| In recent years,due to its unique physical properties and parameters that can be effectively controlled manually,acoustic metamaterials have largely overcome the problem of material property limitations in nature,and have attracted highly attention.Acoustic metasurfaces,regarded as the result of the two-dimensional simplification of acoustic metamaterials,still retain the peculiar characteristics that do not exist in natural materials such as negative bulk modulus,negative mass density,and anisotropy,meanwhile,its thickness becomes more thinner and unit size becomes more smaller,which is so in line with the development trend of small-scale miniaturization and integration of devices as favored by scientific researchers deeply.Acoustic metasurface combined with varied phase distribution can effectively realize acoustic wave manipulation freely.Many important and practical acoustic wave manipulation phenomena and devices have been develo ped,such as acoustic negative refraction,acoustic vortex,acoustic absorption,ventilated acoustic insulation barrier and acoustic focusing lens,etc.Acoustic metasurface focusing plays a highly important role in biomedical ultrasound imaging and medical testing,but it still exists a few limitations such as narrow working bandwidth,large volume,poor tunability,and single function.An acoustic metasurface of a window-shaped structure is proposed in this dissertation and we have carried out specific researches on its focusing characteristics and the diversification of its tunable functions.The main research contents of this dissertation are included as follows:1.A window-shaped structural unit that is easy to construct and possesses high flexibility is designed.The actual length of the sound wave transmission channel is manipulated by varying the length of the two window handles,and the finite element method is employed to calculate and analyze the acoustic transmission performance and phase distribution characteristics of the structural unit with the variation of the length of two window handles.2.An acoustic focusing metasurface lens is proposed based on the window-shaped structure unit combined with the generalized Snell's law and its corresponding model is constructed.The finite element method is employed to calculate and analyze its relationship with frequency,and then a large number of calculations are made for the incident sound waves penetrating the metasurface at different frequencies to obtain the focusing acoustic field and analyze the focusing effect,focus characteristics and effective frequency distribution range.Without changing the metasurface construction unit,set different intervals in the middle of the metasurface along the x-axis direction to explore the axial mobility of the focus position.3.Introduce non-Hermitian complex parameters into the proposed metasurface of window-shaped structure,and only vary the value of the imaginary part of the mass density while keeping other parameters of the structure unchanged.Based on the finite element method,the acoustic intensity field and focus intensity values are obtained,and the variations of the focus characteristics and focus intensity values are compared and analyzed before and after the metasurface is introduced non-Hermitian complex parameters.4.Based on the effective combination of wi ndow-shaped structure unit and digital encoding mechanism,a binary phase encoding metasurface is proposed.Only two kind of window-shaped structure units corresponding to phase ?(logical unit being “1”)and0(logical unit being “0”)respectively are adopted,and study the effective frequency band of focusing and the multi-tunability of the focus,and use the re-encoded metasurface to explore multiple functions such as acoustic beam splitting and deep angle deflection and so on.Use the finite element method to simulate and calculate the acoustic pressure intensity field and acoustic pressure field of different functions,and analyze their respective characteristics in detail. |
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