Vasily Zhakhovsky

Nanoscale surface modifications and formation of conical structures at aluminum surface induced by single shot exposure of soft x-ray laser pulse

We irradiated the soft x-ray laser (SXRL) pulses having a wavelength of 13.9 nm, a duration time of 7 ps, and fluences of up to 27u2002mJ/cm2 to aluminum (Al) surface. After the irradiation process, the modified surface was observed with the visible microscope, the scanning electron microscope, and the atomic force microscope. The surface modifications caused by the SXRL pulses were clearly seen, and it was found that the conical structures having about 70–150 nm in diameters were formed under a single pulse shot. The conical structures were formed in the features with the average depth of about 40 nm, and this value was in accordance with the attenuation length of the SXRL beam for Al. However, those conical structures were deconstructed under the multiple pulse shots exposure. Thermomechanical modeling of SXRL laser interaction with Al surface, which explains nanostructure surface modification, was provided.

Spallative ablation of dielectrics by X-ray laser

A short laser pulse in wide range of wavelengths, from infrared to X-ray, disturbs electron–ion equilibrium and increases pressure in a heated layer. The case where the pulse duration τL is shorter than acoustic relaxation time ts is considered in the paper. It is shown that this short pulse may cause thermomechanical phenomena such as spallative ablation regardless of wavelength. While the physics of electron–ion relaxation strongly depends on wavelength and various electron spectra of substances: there are spectra with an energy gap in semiconductors and dielectrics opposed to gapless continuous spectra in metals. The paper describes entire sequence of thermomechanical processes from expansion, nucleation, foaming, and nanostructuring to spallation with particular attention to spallation by X-ray pulse.

Two-temperature thermodynamic and kinetic properties of transition metals irradiated by femtosecond lasers

We consider the thermodynamic and kinetic properties of Nickel as an example of transition metal in two-temperature state (Te≫Ti,) produced by femtosecond laser heating. Our physical model includes essential processes induced in metals by ultrafast laser energy absorption. Specifically, the electron-ion collision frequency was obtained from recent high-temperature measurements of electrical conductivity and electron-electron screened Coulomb scattering was calculated by taking into account s-s and s-d collisions. In addition, chemical potential, energy, heat capacity, and pressure were obtained from first-principles density functional theory calculations. This model was implemented in two-temperature hydrodynamic code (2T-HD) and combined with molecular dynamics (MD) to determine strength of molten Ni at high strain rates ∼108-109s−1 under conditions of femtosecond laser ablation experiments. The simulated ablation threshold, which depends on material strength, was found to be in good agreement with our e...

Strength of metals in liquid and solid states at extremely high tension produced by femtosecond laser heating

We will discuss results of combined experimental and theoretical investigations of ablation and laser-driven shock-wave phenomena in metal films irradiated by femtosecond laser pulses. The femtosecond interferometric microscopy technique was used to make time-resolved measurements of optical properties as well as record the deformation dynamics at both the rear and frontal surfaces during initial two-temperature electron-ion relaxation and subsequent hydrodynamic expansion. In conjunction with experiment, the formation and propagation of strong tensile and compression waves were investigated by a combination of two-temperature hydrodynamic modeling and molecular dynamics simulations. The experimental tensile strengths of aluminum and nickel in solid and liquid states at extremely high strain rates in range 108÷109s−1 were obtained from the time evolution of rear and frontal surface velocities. Theoretical tensile strengths calculated by atomistic simulations of ablation and spallation using micron-sized f...

Cavitation and formation of foam-like structures inside exploding wires

Large-scale molecular dynamics (MD) simulations are used to study explosions of aluminum wires heated by electric current pulses. It is shown that the observed nonuniform radial expansion of the heated wire is associated with a liquid-vapor phase transition, which is caused by convergence of a radial tensile wave towards the center of the wire. Tension within the wave leads to cavitation in stretched melt that subsequently forms into a low-density foam-like material surrounded by a dense liquid shell. The foam decays into liquid droplets before the outer shell breaks apart. Simulated density profiles demonstrate good qualitative agreement with experimental high-resolution X-ray images showing the complex hollow structures within the long-living dense core.

