Laser ultrasonic measurements are emerging as an efficient means for remote probing of metals. This technique offers the possibility to perform non-contact measurements in a hostile environment, at high temperature, and on any kind of structure. There are two regimes for generating ultrasound in a metal: thermo-elastic and ablative. The former, uses low-energy laser pulses to heat the surface of the sample. The transient expansion of the near surface region launches compression waves into the sample. These displacements are very small (-10 pm) in metals because of the low optical penetration depth, such that lock-in detection techniques must be used for these measurements. In the ablative regime, a higher energy density laser pulse causes partial ablation of the target surface and ionisation of the ablated material. The plasma thus created can reach very high pressures, which causes the plasma to accelerate away from the surface and launches a compressive elastic wave into the sample. The amplitude of these waves is much greater (-10 nm) than those generated in the thermo-elastic regime and they can easily be measured by single shot interferometric techniques. Theoretical simulations of ablation by ultra-short laser pulses [F.Vidal et al., PRL, 86, 2573 (2001)] explore the generation of high frequency ultrasound waves (- GHz) in a thin aluminum film. These simulations indicate that the ultrasonic pulse duration is proportional to the laser pulse duration down to 100 ps, and that below 100 ps, further reduction of the laser pulse duration has little effect on the ultrasound pulse duration.