A Study On Gradient Index Metasurface And Topological Phononic Crystal For Sound Wave Manipulation

In recent years,acoustic metasurfaces have become a hot topic in acoustic regulation due to their advantages of subwavelength,small size and simple design.With the deepening of the research,more and more acoustic metasurface structures have been studied,and more novel and complex phenomena have been realized,as well as the corresponding experiments have been confirmed.Transforming the concept of topological phases into classical waves,such as optics,mechanics,and acoustics,is currently an active field of research.The phenomenon of edge state and topological one-way transmission can also be realized in classical wave system.In this thesis,we mainly study the properties of acoustic metasurface and phononic crystal.We demonstrate that independent and arbitrary modulation of amplitude and phase of acoustic waves can be achieved by introducing tunable losses into the acoustic gradient index metasurface.We use this attenuated acoustic metasurface to achieve high quality Airy beam,and on this basis,we achieve the acoustic phenomenon of multi-focal focus.In addition,we implement an underwater acoustic topological insulator based on a honeycomb phononic crystal with both a pseudo spin topological state and a higher order topological state.The main content includes the following aspects.Firstly,the concept of acoustic metasurface and its research status are briefly introduced.The concept and research status of topological phononic crystal are briefly introduced.Secondly,the related theory and calculation method of acoustic artificial structure are introduced.Next,we demonstrate that independent and arbitrary modulation of the amplitude and phase of the acoustic wave can be achieved by introducing a tunable loss into the acoustic gradient index metasurface.This method has the advantages of simple design,low cost and high efficiency,and provides a new way for fine processing of acoustic waves.And in a wider frequency band can achieve very good results.We use this attenuated acoustic metasurface to achieve high quality Airy beam,and on this basis,we achieve the acoustic phenomenon of multi-focal focus.Next,we propose an underwater acoustic topological insulator with both a pseudo spin topological state and a higher order topological state,which consists of two honeycomb phononic crystals with a special moon shape.To our surprise,we find that there are not only pseudo-spin topological states in this structure,but also high-order topological states.We achieve band inversion by simply rotating the moonlike scatterer of a phononic crystal.In this way,the topological phase transition is achieved under the condition of constant filling rate.The pseudo-time inversion symmetry is constructed by using the space symmetry of the p and d states of ?-point double degeneracy.Topological insulators implemented by acoustics are similar to topological systems in electronic systems,revealing the relationship between time inversion and topology.We then confirm that the numerically simulated boundary states have good robustness and immune defects such as right-angle bending.This kind of phononic crystal is made of common materials,has a wide topological band gap,and has a simple structure,so it has great advantages and potential in practical application.Finally,we realize the higher order topological states of the acoustic system in a phononic crystal with a honeycomb lattice composed of acoustic resonators.We construct the hexagonal and triangular finite acoustic structures and observe the higher order topological states in them.We find that the angular states distribution of the two finite structures is not the same.In order to further understand the characteristics of the high-order topological states,we designed a hexagon composed of mediocre and non-mediocre structures,and we found that topological boundary states and topological Angle states can also be observed in such splices.