The Role of Asymmetric Excitation in Self-organized Nanostructure Formation upon Femtosecond Laser Ablation

Surface pattering upon multi-pulse femtosecond laser ablation is modeled by a nonlinear-dynamic erosion/smoothing model, similar to structure formation during ion sputtering. The model is adopted to account for the influence of laser polarization on nanostructure features. Based on a nonlinear equation of the Kuramoto-Siavshinsky type, it is shown that the directional anisotropy in the pattern formation may result from a spatial anisotropy of the initial excitation/energy-coupling process, such as resonant coupling to surface plasmons/polaritons, or electron diffusion properties. Also, anisotropy of elasto-dynamic surface and diffusion properties may be involved. A comparison of numeric simulations based on the model with corresponding experimental results gives a very good agreement.