Theory of photothermal depth profiling via Rayleigh-type surface acoustic wave detection

Abstract

An analytical description is derived for the frequency spectrum of quasi-Rayleigh waves excited thermoelastically by a pulsed laser in a system composed of a solid film on a solid substrate. The relative magnitude of the film and the substrate contributions to the excited quasi-Rayleigh wave depends on their thermoelastic parameters, namely the relative efficiency of photoacoustic conversion in the film and in the substrate, and also significantly on the frequency, because of the redistribution of absorbed laser energy between the film and the substrate through thermal conductivity. Consequently, the frequency spectrum of the surface acoustic wave pulse contains information on the characteristic time of heat conduction in the film and correspondingly on film thickness and the ratio of thermal effusivities of the layer and the substrate. The theory developed should have applications in non-destructive photoacoustic diagnostics of thin films and coatings.