Femtosecond time-resolved thermomodulation of thin gold films with different crystal structures

A femtosecond laser is used to generate and probe hot electrons in polycrystalline and single-crystalline thin gold films. Transient thermoreflectivity and thermotransmissivity for different heating-laser-pulse fluences are performed. Analysis of the transient thermotransmissivity and thermoreflectivity signals allows us to resolve the modulation to the real and the imaginary parts of the dielectric constant. The latter is found to be dominant at our probe wavelength. The hot-electron energy-loss lifetime is shown to be 1--3 ps and increases with the laser fluence. For film thickness comparable to the optical skin depth, the transient decay time of the reflectivity and of the transmissivity are equal, with the polycrystalline films showing a slightly faster decay time. For thicker films, hot-electron transport to the bulk of the film gives a faster transient-reflectivity decay. Electron transport is slower in the polycrystalline films.