The use of subwavelength structures to control and enhance photoacoustic signals

Goal
The aim of this project is to develop a device with sub-wavelength features that can produce high-amplitude photoacoustic signals from low optical energy.
Objectives
In 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