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

Abstract

A tabletop femtosecond time-resolved soft x-ray transient absorption spectrometer Rev. Sci. Instrum. 79, 073101 (2008);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/cm 2 . 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/cm 2 . 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. V C 2012 American Institute of Physics.

Very low electron temperature in warm dense matter formed by focused picosecond soft x-ray laser pulses

“…9 If the fluence of 65 mJ/cm 2 is supposed for the SXRL pulse, then the total pressure reaches around 15 GPa, which is generated by the electron temperature of around 1 eV. 11 Under a low fluence region of 10-30 mJ/cm 2 for the SXRL pulse, an electron temperature of less than 1 eV is considered reasonable, which is, however, high enough for nanometer scale surface modification on the target surface.…”

“…The model calculations show that the spallation process goes on for more than 100 ps. 3,11 For t em $ 100-1000 ps, for example, the temperature is estimated to be T m < 0.4-0.7 eV.…”

“…Calculations have predicted an electron temperature of 0.1-10 eV, which depends on the laser fluence. [8][9][10][11][12] To consider the radiation from a heated or molten layer, we suppose a black-body radiator as an object. Suppose that the temporal behavior of the target electron temperature, T e , can be described by a Gaussian…”

“…Spallation phenomena are predicted for the interaction between SXRL pulses and material surfaces. 3,[8][9][10][11] The spallation process describing the ablation mechanism is as follows; the nucleation of interatomic size foams occurs under the SXRL pulse irradiation, these foams swell, a free surface separates from the substance, and finally a spallative layer runs away. The driving force for the spallation is an increasing pressure appearing inside the heating layer.…”

“…Under a relatively low fluence region near the ablation threshold, it is considered that the first change of the surface structure is due to a splash of a molten layer, because the a) dependence of the molten depth on the fluence agrees with experimental results. 11,12 The theoretical approach for an interaction model of a SXRL pulse with material surfaces plays a crucial role in the explanation of the nanometer scale surface modification in experiments. However, there is still large gap between the experimental results and the theoretical predictions.…”