Surface morphology after femtosecond laser ablation of insulators

摘要

The crater morphology in transparent insulators upon femtosecond laser ablation was investigated by ex-situ optical and electron microscopy. After multishot irradiation (several thousand shots), a superposition of up to three differently spaced ripple patterns developed at the crater bottom, the finest one running perpendicular and the next larger one parallel to the laser polarization. The ripples periods do not show any relation to the incident laser wavelength. On the contrary, they appear to be strongly influenced by the incident intensity, regardless of the wavelength. The coarsest structure exhibits features of plastic surface waves, reflected at the boundaries of the crater as well as at individual irregularities inside the crater. The finest ripples exhibit strong features of chaotic self-organization and percolation, such as bifurcations. Together with the fact, that ablation under the applied conditions is due to Coulomb explosion of the surface, our observations indicate that local thermal effects can be ruled out as the origin of the ripples formation, in contrast to the classical interference picture of ripples formation. This is further confirmed by two-pulse interference experiments.