V. V. Shepelev

Strength properties of an aluminum melt at extremely high tension rates under the action of femtosecond laser pulses

AbstractThe dynamics of the melting of a surface nanolayer and the formation of thermal and shock waves in metals irradiated by femtosecond laser pulses has been investigated both experimentally and theoretically. A new experimental-computational method has been implemented to determine the parameters of laser-induced shock waves in metallic films. Data on the strength properties of the condensed phase in aluminum films at an extremely high strain rate (

Spallative ablation of dielectrics by X-ray laser

\dot V

Ablation by short optical and x-ray laser pulses

/V ∼ 109 s−1)under the action of a laser-induced shock wave have been obtained.

Pump‐probe method for measurement of thickness of molten layer produced by ultrashort 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 27 mJ/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.

Thin 10–100 nm film in contact with substrate: Dynamics after femtosecond laser irradiation

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.

Ablation of insulators under the action of short pulses of X-ray plasma lasers and free-electron lasers

The separation of a shock wave into an elastic precursor and a plastic wave is a characteristic phenomenon occurring only in solid media. The existence of the elastic shock wave at pressures p ≈ 10 GPa, which is one or two orders of magnitude higher than the dynamic elastic limit, has been detected in recent numerical calculations and a femtosecond laser experiment. The plastic shock wave has no time to be formed in these ultrashort waves at p ≈ 10 GPa. The processes of the formation and propagation of the elastic and plastic waves in aluminum at higher pressures obtained by means of femtosecond lasers have been analyzed in this work. It has been found that the elastic precursor survives even under the conditions when the pressure behind the plastic front reaches a giant value p ∼ 1 Mbar at which the melting of the metal begins. It has been shown that superelasticity should be taken into account to correctly interpret the preceding laser experiments.

Film-substrate hydrodynamic interaction initiated by femtosecond laser irradiation

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.

Ultrashort laser-matter interaction at moderate intensities: two-temperature relaxation, foaming of stretched melt,and freezing of evolving nanostructures

Pump‐probe technique provides an unique method for high precision measurement of ultrafast processes induced in material by ultrashort laser pulses. The time domain and space domain available for measurements are 0.1–1000 ps and 1–1000 nm, respectively. In this respect the pump‐probe technique noticeably extends to smaller scales the well‐known Doppler Velocity Interferometer System for Any Reflector (VISAR) and Optically Recording Velocity Interferometer System (ORVIS) methods, which have micron‐nanosecond accuracy. VISAR and ORVIS are widely used for study of wave dynamics in material plate having thickness from tens microns to several millimeters.For the first time we apply pump‐probe technique for deep probing of ultrasonic melting of micron‐size aluminum film irradiated by ultrashort laser pump pulse. The pump with duration less than ∼1 ps may result in ultra‐high strain rate V/V∼109–1010 s−1 in such thin film. The experimental data and simulation analysis of material response and pressure wave dyna...