Femtosecond IR and UV laser induced periodic structures on steel and copper surfaces

We analyze in detail laser induced periodic surface structures (LIPSS) on steel and copper surfaces. The chosen metals exhibit extreme differences between their thermal conductivity values, which determines the rate of extinguishing of the laser-induced temperature modulation on the surface. Two wavelengths of femtosecond laser radiation sources in IR and UV spectral range were used to inscribe low spatial frequency LIPSS. Regular LIPSS were successfully formed on steel samples for both wavelengths and it was observed that their regularity decreased with shorter wavelength used to induce the structures. For copper samples, LIPSS of considerably lower regularity were formed and even absent for one of the used scanning method at UV wavelength. Experimental results were analyzed in view of the numerical solution of the 2D heat diffusion equation for induced temperature on the surface. It was found that the temperature modulation decreased considerably faster (lasting tens of picoseconds) in copper, and (additionally) the decrease of the modulation is faster at the higher modulation frequency.