Surface Microstructures obtained by Femtosecond Laser Pulses

After machining of steel by femtosecond laser ablation unexpected surface structures are found at the bottom of the machined area. Periodic sub-micrometer spaced ripple patterns are formed after irradiation with several pulses. At a certain number of incident pulses the ripples starts to break up resulting in a more chaotic structure. The point of transition and the morphology of the chaotic structure depend on several parameters. The paper describes the ripple formation and the resulting surface structures dependent on the repetition frequency of the laser and the number of incident pulses.