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 Delta t=0-200ps. The double-pulse beam consisted of 800nm pulses with a duration of 150fs and 400nm pulses with a duration of >150fs. The fundamental-pulse fluence F-800 and the second-harmonic pulse fluence F-400 were set to be near the corresponding ablation thresholds of F-800th=0.108J/cm(2) and F-400th=0.090J/cm(2), respectively. The authors found that uniform LIPSSs could be produced on titanium surfaces using laser fluences of 1.5F(400th)+0.9F(800th) and a delay of Delta t=0-2ps. The periodicity and direction of the LIPSSs were characterized by the wavelength and electric field of the fundamental (800nm) 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.