Focusing hard x-rays with large kinoform lenses of mm size

摘要

This report describes the optimization, first experimental data and the performance outlook for a special refractive lens for the focusing of x-rays. The lens was obtained by applying the strategy of Fresnel to lighten a lens by removing efficiently optically passive material. This strategy was applied to objects, which were produced by deep x-ray lithography and which can thus be shaped in only one dimension. Consequently this class of lenses can focus in only one dimension. While in the normal Fresnel lenses as much material as possible is removed, in the present lens the remaining segments were kept as large as possible, in order to finally obtain a rigid and deep structure. The resultant structure is composed of small prisms of almost identical shape, which are combined such that the final lens looks like an hour glass, i.e. two large prisms touch each other at one of their tips. Such a lens has better transmission than a normal refractive lens of equal focal length. And even though it has worse transmission than the normal Fresnel lenses, it could be produced with an unprecedented geometrical aperture of 2.6 mm for 8 keV photon energy. The uniformly etched structure depth was found to be at least 0.4 mm, over which the lens can focus free of aberrations. A lens with focal length 2.183 m was found to provide after very rapid alignment a focus size of 2.8 μm, while slightly better 1.73 μm were ideally expected.