We present an overview of our systematic studies of the surface modifications resulting from the interactions of both single and multiple picosecond soft x-ray laser (SXRL) pulses with materials, such as gold (Au), copper (Cu), aluminum (Al), and lithium fluoride (LiF). We show experimentally the possibility of the precise nanometer size structures (~10–40 nm) formation on their surfaces by ultra-low (~10–30 mJ/cm2 ) fluencies of single picosecond SXRL pulse. Comparison experimental results with the atomistic model of ablation, which was developed for the single SXRL shot interaction with dielectrics and metals, is provided. Theoretical description of surface nanostructures is considered and is shown that such structures are formed after laser illumination in a process of mechanical spallation of ultrathin surface layer of molten metal. Spallation is accompanied by a strong foaming of melt, breaking of foam, and freezing of foam remnants. Those remnants form chaotic nanostructures, which are observed in experiments. Our measurements show that electron temperature of matter under irradiation of SXRL was lower than 1 eV. The model calculation also predicts that the ablation induced by the SXRL can create the significant low electron temperature. Our results demonstrate that tensile stress created in LiF and metals by short SXRL pulse can produce spallative ablation of target even for drastically small fluencies, which open new opportunities for material nano processing.
To study the ablation process induced by the soft x-ray laser pulse, we investigated the electron temperature of the ablating material. Focused soft x-ray laser pulses having a wavelength of 13.9 nm and duration of 7 ps were irradiated onto the LiF, Al, and Cu surfaces, and we observed the optical emission from the surfaces by use of an optical camera. On sample surfaces, we could confirm damage structures, but no emission signal in the visible spectral range during ablation could be observed. Then, we estimated the electron temperature in the ablating matter. To consider the radiation from a heated layer, we supposed a black-body radiator as an object. The calculation result was that the electron temperature was estimated to be lower than 1 eV and the process duration was shorter than 1000 ps. The theoretical model calculation suggests the spallative ablation for the interaction between the soft x-ray laser and materials. The driving force for the spallation is an increasing pressure appearing in the heated layer, and the change of the surface is considered to be due to a splash of a molten layer. The model calculation predicts that the soft x-ray laser with the fluence around the ablation threshold can create an electron temperature around 1 eV in a material. The experimental result is in good accordance with the theoretical prediction. Our investigation implies that the spallative ablation occurs in the low electron temperature region of a non-equilibrium state of warm dense matter.
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Short pulse x-ray sources are widely used as probing beams for new material development and non-destructive x-ray imaging. The high quality soft x-ray laser (SXRL) source enables us to achieve quite high spatial-resolution as a probe and quite intense x-ray as a pump. As an application using the SXRL, we have observed the spallative ablation process by the interaction with SXRL or femto-second (fs) laser. The dynamical processes of the SXRL and/or the fs laserinduced surface modifications come to attract much attention for the micro processing. However, it is difficult to observe the spallative ablation dynamics, because of non-repetitive, irreversible and rapid phenomena in a small feature size. In the case with SXRL irradiation (13.9 nm, 7ps, ~50 mJ/cm2), we have observed the damage structures and the optical emission from the ablated materials. When focused SXRL pulses were have been irradiated onto the metal surface, we have confirmed damage structures, however no optical emission signal during SXRL ablation could be observed. The electron temperature is estimated to be around a few eV at the ablated surface. In the case with fs laser irradiation (795 nm, 80fs, ~1.5 J/cm2), we have observed the surface morphology of fs laser ablation by the SXRL interferometer and SXRL reflectometer. The time resolved image of nano-scaled ablation dynamics of tungsten surface was observed. The numerical simulation study is underway by using a molecular dynamics code. These results lead not only to understanding the full process of the interaction with the SXRL and/or fs laser, but also to candidate the material of the first wall of magnetic confinement fusion reactors. We also described a preliminary study of radiation effect on culture cells irradiated with the SXRL. Our study demonstrated for the first time that the SXRL induced the DNA double strand breaks
Two-temperature thermal conductivity coefficient κ, and electron-ion coupling parameter o: are obtained using
Boltzmann kinetic equation in the relaxation time approximation. These coefficients are necessary for the quantitative description of the two-temperature state with hot electrons Te much greater than Ti created as result of absorption
of femtosecond laser pulse. Simple, noble, and transition metals are considered. An influence of d- band electrons, which play a significant role, has been evaluated for two latter groups of metals.
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.
To study the interactions between a soft x-ray laser (SXRL) and various materials, we irradiated Al, Au, Cu, and Si with
the SXRL beam pulses having a wavelength of 13.9 nm and duration of 7 ps. Following the irradiation, the induced
structures were observed using a scanning electron microscope and an atomic force microscope. With single pulse
irradiation, conical structures were observed on the Al surface, and ripple-like structures were formed on the Au and Cu
surfaces. The conical structures were destroyed under multiple SXRL pulse irradiation. On the other hand, the
developments of modified structures were observed after multiple pulse irradiations on the Au and Cu surfaces. On the
Si surface, deep holes, that seemed to be molten structures induced by the accumulation of multiple pulse irradiations,
were found. Therefore, it is concluded that the SXRL pulse irradiations of various material surfaces cause different
types of surface modifications, and the changes in the surface behaviors are attributed to the differences in the elemental
properties of each materials, such as the melting point and the attenuation length of x-rays.
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.
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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