Optical properties under exposure to ultrashort laser pulses

For the description of the interaction of a femtosecond laser pulse with a solid, the standard values of the properties of a solid are not suited because they have been experimentally 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 subsystems. For a deeper understanding and for the prediction 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 timescale. In this paper, we derive equations for the optical properties of metals in the case of local thermal nonequilibrium between the electron and phonon systems. 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 behaviour as well as with the experimental findings and close with a short discussion. (author)