Ultrasound generated by a femtosecond and a picosecond laser pulse near the ablation threshold

We have investigated high frequency ultrasound generated by single laser pulses in thin 50m aluminum foils as a function of the laser fluence. Laser-pulse durations of 80 fs and 270 ps were used to compare the ultrasound generated in two very different regimes: thermoelastic and ablation. The measured rear-surface displacement induced by the ultrasound pulse is similar after 50-m propagation through the foils for the two laser-pulse durations in the fluence range of 0.1\u20130.7 J /cm2. For fluences greater than the ablation threshold 0.25 and 0.63J/cm2 for the 80 -fs and 270-ps pulses,respectively, the ultrasound amplitude generated by the 270-ps laser pulse is increased significantly due to absorption of laser energy by the ablating plasma. This is not observed for the 80-fs laser pulse since ablation is produced well after the laser-pulse irradiation of the target. The measured surface displacement as a function of laser fluence is compared to the calculations of a one-dimensional fluid code for both laser-pulse durations. The model calculations agree in many ways with the experimental results, but some discrepancies are observed.Peer reviewed: YesNRC publication: Ye