Mechanisms of femtosecond-laser induced surface structuring of zinc oxide crystals

This study investigates on the laser-induced periodic surface structures (LIPPS) formation in single-crystalline ZnO semiconductors with two distinct types of ripple structures, characterized by the lower and higher spatial frequency LIPPS. Results are shown and interpreted in the frame of a Drude-model including a nonlinear process at the irradiated surface. In addition, results reveal the influence of nonlinear excitation pathways, which is strongly indicated by the detection of surface scattered second harmonic generation. This happens at wavelengths around 400 nm in the regime leading to the high spatial frequency LIPPS (HSFL) formation. These findings directly confirm the earlier assumption of a multiphoton assisted formation mechanism for HSFL. Hence, HSFL structures on ZnO surfaces can be explained by extending the well-established LIPSS theories and by involving nonlinear photo-excitation.