Ultrafast vibration and laser acoustics in thin transparent films.

Picosecond laser pulses are used to excite and detect stress pulses in thin transparent films on opaque substrates. The reflectance variation, measured for silica films, is modeled as a sum of different contributions: an echo contribution from stress-induced modulation of the substrate reflectance, an interference contribution from light reflected by the stress pulse in the transparent film, and a contribution from stressinduced vibrations of the order of 10(-3)nm of the film surface, observed for what is to our knowledge the first time. We show how both the thickness and sound velocity of the film can be determined, provided that its refractive index is known.