Evolution of titanium surfaces irradiated by femtosecond laser pulses with different wavelengths

This paper reports on the evolution of femtosecond laser induced periodic surface structures (LIPSSs) on titanium surface irradiated with different wavelengths. By SEM observations, it is noted that different nanostructures with respective surface features depend highly on the laser wavelength and the laser fluence. The period of LIPSSs formed at the laser fluence just above the ablation threshold is shorter than the laser wavelength, as well as dependence on the incident wavelength. Experiments using wavelength of 600 and 1500 nm, studies are performed in more detail. The period and the depth of the grooves of LIPSSs are increased with the increase of laser fluence at wavelength of 600nm. The created structures on the surface at the laser fluence of 0.42 J/cm2 would significantly influence the field intensity distribution on the surface. The redistribution of the electric field intensity plays a crucial role in the creation of the HSFLs formed on the ridges of the LIPSSs, and the period decreases to half. Another kind of HSFLs whose orientation is perpendicular to the sidewalls of LIPSSs is created at wavelength of 1500nm. These HSFLs lie at the bottom of the valleys between both the LIPSSs and new formed grooves. As compared with the surface nanostructures formed at wavelength of 600 nm, the formation of identical HSFLs is induced with smaller laser fluence at wavelength of 1500 nm.