The main goal of a multilayer coating for the XUV region is to enhance reflectivities in regions of wavelengths and angles of incidence where single surfaces with useful reflectivities are not available and achieve useful reflectivities despite the fact that all materials are absorbing. In wavelength regions where absorption-free materials with different refractive indices are available, the âquarter-wave stackâ is the most efficient design to produce high-reflectivity mirrors because all boundaries add in phase to the reflected wave in this design. Early studies of the influence of absorption on the performance of the quarter-wave stack showed, however, that the performance of this design deteriorates very fast when one of the coating materials becomes slightly absorbing . Efforts to extend multilayer coating technology from the visible to the UV region concentrated on the search for thin film materials that could be made absorption-free. It was also clear that these efforts were doomed to failure for wavelengths below Î» = 1100 Ã , where all materials are strong absorbers.
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