Picosecond transient grating studies of polymeric thin films

Picosecond transient grating experiments have been performed on a 5-μm-thick polystyrene film doped with naphthalene and on a 700-μm-thick pure polystyrene film at room temperature. In both cases, the observed acoustic modulation of the diffracted signal is explained by the formation of a thermal grating. No detectable acoustic attenuation is observed within the time delay of 6 ns, and the speed of sound obtained from the acoustic modulation period is in excellent agreement with that reported by an ultrasonic measurement. The coherent third order nonlinear electronic response observed at zero-time delay is found to be shorter than 2 ps.