The role of asymmetric excitation in self-organized nanostructure formation upon femtosecond laser ablation

The strong influence of laser polarization on the orientation and shape of femtosecond-laser-induced self-organized nanostructures (‘ripples’, LIPSS) still constitutes an open question, taking into account that the laser electric field is present only at the first step of electronic excitation. Based on the explanation of similar structures generated during ion sputtering, we present a theoretical model indicating a possible explanation for this phenomenon. Our model shows that a directional asymmetry in the pattern can result from a spatial asymmetry of the initial excitation, induced e.g. by a corresponding distribution of excited-electron kinetic energies. Numerical simulation of this model yields typical patterns which are compared to experimental observations under appropriate conditions.