Si/SiO2-based filter coatings for astronomical applications in the IR spectral range

Order sorting filters had to be coated for the CRyogenic InfaRed Echelle Spectrograph upgrade (CRIRES+)-instrument, a high-resolution IR spectrograph to be set up at ESO's Very Large Telescope in Chile. Therefore SiO2 was chosen as material with low refractive index. Si and Ge have been investigated as materials with high refractive index, whereby Si has been chosen for the application of the coating. Three types of high-pass filters were deposited with transmission bands starting at 0.96mm, 1.47mm and 2.9mm. These filters need to block effectively all wavelengths between 0.5 mm and the respective band. Therefore, in the blocking range, an optical density above four, or above three for the filter starting at 2.9 mm respectively, had to be achieved. The filter-coatings also needed to survive thermal cycling down to 65K while only introducing a small wave front error. The lower total thickness, compared to coatings consisting of other materials, and the low film-stress are favorable properties for coatings deposited onto prisms and other more complex optical components.