## 3 Results and Discussion  
### Surface Morphology  
Development of typical surface morphology during underwater laser treatment of PET was observed in narrow interval of laser fluencies from 8 to 18 mJ\(\cdot\)cm\({}^{-2}\), regardless of operating voltage applied on laser main storage capacitor. Operating voltage, however, had distinctive impact on the fine microstructure superimposed on the on PET surface, in particular at low applied fluencies. This interesting phenomenon most likely originates from the symmetry of laser efficiency on operating  
Figure 2: Distribution of measuring points within laser treated area during atomic force microscopy (AFM) analysis.  
voltage (see Figure 3), which reaches a maximum at about 25 kV (ca 1%) and then falls evenly to both border values (ca 0.75 %).  
Lower laser efficiency at low and/or high voltages is related to increased instability in laser beam profile at both border values [13], which obviously determines the resulting surface morphology, especially at lower laser fluencies. For clear illustration of this phenomenon, surface morphology in dependence of operating voltages (19-29 kV) is shown in Figures 4-6 for applied laser fluencies of 8, 12, and 16 mJ-cm\({}^{-2}\), respectively. We have chosen these fluencies, since they provide sufficient insight into the nature of morphology changes of underwater laser processed polymer. With increasing flow, massive ablation of the polymer surface took place, until the typical worm-like nanostructure disappeared completely (applied laser fluence > 18 mJ-cm\({}^{-2}\)).  
One can clearly see that at lower laser fluencies and boarder values of applied voltages (Figure 4), the developed surface morphology is reminiscent to that of treated in air [14]. Even though the LIPSS are not precisely developed, typical prolongation of worm-like structures at specific operating voltage into linear ones is obvious. This may originate from the confined plasma plume in water, which expands at a much lower velocity than in air (order of magnitude less). This implicates that spatially confined plasma induces increase of surface temperature in water environment much intensively than in air [10], which may significantly contributes to the development of similar surface structure as in air when processing at sufficiently lower laser fluencies in water [15]. It is also obvious from the Figures 4-6, that the higher the laser fluence applied the more uniform developed nanostructure is and the possibility of modulation of fine microstructure by variation of applied voltage diminishes. This finding may be quantified by corresponding values of surface roughness, which are given in Table 1. Concurrently to AFM images, \(R_{\mathrm{a}}\) values suggest that at higher applied fluencies the effect of voltage on surface roughness is negligible and surface roughness stabilizes, regardless of specific voltage value. At low fluence, when worm-like nanostructure is not fully developed (see Figure 4: 19 and 29 kV), however, the variation of \(R_{\mathrm{a}}\) value corresponds to the possibility to vary surface morphology just by proper setting of applied voltage.  
Figure 3: Dependence of output energy (black) and laser efficiency (red) on charging voltage of laser main storage capacitor of KrF laser.  
Figure 4: AFM images of underwater laser treated PET at laser fluence of 8 mJ\(\cdot\)cm\({}^{-2}\) and different values of operating voltages. Numbers in frames refer to laser operating voltages in kV.  
Figure 5: AFM images of underwater laser treated PET at laser fluence of 12 mJ\(\cdot\)cm\({}^{-2}\) and different values of operating voltages. Numbers in frames refer to laser operating voltages in kV.  
Figure 6: AFM images of underwater laser treated PET at laser fluence of 16 mJ\(\cdot\)cm\({}^{-2}\) and different values of operating voltages. Numbers in frames refer to laser operating voltages in kV.