Far-UV reflectance and stress of narrowband AlF<sub>3</sub>/LaF<sub>3</sub> multilayers
Paloma López-Reyes, Carlos Honrado-Benítez, Nuria Gutiérrez-Luna, Álvaro Ríos-Fernández, Luis Rodríguez-de Marcos, Juan I. Larruquert
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 AlF 3 /LaF 3 FUV MLs prepared by thermal evaporation and compares this performance with MgF 2 /LaF 3 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 CaF 2 crystals, two common optical substrates. AlF 3 /LaF 3 MLs exhibited reduced stress compared with MgF 2 /LaF 3 MLs, resulting in a larger thickness threshold before crack generation. This enables preparing MLs with more layers and hence with higher performance. AlF 3 /LaF 3 MLs underwent lower reflectance decay over time compared with MgF 2 /LaF 3 MLs. Fresh MLs centered at ∼160 nm displayed a peak reflectance close to 100%, and most of the AlF 3 /LaF 3 MLs kept a reflectance of 99% after several months of storage. The bandwidth of AlF 3 /LaF 3 MLs for a given number of layers was found to be somewhat larger than for MgF 2 /LaF 3 MLs.