Global optimization of two-multilayer mirror objective for focusing soft X-ray high harmonics

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.