The homogeneous formation of nucleation centers (nucleation) is regarded as a mechanism for the superfast thermal melting, of solids under the action of femtosecond laser pulses. On the basis of classical nucleation theory, it is shown that, for a high degree of superheating of the solid, the melting process is determined to a greater extent by the time of heating of the crystal lattice as a result of electron-phonon interaction than by the nucleation kinetics. Thus, the melting process can be completed in the course of several picoseconds. On one hand, this time is less than the characteristic time of heterogeneous surface melting and, on the other hand, it is greater than the time for possible superfast nonthermal melting. Experimental data on the melting dynamics of tellurium, obtained by polarization microscopy with femtosecond time resolution, confirm that superfast thermal melting is possible. (C) 2004 Optical Society of America.

Short pulse laser breakdown in a non-uniform vapor cloud is considered as an instant local energy release (point explosion). The breakdown generates a strong shock wave propagating over the cloud. Propagation of the shock is described using an approach developed by A.S. Kompaneets [1]. This approach is generalized to the case of arbitrary density and pressure profiles in the vapor cloud. For this case, the equation for shock wave shape is reduced to the eiconal equation in optics. Exact solutions are obtained for particular density distributions.

Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). Optically sharp surfaces occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials (Si, graphite, Au, Al).

Physical mechanisms and theoretical models of laser ablation are discussed. For various mechanisms, typical associated phenomena are qualitatively regarded and methods for studying them quantitatively are considered. Calculated results relevant to ablation kinetics for a number of substances are presented and compared with experimental data.

The interaction of subpicosecond laser pulses with metals is studied theoretically using phenomenological two-temperature model. A semi-analytical approach to a quantitative analysis of electron and lattice temperatures is presented. Using the nonstationary averaging technique the coupled system of nonlinear heat equations for electron and lattice temperatures is transformed into the set of four ordinary differential equations. Resulting system is convenient for the fast analysis of nonstationary laser heating and laser ablation with ultrashort laser pulses.

We use molecular dynamics to study the structure of steady shock waves with melting transition traveling along the [100], [110], and [111] directions in the Lennard-Jones (L-J) perfect Sec crystal. Unlike shock waves in gases and fluids, solid shocks exhibit oscillatory behaviour of profiles within the front that persists even in the strong shocks with melting. Surprisingly, shock wave along the [100] direction compresses the L-J crystal to the final overheated solid state in contrast to [110] and [111] cases wherein the crystal melted at the same shock velocity mu(s), = 4.39 km/s. Moreover, the [110] and [111] melting shock waves differ widely in the front structure: the [111] profile without oscillations is closely similar to that of the fluid shocks, whereas the (110) shock exhibits a steady precursor of solitary wave train. The evolution of velocity and pair distribution functions across the shock layer are explored to study the shock-induced structure transformations and melting transition occuring within the region of length roughly (10 divided by 30) sigma, sigma = 3.405 Angstrom.

Multi-layered flyer (aluminum–polyimide–tantalum) is designed as a high speed flyer making use of shock impedance matching and reverberation techniques. The designed three layered targets have been irradiated using a 20 J laser beam. Flyer velocities are measured by observing the flyer impact emissions on glass step targets within a 500 μm laser focal spot at laser intensities 5×1012–2×1013 W/cm2. Thin (0.5–1.0 μm) Ta layers of the flyers are accelerated via shock reverberations between the thick polyimide and thin Ta layers for the first time using laser induced shock waves. Their velocities are measured to be more than 13 km/s with a good hydrodynamic stability. The obtained velocity is faster than the ones obtained by a conventional flyer method such as a double gas gun.

The vaporisation of condensed matter is studied theoretically in a wide temperature range. When the temperature is well below the critical point of a substance, the evaporation can be described in terms of a simple model assuming that (i) a sharp boundary separates the condensed and gaseous phase; (ii) the atoms emitted from the phase boundary have nonequilibrium velocity distribution; (iii) the equilibrium is attained within the Knudsen layer, the thickness of which is of the order of the mean free path of atoms. When the temperature of the condensed phase is comparable to or higher than the critical temperature the sharp boundary is transformed to the macroscopic interphase layer. The dynamics of both condensed and gaseous phases are described in this case by the equations of hydrodynamics supplemented by a wide-range semi-empirical equation of state. The vaporisation of strongly superheated metal is analysed.

The effects of temperature and pressure nonuniformities at evaporation on the properties of liquid–gas interface are studied by molecular dynamics (MD) simulation and thermodynamic perturbation method on the basis of the van der Waals theory of capillarity. The structure and properties of the interfacial layer of equilibrium and nonequilibrium Lennard-Jones (12-6) systems are investigated. The surface tension, the two-particle distribution functions, the density fluctuation correlation lengths, and the evaporation coefficients are calculated using MD simulation. It is shown that the presence of the temperature gradient at the interface due to evaporation leads to reduction of the surface tension. The results of MD simulations are in agreement with the results of thermodynamic approach.

