Effect of surface roughness on the ultrashort pulsed laser ablation fluence threshold of zinc and steel

The single and multiple pulse laser ablation threshold of zinc and steel at picosecond laser pulse duration is studied as a function of initial surface roughness at laser wavelengths of 515 and 1030 nm. The initial surface topographies and the resulting crater morphologies are analyzed using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Reflectivity measurements of the initial surfaces show increased absorptivity with increasing surface roughness. It was found that the single pulse ablation threshold increases with increasing effective surface area; the latter resulting from surface roughness. Rougher surfaces tend to have a higher degree of incubation as well. From the experimental and simulation results, it appears that the absorbed energy contributes more to residual heat than to material ablation when effective surface area increases.