Investigation of ablation thresholds of optical materials using 1-µm-focusing beam at hard X-ray free electron laser

Abstract

We evaluated the ablation thresholds of optical materials by using hard X-ray free electron laser. A 1-µm-focused beam with 10-keV of photon energy from SPring-8 Angstrom Compact free electron LAser (SACLA) was irradiated onto silicon and SiO2 substrates, as well as the platinum and rhodium thin films on these substrates, which are widely used for optical materials such as X-ray mirrors. We designed and installed a dedicated experimental chamber for the irradiation experiments. For the silicon substrate irradiated at a high fluence, we observed strong mechanical cracking at the surface and a deep ablation hole with a straight side wall. We confirmed that the ablation thresholds of uncoated silicon and SiO2 substrates agree with the melting doses of these materials, while those of the substrates under the metal coating layer are significantly reduced. The ablation thresholds obtained here are useful criteria in designing optics for hard X-ray free electron lasers.

Fluence thresholds for grazing incidence hard x-ray mirrors

“…We also note that the estimated beam size and shape were consistent with those determined by normal incidence measurements taken on lead(II) iodide 9 and on silicon oxide. 10 It is of note that the fluence damage thresholds are far higher than the values obtained for normal incidence measurements or measurements conducted at lower photon energies. 11,12 This is due to a significant fraction of the pulse energy being reflected by the mirror coatings and not absorbed into the material.…”

Reabsorption of Soft X-Ray Emission at High X-Ray Free-Electron Laser Fluences

“…In the high fluence regime, the studied system considerably deviates from the ground state and our model anticipates water molecules with far more than one valence hole. Effects like Coulomb explosion strongly affecting the geometric structure have been observed for heavier elements, e.g., silicon [31][32][33][34][35] in a similar fluence regime and have been predicted and observed for lighter elements at significantly higher fluences [36,37]. Although a detailed description of the connected effects on the electronic structure and nuclear motions is beyond the scope of this Letter, the created valence holes can still reabsorb x-ray emission at these high fluences.…”

Role of heat accumulation in the multi-shot damage of silicon irradiated with femtosecond XUV pulses at a 1 MHz repetition rate

“…Instead, it was estimated to be 410 mJ/cm 2 , from experimental values obtained for irradiations at the wavelength of 32.5 nm [5,9] and 0.124 nm (10 keV photon energy) [12,13] and theoretical predictions [20].…”

“…For silicon, phase transition mechanisms induced with a single femtosecond and monochromatic pulse of wavelength in the range of XUV and X-ray were studied previously, mainly with an angle of incidence close to surface normal. Bulk crystals [5,[7][8][9][10][11][12][13] together with a-Si layers in Si/C and Mo/Si multilayer systems [14,15] have been examined. These measurements have shown that single shot structural modifications are caused by melting.…”