宽带编码声学超表面的构建理论和方法研究

Acoustic metasurfaces have aroused a tremendous interest in acoustics, since they exhibit unconventional phenomena in ways that could not be mimicked by traditional materials. Acoustic metasurfaces provide an effective approach to manipulate the low-frequency acoustic waves with sub-wavelength structures. However, most of the acoustic metasurfaces only work at a single frequency. Since the gradient phase on the metasurface is approximated by limited elements in practice, obviously it is difficult to allow waves in a wide frequency range transmitting through all the elements with the same amplitudes and the required phases. Therefore, to get a broadband acoustic metasurface still remains challenging. On the other hand, the design of the acoustic metasurface becomes more complicated along with the higher requirement of the wave manipulation. From the practical viewpoint, a broadband metasurfaces with simple design process is useful, but it is difficult to implement. Inspired by the new concept of "digital metamaterial" in the field of electromagnetics, this project introduces the idea of "coding acoustic metasurfaces" into the acoustics. Based on the deigned Boolean elements and coding sequences, acoustic metasurfaces could provide a phase variation on the interface of two media, which can be used to achieve arbitrary cutting of the wave vector. This project will first focus on the basic theory of coding acoustic metasurfaces, including the wave manipulation mechanism, coding method, coding quantization error. And then, based on the resonance structure or fractal structure, a broadband metasurface unit cell will be proposed. Finally, a variety of typical acoustic functional metasurfaces, such as negative refraction, focusing and low scattering, are constructed. The validity of the theory will be verified both by simulation and experiment. According to this project, the wave manipulation behavior of the coding acoustic metasurfaces will be clarified, a simple and flexible coding method for the broadband acoustics metasurfaces will be proposed. The predicted results of the project could provide both theoretical and technical support for the design of broadband coding acoustic metasurfaces.

声学超表面为人类操控声波提供了非常高的自由度，在以小尺寸结构有效调控低频声波方面极具优势从而被广泛关注。但目前大多数声学超表面仍面临工作频带窄、工艺复杂缺乏灵活性两个问题，制约了声学超表面从理论走向实际应用。本项目借鉴电磁学领域中“数字超材料”的新概念，将“编码”的思路引入声学超表面的设计。通过少量几种“码元式”结构单元的组合，在介质分界面上构造出随空间位置变化的任意突变相位，实现对波矢的调控。本项目一方面深入研究编码声学超表面构建的基础理论问题，包括编码调控机理、编码方法、编码量化误差及其影响；另一方面还将提出具有宽带特性的编码式声学单元结构，利用谐振或分形结构拓展单元带宽；最后构建声学负折射、聚焦和低散射等多种典型声学功能表面，通过仿真和实验共同对理论的有效性进行验证，并对性能进行测试。研究成果将同时拓展声学超表面的构建灵活性和工作带宽，为声学超表面的设计提供新颖的理论基础和技术支撑。

声学超表面为人类操控声波提供了非常高的自由度，可以小尺寸结构有效调控低频声波方面。本项目针对编码声学超表面构建的基础理论问题展开了研究，包括调控机理、编码方法、宽带声学码元的设计等。.声学超表面的相位空间函数满足广义斯涅耳方程，其为解析方程且是连续的，利用声学码元模拟连续相位分布时，离散化程度越高，逼近效果自然更好，但也应考虑结构和加工的复杂度。本项目针对负折射、聚焦声学超表面进行了构建，对比了不同的编码方式所带来的逼近误差，优化分析并最终确定所需的码元个数。在宽带码元设计方面，尝试基于确定的声学码元，微调每个结构的尺寸，使得其工作频率稍微偏离中心频点，这样多个码元排列组合后可以实现一定带宽的声学特性。.具体的，针对负折射声学超表面，采用8个阻抗匹配型声学码元结构来覆盖0-360°的相位分布，通过在刚性声学基体之间填充不同比例的介质来构造声学码元结构，各码元具有不同的折射率和阻抗特性，排列形成渐变的阻抗从而与背景阻抗相匹配，实现声波的有效透射。入射声波的中心频率为3430Hz，入射角度分别为30°、45°、60°的入射波经过该声学表面后，可实现折射角分别为-30°、-17.038°、-7.701°的负折射，Comsol仿真结果与理论预期一致。针对聚焦型声学超表面，首先借助Matlab分析了焦点与相位之间的关系，焦点距离超表面越近，突变相位曲线越陡峭，即相位随空间单位距离变化越大，为了有效覆盖0-360°的相位变化，选取单元结构的数量势必增加。我们选取了焦点位置在5倍波长的超表面来进行设计，通过优化分析，用 10个镀膜型声学码元结构可以实现有效逼近，仿真和实验结果表明无论是平面波入射还是高斯波束入射，都可以观察到明显的聚焦效果。.本项目研究成果使得声学超表面的构建更加灵活简单，为功能型声学超表面的设计提供了一定理论指导，有益于声学超表面在噪声控制、声学保密通信、水下声呐隐身、医疗高清超声成像等领域得到更好的应用。