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

The role played by heat accumulation in multi-shot damage of silicon was studied. Bulk silicon samples were exposed to intense XUV monochromatic radiation of a 13.5 nm wavelength in a series of 400 femtosecond pulses, repeated with a 1 MHz rate (pulse trains) at the FLASH facility in Hamburg. The observed surface morphological and structural modifications are formed as a result of sample surface melting. Modifications are threshold dependent on the mean fluence of the incident pulse train, with all threshold values in the range of approximately 36-40 mJ/cm². Experimental data is supported by a theoretical model described by the heat diffusion equation. The threshold for reaching the melting temperature (45 mJ/cm²) and liquid state (54 mJ/cm²), estimated from this model, is in accordance with experimental values within measurement error. The model indicates a significant role of heat accumulation in surface modification processes

Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films

Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump - X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of melting. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state.Comment: main manuscript 33 pages, 9 figures; supplemental material 19 pages, 13 figures - all in one fil

Damage studies of multilayer optics for XUV free electron lasers

We exposed standard Mo/Si multilayer coatings, optimized for 13.5 nm radiation to the intense femtosecond XUV radiation at the FLASH free electron laser facility at intensities below and above the multilayer ablation threshold. The interaction process was studied in-situ with reflectometry and time resolved optical microscopy, and ex-situ with optical microscopy (Nomarski), atomic force microscopy and high resolution transmission electron microscopy. From analysis of the size of the observed craters as a function of the pulse energy the threshold for irreversible damage of the multilayer could be determined to be 45 mJ/cm2. The damage occurs on a longer time scale than the XUV pulse and even above the damage threshold XUV reflectance has been observed showing no measurable loss up to a power density of 1013 W/cm2. A first explanation of the physics mechanism leading to damage is given

Structural changes across thermodynamic maxima in supercooled liquid tellurium: a water-like scenario

Liquid polymorphism is an intriguing phenomenon which has been found in a few single-component systems, the most famous being water. By supercooling liquid Te to more than 130 K below its melting point and performing simultaneous small-angle and wide-angle X-ray scattering measurements, we observe clear maxima in its thermodynamic response functions around 615 K, suggesting the possible existence of liquid polymorphism. A close look at the underlying structural evolution shows the development of intermediate-range order upon cooling, most strongly around the thermodynamic maxima, which we attribute to bond-orientational ordering. The striking similarities between our results and those of water, despite the lack of hydrogen-bonding and tetrahedrality in tellurium, indicate that water-like anomalies may be a general phenomenon among liquid systems with competing bond- and density-ordering.Comment: Main text: 10 pages, 5 figures; supplementary materials: 14 pages, 13 figure

Modelling of damage in Ru thin films induced by femtosecond XUV laser pulses

Survivability of optical elements exposed to high doses of XUV laser radiation is an important issue in the context of rapidly developing x-ray free-electron laser (XFEL) light sources. In order to prevent optics from being damaged, the fundamental mechanisms governing the material response to ultrashort high peak power XFEL pulses must be identified and studied. We present computational study of the interaction of femtosecond XUV (13.5 nm wavelength) laser pulses with 50 nm thin Ru films. With our calculations we model the damage experiments that was performed at Free-Electron LASer in Hamburg (FLASH) [1]–[3]. Ru is chosen as optically favorable material for grazing incidence reflective mirrors. The performed simulations consist of two parts. First, the effect of electron cascading induced after absorption of XUV photons is studied using an event-by-event Monte Carlo code XCASCADE [4]. Time of cascading and ballistic range of non-thermalized electrons are calculated. Second, the evolution of electron and lattice temperatures in the regime of thermal non-equilibrium together with atomic motion in irradiated Ru are modeled with a combination of two temperature hydrodynamics [5] and molecular dynamics [6]. Our calculations showed that the mechanism responsible for the ablation of Ru observed in the experiment is spallation in the stress confinement regime. The processes of melting, cavitation, spallation and recrystallization are modeled. The results show good agreement with the experimental observations