## 5 Conclusions  
This paper is the first to describe a method for bonding optics with sensitive metal containing functional coatings. It thus opens up new possibilities in the production of high-precision beam splitter optics. It is based on surface activation by diffuse helium DBD of high homogeneity. The applied plasma treatment leads to an increase in the achieved bond strengths by a factor of approximately two compared with untreated, classically contacted samples. This fits with the observations of the contact angle measurement that for both polar liquids (water) and predominantly disperse ones (diiodomethane) the contact angle decreases and thus the wettability and bond strength increases. The described increase in shear strength and decrease in contact angle in the manner shown due to plasma treatment is observed in an almost identical manner for uncoated and coated/uncoated specimen pairs. This can be attributed to the same basic surface material (silicon dioxide) of all samples used. The shown behaviour of the samples allows the conclusion that the sample surface is at least cleaned and possibly also activated by the plasma treatment.  
Thermal treatment as part of the annealing process further increases the bond strength achieved, with this increasing in proportion to the increase in temperature. This indicates that the surfaces are activated by the plasma treatment and that the siloxane bonds described in the introduction, whose strength increases with temperature, are formed as a result of the curing process. This means that a bond can be said to be formed. The differences in the strength measurements carried out on the various coated samples can be attributed to the layer properties and the resulting layer stresses.  
Even though it was demonstrated that the presented approach allows for notable increase in bonding strength of optics surfaces with functional coatings further investigations are to be performed in ongoing work. This includes the characterisation of the impact of the plasma treatment on surface roughness and waviness, adhesion energy as well as the chemical composition of the surface and surface-adherent groups. Such investigations are to be performed in order to identify the most important mechanisms such as cleaning or surface activation that contribute to the observed increase in bond strength.  
[PERSON] conceived and designed the experiments, prepared the samples, performed the experiments, analysed the data, and wrote and edited the paper draft; [PERSON] supervised the experiments, validated the obtained results and reviewed and edited the paper draft; [PERSON] acquired the funding and administrated the project and reviewed and edited the paper draft; [PERSON] conceived the experiments, visualised the obtained data and wrote and edited the paper draft. All authors have read and agreed to the published version of the manuscript.  
The support by the Federal Ministry of Education and Research in the frame of the FH-Impuls partnership \"Plasma for Life\", grant number 13 FH6l02 IA is gratefully acknowledged.  
Not applicable.  
Raw data are available on request.  
The authors thank Quoptiq Photonics GmbH & Co.KG for providing the investigated samples. Further, the authors thank [PERSON], [PERSON] and [PERSON] from Qioptiq Photonics GmbH & Co.KG for supporting this work and preparing samples for the bond tests.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.