Ejecta from shocked metals: Comparative simulations using molecular dynamics and smoothed particle hydrodynamics

Experimental methods of observation of early stage of shock-induced ejecta from metal surface with micrometer-sized perturbations are still limited in terms of following a complete sequence of processes having microscale dimensions and nanoscale times. To get an insight into micro-jet formation the ejecting from tin and copper samples is simulated in detail by smoothed particle hydrodynamics (SPH) and molecular dynamics (MD) methods. Such a comparative study shows, with some exception, a good similarity between material flows in nanometer- and micrometer-sized samples. We demonstrate that the jet velocity profiles and mass distributions obtained by both methods are similar, even for a MD sample with the linear size of 100u2005nm. It is shown that the bigger MD sample is simulated, the better agreement with SPH results is achieved for spike velocities of liquid jets due to decrease of surface tension effect. The calculated ejecta masses do nonetheless agree well for all sizes of interest. Material strength eff...

Surface nano-structuring produced by spallation of metal irradiated by an ultrashort laser pulse

It is known that under certain conditions the complex surface nano-structures are formed after irradiation on metals by ultrashort optical and X-ray laser pulses. In the paper the mechanism of formation and final geometry of such surface structures are discussed for the case of single pulse acting on a well-polished metal surface. The typical surface structures observed in our experiments and simulations are different from well-known ripples composing a regular pattern generated by excitation of surface plasmons. By contrast with the plasmon mechanism, the observed structures have spacial scales which are order of magnitude less than the used optical laser wavelengths. We demonstrate that such structures are formed after laser irradiation due to the thermomechanical spallation of ultrathin surface layer of melt, rather than the plasmon effects, which are found to be insignificant in given conditions of a single shot and initially smooth surface. Spallation is accompanied by a strong foaming of melt followed by breaking of the foam. After several nanoseconds the foam remnants freeze up with formation of complex nano-structures on a target surface.

Super-elastic response of metals to laser-induced shock waves

The structure and evolution of ultrashort shock waves generated by femtosecond laser pulses were explored in single-crystal nickel films via molecular dynamics and two-temperature hydrodynamics simulations. Ultrafast laser heating induces pressure build-up in a 100-nm-thick layer below the surface of the film. For low-intensity laser pulses, the stress-confined subsurface layer breaks into a single elastic shock wave with an amplitude that may exceed the conventional Hugoniot elastic limit. Comparative analysis with available experimental data confirms the existence of super-elastic states attainable through ultrashort shock compression. At high laser intensities, the two shock waves, elastic and plastic, form independently from the initial pressure profile. Because the laser heating was isochoric, the pressure and temperature at the melting front was fixed independent of absorbed fluence and results in a fluence-independent amplitude of the elastic wave generated at the liquid-solid interface. Elastic amplitude does not attenuate during propagation due to support from acoustic pulses emitted by the plastic front; whereas the unsupported plastic front undergoes significant attenuation and may fully decay within the metal film.The structure and evolution of ultrashort shock waves generated by femtosecond laser pulses were explored in single-crystal nickel films via molecular dynamics and two-temperature hydrodynamics simulations. Ultrafast laser heating induces pressure build-up in a 100-nm-thick layer below the surface of the film. For low-intensity laser pulses, the stress-confined subsurface layer breaks into a single elastic shock wave with an amplitude that may exceed the conventional Hugoniot elastic limit. Comparative analysis with available experimental data confirms the existence of super-elastic states attainable through ultrashort shock compression. At high laser intensities, the two shock waves, elastic and plastic, form independently from the initial pressure profile. Because the laser heating was isochoric, the pressure and temperature at the melting front was fixed independent of absorbed fluence and results in a fluence-independent amplitude of the elastic wave generated at the liquid-solid interface. Elastic am...

Ablation by short optical and x-ray laser pulses

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.

Evolution of Elastic Precursor and Plastic Shock Wave in Copper via Molecular Dynamics Simulations

Large-scale molecular dynamics (MD) simulations are performed to investigate shock propagation in single crystal copper. It is shown that the P-V plastic Hugoniot is unique regardless of the samples orientation, its microstructure, or its length. However, the P-V pathway to the final state is not, and depends on many factors. Specifically, it is shown that the pressure in the elastic precursor (the Hugoniot elastic limit (HEL)) decreases as the shock wave propagates in a micron-sized sample. The attenuation of the HEL in sufficiently-long samples is the main source of disagreement between previous MD simulations and experiment: while single crystal experiments showed that the plastic shock speed is orientation-independent, the simulated plastic shock speed was observed to be orientation-dependent in relatively short single-crystal samples. Such orientation dependence gradually disappears for relatively long, micrometer-sized, samples for all three low-index crystallographic directions 100, 110, and 111, and the plastic shock velocities for all three directions approach the one measured in experiment. The MD simulations also demonstrate the existence of subsonic plastic shock waves generated by relatively weak supporting pressures.