A theoretical model of the linear thermo-optical response of an absorbing particle immersed in a liquid

This paper is devoted to a theoretical analysis of the linear optoacoustic effect in an inhomogeneous medium consisting of an absorbing spherical particle in an immersion (nonabsorbing) liquid irradiated with laser pulses. It is shown that the problem of determining the optoacoustic response in an inhomogeneous medium can be separated into three sequential steps: determination of the distribution of the light flux in a medium, calculation of the temperature field distribution related to the absorption of optical energy, and the solution of the acoustic problem of thermo-optical excitation of sound by thermal sources. The cases of thermally large (the length of heat diffusion during the time of laser action is much less than the size of a particle) and thermally small particles are analyzed. It is demonstrated that, in the case of a thermally large particle, the time profile of the pressure of the acoustic signal strongly depends on the relation between the coefficient of light absorption by a particle and its radius, as well as on the duration of the laser pulse. In the case of a thermally small particle, the pressure pulse follows the derivative of the temporal envelope of the laser pulse intensity.