## 4 Discussion  
In Figure 2c,d, we see two types of ripple structures for laser-structured PC, one with a periodicity \(\Lambda\) of about 750 nm and a second one with a periodicity \(\Lambda\) of about 200 nm. They are oriented perpendicular to each other and parallel and perpendicular to the linear polarization of the laser light, respectively. In accordance with the classification reported for the ps-laser-structuring of PC [19], we address the first type as low-spatial-frequency (LSF) LIPSS and the second type as high-spatial-frequency (HSF) LIPSS. The latter also occurred inside the craters and on the sidewalls of the samples shown in Figures 3, 5a-c and 7a,b. However, we want to point out that the sequence of the LIPSS types is different in our work in comparison to that in [19]. In our case, the LSF-LIPSS are outside of areas where the HSF-LIPSS are observed, while in [19] it is the other way round. This means that, in our case, due to the Gaussian beam profile, the HSF-LIPSS occur at higher laser fluences than the LSF-LIPSS. This discrepancy with the results in [19] may be related to the fact that they used a UV laser light, where the PC absorbs considerably more light than at our laser wavelength in the NIR region. Therefore, we needed fluences \(F_{0}\) of at least 500 mJ/cm\({}^{2}\) for the structuring of the PC, while the experiments in [19] were performed with fluences \(F_{0}\) of only a few tens of mJ/cm\({}^{2}\). These different fluence ranges may cause different mechanisms of ripple formation. For both studies, rather higher pulse numbers per area were used, especially for the lines with multiple overscans. These high pulse numbers may lead to photodegradation effects on the PC, as is described, for instance, in [27; 28]. This photodegradation can result in a step-wise change in the optical properties of the samples during multi-pulse laser irradiation is often addressed as the incubation effect. For this reason, we have discussed our results in terms of laser power instead of laser fluence, as these incubation effects would probably scale with the power. In [19], the effect of an increased bulk temperature on structure formation is also discussed. In this paper, it is mentioned that, in the case of PC, the LIPSS, especially the LSF-LIPSS subvariant, are not formed at the surface but in a sub-layer at a certain temperature. This temperature lies, in the case of polycarbonate, near the glass transition temperature (\(\sim\)150 \({}^{\circ}\)C). We also performed laser-irradiation experiments at elevated bulk temperatures. Overall, these experiments showed that the heating of the sample during the structuring process in the NIR region and for fs pulses also leads to a lower necessary laser power for the formation of the same amount of LIPSS, as was also observed in the UV region. The effect in the NIR region is far smaller than in the UV region, probably due to the lower absorption of PC in this wavelength range. That the effect is rather small means, for instance, that for \(p\) = 9 mW at room temperature we obtained roughly the same amount of LIPSS for 100-overscan lines as for \(p\) = 7 mW lines at 50 \({}^{\circ}\)C.  
For PC as well as for CAB, the sponge structures appear at relatively high laser powers. It seems that the material is becoming hot enough to make the transition to the gas state in some local areas. This gas may then leave the sample and leave behind this sponge-like irregular structure. This explanation is supported by the fact that, at least for CAB, lower laser powers led to lines with molten sidewalls, as the temperature may not have been high enough for evaporation. In the NIR spectral range, CAB exhibits a very low absorption of laser light. This could be one reason that some characteristic laser structures, like the HSF-LIPSS on PC, do not appear in the experiments with CAB, as they require an appropriate amount of laser light to be absorbed.