Simulations of layer effects in confocal photothermal microscopy

Abstract

This work presents a detailed study of how the thermal diffusivity measurement of a sample is affected by the deposition of an absorbent layer on its surface, when analyzed by confocal photothermal microscopy. First, the temperature distribution and surface curvature of a solid, which are related to the two basic mechanisms that modify the signal measured with that technique, are solved by means of the spatial Fourier transform, considering a semi-infinite substrate with a surface layer, which is heated by a modulated pump beam. Based on the calculated solutions, the typical signals of this technique are simulated, varying parameters such as the thickness of the layer, the material, and the pump beam radius. These signals are fitted by means of usual theoretical photothermal models that allow the retrieving of the substrate thermal diffusivity, but ignore the existence of the layer. In this way, the zones of validity of the same models are identified from the analysis of the fitting error, and the shift between the recovered and expected value.