Switching between normal and anomalous Laser Induced Periodic Surface Structures

We report on the studies of switching mechanism between normal and anomalous laser induced periodic surface structures. We have shown that for high loss metals the switching mechanism between normal and anomalous modes relays on an interplay between two different feedbacks inherent into the structure formation process: long range, low intensity dipole-like scattering of light along the surface, which governs anomalous ripples parallel to the laser polarization, and short range, high intensity plasmon-polariton wave, which is initiated by near field dipole radiation and responsible for creation of ripples perpendicular to the polarization, i. e. normal structure. By managing these two feedbacks, we demonstrated creation of both normal and anomalous laser induced periodic surface structures on the same surface. In contrast to the previous studies, we have shown that the thermal oxidation mechanism can form both normal and anomalous types of the structure, while the ablation mechanism is involved only during the normal structure formation. Unlike the formation of oxidation type anomalous structure, the formation of ablation type normal structure does not have inherent negative feedback, which is self-regulating nano-ripples formation. This feedback needs to be applied artificially, by regulating scanning speed and pulse energy at a given repetition rate of the laser. With implementation of nonlinear laser lithography technique for ablation type of normal laser induced periodic surface structures we demonstrated large area high regular nano-grating formation on different surfaces with extremely high industrially acceptable speed.