Narrowband mirrors based on fluorides tuned at vacuum ultraviolet wavelengths

The interest of the astrophysics community to make space observations in the vacuum ultraviolet (VUV, 100-200 nm) triggers the research to overcome the challenges to develop efficient VUV optics due to material absorption and the limited knowledge of optical constants. Future space observatories like the "Habitable Worlds Observatory, HWO/NASA" require efficient coatings capable of providing high-throughput image bandpasses in the VUV which is currently included as a NASA technology gap. Among the scarce transparent materials in this range, some metal fluoride materials exhibit the shortest cutoffs. Typically, high-reflective narrowband coatings consist of periodic combinations of fluoride multilayers (MLs), with relatively high contrasting refractive index, such as MgF2/LaF3 or AlF3/LaF3 MLs. In this context, GOLD-IO-CSIC group has been developing high-performance all-dielectric VUV coatings, with special emphasis on short wavelengths down to 120 nm. This short VUV range is relatively unexplored and shows a comparatively lower performance due to the increased absorption of fluorides towards shorter wavelengths. Here, we present coatings based on combinations of MgF2/LaF3 and AlF3/LaF3 MLs, which can be tuned at any VUV wavelength >120 nm with a remarkable performance above 85% at H Ly-alpha (121.6 nm) for AlF3/LaF3 MLs; this improves the state-of-the-art at such short wavelengths. We also present a comparative study on the nanostructural morphologies of the two sets of MLs. Both the selection of the ML materials and the introduction of some aperiodicity on the ML designs allow choosing the bandwidth or the desired optical profile with remarkable freedom. Below similar to 120 nm there is no suitable combination of fluorides since all fluorides but LiF turn absorbing. We, then, present narrowband coatings based on Al, LiF, and SiC films, tuned at similar to 100 nm, with a strong rejection at the close H Ly-a line that could mask the observations.