## 3 Results and Discussion
### Direct Laser Interference Patterning of Dielectric Polymers  
A possible processing strategy in DLIP is to employ a pixel-wise structuring, that means making use of slightly focused interfering beams and irradiating areas with an interference area with dimensions ranging from tens to a few hundreds of \(\upmu\)m [25; 28; 29]. It has been also demonstrated that under specific conditions polymers can be structured by DLIP obtaining unusual ablation profiles [28]. This is the case of transparent polycarbonate, which for low special periods and high laser fluence, shows a high amount of unselectively ablated material in the upper part of the treated DLIP pixel volume, creating a concave-like shape. If this phenomenon is controlled, hierarchical structures can be realized by correctly distributing the amount of energy on the material's surface [30]. Preliminary investigations aiming to explore the behavior of PC under high pulse-overlap structuring has been carried out by irradiating polycarbonate sheets with a UV (266 nm) laser, a spatial period of 2.0 \(\upmu\)m and a pulse-overlap varying from 90% to 99.57%, with a beam size of \(\sim\)20 \(\upmu\)m. The results were analyzed by monitoring how the non-structured depth changes with the pulse-overlap, aiming to investigate the saturation depth of the method. The large-periodicity depth increases linearly with the number of laser pulses up to approximately 200 pulses, corresponding to 22 \(\upmu\)m in depth. On the other hand, the height of the upper interference-generated structures remains constant after 100 pulses, reaching the value of about 1.2 \(\upmu\)m, due to saturation effects associated with the plasma formation during the ablation process [30]. An example of the structures produced with this method is presented in Figure 3a, where an overlap corresponding to 194 pulses per pixel area with a fluence of 1.63 J/cm\({}^{2}\) was employed (5.4 \(\times\) 10\({}^{8}\) W/cm\({}^{2}\)) [30]. As it can be noticed, the upper part of the structures consists of DLIP line-like structures, which due to the high overlap have a strong modulation of the depth in the horizontal direction (perpendicular to the pattern). The shape of these structures resembles the multi-level structure of the rice leaf, in which the grooves act as a major role in the anisotropic spreading of the water droplet along the lines. If a second irradiation is performed after the first structuring on the 90\({}^{\circ}\) rotated sample, pillars-like structures with rectangular symmetry can be obtained and its dual-scale geometry can be varied through the laser fluence of the second process. An example of the results for a double-irradiation DLIP is presented in Figure 3b-d, where the first structuring process has been carried out with 1.48 J/cm\({}^{2}\) and a structure period of 2 \(\upmu\)m, with an overlap variation corresponding to 90 pulses per pixel. If the same pulse-overlap is employed after a 90\({}^{\circ}\) rotation, but using a fluence of 0.51 mJ/cm\({}^{2}\), the result is a hierarchical structure with lines perpendicularly orientated line-like structures (Figure 3b). In this case, the second irradiation has erased the first DLIP-lines, but the large-period lines have not been strongly affected. Figure 3c,d shows instead hierarchical structures consisting of lines on pillar-like structures, obtained with a second irradiation fluence of 1.63 mJ/cm\({}^{2}\). In this case the second irradiation has not only erased the first DLIP structures, but also ablated the large-periodicity structures, leaving pillars in the lower hierarchy level. It is worth noticing that the high-overlap structuring produces a high amount of polymeric particles which re-arrange and deposit on the pillars crests (where the Gaussian-beam distribution is lower, Figure 3c). After an ultra-sound bath in ethanol (1 h) no particle is visible on the resulting structures (Figure 3d).