Narrowband multilayer mirrors for the extreme ultraviolet spectral range of 50 to 95 nm

The spectral range of 50 to 115 nm in the extreme ultraviolet (EUV) is characterized by the high absorption and low normal incidence reflectance of most materials, which make difficult the development of high reflectance multilayer mirrors at normal incidence angles. The availability of efficient mirrors would have a great impact on the performance of EUV space telescopes and other EUV instruments at these wavelengths. In order to obtain high normal-incidence reflectance coatings, it is necessary to find materials with absorption as low as possible at 50 - 115 nm. Recently, lanthanides and close elements have attracted the attention of researchers because of their relatively low absorption at bands in the EUV, and several studies on the optical constants of lanthanides in the EUV have been published. As a result of these investigations we have identified those lanthanides which better match the low absorption requirement at wavelengths in the 50 - 115 nm range. In this work we present the use of the lanthanide Yb combined with other materials and protective capping layers in multilayers designed to have a reflectance maximum at a wavelength selected within the spectral range of 50 to 95 nm. Experimental results for the case of a multilayer composed of Yb, Al and SiO layers confirm the adequacy of this approach, providing a peak reflectance of 0.276 at 80 nm with FWHM of 14.5 nm for samples not exposed to the atmosphere. A decrease in peak reflectance from 0.276 to 0.209 was observed after 2 years of storage in a dessicator.