Microscopy study of ripples created on steel surface by use of ultra short laser pulses

This paper concentrates on ripples on the surface of steel that arise from lasermaterial interaction. In particular we have observed two different sets of ripples on steel samples that were machined by 210 fs laser pulses with 800 nm wavelength at normal incidence. Small ripples were found with spacing of about 250 nm lying longitudinal to the vector of laser beam polarization. Big ripples exhibited at a much larger distance of about 500 nm and they are perpendicular to the polarization vector. The laser treated surfaces were investigated with Scanning Electron, Confocal and Atomic Force Microscopy. The laser-material interaction could be divided into three subsequent steps: absorption of laser light via electron gas excitations, transfer of heat into the lattice followed by a thermal expansion of material. From our microscopic observations it is concluded that the small ripples are formed by solidification of liquid material present as a thin layer near the interface of solid bulk material.