Analytical photoacoustic model of laser-induced ultrasound in a planar layered structure

Abstract: A way of describing photoacoustic measurement techniques is listening to light. Another way is as sensor techniques for characterisation of materials by means of light and sound interacting with the material under investigation. Photoacoustic measurements are emerging techniques and examples and suggestions of applications can be found foremost in diagnostics for medical purposes but also for industrial materials.The photoacoustic measurement techniques used in present thesis is based on the thermoelastic effect. A short light pulse is sent into a material, and energy from the light being absorbed leads to a localised heating. If the duration of the heating is short enough, a local pressure wave is built up, that propagates through the material as an ultrasonic pulse which can be detected by an ultrasound transducer and subsequently analysed.In photoacoustic measurement techniques, the advantage of high spatial resolution from measuring with ultrasound is combined with the advantage of high contrast from measuring with light. Ultrasound propagating in for example human tissue scatters less than what light does, enabling more precise positioning of the origin of the information in the measured signal. High spatial resolution is thereby achieved from measuring with ultrasound. High contrast can be reached from dierent responses to light in materials with diverse light absorption coecients. However, a disadvantage of the technique is that in biological tissue, light scattering is limiting the practically usable penetration depth.Besides for photoacoustic measurements, the thermoelastic effect can be utilised to generate laser-induced ultrasound for measurement applications. The present thesis' contribution is studies of laser-induced ultrasound in a thin semitransparent light-absorbing layer, experimentally and theoretically. To draw conclusions from measurements, one wish to know more about what happens in the absorbing layer, knowledge that could also be applied on corresponding processes in a material under study. An analytical model could thereto constitute a base for ultrasound pulse shaping, for adjusted measurements for specific applications.Through experiments on dyed polymer film, frequency spectra of laser-induced ultrasound were studied, varying thickness and light absorption coecient of the film layer structure. In the outcome of the experiments, decreased thickness was related to increased centre frequency as well as increased bandwidth of the generated ultrasound, and increased absorption together with decreased thickness was related to increased ultrasound amplitude.A one-dimensional analytical photoacoustic wave model of generation and propagation of heat-induced ultrasonic pressure wave in three material layers has been developed. From an existing model of three layers, this is an expansion enabling the two materials surrounding the light absorbing layer to be dierent. The analytical solution to the photoacoustic wave equation problem was based on a Laplace transform approach. Pressures resulting from the analytical model corresponds well to results from simulations. Analytical pressures were compared to ultrasound transducer experimental voltages, through system identication of the transducer system conversion between pressure and voltage. The system identication was however unstable. For ultrasound reception, experiments were performed on piezoelectric film with electrically conductive coatings as a transducer.