Low spatial frequency surface feature formation during femtosecond laser deep engraving: origins and mechanisms.

In this study, we explore the formation of high-amplitude, low-spatial-frequency surface features - also commonly referred to as waviness - during laser deep engraving of dielectric materials. This unique phenomenon significantly increases surface roughness and undermines the ability to control it during deep engraving, limiting the technique's applicability. Here we establish the experimental conditions under which these features are formed and demonstrate how they depend on various processing conditions. Furthermore, we develop a numerical model that considers acoustic wave propagation in a confined cavity. Combining a numerical model with experimental results we demonstrate that the formation of low-spatial-frequency features is governed by the interaction between incident laser pulses and the residual effects of preceding pulses. Finally, building on this insight, we developed strategies to suppress the formation of such surface features, reducing the final surface roughness by a factor of ten.