Coherent Phonon Generation and Detection by Picosecond Light Pulses

Using the picosecond pump and probe technique we have detected oscillations of photoinduced transmission and reflection in thin films of $a$-${\mathrm{As}}_2$ ${\mathrm{Te}}_3$ and cis-polyacetylene. These oscillations are due to the generation and propagation of coherent acoustic phonons in the film. We discuss the generation and detection mechanism, and we use this effect to measure the sound velocity in a film of $a$-Si${\mathrm{O}}_2$.