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 parameters of the shock adiabat of a porous material are calculated in the mesoscopic setting with the developed method of a moving window. The porous material is considered a framework of solid material that fills the space between pores, the mechanical properties and shock adiabat of which are often well known. The essence of the method is as follows: uncompressed material flows into the computational box with a constant velocity, while the outflow velocity from the box is chosen based on iterations so as to make the wave front immobile relative to the window, because the stationary mode of shock-wave propagation must be achieved to calculate the shock adiabats. The simulation of shock waves is performed both in the standard setting with an immobile piston (inverse-motion method) and in the system of a moving window. It is demonstrated that the wave profiles obtained with both methods are identical after the stationary mode is achieved. As an example, the shock adiabats of porous copper, which adequately reproduce experimental data for different porosities, are calculated. The proposed mesoscopic method of calculating the response of porous materials to shock compression in a moving window enables direct calculation of desired shock adiabats for porous materials that have not been studied experimentally.

Воздействие двух последовательных лазерных импульсов на кремний, помещенный в глицерин, исследовано экспериментально и численно с помощью программ электромагнитного, гидродинамического и атомистического моделирования. Показано, что после первого импульса на поверхности образуется микропузырек в жидкости, на котором затем происходит дифракция второго импульса, ширина светового пучка которого сравнима с диаметром микропузырька. Мы рассчитали дифракционную картину и распределение интенсивности света на поверхности кремния, и оказалось, что максимальная интенсивность в дифракционных пиках может заметно превышать интенсивность на оси падающего гауссова пучка. В результате усиления интенсивности, сконцентрированной в одном ярком узком кольце вокруг микропузырька, на кремнии образуется характерная канавка, окруженная валиками. Мы продемонстрировали в молекулярно-динамическом расчете, что интенсивный нагрев в дифракционном пике вызывает плавление и вытеснение расплава от центра прогрева. Это приводит к формированию канавок с валиками, имеющими профиль, подобный измеренному в эксперименте.

Evaporation and condensation processes are intensely used in various fields of technology. Efforts to understand the features of film boiling of various liquids, primarily superfluid helium, inevitably lead to studying the strongly nonequilibrium processes of heat transfer from the heating surface through the vapor to the condensed phase. Theoretical studies of evaporation and condensation of single-component substances are briefly reviewed. Corresponding experimental data are analyzed and compared with calculations. We explore the important, yet unresolved and actively studied problems of condensation from vapor–gas media, the formulation of boundary conditions, and the application of molecular dynamics and kinetic theory methods to the study of heat and mass transfer at phase interfaces.

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.

The machining of materials produces regular micrometer-sized surface corrugations. The microscopic cumulative jets can be generated from such surface under shock loading. It is too difficult to resolve space-time evolution of such jets in experimental conditions. The details of jet formation can be observed in molecular dynamics (MD) and smoothed particle hydrodynamics (SPH) simulations. We demonstrate that for strong enough shocks the scaling provides the similar jet velocity profiles and mass distributions obtained by both methods. But for weaker shocks and small surface perturbations the scaling does not work well which may lead to discrepancies between MD and SPH results.

We present the results of studying the morphology of the modified surface of aluminium, nickel and tantalum after ablation of the surface layer by a femtosecond laser pulse. The sizes of characteristic elements of a cellular nanostructure are found to correlate with thermo-physical properties of the material and the intensity of laser radiation.

It is well known that during ablation by an ultrashort laser pulse, the main contribution to ablation of the substance is determined not by evaporation, but by the thermomechanical spallation of the substance. For identical metals and pulse parameters, the type of spallation is determined by film thickness d f . An important gauge is metal heating depth d T at the two-temperature stage, at which electron temperature is higher than ion temperature. We compare cases with d f < d T (thin film) and d f ≫ d T (bulk target). Radius R L of the spot of heating by an optical laser is the next (after d f ) important geometrical parameter. The morphology of film bulging in cases where d f < d T on the substrate (blistering) changes upon a change in radius R L in the range from diffraction limit R L ∼ λ to high values of R L ≫ λ, where λ ∼ 1 μm is the wavelength of optical laser radiation. When d f < d T , R L ∼ λ, and F abs > F m, gold film deposited on the glass target acquires a cupola-shaped blister with a miniature frozen nanojet in the form of a tip on the circular top of the cupola (F abs and F m are the absorbed energy and the melting threshold of the film per unit surface area of the film). A new physical mechanism leading to the formation of the nanojet is proposed.

