Research On Asymmetric Phenomenon Based On Acoustic Metamaterials

Acoustic metamaterials are artificially engineered media made from subwavelength building blocks.Through careful design of the material’s composition,size,and structure,the sound path and resonance can be adjusted,thus realizing many intriguing properties which cannot be found in nature materials.In recent years,due to its broad development prospect and application value,there are various structures proposed to realize versatile properties.It has not only realized a series of acoustic functions with direct application scenarios,such as cloak,low-frequency sound insulation and superresolution,but also developed rapidly in the emerging fields of topology and PT symmetry.Metamaterial has become a research hotspot in acoustics.In this dissertation,several new acoustic devices of metamaterials are proposed and the transmission behaviors of sound waves are systematically studied.We focus on acoustic spin Halllike effect,asymmetric acoustic transmission with a lossy metasurface,acoustic accelerating beam based on a curved metasurface,and PT-symmetric coherent perfect absorption with a coupled-Mie-resonators system.Acoustic spin Hall-like effect and asymmetric acoustic transmission with a lossy metasurface study the asymmetric propagation of sound waves.Acoustic accelerating beam based on a curved metasurface studies the asymmetric structure of the metasurface.PT-symmetric coherent perfect absorption with a coupled-Mie-resonators system studies the asymmetry of the gain and loss in the system.In conclusion,the asymmetric phenomena in metamaterials are studied from different perspectives in this dissertation.In chapter one,we give an introduction for the proposal and classification of metamaterials,the research progress,and a brief outline of the dissertation.In chapter two,we investigate the spin Hall-like effect in hyperbolic metamaterials controlled by the helical wave.Acoustic helical wave with different helical directions is taken as a “spin-like” degree of freedom.The hyperbolic metamaterials are constructed from string arrays and membrane arrays.There is the pseudospin-orbit coupling effect when the acoustic helical wave emitter is situated at the boundary of the hyperbolic metamaterials.Thus,the unidirectional excitation is observed,and the direction of excitation is completely related to the direction of the helical wave.This phenomenon is very similar to the spin Hall effect in electronics,so it is called the spin Hall-like effect in acoustics.This phenomenon is verified by the finite element numerical simulation and circuit simulation.In chapter three,we study the asymmetric acoustic transmission with a lossy metasurface.The acoustic waves are little attenuated for positive incidence(PI)and strongly attenuated for negative incidence(NI).The lossy metasurface has the effects of the phase gradient and period grating.The total diffraction order takes a value of-1 for PI and +1 for NI.The diffraction order associated with the period grating takes a value of 0(without multiple reflections)for PI and 2(with multiple reflections)for NI.There are multiple reflections in the metasurface for high-order diffraction waves associated with the period grating.In other words,there are multiple reflections in the lossy metasurface only for NI.The sound waves for NI stay longer than for PI in the lossy metasurface.Thus,the asymmetric acoustic transmission is obtained.In chapter four,we study the acoustic accelerating beam based on a curved metasurface.By extending the design method of flat metasurfaces to curved metasurfaces,the multi-directional sound transmission with a curved metasurface is obtained.Based on this design method,we take an analog of the curved spacetime theory in general relativity: the different motions of particles in different curved spacetime follow the same physical laws.In the metasurface,it is expressed as: different curved metasurfaces with different phase shift distributions can generate an identical target acoustic field.In other words,covariant structures can produce an identical acoustic field by the covariant transformation between different curved metasurfaces.First,we demonstrate the covariant transformation between the accelerated velocity metasurface and the constant velocity metasurface.Then,we employ the Rindler metric in the design of covariant metasurfaces to generate a new kind of acoustic accelerating beam,i.e.the Rindler beam.In chapter five,we study the parity-time-symmetric coherent perfect absorption with an acoustic passive-coupled-resonators system.The system is consisted of two Mie resonators distributed spatially symmetric in their waveguides at both sides of the aperture,which is designed to connect the two waveguides.The waveguides of the Mie resonators probe the system’s resonances and provide the incident waves.In our design,the effective gain is realized by the incident waves from two opposite directions of one waveguide,and the loss is realized by the radiation boundaries of the other waveguide.Thus,the loss and gain are balanced with only passive materials.We observe coherent perfect absorption which can completely absorb sound in its exact PT-symmetry phase(two Mie resonators are situated closely)but not in its broken PT-symmetry phase(two Mie resonators become far away).The last chapter presents a summary of this dissertation,and then gives some outlook for future investigation.