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

• Published in: Journal of applied physics 2005-08, Vol.98 (3), p.033104-033104-7
• Main Authors: Hébert, H., Vidal, F., Martin, F., Kieffer, J.-C., Nadeau, A., Johnston, T. W., Blouin, A., Moreau, A., Monchalin, J.-P.
• Format: Article
• Language: English
• Subjects: ABLATION; ABSORPTION; ALUMINIUM; AMPLITUDES; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; FLUIDS; FOILS; LASERS; LIGHT TRANSMISSION; ONE-DIMENSIONAL CALCULATIONS; PLASMA; PULSED IRRADIATION; PULSES; SURFACES; THERMOELASTICITY
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.1999827

We have investigated high-frequency ultrasound generated by single laser pulses in thin ( 50 μ m ) 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 - μ m propagation through the foils for the two laser-pulse durations in the fluence range of 0.1 - 0.7 J ∕ cm 2 . For fluences greater than the ablation threshold (0.25 and 0.63 J ∕ cm 2 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.