Self-organized structure formation on the bottom of femtosecond laser ablation craters in glass

Using femtosecond laser pulses (150 fs duration at λ=400 nm) for ablation experiments on glass samples with and without enclosed silver nanoparticles, characteristic sub-micrometer surface topologies are observed on the flat bottom of the ablation craters produced. The structures show increasing order towards periodic ripple-like features with an increasing number of successive pulses applied. Depending on sample and experimental conditions, the spatial periodicity varies between 340 nm and 1900 nm, despite a constant laser wavelength and incidence angle. An analysis based on electron and atomic force microscopy of the structures indicates that the formation of the ripples in this work is due to instabilities and self-organization of the surface relaxation after ablation. PACS 81.16.Rf; 42.70.-a