Influence Of Stress Confinement, Particle Shielding And Re-Deposition On The Ultrashort Pulse Laser Ablation Of Metals Revealed By Ultrafast Time-Resolved Experiments

The ablation rate in double-pulse material processing is strongly influenced by the pulse separation. For pulse separations exceeding 3 ps a significant decrease in ablation volume has been observed. This was attributed to three mechanisms: rarefaction wave interaction, shielding by ablation plume and material re-deposition. Here we present carefully designed double-pulse ablation experiments on three industrially relevant metals: stainless steel, aluminum and copper and interpret them based on ablation dynamics derived from ultrafast time-resolved pump-probe microscopy and ellipsometry. Adjustment of the fluence distribution within the double-pulse allows us to separate effects arising from rarefaction wave suppression and shielding/re-deposition. We found that the rarefaction wave contribution to the double-pulse ablation volume is about 25% for aluminum and stainless steel and about 40% for copper. A pronounced re-deposition was observed for stainless steel and aluminum, while for copper shielding by the ablation plume plays a dominant role. It was revealed that the ablation volume stays maximal, if the pulse energy is deposited within the mechanical relaxation time of (2 - 5) ps. According to our findings the ablation process exerts maximum efficiency and precision, when energy is coupled into the material before mechanical relaxation or after material surface equilibration.