Application of the picosecond ultrasonic technique to the study of elastic and time-resolved thermal properties of materials

We describe a technique - the Picosecond Ultrasonic Technique - making use of a femtosecond laser source, suitable to generate and to detect high frequency elastic waves in submicron samples. The optically-generated acoustic pulse has a broad spectral contents extending over several hundreds of GHz. Then, we present results obtained in thin layers and in bulk samples as well. Using this technique, we were able to measure thickness Variations of a few nanometers in a 100 nm thick LiNbO3 sample. We present also results obtained in a set of periodic Cu/W multilayers. In these samples, we measured the effective sound velocity of the bulk longitudinal modes propagating within the heterostructure as a function of the superlattice period. For the shortest periods, we observe a decrease of the sound velocity by similar to 10%, corresponding to a softening of the heterostructure. This elastic anomaly is related to interfacial effects. Then, we were able to put in Vibration the surface of Cu/W multilayers at frequencies up to 0.81 THz. We attribute this vibration to a surface mode localized in the second frequency band gap of the dispersion curve. Finally, we show how time-resolved thermal properties can be deduced from a picosecond ultrasonic experiment and we present results obtained in a macroscopic aluminium sample. (C) 1997 Elsevier Science B.V.