Nanomodification of gold surface by picosecond soft x-ray laser pulse

We show experimentally the possibility of nanostructuring (about 20 nm) of gold surface by picosecond soft x-ray single pulse with low fluence of ∼20 mJ/cm2. The nanometer-scale changes of the surface structure are due to the splash of molten gold under fluence gradient of the laser beam. In addition, the ablation process occurs at slightly higher fluence of ∼50 mJ/cm2. The atomistic model of ablation is developed which reveals that the low threshold fluence of this process is due to the build-up of the high electron pressure and the comparatively low electron-ion energy relaxation rate in gold. The calculated ablation depths as a function of the irradiation fluence are in good agreement with the experimental data measured for gold surface modification with ultra-short duration soft x-ray and visible lasers.