The action of an ultrashort laser pulse transforms a metal into a two-temperature (2T) state with different temperatures of the electron and ion subsystems (T _e ≫ T _i ). The metal stays in this state in a rather long time interval (from several to several tens of picoseconds depending on the metal). The 2T stage is very important since it includes 2T relaxation, in which laser energy is transferred to ions, and the formation of a heated layer, which plays a key role in the subsequent dynamics. The kinetic coefficients of a condensed medium with hot electrons are poorly known: researchers use phenomenological dependences consisting of asymptotics at low and high temperatures T _e . However, it is impossible to perform a numerical simulation of the interaction of laser radiation with a substance without these coefficients. In this work, the thermal conductivity is calculated using a kinetic equation for the first time. This calculation is valid at low (T _e ≪ T _F = E _F/k _B, where E _F is the Fermi energy and k _B is the Boltzmann constant) and moderate (T _e < T _F) temperatures. The earlier kinetic calculations are related to the case of T _e < T _F, where the calculations of, e.g., the electron-electron collision frequency are significantly simplified due to the reduction of multiple integration to integration in a small neighborhood of the Fermi sphere. In our case, integration at moderate temperatures should be performed over the entire volume of momentum space.

Laser-produced surface nanostructures show considerable promise for many applications while fundamental questions concerning the corresponding mechanisms of structuring are still debated. Here, we present a simple physical model describing those mechanisms happened in a thin metal film on dielectric substrate irradiated by a tightly focused ultrashort laser pulse. The main ingredients included into the model are (i) the film–substrate hydrodynamic interaction, melting and separation of the film from substrate with velocity increasing with increase of absorbed fluence; (ii) the capillary forces decelerating expansion of the expanding flying film; and (iii) rapid freezing into a solid state if the rate of solidification is comparable or larger than hydrodynamic velocities. The developed model and performed simulations explain appearance of microbump inside the focal spot on the film surface. The model follows experimental findings about gradual transformation of the bump from small parabolic to a conical shape and to the bump with a jet on its tip with increasing fluence. Disruption of the bump as a result of thinning down the liquid film to a few interatomic distances or due to mechanical break-off of solid film is described together with the jetting and formation of one or many droplets. Developed theory opens door for optimizing laser parameters for intended nanostructuring in applications.

The understanding of fracture phenomena of a material at extremely high strain rates is a key issue for a wide variety of scientific research ranging from applied science and technological developments to fundamental science such as laser-matter interaction and geology. Despite its interest, its study relies on a fine multiscale description, in between the atomic scale and macroscopic processes, so far only achievable by large-scale atomic simulations. Direct ultrafast real-time monitoring of dynamic fracture (spallation) at the atomic lattice scale with picosecond time resolution was beyond the reach of experimental techniques. We show that the coupling between a high-power optical laser pump pulse and a femtosecond x-ray probe pulse generated by an x-ray free electron laser allows detection of the lattice dynamics in a tantalum foil at an ultrahigh strain rate of ~2 × 10⁸ to 3.5 × 10⁸ s⁻¹. A maximal density drop of 8 to 10%, associated with the onset of spallation at a spall strength of ~17 GPa, was directly measured using x-ray diffraction. The experimental results of density evolution agree well with large-scale atomistic simulations of shock wave propagation and fracture of the sample. Our experimental technique opens a new pathway to the investigation of ultrahigh strain-rate phenomena in materials at the atomic scale, including high-speed crack dynamics and stress-induced solid-solid phase transitions.

Using the gold–water pair as an example, we analyze the problems related to an ultrashort laser action on a metallic target submerged in a transparent liquid.

Laser ablation (i.e., the removal of target material upon irradiation) is under consideration; it is widely applied in some technologies. Physical models needed for an understanding of ablation have been developed since the invention of the first lasers. Some phenomena have been thoroughly investigated; however, there are still many problems that should be analyzed additionally. These poorly studied phenomena include surface structuring. The problems concerning ablation in liquid and laser forging/peening are not yet completely understood. They represent two sides of the same process: in the first case, the emphasis is on the description of motion of the substance beyond the target, whereas, in the case of laser forging, one abstracts from the plume and considers the elastoplastic transformations caused by a shock wave inside a target. The hydrodynamics of these processes differs radically with the transition from ultrashort to long pulses.