Optical properties under exposure to ultrashort laser pulses

Abstract

For the description of the interaction of a fs-laser pulse with a solid, the standard values of the properties of a solid are not suited because they have experimentally been determined or theoretically derived under the assumptions of a steady state and local thermal equilibrium. Depending on the property considered and the laser pulse duration either one or both of these conditions may be violated. This is caused by the appearance of a local thermal nonequilibrium, a nonsteady state, and the possibly large difference between the temperatures of the electron and phonon subsystem. For a deeper understanding and for the predictability of experimental results, it is essential to have knowledge of the optical properties and especially of the absorption and of the optical penetration depth on the fs time scale. In this paper, we derive equations for the optical properties of metals in the case of local thermal nonequilibrium between the electron and phonon system. For given laser intensities, we calculate, as an example, the optical properties for gold. For this purpose, we need the time dependent temperatures of the electrons and phonons. They are evaluated by means of our extended two temperature model. Finally, we compare the results with the standard equilibrium behavior as well as with the experimental findings and close with a short discussion.