Homogeneous periodic surface nanostructures formed by interference of square flattop femtosecond laser pulses

We report on the formation of homogeneous LIPSS by using the two-step ablation process with a square flattop beam of femtosecond (fs) laser pulses. The Gaussian beam from a Yb fs laser system was converted to a square flattop beam by passing through a refractive beam shaper and a square metal mask. This beam was split into two beams with a diffraction optical element and focused on a titanium surface. Firstly, the interference of these two beams formed an interference pattern with a period of 1.8 µm through ablation. Next, the single beam was irradiated onto the surface to form a homogeneous nanostructure with a period of 450 nm in 25 μm square area.