Understanding the physical and material dynamics of multipulse femtosecond laser interactions with surfaces

We describe the evolution of laser damage spots on bulk nickel generated by multipulse femtosecond laser irradiation with a 100 μm x 100 μm square flat-top beam profile as a function of the laser fluence and the number of pulses incident on the target. This large-area irradiation simulates conditions associated with the interaction of femtosecond laser pulses on a remote target. The larger area laser damage sites are characterized either by a series of self-organized surface structures with micro- and nanoscale features or a deep circular pit rather than a crater that mirrors the beam profile. Furthermore, the ablation rate of the deepest feature sharply increases above a laser fluence of 2 J/cm2; this increase is associated with the creation of a deep circular ablation pit generated during ablation with the first few pulses on the sample that continuously grows upon multipulse irradiation due to the focusing of incident laser energy into the pit by the sloped pit surfaces.