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In ultra-short pulsed laser micromachining of metals shorter pulses generally lead to higher throughput e.g. for copper the specific removal rate drops to about 25 % if the pulse duration is raised from 10 ps to 50 ps and it is increases by about 35 % for a decreased pulse duration of 350 fs. In contrast, a significantly higher increase of 100 % was observed for stainless steel AISI 304 when the pulse duration is decreased from 10 ps to 350 fs, whereas the drop for the longer pulse duration of 50 ps was of the same order of magnitude as for copper. Recent results of a two-pulse burst experiment on copper with temporal varying intra-burst pulse distance have shown, that pulse distances from 10 ps to 100 fs cause only a slight increase in efficiency of approximately 10 % whereas for pulse distances from 10 ps to 500 ps the efficiency is reduced to nearly 50 %. Although, these factors differ from the well-studied pulse duration experiments, they show a similar tendency. Therefore, we assume a dominant common cause.
In literature the diminished efficiency is often referred to particle- or plasma shielding for tens of picoseconds pulse durations. In this sense, the ablation of the material during the pulse is faded by itself for longer pulses as well as the ablation of the material from the first pulse is restricted by the second pulse in the burst experiments. Additional investigations concerning reflection and calorimetric considerations will help to underline this hypothesis.
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