Improving optical response and hydrolyzation resistance of silverbased astronomical telescope mirrors via filtered cathodic arc deposited diamond like carbon

Silver based mirrors (Ag-mirrors) boast high reflectivity in the visible spectral range and low emissivity in the thermal infrared, but tend to suffer from low durability; requiring regular recoating. Conventional coatings extend the operational life of Ag-mirrors, but significantly reduce the mirror's efficacy in the blue-UV spectral range. The solution our study proposes is a protective coating of diamond-like carbon (DLC); known for its hydrophobicity and abrasion resistance. DLC coating were produced at room temperature using filtered cathodic arc (FCA) deposition, which results in a high sp3 to sp2 bond ratio. Raman spectroscopy characterization reveals that the sp3 to sp2 ratio can be controlled by tuning the applied substrate bias. Optical characterization in turn shows only minor impairment to the Ag-mirror's overall spectral response in the UV to NIR spectral range with a more substantial decrease in performance around the surface plasmon polariton region. FCA-deposited DLC shows promise as an alternative protective coating providing an increase in durability over conventional coatings without significantly impairing the UV response.