The use of subwavelength structures to control and enhance the photoacoustic effect

Thesis

The photoacoustic effect describes the conversion of light to sound, via thermoelastic expansion, and is used extensively in cross-media applications as it combines the favourable properties of light in one medium with the favourable properties of sound in the other. Photoacoustic composites are used for efficient photoacoustic conversion and can be made from planar multilayered structures, comprised of thin layers of optically absorbing media, separated by layers of a thermally-responsive medium. The aim of this project was to formulate a one-dimensional model capable of simulating the acoustic signal generated from a multilayered structure with several absorbing layers. Then, using the model, the effect of subwavelength structures on the generated acoustic signal was investigated for weakly- and strongly-absorbing layers. The transfer-matrix method was used to solve the governing photoacoustic wave equation in the frequency domain and the source function of the second-order differential equation was kept as general as possible so that the user could choose any function to describe the optical absorption profile of the structure. The model was verified by comparing its results with those from previous studies. It was found that acoustic resonant effects could be induced in structures with optically-thin or optically-thick absorbers but that the effect was more pronounced in the spectra of optically-thick absorbers. Furthermore, the features of the frequency spectrum could be explained by considering the poles and zeros of the transfer-matrix when source terms were included. Thus, these observations stimulated the design of a photoacoustic structure with enhanced directivity by using an acoustic high-reflectance coating. The simulated structure was capable of increasing the amplitude of the generated acoustic signal by about 15% at a target frequency, compared to previous methods. These findings indicate that as new photoacoustic composites have reduced dimensions (<10nm), the resonant properties of both light and sound could be utilised to enhance the conversion from optical to acoustic energy.

Authors: Patterson, I

• DOI: 10.5287/ora-9g1qmkz57
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: transfer-matrix method; photoacoustics; subwavelength structures
• Subjects: Metamaterials; Acoustooptical devices