The use of subwavelength structures to control and enhance photoacoustic signals

本项目的目的是开发一种具有亚波长特征的器件，可以从低光能中产生高振幅的光声信号。为了实现这一目标，该项目的目标是：回顾亚波长结构的性质，确定优化光吸收和热传递的方法。2.创建一个电磁场模型，以最大限度地提高具有亚波长特征的材料的时空光吸收。创建由吸收光能产生的声信号的一维模拟。使用模型优化原型设计，然后构建和评估优化后的设计。背景PA波可以由脉冲入射光产生。光的吸收引起介质的热膨胀，体积的变化导致压力波[1]。为了实现高效的PA转换，材料必须具有高光吸收和高热声转换。不幸的是，大多数具有良好光吸收率的材料具有较低的热膨胀系数。因此，光学超声发射机越来越多地由复合材料[2]制成。具有良好光吸收性能的碳纳米管（CNT）和热膨胀系数大的聚二甲基硅氧烷（PDMS）是常用的bb0材料。亚波长结构或超材料的使用可以改善聚合物层的光吸收和热传递。术语“亚波长”或“超材料”指的是一类具有小于入射光波长特征的材料。众所周知，这种结构可以在最小的空间距离内增强入射光场，无论是共振还是与表面等离子激元极化，这是有效产生PA的基本要求。项目的初始阶段将是确定PA生成所需的特性，评估可行的材料特性并审查当前使用的方法。由此产生的理解和知识将用于在一维中部署电磁模型。最初的文献检索显示，只有一些简单的模拟存在[3]，b[4]，因此，该模型的开发将使PA器件的设计系统优化。在开发了一个模型来模拟电磁场之后，还将创建一个一维模拟所产生的压力波。通过传递矩阵法，可以探索不同材料和结构设计对输出压力信号的影响。通过这种方式，建模将指导新的设计，专门为PA生成定制结构。该模型的准确性将通过实验工作进行验证。这些实验将集中于量化这些（空间色散）结构的吸收特性，利用已经可用的一系列激光器（脉冲宽度为ps和ns）来探测时间吸收特性。在实践中，PA效应在成像和传感方面显示出巨大的前景。然而，目前的PA成像系统使用昂贵的激光源，并要求严格的安全标准。这限制了它们在临床环境中的使用。通过精心的材料选择和结构设计，可以用较低的光能源产生高振幅的扩音信号。该项目属于EPSRC工程研究领域高峰等，“光声和热声生成效率对造影剂和薄膜设计优化的分析研究”，李涛，李强，郭立军，“光学纳米材料及其复合材料的高效光声转换”，光学材料，2018.[3]N. badour和A. Mandelis，“基于一维频域光声的地下吸收器几何重建的声阻抗影响”，《光声学》，2015年12月

GoalThe aim of this project is to develop a device with sub-wavelength features that can produce high-amplitude photoacoustic signals from low optical energy. ObjectivesIn order to achieve this aim, the objectives of the project are to:1.Review the properties of subwavelength structures, identifying methods to optimise optical absorption and heat transfer. 2.Create a full-field electromagnetic model to maximise the spatial and temporal optical absorption in materials with subwavelength features.3.Create a 1D simulation of the acoustic signal generated from absorbed optical energy.4.Optimise a prototype design using the model and then build and evaluate the optimised design.Background PA waves can be produced from pulsed incident light. The absorption of light causes thermal expansion of the medium and the change in volume results in a pressure wave [1]. For highly efficient PA conversion, a material must have high optical absorption and high heat-to-sound conversion. Unfortunately, most materials with good light absorption have a low thermal expansion coefficient. Therefore, optical ultrasound transmitters are increasingly being made from composites [2]. Carbon nanotubes (CNT) with good optical absorption and polydimethylsiloxane (PDMS) with a large thermal expansion coefficient are commonly used [2].The use of subwavelength structures, or metamaterials, could improve optical absorption and heat transfer to the polymer layer. The term 'subwavelength' or 'metamaterial' refers to a general class of materials with features that are smaller than the wavelength of incident light. Such structures are known to enhance incident optical fields, either resonantly or with surface plasmon polaritons, over a minimal spatial distance - an essential requirement for efficient PA generation. Outline of research methodology The initial stage of the project will be to identify the desirable properties for PA generation, evaluate viable material properties and review the current approaches used. The resultant understanding and knowledge will be used to deploy an electromagnetic model in one dimension. An initial literature search has revealed that only a few simple simulations exist [3], [4], and hence, this model development will enable the systematic optimisation of the design of PA devices. Having developed a model to simulate the electromagnetic fields, a 1D simulation of the resultant pressure wave will also be created. By using a transfer-matrix method, the effect of different materials and structure design on the output pressure signal can be explored.In this way, the modelling will guide the design of new, bespoke structures specifically for PA generation. The accuracy of the model will be validated with experimental work. These experiments will focus on quantifying the absorption properties of these (spatially dispersive) structures, utilising a range of lasers (ps and ns pulse widths) already available to probe the temporal absorption properties. In practice, the PA effect shows great promise for imaging and sensing. However, current PA imaging systems use the laser sources that are expensive and require strict safety standards. This limits their use in clinical settings. By careful material selection and structure design, high-amplitude PA signals could be produced by lower optical energy sources. This project falls within the EPSRC Engineering research area.[1] F. Gao et al., "An analytical study of photoacoustic and thermoacoustic generation efficiency towards contrast agent and film design optimization," Photoacoustics, 2017, [2] T. Lee, H. W. Baac, Q. Li, and L. J. Guo, "Efficient Photoacoustic Conversion in Optical Nanomaterials and Composites," Advanced Optical Materials, 2018.[3] N. Baddour and A. Mandelis, "The Effect of Acoustic Impedance on Subsurface Absorber Geometry Reconstruction using 1D Frequency-Domain Photoacoustics," Photoacoustics, Dec. 2015