Measurement of the electron-phonon coupling factor dependence on film thickness and grain size in Au, Cr, and Al

Femtosecond thermoreflectance data for thin films and bulk quantities of Au , Or, and Al are compared with the parabolic two-step thermal diffusion mod el for the purpose of determining the electron-phonon coupling factor. The thin films were evaporated and sputtered onto different substrates to produ ce films that vary structurally. The measurement of the electron-phonon cou pling factor is shown to be sensitive to grain size and film thickness. The thin-film thermoreflectance data are compared with that of the correspondi ng bulk material and to a theoretical model relating the coupling rate to t he grain-boundary scattering and size effects on the mean free path of the relevant energy carrier. (C) 1999 Optical Society of America.