Photothermal properties of bulk and layered materials by the picosecond acoustics technique

The picosecond acoustic technique is a pump and probe method associated with a lock-in amplification scheme. It involves the excitation of the sample by an ultrashort optical pulse and the monitoring of the subsequent relaxation processes by a weaker pulse delayed with respect to the former. In particular, information about the temperature evolution a short duration after the pump pulse is absorbed can be obtained. We examine here the different time scales involved in such an experiment. We show that the photothermal signal is superimposed on a continuous background originating from the modulation of the pump beam for synchronous detection. We also derive expressions for the cooling rate of both bulk and layered specimens after an instantaneous heating and we show experimentally that this macroscopic approach is valid after a delay of several picoseconds. (C) 1998 American Institute of Physics.