Sub-100 nanometer transverse gratings written by femtosecond laser pulses on a titanium surface

One-dimensional transverse (perpendicular to the laser polarization) gratings with periods Λ ≈ 50–60 nm were observed on a titanium surface within 150 nm wide, micrometer-long regular surface modification longitudinal stripes fabricated by multiple 744 nm Ti:sapphire femtosecond laser shots, occurring at a repetition rate of 10 Hz. In the center of the surface laser spot these stripes are oriented strictly perpendicular to the laser polarization, in accordance with the plasmon-polaritonic model, and appear as ablative longitudinal trenches centered along the main stripe axes, which are precursors of longitudinal common ripples with a 500 nm period. At the low-fluence periphery of the laser spot, the stripes appear not as ablative longitudinal trenches, but as linear arrays of sub-ablative transverse nanoripples with periods down to 50 nm. The appearance of such superfine transverse nanoripples is related to incomplete spallation of the laser–molten surface layer, periodically modulated at the nanoscale through coherent sub-surface cavitation.