Impact of molten pool dynamics on resultant surface structures during direct laser interference patterning

Direct laser interference patterning (DLIP) is a well-established technology, which is often used for material surface structuring when both high-throughput and resolution are needed. In this technique, the periodical features of the surface structures are produced simultaneously within a single laser spot. This requires taking into consideration the interaction between the ablation processes of each feature, which includes heat transfer and the interaction between the molten material. In this study, we demonstrate how the molten material dynamics during the four-beam picosecond DLIP of the steel surface affect the topography of the fabricated structure. The detailed analysis of the structures produced with various laser fluences reveals three distinct mechanisms of structure formation, which lead to the structure consisting not only of periodically distributed craters but also of pillars, formed due to the molten material flow dynamics. The topography of the structures formed with different laser fluences and the number of laser pulses is investigated using confocal and scanning electron microscopy techniques.