Laser Picosecond Acoustics in Thin Films: Effect of Elastic Boundary Conditions on Pulse Generation

Using the pump-probe technique with laser noise compensation, laser picosecond acoustic measurements were carried out for the amorphous sputtered thin films a-Ge and a-InSb on sapphire substrates with both pump and probe beams incident on the front side of the film. The contraints at the stress generation region were then changed using a pump beam incident on the substrate side of the a-Ge film, allowing background signals from non-acoustic sources to be greatly reduced. The symmetry of the detected echo pulse shapes are interpreted with a theoretical model.