Published online by Cambridge University Press: 10 February 2011
We have developed a new technique for the study of the onset of plastic flow in thin metal films. A sub-picosecond light pulse is absorbed at the upper surface of the film. This generates a strain pulse that propagates through the film to the substrate and then back to the upper surface of the film where it is detected. When the temperature of the film is changed, there is a change in the transit time of the strain pulse. This change arises from the variation of the sound velocity with temperature, and also has a contribution from thermal stress in the film. By making accurate measurements of the transit time, we are able to identify the onset of plastic flow in the film.