The spatial emergence of laser-induced periodic surface structures
Within the NanoClean project the University of Twente, represented by the Chair of Applied Laser Technology, studied the spatial emergence of laser-induced periodic surface structures (LIPSS, nano-structures), upon irradiation with linearly polarized femtosecond and picosecond laser pulses theoretically and experimentally, under lateral displacement conditions. This study is required for the application of LIPSS in the field of surface functionalization.
An Irradiation-Model (IM) has been developed, which analyses the laser radiation transmitted to the sample plane without considering a laser-material interaction yet. The input of the model considers parameters of the laser source, optical and kinematical system. The output of the model is the fluence accumulation process in the irradiation plane of the specimen.
Figure 1 Examples of characteristic fluence accumulation profiles, for = 1 J/cm2, Nr = 1 and (a) φ = -0.75, (b) φ = -0.10 and (c) φ = 0.55. The grey scale represents the accumulated fluence in J/cm2. Brighter grey tones represent larger accumulated fluence.
Furthermore a theoretical concept has been developed, referred to as Fluence-Domain-Theory (FDT), which explains the spatial emergence of laser-induced periodic surface structures. Based on this theory, an experimental approach has been developed for the determination of irradiation parameters of extended surface areas homogenously covered with LIPSS. The approach is based on accumulated fluence and consists of two steps, first the empirical determination of accumulated fluence domain boundaries and second the approximation of irradiation parameters.
Figure 2 Left: Schematic diagram of the irradiation with laterally displaced and intersected pulses of Gaussian fluence distribution
In order to evaluate the impact of errors sources regarding a specific laser machining system design, the irradiation model can be extended by introducing additional kinematical systems or by implementation of technical errors, such as beam ellipticity or position errors. This allows to analyse the irradiation of extended areas under practical machining conditions and deduce optimization and handling strategies.
Figure 3 Numerical energy density simulations showing exemplarily the influence of stitching errors, beam ellipticity and position errors on the irradiation.
The approach was successfully applied for structuring extended surface areas, which were homogenously covered with LIPSS. The areas, obtained by different irradiation parameter combinations, satisfying accumulated fluence boundary conditions, show the same type of LIPSS. This observation provides evidence, that the accumulated fluence has a decisive role in the spatial emergence of LIPSS.
Figure 4Scanning electron images of steel (Mirrax, Uddeholm) surface irradiated with multiple pulses (top, left) and displaced pulses (top right and bottom)
Figure 4 Scanning electron images of steel (Mirrax, Uddeholm) surface irradiated with multiple pulses (top, left) and displaced pulses (top right and bottom)