Lyman-β narrowband coatings with strong Lyman-α rejection.

Novel narrowband multilayer coatings efficient at a wavelength as short as 100 mn are presented, which pushes shortwards the existing limit of reported narrowband multilayers. Such limit had been established at similar to 120 nm, close to the MgF2 cutoff wavelength. The new multilayers combine layers of Al, LiF, and SiC, in an Al/LiF/SiC/LiF multilayer design (four layers, starting with the innermost layer). Among these materials, Al and LiF are deposited by evaporation and SiC by ion-beam-sputtering. In addition to a high, narrow peak close to H Lyman beta (102.6 nm), these multilayers simultaneously provide a very small reflectance at H Lyman alpha (121.6 nm). This combined performance is demanded in space instrumentation for astrophysics and solar physics observations among others, where imaging the sky at the important diagnostic spectral line of Lyman beta line requires rejecting the frequently much more intense background at Lyman alpha line. Such is the case for solar corona observations at Lyman beta, which is masked by the strong Lyman alpha line. The multilayer peak is placed close to another important diagnostic tool: the OVI doublet at 103.2 and 103.8 nm. The target of small reflectance at 121.6 nm was seen to be the most critical. The best strategy in multilayer preparation was to prepare it with such minimum reflectance at slightly shorter wavelengths so that the coating evolved to shift it longwards over time. Multilayers kept a remarkable 102.6 nm/121.6 nm reflectance ratio over time in spite of some performance degradation. Hence, a multilayer coating aged of 4 years kept a reflectance of 43% at 102.6 nm and 0.2% at 121.6 nm. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement