Layered synthetic microstructures (LSMs), a subset of multilayer coatings, are coming into widespread use in the soft x-ray region because of the favorable optical properties of materials at those wavelengths. In addition to the simple multilayered structures, Fabry-Perot etalons are also being designed and constructed. The design possibilities are not so favorable at the longer wavelengths (VUV) because of the increased absorptance of the spacer materials. A small number of elements are potentially useful as spacer materials in the VUV. Magnesium, with its critical wavelength at about 1200 Å may be useful in designing LSMs for wavelengths somewhat shorter. It is a difficult material to evaporate in vacuum, however, and its tendency for interdiffusion with materials used for nodal layers has yet to be determined. Aluminum, germanium, and silicon have their critical wavelengths grouped around 800 Å so that the choice between them might depend heavily on their tendency to interdiffuse with the nodal layer material rather than their optical properties. Beryllium could be used for LSMs to wavelengths as long as about 500 Å but the difficulties and dangers in using that element, plus the availability of other elements that can be used to even longer wavelengths, militates against it. This paper will review the principles of LSMs, discuss the possible materials for use in LSMs in the VUV, and will show some of the design efforts currently being carried out.
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