Pulsed laser ablation of solids: transition from normal vaporization to phase explosion

We present experimental data on mass removal during 1064-nm pulsed laser ablation of graphite, niobium and YBa2Cu3O7-δ superconductor. Evidence for the transition from normal vaporization to phase explosion has been obtained for these materials, showing a dramatic increase in the ablation rate at the threshold fluences of 22, 15 and 17.5 J/cm2, respectively. A numerical model is used to evaluate the ablation rate and temperature distribution within the targets under near-threshold ablation conditions. The results are analyzed from the viewpoint of the vaporized matter approaching the critical point with increasing laser fluence. A possible means of the estimating the thermodynamic critical temperature from the data for nanosecond laser ablation is discussed. It is suggested that the critical temperature of refractory metals is higher than that estimated with the traditional methods due to plasma effects. An analogy with the boiling crisis (the transition from nucleate to film boiling) is drawn to explain the formation of ablation craters with spallated edges.