM and then control the emergent waves from NZIM to propagate along the original incident direction.Thus the acoustic cloaking can be achieved.The NZIM is constructed by a square labyrinth structure,and there is a flat band in the center of the first Brillouin zone of its band structure.Through the verification of effective parameters,it is found that the square labyrinth structure has the property of zero refractive index at the corresponding frequency of the flat band,and can be used as the NZIM after periodic arrangement.The PCM is realized by the coated labyrinth structure.Finally,two structures are assembled to verify the transmission-type stealth effect of the acoustic cloak against square and round obstacles.In Chapter 5,a new phase-control method based on the broadband metagrating is proposed.The metagrating is composed of subwavelength rectangular waveguides(SRWs)and can concentrate the transmitted waves only in the ±1st diffraction orders by reasonable design of SRWs.Then "sound prism" can be realized by the metagrating to split acoustic wave beams of mixed frequencies.Choosing six SRWs as the "effective aperture".Then the metagrating can also provide a group of phase delay from 0 to 2?,which is only related to the grating constant and the displacement of effective apertures but independent of the wavelength.Thus,the metagrating can modulate the wave trace in a wide frequency band,and convert the plane waves to the Gaussian beams.By rotating the periodic elements into a two-dimensional structure,the Bessel-like acoustic beam is further obtained.In the sixth chapter,a brief summary of the thesis and the prospect of the future work are given.