The time dynamics and morphology of a spall fracture of a micron-thick vanadium film sample under shock-wave loading by femtosecond laser pulses under various radiation focusing conditions were studied using high-speed laser interferometry, scanning electron microscopy, and atomic force microscopy. It is shown that near the destruction threshold at a duration of the generated compression pulse of 50 ps and a deformation rate of 1.4 × 109 s–1 a crater 170–190 nm depth and nanoscale roughness is formed on the back surface. The results may be of interest for studying the physics and modeling of high-speed metal destruction processes.

The evolution of the optical properties of a 25-nm-thick nickel film on a glass substrate excited by a subpicosecond terahertz pulse with a field strength of 11 MV/cm has been studied by femtosecond interference microscopy with a time resolution of 10–13 s. The measurements of the complex reflection coefficient and the permittivity in the visible range of the spectrum indicate the non-equilibrium heating of the electron subsystem of nickel to several thousands of degrees, which is accompanied by the induced increase in reflection at the initial time of irradiation and by the subsequent melting after 5–10 ps. Scanning electron and atomic force microscopy studies of the morphology of the modified surface have indicated the local melting of the nanofilm and its delamination from the substrate at this field strength.

The behavior of molybdenum under the action of load pulses of picosecond duration is studied in an experiment. Using the method of spectral interferometry in the single-exposure mode in the picosecond range, changes in the phase and amplitude of the diagnostic pulse reflected from the free surface of the sample are recorded. In a film sample of molybdenum of submicron thickness, compressive stresses reaching 89 GPa are realized and are accompanied by a significant increase in the surface reflectance.

The movement of the rear free surface of submicron cobalt film samples exposed to picosecond laser pulses with different energy densities was studied experimentally. The displacement of the free rear surface of the sample determined in the picosecond range in single-pulse mode using spectral interferometry. Data were obtained on the spall strength cobalt in a condensed state for a strain rate of 109 s1.

The method of interference microscopy is used to study the features of iron ablation under single exposure to laser pulses with a duration of 60 fs of moderate intensity 10 12 –10 13 W/cm 2 . The value of the reflection coefficient is measured and the value of the threshold of thermomechanical ablation is determined from the absorbed energy density. The evolution of the crater bottom morphology and the dependence of their depth on the energy density of laser pulses are studied.

Laser-driven shock wave phenomena in a sub-micrometer Mg–4Al–2Zn alloy film are studied using spectral interferometry with spatial and temporal (1 ps) resolution. Upon irradiating the film through a glass substrate by 500 fs laser pulses, the ultrashort elastic compression pulses with the peak stress up to 4.6 GPa at a propagation distance of 0.5 μm were generated. Depending on the laser fluence, either spall fracture near the rear surface in the solid state or cavitation near the metal–glass interface in the liquid state was observed. The spall strength of the solid Mg alloy and the upper limit of the cavitation threshold in the melt at the strain rate of ∼109 s−1 were extracted from the free surface velocity history. The depth of fracture initiation was retrieved from the instant of the spall pulse exit, and the thickness of the molten layer was estimated to be 100–160 nm depending on laser fluence. The investigation of the residual morphology by scanning electron and atomic force microscopies revealed the presence of melting and nucleation within the irradiated area. The experimental findings are of interest for predicting the behavior of magnesium alloys in the condensed state at extremely high strain rates, for studying the physics of metastable states and for simulating the interaction of ultrashort laser pulses with thin film materials.

В эксперименте исследовано поведение молибдена под действием импульсов нагрузки пикосекундной длительности. Методом спектральной интерферометрии в режиме однократного воздействия в пикосекундном диапазоне регистрировалось изменение фазы и амплитуды диагностического импульса, отраженного от свободной поверхности образца. В пленочном образце молибдена субмикронной толщины реализованы напряжения сжатия, достигающие 89 ГПа и сопровождающиеся существенным ростом коэффициента отражения поверхности.

The data is obtained on the effect of high-intensity pulses of terahertz (THz) radiation with a broad spectrum (0.2-3 THz) on cell cultures. We have evaluated the threshold exposure parameters of THz radiation causing genotoxic effects in fibroblasts. Phosphorylation of histone H2AX at Ser 139 (gamma H2AX) was chosen as a marker for genotoxicity and a quantitative estimation of gamma H2AX foci number in fibroblasts was performed after cell irradiation with THz pulses for 30 min. No genotoxic effects of THz radiation were observed in fibroblasts unless peak intensity and electric field strength exceeded 21 GW cm(-2) and 2.8 MV cm(-1), respectively. In tumor cell lines (neuroblastoma (SK-N-BE (2)) and glioblastoma (U87)), exposure to THz pulses with peak intensity of 21 GW cm(-2) for 30 min caused no morphological changes as well as no statistically significant increase in histone phosphorylation foci number.

