## 4 Discussion  
The results presented in Figure 3 show that the applied diffuse DBD plasma using helium as process gas allows the treatment of NPBS coatings without layer destruction. It is crucial that the DBD is in a diffuse mode with as few stationary filaments as possible. The generation of a diffuse DBD after [21; 23] can be made possible by the appropriate choice of electrode geometry. Diffuse DBDs can activate surfaces simultaneously in a particularly gentle but highly effective manner [22]. The influence of different process gases on the homogeneity of the DBD can be seen in Figure 2 in the current curves. For the helium DBD, the current curve is the smoothest, which indicates the highest homogeneity. The positive influence of helium on plasma homogeneity has already been observed in [29; 30] where a homogeneous DBD at atmospheric pressure was produced with helium as process gas. The stationary filaments of the current curves of the other investigated gases are very pronounced and have a greater power density due, among other things, to the higher applied voltage than for helium. Thus, such filaments lead to the destruction of the NPBS functional layer for all gases except helium. As the contact angle measurements show, significant modification of the surfaces takes place. With the presented approach, a surface conditioning for bonding of very sensitive optical functional layers becomes possible. In Figures 5-7 an increase in shear strength only induced by plasma treatment could be observed. The influence of the plasma treatment on the surface is also reflected in the contact angle measurement shown in Figure 4. As can be seen there, the plasma affects the surface very quickly. After 5 s, the occurrence of saturation can already be observed. After this, the variation of values of the contact angle measurement takes place only in the range of the standard deviation. The increase in shear strength and the decrease in the contact angle can be explained by plasma-induced cleaning. In addition, the occurrence of surface activation is probable. The rapid decline of contact angle indicates a very superficial contamination in the area of a few atomic layers. This contamination may come from the storage period of the optics after manufacturing, even under clean conditions. Thus, small amounts of carbon and hydrocarbons can accumulate on the surface. This type of contamination forms in a short time due to the storage of optics and cannot be  
Figure 7: Shear strength of bond surfaces; pure fused silica and fused silica coated with a polarising beam splitter (PBS) coating, without any plasma treatment and after plasma treatment for 30 s and curing at different annealing temperatures. removed by manual cleaning with acetone or ethanol. Plasma treatment removes such carbon groups [25] and may furthermore remove residues of the used polishing agent [26], which is adsorbed on the surface and also cannot be removed by classical manual cleaning that is applied in optics manufacturing. This leads to an increase in surface energy, which is reflected by a decrease in the measured contact angle. The removal of carbon groups also shifts the ratio of oxygen present at the surface from the siloxane bonds of the fused silica surface and attached carbons. In [31], a direct relationship between the oxygen-carbon ratio and the contact angle was demonstrated. It is most likely that this relationship also applies to the glass samples here as indicated by the contact angle measurement shown in Figure 4.  
It is known from the literature [8; 32] that plasma treatment of Silicon dioxide surfaces has both cleaning and activation effects. It is likely that such an activation effect contributes to the achievement of bond strength. This should be investigated as part of further work. As in chemical activation, e.g., by RCA-SC1, the siloxane bonds located on the surface are broken, and other functional groups, such as hydroxyl or amino groups [8; 11], attach to the open bonds according to the environment in which the activation takes place.