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性的器件，可以从低光能中产生高振幅的光声信号。为了达到这个目的，本计画的目标是：1.回顾次波长结构的特性，找出最佳化光吸收与热传递的方法。2.创建一个全场电磁模型，以最大化具有亚波长特征的材料的空间和时间光吸收。3.创建由吸收的光能产生的声学信号的一维模拟。4.使用模型优化原型设计，然后构建和评估优化的设计。背景PA波可以由脉冲入射光产生。光的吸收导致介质的热膨胀，体积的变化导致压力波[1]。为了实现高效的PA转换，材料必须具有高的光吸收和高热-声转换。不幸的是，大多数具有良好光吸收的材料具有低的热膨胀系数。因此，光学超声发射器越来越多地由复合材料制成[2]。通常使用具有良好光吸收的碳纳米管（CNT）和具有大的热膨胀系数的聚二甲基硅氧烷（PDMS）[2]。使用亚波长结构或超材料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”是指具有小于入射光的波长的特征的一般类别的材料。已知这样的结构在最小空间距离上共振地或利用表面等离子体激元增强入射光场，这是高效PA生成的基本要求。该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部署一维电磁模型。最初的文献检索显示，只有少数简单的模拟存在[3]，[4]，因此，该模型开发将使PA设备的设计的系统优化。在开发了模拟电磁场的模型之后，还将创建合成压力波的1D模拟。通过使用传递矩阵方法，可以探索不同材料和结构设计对输出压力信号的影响，从而为设计新的、专门用于PA生成的定制结构提供指导。模型的准确性将通过实验工作进行验证。这些实验将侧重于量化这些（空间色散）结构的吸收特性，利用一系列激光（ps和ns脉冲宽度）已经可用于探测时间吸收特性。在实践中，PA效应显示出成像和传感的巨大前景。然而，当前的PA成像系统使用昂贵且需要严格安全标准的激光源。这限制了它们在临床环境中的使用。通过材料选择和结构设计，可以用较低的光能量产生高幅度的光放大信号。该项目属于EPSRC工程研究领域的福尔斯。[1]F. Gao等人，“对造影剂和薄膜设计优化的光声和热声生成效率的分析研究”，光声，2017年，[2] T。Lee，H. W. Baac，Q. Li和L. J. Guo，“光学纳米材料和复合材料中的高效光声转换”，先进光学材料，2018年。[3]N. Baddour和A. Mandelis，“声阻抗对使用1D频域光声重建地下吸收器几何形状的影响”，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