Uniform LIPSS on titanium irradiated by two-color double-pulse beam of femtosecond laser

The authors have investigated the uniformity of laser-induced periodic surface structures (LIPSSs) generated on titanium surfaces irradiated with a two-color double-pulse cross-polarized beam with a time delay of Δt = 0–200 ps. The double-pulse beam consisted of 800 nm pulses with a duration of 150 fs and 400 nm pulses with a duration of >150 fs. The fundamental-pulse fluence F800 and the second-harmonic pulse fluence F400 were set to be near the corresponding ablation thresholds of F800th = 0.108 J/cm2 and F400th = 0.090 J/cm2, respectively. The authors found that uniform LIPSSs could be produced on titanium surfaces using laser fluences of 1.5F400th + 0.9F800th and a delay of Δt = 0–2 ps. The periodicity and direction of the LIPSSs were characterized by the wavelength and electric field of the fundamental (800 nm) pulse. The results suggest that the longer-wavelength pulse influences surface plasma wave generation and improves uniformity by the second harmonic pulse even though laser plasma is produced on the surface.