Femtosecond laser surface processing (FLSP) is an advanced manufacturing technique that permanently modifies material properties by generating self-organized quasi-periodic micro- and nanostructures. FLSP enables creation of surfaces with tailored structures for diverse applications, including heat transfer enhancement, corrosion resistance, broadband light absorption, and super-wicking capillary channels. These structures and their properties can be precisely controlled by adjusting laser processing parameters, where laser fluence and pulse count are known to be critical factors in determining surface morphology. However, while the effects of fluence and pulse count are well understood, the roles of pulse overlap (PO) and raster line overlap (LO) remain relatively unexplored.

In this work, we systematically decouple the effects of PO (99.8% to -42.8%) and LO (0% to 99.8%) on structure formation in silicon while maintaining a constant pulse count. The high repetition rate femtosecond laser with a high-precision galvo-scanner system enables us to explore unprecedented processing regimes, including PO below -42.8% and LO exceeding 99.8%, as well as negative overlap conditions where pulses are spaced farther apart. In negative overlap regimes, FLSP structure formation exhibits a unique sensitivity to slight adjustments in PO and LO, resulting in intriguing self-organized patterns. For example, surfaces processed with negative LO and PO develop row-like structures: negative LO produces aligned features along the raster scan direction, while negative PO generates structures perpendicular to the scan direction. By correlating LO and PO with surface and subsurface morphology, we provide fundamental insights into micro- and nanostructure formation mechanisms under these extreme processing conditions.