Aperiodic normal-incidence antimony-based multilayer mirrors in the 8 — 13-nm spectral range

The optical properties of several materials were analysed from the standpoint of fabrication of broadband normal-incidence multilayer mirrors possessing maximal uniform reflectivity in the 8—13-nm range. By solving the inverse problem of multilayer optics we show that aperiodic Sb/(B₄C, Sc, Si) multilayer structures optimised for maximum uniform reflectivity in the 8—13-nm range are able to afford a normal-incidence reflectivity R~10 % throughout this range. The best results are exhibited by the pair Sb/B₄C, for which the average reflection coefficient amounts to about 13%. The dependence of optimisation result on the programmable limitation on the minimal layer thickness in the multilayer structure was numerically investigated. An empirical rule was established whereby setting the lower bound for a layer thickness at a level ~λ_min/4 (in this case, λ_min = 8 nm) does not result in an appreciable lowering of attainable uniform reflectivity.