Generation and detection of ultrasonic waves in micrometric and sub-micrometric films using picosecond laser pulses

In this paper, the effects of the laser pulse duration (100 fs - 12 ps) are investigated on the generation and the detection of photo-acoustic signals. A model of generation and detection of ultrasonic waves with ultra-shorts laser pulses is presented. An interferometric detection technique is used to probe the amplitude and the phase changes of the perturbed optical reflectivity. Measurements are performed on two samples: a sub-micrometric metal layer deposited on a semiconductor substrate and a GaAs semiconducting film. It is shown that in femtosecond and picosecond regimes the acoustic waves can be measured through amplitude and phase changes of the reflectivity.