Effect of the laser pulse duration on picosecond ultrasonic signals

In this paper, the effects of the laser pulse duration on the generation and the detection of picosecond ultrasonic signals are investigated. A model accounting for both the photothermal generation and the optical detection of ultrasonic waves with ultra-short laser pulses is presented. An interferometric detection technique is used to probe the amplitude and phase changes of the perturbed optical reflectivity of tested samples. Measurements are performed on two samples. The first one consists of an absorbing tungsten layer deposited on a silicon substrate, whereas the second one is a semi-transparent GaAs semi-conducting film. It is shown that, with either 100 fs or 12 ps laser pulses, acoustic waves can be measured through changes of reflectivity, and that Brillouin oscillation frequencies can be measured as well.