Modifications of optical properties by ultrashort laser pulses

Abstract

From both a theoretical and an experimental point of view, the properties of fs laser pulses are fascinating. For a deeper understanding and for the prediction of the experimental results, the knowledge is necessary of absorption and of the optical penetration depth on the fs scale. 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 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. 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 a given laser intensity, we calculate, as an example, the absorption and optical penetration depth 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.