Photoacoustic determination of thin-film thermal properties

Photoacoustic measurements of the thermal diffusivity and thermal conductivity of a thin-film optical coating are described. The figures of merit characterizing the thin-film/substrate system studied are as follows: ratio of substrate to thin-film effusivity g=5.0%±10%, critical frequency (equal to thin-film thermal diffusivity divided by the square of the thin-film thickness) fc=98 kHz±3%. Measured values of thin-film thermal diffusivity and thermal conductivity are comparable to tabulated bulk values. The ability to directly measure thin-film thermal properties may be particularly valuable in investigating laser damage of optical coatings.