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, Cr, and Al are compared with the parabolic two-step thermal diffusion model for the purpose of determining the electron-phonon coupling factor. The thin films were evaporated and sputtered onto different substrates to produce films that vary structurally. The measurement of the electron-phonon coupling factor is shown to be sensitive to grain size and film thickness. The thin-film thermoreflectance data are compared with that of the corresponding bulk material and to a theoretical model relating the coupling rate to the grain-boundary scattering and size effects on the mean free path of the relevant energy carrier.