Fig. 5. Electron-phonon interactions at non-equilibrium conditions.

a Normalized thermoreflectance as a function of pump-probe delay time for as-deposited 27, 57, and 118 nm Cu films at probing energy of 0.775 eV. b Two-temperature model derived electron–phonon coupling factor (G) of the as-deposited films, with grain sizes indicated for selected films. G across the film thicknesses remain constant within uncertainty. Our TTM-derived electron-phonon coupling factor is in agreement with our first-principles calculations (green dashed line) and reported values from the literature^42,45,46 (c, d) normalized thermoreflectance as a function of pump-probe delay time for as-deposited and annealed for 27 and 43 nm Cu films at probing energy of 0.775 eV. The films exhibit almost identical transient reflectivity rises at as-deposited and different annealing conditions.