Optically induced temperature variations in a two-layer volume absorber including thermal memory effects

Abstract

In this paper, a theoretical model of temperature variations is derived for a two-layer optically absorbing structure, including thermal memory effects. It is considered that the two-layer structure is surrounded by gas and illuminated on the front side by a harmonically modulated laser beam. This model is based on the hyperbolic theory of heat conduction and Beer–Lambert’s law of absorption neglecting multiple optical reflections in each layer. The derived model represents the generalization of the current models in two aspects. First, the influence of thermal memory properties of both layers is accounted for, and second, both layers are regarded as volume absorbers of the incident light. Based on the derived model, the expressions for surface temperature variations are given and discussed for the special type of two-layer structures, irradiated on the coated side, which is a prominent configuration in photoacoustic and photothermal experiments. It is shown that there exists a frequency range in which the influence of the thermal properties of the coating cannot be neglected, especially at the high-frequency range in which thermal memory of coating becomes significant. It is an important result in terms of understanding experimentally measured photothermal and photoacoustic response and, consequently, accurate characterizations of various high optically reflected and/or optically transparent samples by using these experimental techniques.