We report the results of an optical design study of a multilayer mirror that provides high reflectivity in the 400 eV region. First, the material pair selection rule proposed by Yamamoto was applied to examine the coating materials. Using the optical constants table by Palik, we calculate the Fresnel coefficients for various materials at angle of incidence of 60 deg. Following the selection rule, we looked for two materials yielding strong reflection at interfaces where the distance between two points of the Fresnel coefficients on the complex plane is far apart, as well as small absorption in the multilayer structure. Then film thicknesses of the multilayer structure were optimized by numerical calculation using IMD software, which results in practical high reflectivity between 43 to 50% on the Sc/Si, Sc/Mg, and Sc/Cr multilayer mirrors at the photon energy of 397.5 eV. In this presentation, we also report grazing incidence x-ray reflectivity results for multilayer mirrors deposited by a magnetron sputtering method.
To bring global optimization capability on optical design of a focusing objective in soft X-ray region, we describe novel design approach by combining analytical and numerical methods based on geometric optics. We assume two-spherical mirror objective of glazing-incidence configuration. First, relationship between system layout parameters, i.e., radii of curvatures and mirror separations, and focus length is derived with paraxial approximation, to describe all feasible mirror layouts with constant focal length. Then, focusing property of the mirror objectives are computed by applying numerical raytracing method, to seek practical optical designs with low aberrations. As design examples, the proposed method is applied to a two-spherical-mirror objective for one-dimensional focusing of soft X-ray high harmonics. We successfully find three kinds of practical designs with low aberration. The calculation results show that we can expect small spot size close to 100 nm on the focal plane, for the case that the objective has moderate focal length (f = 100 mm) and numerical aperture (NA = 0.02). These results indicate that the proposed approach is capable of global optimization of the mirror objective with glazing-incidence configuration.
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