Observation of dynamics and modification of solid surface using a picosecond soft x-ray laser

Short pulse x-ray sources are widely used as probing beams for new material development and non-destructive x-ray imaging. The high quality soft x-ray laser (SXRL) source enables us to achieve quite high spatial-resolution as a probe and quite intense x-ray as a pump. As an application using the SXRL, we have observed the spallative ablation process by the interaction with SXRL or femto-second (fs) laser. The dynamical processes of the SXRL and/or the fs laser-induced surface modifications come to attract much attention for the micro processing. However, it is difficult to observe the spallative ablation dynamics, because of non-repetitive, irreversible and rapid phenomena in a small feature size. In the case with SXRL irradiation (13.9 nm, 7ps, ∼50 mJ/cm²), we have observed the damage structures and the optical emission from the ablated materials. When focused SXRL pulses were have been irradiated onto the metal surface, we have confirmed damage structures, however no optical emission signal during SXRL ablation could be observed. The electron temperature is estimated to be around a few eV at the ablated surface. In the case with fs laser irradiation (795 nm, 80fs, ∼1.5 J/cm²), we have observed the surface morphology of fs laser ablation by the SXRL interferometer and SXRL reflectometer. The time resolved image of nano-scaled ablation dynamics of tungsten surface was observed. The numerical simulation study is underway by using a molecular dynamics code. These results lead not only to understanding the full process of the interaction with the SXRL and/or fs laser, but also to candidate the material of the first wall of magnetic confinement fusion reactors. We also described a preliminary study of radiation effect on culture cells irradiated with the SXRL. Our study demonstrated for the first time that the SXRL induced the DNA double strand breaks.