We review the results of our research on damage mechanisms in materials irradiated with femtosecond free-electron-laser (FEL) pulses. They were obtained using our hybrid approach, X-ray-induced thermal and non-thermal transitions (XTANT). Various damage mechanisms are discussed with respect to the pulse fluence and material properties on examples of diamond, amorphous carbon, C₆₀ crystal, and silicon. We indicate the following conditions: those producing thermal melting of targets as a result of electron-ion energy exchange; non-thermal phase transitions due to modification of the interatomic potential; Coulomb explosion due to accumulated net charge in finite-size systems; spallation or ablation at higher fluences due to detachment of sample fragments; and warm dense matter formation. Transient optical coefficients are compared with experimental data whenever available, proving the validity of our modeling approach. Predicted diffraction patterns can be compared with the results of ongoing or future FEL experiments. Limitations of our model and possible future directions of development are outlined.

The mechanism of surface restructuring by ultrashort laser pulses involves a number of fast, nonequilibrium, and interrelated processes while the solid is in a transient state. As a result, the analysis of the experimental data cannot address all of the mechanisms of nanostructuring. In this paper, we present a direct comparison of a simulation and the experimental results of surface nanomodifications induced by a single laser pulse. The experimental results are obtained by using a mask-projection setup with a laser wavelength of 248 nm and a pulse length of 1.6 ps. Two-beam interference of this short wavelength allows for producing a large-area intensity grating of $40\mu \mathrm{m}$ in diameter on a gold surface with a sinusoidal shape and a period of 500 nm. The formed structures are analyzed at the surface and in a cross section by a scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. Then a hybrid atomistic-continuum model capable of capturing the essential mechanisms responsible for the nanostructuring process is used to model the interaction of the laser pulse with a thick gold target. The good agreement between the modeling results and the experimental data justifies the proposed approach as a powerful tool revealing the physics behind the nanostructuring process at a gold surface and providing a microscopic insight into the dynamics of the structuring processes of metals in general. The presented model, therefore, is an important step towards a computational tool for predicting a materials response to an ultrashort laser pulse on the atomic scale. This detailed understanding of the dynamics of the process will pave the way towards predesigned topologies for functionalized surfaces on nanoscales and microscales.

We present an atomistic-continuum model to simulate ultrashort-pulse laser melting processes in semiconductor solids on the example of silicon. The kinetics of transient nonequilibrium phase transition mechanisms is addressed with a molecular dynamics method at atomic level, whereas the laser light absorption, strong generated electron-phonon nonequilibrium, fast diffusion of and heat conduction due to photoexcited free carriers are accounted for in the continuum. We give a detailed description of the model, which is then applied to study the mechanism of short laser pulse melting of freestanding Si films. The effect of laser-induced pressure and temperature of the lattice on the melting kinetics is investigated. Two competing melting mechanisms, heterogeneous and homogeneous, were identified. Apart from the classical heterogeneous melting mechanism, the nucleation of the liquid phase homogeneously inside the material significantly contributes to the melting process. The simulations showed, that due to the open diamond structure of the crystal, the laser-generated internal compressive stresses reduce the crystal stability against the homogeneous melting. Consequently, the latter can take a massive character within several picoseconds upon the laser heating. Due to the negative volume of melting of modeled Si material, −7.5%, the material contracts upon the phase transition, relaxes the compressive stresses, and the subsequent melting proceeds heterogeneously until the excess of thermal energy is consumed. The threshold fluence value, at which homogeneous nucleation of liquid starts contributing to the classical heterogeneous propagation of the solid-liquid interface, is found from the series of simulations at different laser input fluences. For the example of Si, the laser melting kinetics of semiconductors was found to be noticeably different from that of metals with a fcc crystal structure.

Ruthenium is a perspective material to be used for XUV mirrors at free-electron laser facilities. Yet, it is still poorly studied in the context of ultrafast laser-matter interaction. In this work, we present single-shot damage studies of thin Ru films irradiated by femtosecond XUV free-electron laser pulses at FLASH. Ex-situ analysis of the damaged spots, performed by different types of microscopy, shows that the weakest detected damage is surface roughening. For higher fluences we observe ablation of Ru. Combined simulations using Monte-Carlo code XCASCADE(3D) and the two-temperature model reveal that the damage mechanism is photomechanical spallation, similar to the case of irradiating the target with optical lasers. The analogy with the optical damage studies enables us to explain the observed damage morphologies.