2.1.2. LBIA

Beside the LSFL process, the test disc was structered with high definition periodic line gratings of a single orientation across the whole disc. For the fabrication of these surface structures, laser beam interference ablation (LBIA) was used as shown in Figure 1b, applying a special interferometer-based technique developed at LLG. In this approach, the key component is a so-called grating interferometer [9,31], which creates a well-defined, high quality interference pattern, projected onto the sample surface. This interferometer consists of two transmission phase gratings made of fused silica with their grating lines lined up parallel to each other. In the present experiment, the resulting period of the linear grating-like structures projected onto the sample surface was 1.5 μm. A fast scanning system (intelliSCANde 14, SCANLAB GmbH, Puchheim, Germany) with a focusing optic is applied to ablate arbitrary patterns on the sample, while allowing full coverage of the entire processed area with a highly deterministic periodic structure. The modulation depth of the resulting relief structure can be varied within a wide range by adjusting the laser energy and the scanning speed. In the present study, the modulation depth was adjusted to approx. 1.6 μm on the surface of the test discs, in order to achieve an aspect ratio of roughly 1:1. The Yb:KGW femtosecond laser source (Pharos 20-1000-PP, Light Conversion, Vilnius, Lithuania) used for the ablation process provides pulses at 300 kHz with a duration of 250 fs at a wavelength of 1030 nm. For the presented investigation the laser system was equipped with a third harmonics module which converted the wavelength to 343 nm. Those ultrashort laser pulses are focused to approx. 20 μm and scanned across the sample area.
