Modelling of ultrashort laser pulse material processing

Summary form only given. We have used a modified version of the hydro code HYADES to simulate the interaction of intense laser pulses with various materials, both metals and dielectrics. Here we discuss two effects. Because the ultrashort laser pulse (USLP) energy is absorbed in a very thin layer, at high laser intensities, the plasma temperature in this layer becomes so high during the pulse that the plasma layer, which hasn´t had time to expand, works as an ideal plasma mirror. Our experiments and simulations indicate >90% reflectivity for 0.5-ps pulses with fluence over about 200 J/cm/sup 2/. We find that, as a result of this plasma mirror effect, the optimal fluence for material processing is not very high compared with the ablation threshold. Optimization of laser parameters for material processing will be discussed. Very often prepulses materially affect USLP material processing. Prepulses produce an initial plasma, which can expand and shield the underlying material. On the other hand, a small-scale-length plasma layer increases absorption near the ablation surface and can compensate the plasma mirror effect. Optimization of the prepulse will also be discussed. Finally, simulation results are compared with the experimental data.