Thresholds of plasma formation in silicon identified by optimizing the ablation laser pulse form

Using an evolutionary algorithm combined with pulse shaping, we have identified that rapid plasma formation in Silicon can occur already at a fluence of about 150 mJ/cm(2) if a substantial part of the laser energy is deposited efficiently around 200 fs after an exciting laser pulse. Nonthermal solid-to-liquid phase transition leads to the increase of the deposited energy in the material. Highly charged ions have been observed in the mass spectrum. While the pulse optimization procedure allowed us to identify the plasma formation, further experiments where the influence of the laser pulse width on the ablation yield was studied and Two-Pulse-Correlation experiments provided additional proof for the appearance of rapid plasma formation.