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

Abstract

The formation of laser-induced periodic surface structures (LIPSS, ripples) was studied since four decades for a variety of materials including semiconductors. Recently, it was demonstrated that even a structuring of materials with high-energetic band edges like ZnO is enabled by highly intense femtosecond laser pulses at center wavelengths of 800 nm and pulse durations between 10 fs and 200 fs [1–3]. Because of its transparence and biocompatibility, ZnO is of increasing interest for UV optoelectronics, photovoltaics, spintronics and biosensors. Although the LIPSS phenomenon is well-known for a long time, the discussion about the relevant mechanisms is still controversial. Usually, the formation of periodic ripples is explained by the interference of a scattered surface electromagnetic wave with the incident laser pulse. For high intensity ultrashort laser pulses, however, the laser-material interaction is rather complex because of contributions from nonlinear optical excitation pathways. Here, we report on recent investigations on LIPSS formation in single-crystalline ZnO with two distinct types of ripple structures characterized by lower and higher spatial frequency LIPSS. The results are interpreted in frame of a simple Drude-model including a nonlinear process at the irradiated surface. On the basis of this approach it can be well understood that high frequency ripples are only observed with sub-picosecond laser pulses for a below band-gap excitation of dielectrics and semiconductors.