Ultrashort heating of substances converts them into the two-temperature (2T) state with hot electrons. The thickness of the heated layer rapidly increases into the depth of the metal in this state (by comparison with the thickness of the skin layer), and the pressure in the heated layer rises sharply because of the high rate of heating (inertial confinement). A technique has been developed for taking into account 2T, thermomechanical, and multidimensional (target-structuring) phenomena. It is based on quantum-mechanical computations by means of the density functional, the solution of kinetic equations, and 2T hydrodynamic and molecular-dynamic calculations. The mechanism for forming superelastic shock waves and for constructing complex surface structures has been studied. The corresponding results have great significance for developing promising nanometallurgical technologies associated with laser pinning, for increasing corrosion resistance, and for altering optical surface characteristics. (C) 2014 Optical Society of America.

A new model has been developed and calculations have been performed for the formation of nanodroplets after action of an ultrashort laser pulse on a thin (10–100 nm) gold film deposited on a glass substrate. The action of a laser results in the melting of the film in the region of a laser spot and in its thermomechanical separation from the substrate. The separated film acquires a dome shape because of a decrease in the temperature in the direction from the center of the laser spot. This theoretical model provides the explanation of the formation of nanodroplets. It has been established that, first, the separation speed of a gold film from glass decreases sharply because the acoustic impedance of gold is much larger than that of glass. Second, nanodroplets are formed owing to the capillary focusing of the substance, which is manifested in the appearance of the drag component directed toward the axis of symmetry of the dome. The surface tension becomes dynamically significant because of the indicated sharp decrease in the separation speed from glass and of the smallness of the diameter of the focal spot (D ∼ 1 μm), which is determined by the diffraction limit of optical radiation.

Using an interferometric continuous monitoring technique, we have investigated the motion of the surface of an aluminium target in the case of femtosecond laser ablation at picosecond time delays relative to the instant of laser exposure. Measurements of the temporal target dispersion dynamics, molecular dynamics simulation results and the morphology of the ablation crater have demonstrated a thermomechanical (spall) nature of the disruption of the condensed phase due to the cavitation-driven formation and growth of vapour phase nuclei upon melt expansion, followed by the formation of surface nanostructures upon melt solidification. The tensile strength of heated aluminium in a condensed state has been determined experimentally at an expansion rate of .

Хорошо известно, что при абляции ультракоротким лазерным импульсом основной вклад в унос вещества определяется не испарением, а термомеханическим отрывом вещества. При одинаковых металлах и параметрах импульса характер процесса отрыва зависит от толщины пленки d f. Имеется важный масштаб, равный глубине прогрева металла d т на двухтемпературной стадии, на которой температура электронов выше температуры ионной подсистемы. Мы сравниваем случаи d f ˂ d т(тонкая пленка) и d f ˃ d т(объемная мишень). Радиус R L пятна нагрева оптическим лазером является вторым (после d f ) важным геометрическим параметром. Морфология вспучивания пленки в случае d f ˂ d т на подложке (блистеринг) меняется при изменении радиуса R L в диапазоне от дифракционного предела R L F m на пленке золота, нанесенной на стеклянную подложку, появляется куполообразное вспучивание с миниатюрной замороженной наноструйкой, имеющей вид заострения, на округлой вершине купола (F abs и F m - поглощенная энергия и порог плавления пленки в расчете на единицу поверхности пленки). Предложен новый физический механизм, который приводит к образованию наноструйки.