B. Rethfeld

Theory of ultrashort laser pulse interaction with a metal

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.

A self-consistent model for the cathode fall region of an electric arc

A one-dimensional model of the non-equilibrium plasma region adjacent to an electric arc cathode is developed. The collisionless space-charge zone (sheath) and the hydrodynamically described ionization zone (pre-sheath) are connected to a unified model. In the pre-sheath, ionization, diffusion and decoupling of electron and heavy-particle temperature are considered. The voltage drop in the space-charge zone is computed self-consistently including thermionic electron emission at the cathode surface. This unified model yields the overall cathode potential drop (cathode fall), the extent of the non-equilibrium region and the net energy flux towards the cathode surface. In this paper, the model is applied to an argon arc plasma at atmospheric pressure with a thoriated tungsten cathode, as is typically used for welding of stainless steel materials.

Laser damage in silicon: Energy absorption, relaxation, and transport

Silicon irradiated with an ultrashort 800 nm-laser pulse is studied theoretically using a two temperature description that considers the transient free carrier density during and after irradiation. A Drude model is implemented to account for the highly transient optical parameters. We analyze the importance of considering these density-dependent parameters as well as the choice of the Drude collision frequency. In addition, degeneracy and transport effects are investigated. The importance of each of these processes for resulting calculated damage thresholds is studied. We report damage thresholds calculations that are in very good agreement with experimental results over a wide range of pulse durations.

Driving force of ultrafast magnetization dynamics

Irradiating a ferromagnetic material with an ultrashort laser pulse leads to demagnetization on the femtosecond timescale. We implement Elliott–Yafet-type spin-flip scattering, mediated by electron–electron and electron–phonon collisions, in the framework of a spin-resolved Boltzmann equation. Considering three mutually coupled reservoirs, (i) spin-up electrons, (ii) spin-down electrons and (iii) phonons, we trace non-equilibrium electron distributions during and after laser excitation. We identified the driving force for ultrafast magnetization dynamics as the equilibration of temperatures and chemical potentials between electronic subsystems. This principle can be used to easily predict the maximum quenching of magnetization upon ultrashort laser irradiation in any material, as we show for the case of 3d-ferromagnetic nickel.

Nanocrystalline structure of nanobump generated by localized photoexcitation of metal film

The extreme cooling rates in material processing can be achieved in a number of current and emerging femtosecond laser techniques capable of highly localized energy deposition. The mechanisms of rapid solidification of a nanoscale region of a metal film transiently melted by a localized photoexcitation are investigated in a large-scale atomistic simulation. The small size of the melted region, steep temperature gradients, and fast two-dimensional electron heat conduction result in the cooling rate exceeding 1013 K/s and create conditions for deep undercooling of the melt. The velocity of the liquid/crystal interface rises up to the maximum value of ∼80 m/s during the initial stage of the cooling process and stays approximately constant as the temperature of the melted region continues to decrease. When the temperature drops down to the level of ∼0.6Tm, a massive homogeneous nucleation of the crystal phase inside the undercooled liquid region takes place and prevents the undercooled liquid from reaching th...

The role of mass removal mechanisms in the onset of ns-laser induced plasma formation

The present study focuses on the role of mass removal mechanisms in ns-laser ablation. A copper sample is placed in argon, initially set at standard pressure and temperature. Calculations are performed for a 6 ns laser pulse with a wavelength of 532 nm and laser fluences up to 10 J/cm2. The transient behavior in and above the copper target is described by a hydrodynamic model. Transmission profiles and ablation depths are compared with experimental results and similar trends are found. Our calculations reveal an interesting self-inhibiting mechanism: volumetric mass removal in the supercritical region triggers plasma shielding and therefore stops proceeding. This self-limiting process indicates that volumetric mass removal does not necessarily result in large ablation depths.

Space charge corrected electron emission from an aluminum surface under non-equilibrium conditions

A theoretical study has been conducted of ultrashort pulsed laser induced electron emission from an aluminum surface. Electron emission fluxes retrieved from the commonly employed Fowler-DuBridge theory were compared to fluxes based on a laser-induced non-equilibrium electron distribution. As a result, the two- and three-photon photoelectron emission parameters for the Fowler-DuBridge theory have been approximated. We observe that at regimes where photoemission is important, laser-induced electron emission evolves in a more smooth manner than predicted by the Fowler-DuBridge theory. The importance of the actual electron distribution decreases at higher laser fluences, whereas the contribution of thermionic emission increases. Furthermore, the influence of a space charge effect on electron emission was evaluated by a one dimensional particle-in-cell model. Depending on the fluences, the space charge reduces the electron emission by several orders of magnitude. The influence of the electron emission flux pr...

Modelling ultrafast laser ablation

This review is devoted to the study of ultrafast laser ablation of solids and liquids. The ablation of condensed matter under exposure to subpicosecond laser pulses has a number of peculiar properties which distinguish this process from ablation induced by nanosecond and longer laser pulses. The process of ultrafast ablation includes light absorption by electrons in the skin layer, energy transfer from the skin layer to target interior by nonlinear electronic heat conduction, relaxation of the electron and ion temperatures, ultrafast melting, hydrodynamic expansion of heated matter accompanied by the formation of metastable states and subsequent formation of breaks in condensed matter. In case of ultrashort laser excitation, these processes are temporally separated and can thus be studied separately. As for energy absorption, we consider peculiarities of the case of metal irradiation in contrast to dielectrics and semiconductors. We discuss the energy dissipation processes of electronic thermal wave and lattice heating. Different types of phase transitions after ultrashort laser pulse irradiation as melting, vaporization or transitions to warm dense matter are discussed. Also nonthermal phase transitions, directly caused by the electronic excitation before considerable lattice heating, are considered. The final material removal occurs from the physical point of view as expansion of heated matter; here we discuss approaches of hydrodynamics, as well as molecular dynamic simulations directly following the atomic movements. Hybrid approaches tracing the dynamics of excited electrons, energy dissipation and structural dynamics in a combined simulation are reviewed as well.

A comprehensive model for the ultrashort visible light irradiation of semiconductors

We present a new model describing the irradiation of semiconductors with ultrashort laser pulses. Based on the earlier developed multiple rate equation [Rethfeld, Phys. Rev. Lett. 92, 187401 (2004)], the model additionally includes the interaction of electrons with the phononic subsystem of the lattice and allows for the direct determination of the conditions for crystal damage. In contrast to commonly used approaches based on the thermodynamic description of the material, our model is applicable to nonequilibrium electronic conditions. Accounting for the dynamic changes in the optical properties of the target (i.e., reflectivity, photoabsorption coefficients), the developed model allows for a comprehensive evaluation of the damage by tracing the changes in the optical parameters, lattice heating and subsequent melting. Our model effectively describes the dynamics of the electronic subsystem and lattice heating and the results are in very good agreement with experimental measurements on the transient refl...

Energy dissipation in dielectrics after swift heavy-ion impact: A hybrid model

The energy dissipation after irradiation of dielectrics with swift heavy ions is studied applying a combination of the Monte Carlo (MC) method and the two-temperature model (TTM). Within the MC calculation the transient dynamics of the electrons in the excited dielectric is described: the primary excitation and relaxation of the target electrons as well as the creation of secondary electrons. From the MC data, it was observed that the electron system can be considered as thermalized after a time of t{approx_equal}100 fs after the ion impact. Then the TTM is applied to calculate the spatial and temporal evolution of the electron and lattice temperature via the electron-phonon coupling using the MC data as initial conditions. Additionally, this MC-TTM combination allows to compute material parameters of strongly excited matter.