Observation of the nano-scale surface dynamics of femtosecond laser ablation by time-resolved soft x-ray imaging technique

We have developed the pump and probe soft x-ray imaging technique of the metal surfaces during the femtosecond laser ablation by using the laser-driven soft x-ray laser at the wavelength of 13.9 nm. The pumping laser used for the ablation was a Ti: Sapphire laser pulse with the duration of 80 fs pulse at a central wavelength of 795 nm, and had a Gaussian spatial profile. By using the x-ray interferometer, the time resolved image of nano-scaled ablation dynamics of the platinum was obtained. At the timing of 36 ps after the femtosecond laser irradiation, the maximum surface expansion and expansion speed were measured to be about 60 nm and 1,700 m/s, respectively. We have compared between the experimental result of the surface expansion and the profile of the ablated hole measured by the atomic force microscopy, and discussed the fluence dependence of the femtosecond laser ablation. These results lead to better understanding of the initial process of the laser ablation dynamics.