Supermirrors are multilayer structures where the thickness of the layers down through the structure changes so that wide-band reflection occurs. The principles were developed in the mid-70's and have been used extensively for neutron optics. Absorption in the upper layers limits the attainable reflectivity for X-rays. For hard X-rays (>= 15 keV), the absorption, however, is low enough that it is possible to design supermirrors with 10 - 70% reflectivity in a band approximately equals 3 times the width of the total reflection regime. Supermirrors of W/Si and Ni/C have been successfully fabricated and characterized. The measured X-ray reflectivities are well accounted for by the standard dynamical theories of multilayer reflection. Hard X-ray applications that could benefit from X-ray supermirror coatings include focusing and imaging instrumentation for astrophysics, and collimating and focusing device for synchrotron radiation.
Proceedings of Spie, the International Society for Optical Engineering, 1994, Issue 2253, p. 299-308