The use of subwavelength structures to control and enhance photoacoustic signals

目标该项目的目标是开发一种具有亚波长特征的设备，该设备可以从低光能产生高幅度光声信号。目的为了实现这一目标，该项目的目标是：1.回顾亚波长结构的特性，确定优化光吸收和热传递的方法。2.建立全场电磁模型，以最大化具有亚波长特性的材料的空间和时间光吸收。3.创建从吸收的光能量产生的声信号的一维模拟。4.使用该模型优化原型设计，然后建立和评估优化的设计。背景PA波可以由脉冲入射光产生。光的吸收引起介质的热膨胀，体积的变化引起压力波[1]。要实现高效的功率放大器转换，材料必须具有高的光吸收和高的热声转换。不幸的是，大多数具有良好光吸收的材料的热膨胀系数都很低。因此，光学超声发射机越来越多地由复合材料制成[2]。碳纳米管(CNT)具有良好的光吸收性能，聚二甲基硅氧烷(PDMS)具有较大的热膨胀系数[2]。亚波长结构或超材料的使用可以改善光吸收和热传递到聚合物层。术语“亚波长”或“超材料”指的是特征小于入射光波长的一类材料。众所周知，这种结构可以在最小的空间距离上共振地或与表面等离子体激元一起增强入射光场--这是有效产生PA的基本要求。研究方法概述该项目的初始阶段将是确定产生PA所需的特性，评估可行的材料特性，并审查目前使用的方法。由此产生的理解和知识将用于在一个维度上部署电磁模型。最初的文献搜索显示，只有少数几个简单的模拟存在[3]、[4]，因此，这种模型的发展将使功放器件设计的系统优化成为可能。在开发了模拟电磁场的模型后，还将创建所产生的压力波的一维模拟。通过传递矩阵法，可以探讨不同材料和结构设计对输出压力信号的影响，从而指导专门针对功率放大器的新的定制结构的设计。模型的准确性将通过实验工作得到验证。这些实验将集中于量化这些(空间色散的)结构的吸收特性，利用已有的一系列激光(ps和ns脉冲宽度)来探测时间吸收特性。在实际应用中，PA效应在成像和传感方面显示出巨大的应用前景。然而，目前的PA成像系统使用的是昂贵且要求严格的安全标准的激光光源。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，可以用较低的光能产生高幅度的功放信号。本项目属于EPSRC工程研究领域。[1]F.高等人，“面向对比剂和薄膜设计优化的光声和热声产生效率的分析研究”，光声学，2017，[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。[3]N.Baddour和A.Mandelis，“声阻抗对使用一维频域光声学进行亚表面吸波几何重建的影响”，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