Modelling of laser matter interaction: response of the electronic and atomic subsystems to ultra short UV radiation

When irradiating material with ultrashort laser pulses in the UV regime, mainly the electronic system of the solid is excited. The subsequent energy transfer to the atomic subsystem may initiate a number of interesting structural modifications. They reach from melting of the solid and/or cavitation (spallation) to multifragmentation and the so-called phase explosion. These processes occur in the case of ultrafast excitation on a very short timescale of picoseconds and are therefore likely to occur in strong nonequilibrium conditions which cannot be described by standard thermodynamic models.

The coupling between the electronic system and the atom lattice is usually described in the framework of a continuum model. Here the electron-phonon coupling constant is the basic quantity joining the two subsystems. The applicability of this description for ultrashort excitation is questionable. In this project we plan to describe the dynamics of electrons and phonons under UV irradiation with the help of a time and space dependent kinetic approach (AG Rethfeld). The focus of this part will be the influence of the nonequilibrium state of the electronic system on material parameters such as the electron-phonon coupling - which cannot be assumed to be constant for the case of strong excitation. In particular we want to study the influence of high-energy electrons on the heat transport. The kinetic model thus provides the initial conditions for the description of the lattice dynamics. The reaction of the atomic subsystem on the excitation can be modelled with the help of a molecular dynamics approach (AG Urbassek). This method allows to study spatially inhomogeneous nonequilibrium dynamical processes with atomistic resolution. It is planned to develop a hybrid model to combine both approaches.

As a result of this project we expect a temporally and spatially resolved description of the dynamics of energy dissipation in the material during and after laser absorption, insight into the induced phase transitions (melting, phase explosion) or fracture processes due to tensile stresses (cavitation in the liquid phase, spallation of the solid) as well as a model for the resulting ablation and potential applications for material processing.

Principal Investigators:

Dr. Bärbel Rethfeld(Department of Physics, TU Kaiserslautern)

Prof. Dr. Herbert Urbassek (Department of Physics, TU Kaiserslautern)