The optical properties of opaque sputtered silver films are investigated and related to their surface morphology. As reference, an evaporated sample produced by thermal flash evaporation has been studied, too. Standard optical and electrical characterizations as well as scanning electron microscopy (SEM) and X-ray reflectometry have been carried out with unprotected silver films directly after deposition and reveal strong correlations between the corresponding characterization results. The aging behavior of the normal incidence reflectance has been studied over a time period of 36 weeks. Protected silver mirrors have been prepared by sputtering using silica and alumina as the protective coating. It is shown by model calculations that the main spectral features occurring in the reflectance spectra of as-deposited unprotected coatings, unprotected coatings after aging, and protected coatings can be reproduced assuming a rough silver surface covered by a corresponding ultrathin absorbing interface layer or overlayer.
Optical coatings are an integral part of superior optical components. Astronomical applications (ground- and space-based) place especially high demands on these coatings, not only with regard to their optical performance but also to their mechanical and environmental stability, their thermal properties, and their radiation resistance. This article presents a short overview of several coating solutions developed in recent years at Fraunhofer IOF in order to meet the challenging demands of astronomical applications. The focus is placed on high reflective coatings for different wavelength regions including coatings for the VUV range below 100nm, coatings for the DUV wavelength range above 100nm and VIS/NIR/IR coatings. Further, amorphous silicon layers will be introduced which can be polished to very low roughness values and therefore can act as polishing layer for the manufacture of ultraprecise optical components from metal substrates
In the visible to infrared spectral range, highly-reflective silver mirrors are applied in the manufacture of optical instruments such as telescopes. However, it is still difficult to combine high reflectivity and long-term stability of the protected silver coating. We show that the deposition of impervious protective layers is necessary but often not sufficient for long-term environmental stability. Hygroscopic air borne particles absorbed by the protections surface attract water molecules and form a solution. This solution first damages the protection, subsequently permeates the protection and finally damages the silver whereby the reflectivity is reduced. We demonstrate this particular damage mechanism with different experiments and describe this mechanism in detail.
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