Evaluation of thermal diffusivity for thin gold films using femtosecond laser excitation technique

We evaluated the dependence of the thermal diffusivity upon the thickness of the thin films and the substrates by means of the transient reflecting gratings (TRGs) method. The wave length of the laser beam was 800 nm, and the width of the pulse 200 fs. The materials of the samples are pure gold thin films on silicon wafer or quartz glass substrates. We have performed TRG measurements for various thickness thin films, and made analysis on changes in the thermal diffusivity and the acoustic wave. The result of our analysis shows that the change in the thermal diffusivity, in the thickness range of 25 and 500 nm, depends upon both the thin film thickness and the substrates. It is also revealed that the acoustic-wave frequency changes depend upon both the thin film thickness and the substrates. These dependencies suggest that information on both the thin films and substrates are mixed up in the TRG data.