Time-resolved study of vibrations ofa-Ge:H/a-Si:H multilayers

Abstract
We describe experiments in which the vibrations of a-Ge:H/a-Si:H multilayer structures on silica substrates are studied in real time. The vibrations are generated when a picosecond light pulse is absorbed in the structure, thereby setting up an elastic stress. The resulting motion is studied through a measurement of the change in the optical reflectivity as a function of time. We analyze the results in terms of the spectrum of normal modes of the multilayers and show that there exist surface modes at the free surface of the multilayer.