Characterization of nanostructured metal films by picosecond acoustics and interferometry

Picosecond interferometry is used to study the acoustics waves created by heating Pt films with a subpicosecond laser pulse. Both the period of the initial oscillations in the metal film and the amplitude of the sound wave in the substrate are measured quantitatively. The platinum films are roughened by irradiation with energetic ions. The amplitude of the sound wave is doubled at those irradiation levels where the platinum coverage has been reduced by about one-half. A theory for the amplitude of the launched acoustic wave predicts that the acoustic amplitude is proportional to the mean square film thickness. Thus changes in the morphology of a partially perforated metal film can be observed using a simple, nondestructive optical technique.