Far-UV reflectance and stress of narrowband AlF3/LaF3 multilayers

Abstract

Upcoming space instrumentation, such as LUMOS (LUVOIR Ultraviolet Multi-Object Spectrograph) in LUVOIR (Large Ultraviolet Optical Infrared Surveyor) mission, demands efficient narrowband coatings centered in the far UV (FUV). Narrowband FUV coatings can be prepared with all-dielectric multilayers (MLs) based on two fluorides. This research evaluates the performance of AlF3/LaF3 FUV MLs prepared by thermal evaporation and compares this performance with MgF2/LaF3 MLs, which were previously investigated. FUV reflectance, stress, and the influence of substrate materials have been investigated, along with ML stability over time when stored in a desiccator. Coatings were deposited both on fused silica and on CaF2 crystals, two common optical substrates. AlF3/LaF3 MLs exhibited reduced stress compared with MgF2/LaF3 MLs, resulting in a larger thickness threshold before crack generation. This enables preparing MLs with more layers and hence with higher performance. AlF3/LaF3 MLs underwent lower reflectance decay over time compared with MgF2/LaF3 MLs. Fresh MLs centered at ∼160 nm displayed a peak reflectance close to 100%, and most of the AlF3/LaF3 MLs kept a reflectance of 99% after several months of storage. The bandwidth of AlF3/LaF3 MLs for a given number of layers was found to be somewhat larger than for MgF2/LaF3 MLs.