## 2 Materials and Methods
### Experimental Setup and Testing  
For plasma treatment, a plasma source with a flat copper electrode and an aluminosilicate glass plate as dielectric separation was used. As shown in Figure 1, the samples were placed in a plastic housing made of photopolymer resin. The plasma was ignited between the high-voltage electrode and an external ground electrode located below the housing. The purpose of the dielectric on the ground electrode is to mitigate accidental flashovers in the laboratory setup. The 1 mm wide discharge gap was flushed with the process gas. The used input gases were synthetic air, nitrogen (N 5.0), argon (Ar 5.0) and helium (He 5.0; each was from Linde Gas AG). All used process gases contained a small proportion of approx. 0.5% ambient air, resulting from the intended final use in ambient air in classical optics production without an additional enclosure. The gas flow rate was 5 L per minute since the highest plasma homogeneity was achieved at this setting. Due to the dielectric separation, a direct DBD plasma was ignited. As is common for DBD plasmas, an alternating high voltage was applied to power the discharge. The voltage pulses are generated according to the Fourier synthesis principle. The chosen peak voltage as well as the pulse repetition rates depended on the ignition threshold of the used process gases and their respective behaviour concerning the formation of stationary filaments. In general, the voltages were chosen to be as low as possible for each process gas to allow plasma ignition on the one hand and to avoid layer destruction on the other hand. Due to the different ignition voltages of the various process gases, all plasmas operated at different voltages. Since they are all used directly near their respective ignition limit, a comparison is nevertheless useful for the intended application. The different current-voltage characteristics are shown in Figure 2. For each of the different process gases, the mean power was determined using the Electric Current method from [28]. The mean powers, together with the respective amplitudes, pulse repetition rates, voltages and  
Figure 1: Schematic of the setup used for plasma treatment. currents are shown in Table 1. The pulse duration of each high-voltage pulse was about 1 \(\upmu\)s (FWHM). Since DBD plasmas feature low gas temperatures [20], the maximum observed sample temperature was 23 \({}^{\circ}\)C.  
The plasmas using the different process gases were first applied to the functional NPBS-layer containing silver, as this is the most sensitive coating. Microscope images were taken to check whether the plasma treatment causes coating damage. Moreover, contact angle measurements were carried out to detect whether plasma-induced surface conditioning already occurs before coating damage. Using this setup with helium as process gas enables both the highest homogeneity of the plasma and the gentlest surface conditioning, contact angle measurements were performed to determine the optimal treatment time. Due to better contrast, those measurements were performed on uncoated samples. Since both coatings have a silicon dioxide top layer, no differences are to be expected between coated and uncoated samples. Bond tests were then carried out with the parameters determined in that vein. In order to investigate the impact of such plasma treatment on the stability of bonded doublets, pairs of uncoated and uncoated or uncoated and coated samples were brought into direct contact and pressed at a mechanical pressure of 0.2 MPa. Such contacting was performed in deionised and distilled water. This approach is the standard wringing process in optics manufacturing since it allows for a final fine adjustment of the components after contacting due to the thin water film between the surfaces. Moreover,it helps to meet the cleanliness requirements of the optics industry. Subsequently, the samples were cured at different temperatures in order to remove this water film.