Ab initio determination of transient electronic properties of an ultrafast laser irradiated metal surface. Consequences for LIPSS formation

Abstract : Nanostructuring features under ultrafast laser excitation of metallic surfaces are strongly influenced by light coupling and the associated material response under conditions of electron-phonon nonequilibrium. This is nowadays imperfectly described with uncertainties on the transient variation of optical and electronic properties during irradiation. In that context, dedicated ab initio calculations were carried out in the framework of the Density Functional Theory to elucidate some of the primary aspects of material response. Ground-state calculations and molecular dynamic simulations have been thus conducted to derive electronic structure and associated transport properties under nonequilibrium conditions. We observe that electronic temperature leads to strong modifications of the electronic screening. This displaces in turn the electronic structure, affecting transient electronic properties such as free electron number, specific heat and thermal pressure. Finally, we evaluate the optical index under different electronic temperatures based on the Kubo-Greenwood formalism. In addition to providing insights into the dynamics of optical response of a metallic surface, these transport properties also shines a new light on a recurring problem concerning periodicity variations of LIPSS (ripples) under ultrashort excitation. Accordingly, the consequences of thermal nonequilibrium on inhomogeneous electric field distribution on a rough metallic surface will be also addressed.