Abstract

High reflectivity normal incidence mirrors do not exist for wavelength λ < 1100 Å. There are no absorption-free dielectrics available which can protect a mirror from oxidation or can be incorporated into a loss-free multilayer. Furthermore the reflectivity of all materials decreases with decreasing wavelength to values around 1% at λ = 200 Å for the best materials. The reflectivity drops proportional to λ⁴ at still shorter wavelength with values less than 10⁻⁵ at λ = 50 Å. However, the loss in reflectivity from a single boundary can be compensated by using multilayer structure with many boundaries. Due to the absorption of all materials the coating designs differ from those used for visible light. Peak reflectivities of 20% or more can be obtained at nearly all wavelengths. The theoretically possible peak reflectivity increases with decreasing wavelengths and reaches very high values (R > 80%) around λ = 1 Å. Fabrication of normal incidence multilayer mirrors has concentrated on the wavelength region from λ = 50 - 250Å and peak reflectivities around R ≈ 25% have been obtained. The obtained results will be reviewed and the expected performance for other wavelengths will be discussed.

© 1984 Optical Society of America