Interaction of ultrashort laser pulse with metals is considered. Ultrafast heating in our range of absorbed fluences Fabs > 10 mJjcm2 transfers matter into two-temperature (2T) state and induces expressed thermomechani cal response. To analyze our case, where 2T, thermomechanical, and multidimensional (formation of surface
structures) effects are significant, we use density functional theory (DFT), solutions of kinetic equations in τ- approximation, 2T-hydrodynamics, and molecular dynamics simulations. We have studied transition from light absorption in a skin layer to 2T state, and from 2T stage to hydrodynamical motions. We describe (i) formation of very peculiar (superelasticity) acoustic wave irradiated from the laser heated surface layer and (ii) rich com plex of surface phenomena including fast melting, nucleation of seed bubbles in hydrodynamically stretched fluid, evolution of vapor-liquid mixture into very spatially extended foam, mechanical breaking of liquid membranes in foam (foam disintegration), strong surface tension oscillations driven by breaking of membranes, non-equilibrium freezing of overcooled molten metals, transition to nano-domain solid, and formation of surface nanostructures.
The paper is devoted to experimental and theoretical studies of ablation of condensed matter by optical (OL),
extreme ultraviolet (EUV) and X-ray lasers (XRL). Results obtained at two different XRL are compared. The
first XRL is collision Ag-plasma laser with pulse duration τL = 7 ps and energy of quanta hv=89.3 eV, while
the second one is EUV free electron laser (EUV-FEL) and has parameters τL = 0.3 ps and energy of quanta 20.2
eV. It is shown that ablation thresholds for these XRL at LiF dielectric are approximately the same. A theory is
presented which explains slow growth of ablated mass with fluence in case of XRL as a result of transition from
spallative ablation near threshold to evaporative ablation at high fluencies. It is found that the metal irradiated
by short pulse of OL remains in elastic state even in high shear stresses. Material strength of aluminum at very
high deformation rates V/V ~ 109 s-1 is defined.
The motion of both Lennard-Jones solids and metals induced by ultrashort laser irradiation near the ablation threshold is
investigated by molecular dynamics simulation. The universality of the ablation threshold fluence with respect to the
cohesion energy of solids irradiated by femtosecond laser pulses is demonstrated for Lennard-Jones solid and metals
simulated by many-body EAM potentials.
Theoretical consideration of the ablation of laser heated metal target based on two-temperature hydrodynamic calculation
is performed for aluminum and gold targets. Concurrent with the hydrodynamic calculation the molecular dynamics
simulation of the ablation was carried out in the case of aluminum. The initial state of matter for the molecular dynamics
is taken as a final state of hydrodynamic calculation. Molecular dynamics simulation is extended to cover late stages of
the evolution of two-phase foam placed between the crater and spalled cupola. Theoretical results are in a good
agreement with the experimental data obtained by the microinterferometer diagnostics of the femtosecond laser ablation
both for aluminum and gold.
In the present work phenomena are considered related to the interaction of ultra-short laser pulses, τL~0.1 ps, with metallic targets. The absorption of laser pulse results in formation of thin layer of hot electrons strongly superheated (Te>>Ti) relative to the ion temperature, Ti. Initial thickness of the layer dheat is small, dheat~δ, where δ~10 nm is the skin layer thickness. Subsequent developments include the following stages: (1) Propagation of electron thermal wave which expands the hot layer dheat; (2) Cooling of electrons due to energy transfer to cold ions; (3) Onset of hydrodynamic motion that constitutes the rarefaction wave with positive pressure; (4) Further expansion of target material leading to the appearance of negative pressure; and (5) Long separation process which begins with nucleation of voids and goes on to the total separation of spallation plate. The thickness of the plate is ~10 nm (we call it nanospallation). Theoretical model involves two-temperature hydrodynamic equations with semiempirical EOS for a metal, electron heat conduction and electron-ion energy exchange. The decay of metastable strongly stretched matter is described by molecular dynamics (MD) simulation with extremely large number of atoms. The experimental setup includes femtosecond chromium-forsterite laser operating in the pump-probe regime. The experiments are performed with gold target. Measured ablation threshold for gold is 1.35 J/cm2 of incident pump light at inclination 45°, p-polarization. Calorimeter measurements give for the absorbed fluence Fabs=0.3Finc, therefore the threshold value of Fabs is 0.4 J/cm2.
The advantages of the modified Nakatani method for diagnostics of energetic condition of meridians are described. Also
the perspectives of its clinical application for dynamic patient monitoring in treatment process are considered. The
model of a biological feedback through measurement of electric conductivity of a skin at pulse current is represented
with its mathematical and software base. The results of clinical tests of the treatment-and-diagnostic complex (TDC)
functioning in correspondence with the modified Nakatani method are analyzed.
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. 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).
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.
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.
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.
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, and 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 approximately 10-10 s.
The photophysical mechanism of laser ablation related to the change in activation energy for desorption (or destruction and further elimination) of electronically excited molecules is discussed. Despite the obvious physical reasons, up to now there is no experimental confirmation that this mechanism plays an important role with nanosecond laser ablation where duration of the laser pulse is longer than the corresponding relaxation time (typically, tens of picoseconds). On the other hand there was no special study of this mechanism for Ultra Short Laser Pulses (USLP) with pulse duration significantly shorter than the electronic energy relaxation time. Here we present the theoretical investigation of the photophysical ablation features with USLP and compare predictions which follow for the photophysical and pure thermal models.
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.
The joint influence of electronic excitations and mechanical stresses originated from UV laser action on solids (primarily polymers) decomposition kinetics is discussed.
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.
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