Formation of a liquid phase with subsequent transition to a uniform amorphous state of surface layer upon solidification is observed under action of picosecond laser pulses on microcrystalline graphite. This phenomenon is registered on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, an only for certain parameters of laser pulse. A study of melting and solidification of graphite is performed using a new method based on partial transmission of incident p-polarized wave into s-polarized reflected wave due to optical anisotropy. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. Periodic surface structures with a period of the order of the wavelength of the heating pulse was detected on amorphous graphite region. The orientation of the structures correlates with polarization of incident laser pulse. The instability to formation of these structures is assumed to be connected with surface electromagnetic wave excitation. The characteristic time of existing of liquid phase and of solidification processes is determined to be similar to 10(-10) s.

The condensation of vapor within the expanding plume produced by ns-laser ablation is discussed in the frame of Zeldovich and Raizer theory of condensation. The calculations have been performed for Si, Ge and C-vapors. It is shown that the size of clusters formed during the condensation is very small, typically of the order of few nanometers. The averaged cluster radius is calculated for different temperatures and densities of the initial plume. The generalization of the theory is made for inhomogeneous plume where the rates of nucleation as well as condensation times are different at different parts of the plume. The size distribution function is calculated for the plume expansion into vacuum. For the clusters moving together with vapor one can distinguish three different waves propagating through the plume: (1) The saturation wave, where the vapor becomes saturated, (2) The supercooling wave, where the highest supercooling is reached, and (3) The quenching wave, where the growth of cluster stops. The last stage of cluster formation is related to cooling of clusters and their crystallization. This leads to delay in photoluminescence signal with typical delay time from 0.1 to 7 ms depending on the type of the background gas and its pressure.

Short pulse laser ablation of semiconductors and metals is studied by means of ultrafast time-resolved microscopy. The characteristic stages of the conversion of solid material into hot fluid matter undergoing ablation are identified, initially metallic material transforms during the expansion into a transparent state with a high index of refraction.

A new method of crystalline order detection in highly absorbing anisotropic crystals is worked out. The method is based on partial transformation of incident p-polarized electromagnetic wave into s-polarized reflected wave due to optical anisotropy. The method makes possible to follow changes of crystalline structure in thin (10(-6)-10(-5) cm) surface layers of solids. Using picosecond laser pulses and streak camera "Agat", surface melting and evaporation of Zn and C (graphite) are studied. Direct observation of the melting of graphite subjected to picosecond laser pulse is performed. The kinetics of solidification of fused surface layers are studied.

The interaction of subpicosecond laser pulses with metals is studied theoretically using phenomenological two-temperature model. Wide-range approximations for electron thermal conductivity and electron-ion energy exchange rate are proposed. Effects of temperature dependence of the thermophysical characteristics on lattice heating dynamics are discussed. Melting and evaporation kinetics are incorporated into the model to describe the metal ablation. Damage threshold and ablated layer thickness are calculated.

Dynamics of dense laser‐produced vapor plume are studied. The analysis is based on the well‐known special solution of gas dynamics equations that describes the expansion of an ellipsoidal gas cloud into vacuum. This solution is employed to explain the apparent ‘‘rotation’’ of the cloud (so called ‘‘flip‐over effect’’). The flux of atoms to the substrate and the film thickness profiles are calculated using the model. It is shown that the cloud expansion into vacuum and into an ambient gas both are unstable. For short‐wavelength perturbations, estimates of the instability growth rate are presented.

Dynamic of a dense, laser-produced vapor plume is studied. The analysis is based on the special solution of gas dynamics equations that describes the expansion of an ellipsoidal gas cloud into vacuum. This solution is employed to explain the apparent 'rotation' of the cloud — the so called 'flip-over effect'. The flux of atoms onto the substrate and the film thickness profile are calculated using the model. The present model can be used for the interpretation of the time-of-flight spectra of atoms in laser ablation, and for the description of the shape of vapor cloud expanding into an ambient gas.

In this present report we consider dust particle striking surface of a target. Swift impact is a reason of a whole sequence of mechanical and physical phenomena. It generates spolation and crater formation, emission of matter, plasma creation and radiation flash. We have considered also dynamics of this plasma cloud in an external electric field (charge separation and charge extraction). This consideration is important from an experimental point of view for impact detection and analysis.

An evaporation mechanism involving fluctuations in the binding energy of atoms in a surface layer is discussed. An evaporation of clusters of several atoms becomes likely according to this mechanism. A qualitative analysis is supported by the results of a numerical simulation of a phase transition by a molecular-dynamics method.