Manipulation Of Reflected Waves In The 3D Space By Tunable Acoustic Metasurfaces

As a two-dimensional analog of metamaterials,metasurfaces can freely engineer the reflected or transmitted wavefront.The crucial concept is to introduce discontinuous phase shift.Specifically,by redistributing the phase on the metasurface,the acoustic wavefront will be engineered after interacting with the metasurface.In this thesis,the generalized Snell’s law(GSL)applicable to the three-dimensional space is deduced,which extends the application realm of the metasurfaces enormously.The micro-structure unit with the ability to tailor the phase shift continuously is designed,and the abnormal reflection,focusing,non-diffractive beams,ground illusion and other extraordinary characteristics for acoustic waves in the three-dimensional space are studied.In addition,the strategy to improve the performance of metasurfaces by introducing some kind of impedance boundaries is discussed,and it is shown that the performance of the metasurface based on the impedance distribution is better than that based on the GSL.The main content and conclusions are as follows:1.The generalized Snell’s law(3D-GSL)on a flat surface in the three-dimensional space is derived,and a kind of micro-structure unit that can continuously tailor the phase shift is designed.The unit can be regarded as a stack of multiple Helmholtz resonators.By injecting different volumes of liquid into the unit,the phase shift of the reflected wave can be continuously changed.The effect of the aperture size of the unit is analyzed in detail.It is pointed out that the behavior of the unit with a small aperture is similar to a resonant structure,while it is similar to a tube when the unit has a large aperture.In addition,the thermal-viscosity effect is also discussed.With the help of 3D-GSL,the phase distribution functions required for different acoustic features such as abnormal reflection,focusing,and non-diffractive beams in the three-dimensional space are deduced,and a metasurface is constructed to realize the above functions.It is worth mentioning that,the same metasurface can be applied to multiple scenarios owing to the continuously tunability.2.The GSL needs to be modified if the shape of the reflective surface is not a plane.In this thesis,the generalized Snell’s law suitable for an arbitrarily curved surface is derived with its the mathematical form including the geometric information of the reflective surface.By using the 3D-GSL on a curved surface,the phase distribution functions for anomalous reflection,focusing and ground illusion with a curved reflective surface are presented.A metasurface with a curved shape is constructed and realized for above applications.In addition,the reason of the deviation is analyzed,and it is found that not only the discretization of the reflective surface but also the complexity of the curved surface causes the error.3.Combining the theory of caustics and GSL on curved surfaces,the design principle using curved metasurfaces to realize self-accelerating beams is given.The phase distribution function required for the reflective metasurface with a hyperbola to focus the waves along a circular arc or hyperbola trajectory is deduced in detail;and the differences between the plane wave incidence and point source incidence are discussed.A tunable 2D unit is designed and a metasurface is constructed to verify the design numerically.The experiment is performed to measure the self-accelerating beams with the circular arc shape;and the measured acoustic field is consistent with the simulation result.Finally,the axisymmetric 3D self-accelerating beams is studied and verified numerically.4.Four different impedance boundary conditions for improving the performance of the metasurface are derived.No matter which impedance boundary it is,the reflection efficiency is significantly higher than that with the phase distribution based on GSL.Some impedance conditions required the construction unit can absorb or provide energy,which causes the difficulties in applications.In order to use general passive unit to construct the impedance boundary,a conversion method is proposed.With the help of the deduced impedance boundary,the impedance distributions of the planar and the curved reflective surfaces are studied.Finally,we realize the high efficiency abnormal reflection and carpet cloaking.In this thesis,the design metasurfaces with planar and curved geometric shapes for engineering reflected acoustic waves in the three-dimensional space is systematically studied;and the strategy for improving the reflection efficiency of the metasurface is proposed.The related research greatly expands the application realm of the metasurface and pave the way to design high-performance planar or curved acoustic devices.