Near-surface laser–vapour coupling in nanosecond pulsed laser ablation

The evaporation mechanism at high temperatures is one of the disputable points of the conventional thermal model of laser ablation connecting heat transfer in the target with gas-phase dynamics. A model of supercritical ablation is proposed where the vapour density is limited by a transparency condition. The model is basically laser–vapour coupling in a thin near-surface layer. The experiments on ablation of Au at 193 and 266 nm, Al at 266 nm, and graphite at 1.06 µm can be described by the proposed model. However, optical breakdown is important in graphite ablation at 193 and 248 nm. When the target surface temperature is less than the critical one (typically at fluences less than or about several J cm −2 ) the thermal evaporation mechanism works. At higher laser fluences it has been found that a combination of the proposed supercritical ablation mechanism with optical breakdown in the volume of the gas phase is more realistic.