Brian Demaske

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...

Ultrafast lasers and solids in highly excited states: results of hydrodynamics and molecular dynamics simulations

Action of ultrafast optical and X-ray lasers on metals is considered. It is known that under certain conditions surface structures appear as result of irradiation. Generation of nano-structures is usually associated with excitation of surface plasmons. But often structures do not have forms of ripples, and their spacial scales are order of magnitude less than optical wavelength. In the paper full description of surface nano-structures is given for the case of single shot laser action onto well polished boundaries. Plasmon effects are insignificant for this case and also for X-ray pulses. It is shown that 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 nano-structures observed in experiments.

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...

Molecular Dynamics Simulations of Femtosecond Laser Ablation and Spallation of Gold

The behavior of micron‐sized gold films irradiated by a femtosecond laser is investigated via molecular dynamics simulations. The ultrafast laser heating induces a stress‐confined state near the surface of the film. For sufficient laser fluences, decomposition of the stress‐confined state leads to ablation of molten material in the frontal and spallation of crystalline solid at the rear sides of the sample. Simulations for thick films (>0.5 μm) give distinct fluence thresholds for ablation and spallation of 137 and 193 mJ/cm2, respectively, whereas for thin films only a single fracture process is observed. For thin films, it is shown that absorbed fluence at fracture threshold is a function of foil thickness. As foil thickness is decreased to the limit of very thin foils, the stress‐confinement picture becomes increasingly less valid.

Elastic-plastic collapse of super-elastic shock waves in face-centered-cubic solids

Shock waves in the [110] and [111] directions of single-crystal Al samples were studied using molecular dynamics (MD) simulations. Piston-driven simulations were performed to investigate the split shock-wave regime. At low piston velocities, the material is compressed initially to a metastable over-compressed elastic state leading to a super-elastic single shock wave. This metastable elastic state later collapses to a plastic state resulting in the formation of a two-wave structure consisting of an elastic precursor followed by a slower plastic wave. The single two-zone elastic-plastic shock-wave regime appearing at higher piston velocities was studied using moving window MD. The plastic wave attains the same average speed as the elastic precursor to form a single two-zone shock wave. In this case, repeated collapse of the highly over-compressed elastic state near the plastic shock front produces ultrashort triangle pulses that provide the pressure support for the leading elastic precursor.

Ultrashort elastic and plastic shockwaves in aluminum

Ultrashort shock waves in aluminum films generated by femtosecond laser pulses were studied using two-temperature hydrodynamics and molecular dynamics methods. We observed double wave breaking characterized by an independent formation of leading elastic and trailing plastic shock waves. Both the amplitude and speed of the plastic shock decrease quickly in time due to hydrodynamic attenuation, while the elastic shock front slowly decays during propagation. When the pressure in the plastic front becomes equal to the normal component of pressure tensor in the elastic zone, the plastic wave disappears. Therefore, the distance between elastic and plastic fronts first decreases, then increases, with time. The elastic shock uniaxially compresses the crystal to pressures higher than the Hugoniot elastic limit (HEL). For a short time, the crystal within the elastic zone remains in a metastable state, that lies on an extension of the elastic branch of the Hugoniot beyond the HEL. Our theoretical results explain the seemingly puzzling experimental findings, where high-pressure elastic shock waves were observed with normal pressures up to 10-20 GPa.

MD simulations of laser-induced ultrashort shock waves in nickel

The dynamics of ultrashort shock waves induced by femtosecond laser pulses were explored in nickel-glass and free-standing nickel films by molecular dynamics simulations. Ultrafast laser heating causes stress-confinement, which is characterized by formation of a strongly pressurized 100-nm-thick zone just below the surface of the film. For low-intensity laser pulses, only a single elastic shock wave was formed despite pressures several times greater than the experimental Hugoniot elastic limit. Because the material remains uniaxially compressed for < 50 ps, comparatively slow processes of dislocation formation are not activated. For high intensity laser pulses, the process of double wave breaking was observed with formation of split elastic and plastic shock waves. Presence of a trailing rarefaction wave acts to attenuate the plastic wave until it disappears. Agreement between the experimental and simulated Hugoniot was facilitated by a new EAM potential designed to simulate nickel in a wide range of pressures and temperatures.The dynamics of ultrashort shock waves induced by femtosecond laser pulses were explored in nickel-glass and free-standing nickel films by molecular dynamics simulations. Ultrafast laser heating causes stress-confinement, which is characterized by formation of a strongly pressurized 100-nm-thick zone just below the surface of the film. For low-intensity laser pulses, only a single elastic shock wave was formed despite pressures several times greater than the experimental Hugoniot elastic limit. Because the material remains uniaxially compressed for < 50 ps, comparatively slow processes of dislocation formation are not activated. For high intensity laser pulses, the process of double wave breaking was observed with formation of split elastic and plastic shock waves. Presence of a trailing rarefaction wave acts to attenuate the plastic wave until it disappears. Agreement between the experimental and simulated Hugoniot was facilitated by a new EAM potential designed to simulate nickel in a wide range of pres...

Evolution of metastable elastic shockwaves in nickel

Shock waves in nickel were simulated by molecular dynamics using a new EAM potential. Three distinct regimes of shock propagation were observed, including a single elastic shock wave, two split elastic and plastic shock waves, and a single two-zone elastic-plastic shock wave, in order of increasing piston velocity. The single two-zone wave consists of a leading lower-pressure elastic zone, followed by a higher-pressure plastic wave, both moving with the same average speed. The elastic zone of this wave is compressed to a metastable state having an average pressure above the Hugoniot elastic limit. Similar metastable states appear in the elastic precursor during the early stage of split wave development. The mechanism for relaxation of the metastable elastic state is discussed.

A new nickel EAM potential for atomistic simulations of ablation, spallation, and shockwave phenomena

A new embedded atom method (EAM) interatomic potential for nickel was developed with the goal of improving the predictive power of atomistic simulations of materials at extreme conditions. In contrast to standard development approaches, our methodology focuses on accurate sampling of stress tensor components calculated for wide ranges of isotropic and uniaxial compressive and tensile strains. Also included in the fitting database are experimental properties at normal conditions, such as a lattice parameter and elastic constants, as well as cohesive, vacancy formation, and stacking fault energies. After initial fitting, several candidate potentials were further screened to select the final potential, which best reproduces the experimental melting temperature. Verification of the new EAM potential against the liquidvapor coexistence line and shock Hugoniot ensures an accurate description of properties of Ni near the vicinity of phase transitions and under shock compression.

Two-temperature hydrodynamic expansion and coupling of strong elastic shock with supersonic melting front produced by ultrashort laser pulse

Ultrafast processes, including nonmonotonic expansion of material into vacuum, supersonic melting and generation of super-elastic shock wave, in a surface layer of metal irradiated by an ultrashort laser pulse are discussed. In addition to the well-established two-temperature (2T) evolution of heated layer a new effect of electron pressure gradient on early stage of material expansion is studied. It is shown that the expanding material experiences an unexpected jump in flow velocity in a place where stress exceeds the effective tensile strength provided by used EoS of material. Another 2T effect is that supersonic propagation of homogeneous melting front results in distortion of spatial profile of ion temperature, which later imprints on ion pressure profile transforming in a super-elastic shock wave with time.