Strong effect of interband transitions in the picosecond ultrasonics response of metallic thin films

We report on the strong laser wavelength effects in the shape of acoustical pulses generated and detected in thin metallic films using ultrashort optical pulses. The experiments have been performed on aluminum and copper. In both cases, we have observed a sign change of the acoustical part in the detected signals in the vicinity of a well-known interband transition. We identify the origin of the phenomenon in the detection mechanism. These results let us demonstrate the role picosecond ultrasonics may play in the study of the electronic structure of thin films.