Formation Mechanism of Nanostructures on the Stainless Steel Surface by Femtosecond Laser Pulses

In this paper, we report on the formation of a variety of micro/nano-metric structures on the stainless steel surfaces by single-beam direct femtosecond laser writing. Periodic microgratings of different line widths and period are fabricated on the sample surfaces. Random nanostructures are observed inside the microgratings, when laser pulses with low laser energies are applied on the samples. Unlike random nanostructures, periodic nanostructures are produced inside the microgratings, when laser pulses with high average pulse energy are applied. Besides, self-organized nanogratings, with variable periods, are formed all over the micro-machined area on the stainless steel surfaces due to the overlap of the microgratings, where the orientation of the nanogratings is parallel to the incident light polarization. Finally, we illustrate the formation mechanism of various kinds of nanostructures, produced on the stainless steel surfaces during our experiment.