Multi-timescale observation of ultrashort pulse laser ablation of copper

Ultrashort pulse lasers have attracted attention because they can be used to fabricate various types of solid materials, including metals, semiconductors, and insulators. To further improve ultrashort pulse laser processing, it is important to understand the phenomena that occur during processing. Optical-delay pump- probe observation is a powerful tool for investigating the ultrafast dynamics of ablation on a timescale ranging from sub-picoseconds to a few nanoseconds. However, the discrepancy in the timescales between ultrafast phenomena and the final processed shape is too large to understand the decisive process leading to the final processed shape. In this study, we combine optical-delay with electrical-delay pump-probe shadowgraphy to capture phenomena over a wide range of timescales ranging from a few nanoseconds to hundreds of nanoseconds to clarify the sequence of processes leading to static processing results. Particularly, we found that a ring-shaped molten structure was formed when the pulse duration was 9 ps or longer and revealed the mechanism of the formation by visualizing shock wave propagation and particle ejection from the ring position. This research is expected to contribute to the further understanding and improvement of ultrashort pulse laser processing.