THEORY OF PHOTOTHERMAL DEPTH PROFILING VIA RAYLEIGH-TYPE SURFACE-ACOUSTIC-WAVE DETECTION

An analytical description is derived for the frequency spectrum of qua si-Rayleigh waves excited thermoelastically by a pulsed laser in a sys tem composed of a solid film on a solid substrate. The relative magnit ude of the film and the substrate contributions to the excited quasi-R ayleigh wave depends on their thermoelastic parameters, namely the rel ative efficiency of photoacoustic conversion in the film and in the su bstrate, and also significantly on the frequency, because of the redis tribution 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 characteri stic time of heat conduction in the film and correspondingly on film t hickness and the ratio of thermal effusivities of the layer and the su bstrate. The theory developed should have applications in non-destruct ive photoacoustic diagnostics of thin films and coatings.