Detection and simulation of optoacoustic signals generated in layered tissues

The absorption of electromagnetic waves by media induces a spatial pressure distribution, proportional to the density of the deposited energy, followed by thermoelastic expansion and emission of acoustic waves. Assuming instantaneous energy deposition, the resulting mathematical problem is governed by an inhomogeneous wave equation featuring an optoacoustic source term. Here, we consider optically inhomogeneous, i.e. layered media, where scattering is effectively negligible and the absorbed energy density follows Beer-Lambert's law, i.e. is characterized by an exponential decay within the layers and discontinuities at interfaces. We complement test experiments on samples where the material properties are known a priori, with numerical simulations based on solving the optoacoustic wave equation, tailored to suit our experimental setup. Experimentally we characterize the acoustic signal observed by a piezoelectric detector in the acoustic far-field in backward mode and we discuss the implication of acoustic diffraction on our measurements as well as possibilities to retrieve the absorption coefficient from measurements in the forward mode.