Spectral interferometry is used to study picosecond acoustic pulse propagation in a submicron titanium film heated by femtosecond laser pulse through a glass substrate. The melting depth was estimated from a moment of spall pulse arrival, formed due to cavitation in melt during tensile stress relaxation.

The temporal dynamics of the change in the thermally-induced induced reflection coefficient of polycrystalline gold, aluminum, and molybdenum at a wavelength of 800 nm as a result of exposure to a femtosecond laser pulse (at a wavelength of 400 nm) with an intensity of up to 0.2 TW in the in the picosecond time range is studied using pump-probe optical scheme with phase-sensitive detection. New experimental data are obtained on the change in the reflection coefficient under strongly nonequilibrium conditions when the electron subsystem is heated to temperatures of 10 kK, which are relevant for the development of the experimental base for numerical simulation.

The article studies the motion of the rear free surface of a film sample of a magnesium alloy of submicron thickness under the action of subpicosecond laser pulses. Continuous diagnostics of the displacement of the free rear surface of the sample were carried out in the picosecond range in a single-pulse mode using spectral interferometry. The value of the spall strength at a strain rate of 109 s–1, the value of which is about 53

Проведено экспериментальное исследование движения тыльной свободной поверхности субмикронных пленочных образцов кобальта при воздействии лазерными импульсами пикосекундной длительности с различной плотностью энергии. Диагностика смещения свободной тыльной поверхности образца осуществлялась в пикосекундном диапазоне в одноимпульсном режиме методом спектральной интерферометрии. Получены данные об откольной прочности кобальта в конденсированном состоянии при скорости деформирования ~10 9 с –1 .

The results of experimental studies of the laser shock waves initiated by a picosecond pulse in iron are presented. Experimental measurements are processed and analyzed using theoretical approaches and numerical simulation. Interest in picosecond actions is caused by uniquely high strain rates, in particular, the dependence of the thresholds of elastoplastic and polymorphic transformations on the strain rate. Investigations are necessary for the development of the field of laser hardening of metals. The first steps in this direction have been taken, although this kind of approach to laser forging hardening is already widely used in practice. Modern developments in the field of shock wave generation and their experimental diagnostics are used, and the related methods of theoretical interpretation of experimental data are being developed. The difficulty lies in the picosecond time scale, since the diagnostics of experiments is limited by kinematics, namely, measuring the coordinates of the free surface. To elucidate the polymorphic transformation kinetics on picosecond time scales, the technique of inverse analysis of the free surface velocity is used for the first time. This technique is validated using the results of hydrodynamic and molecular dynamics simulation with direct extraction of mechanical stresses and strains. A theoretical study of reconstructed free surface velocity profiles by traditional methods confirms the results obtained in the field of their applicability, specifically, on elastic and plastic shock wave fronts. The transformation of iron into the ε phase takes place in the initial region of shock wave propagation, as long as a shock wave has a sufficient amplitude. The cause is a pressure limitation of 40 GPa because of optical breakdown in glass and shock wave attenuation during wave propagation.

The dynamics of the change in the complex-reflection coefficient of tantalum under the action of femtosecond laser pulses of moderate intensity is studied by the method of spectral microinterferometry with picosecond resolution. The features of the behavior of the amplitude and phase of the diagnostic wave in the modes of spall and fragmentation ablation are studied. The threshold and characteristic time of the development of explosive boiling of a tantalum target are estimated based on the analysis of the picosecond expansion dynamics.

Multilayer products made of ultra-thin layers are widely used in modern science and technology. Laser exposure is used as one of the promising methods of processing such products. In this regard, we study the ablation of a layered target. A physical model is constructed, numerical simulation is performed, and experiments are carried out. The experiments were conducted with two different lasers and various diameters of the focal spot. To estimate the absorbed energy the reflection coefficient was measured. The results of calculations and experiments are consistent with an accuracy of about 10%. This allowed us to refine the model of two-temperature states and determine the strength of nickel. It is explained why, with an increase in the absorbed fluence, first the upper layer breaks in the multilayer.

The selective modification of upper layers of a Ni/Al multilayer nanostructure irradiated by a single femtosecond laser pulse has been studied. The analysis of surface topography indicates that either partial or complete removal of two upper layers is possible depending on the absorbed energy. The surface has been scanned by an atomic force microscope. The numerical simulation of the phenomenon with a two-temperature hydrodynamic code has revealed an asynchronous dynamics of the electron subsystems of Ni and Al and an inhomogeneous heating of ion subsystems. As a result, a complex combination of compression and rarefaction waves is initiated in the multilayer target. It has been shown numerically that, as the absorbed energy increases, the first nickel layer is initially ruptured because of the localization of tensile stresses. The experimental and numerical thicknesses of the separated layer and the threshold energy are in agreement with each other. Consequently, the parameters of two-temperature models of nickel and aluminum are selected correctly.