High numerical aperture zone plates using high orders of diffraction

Abstract

The implementation of laboratory X-ray microscopes with isotropic sources like laser generated plasmas requires condenser elements which collect photons from large solid angles, match the aperture of the micro zone plate objective to insure high spatial frequency transfer and allow to work at different wavelengths. All these requirements can be met by a zone plate condenser used in a high order of diffraction. Recent progress in nanostructuring allows the production of galvanoforms with aspect ratios >14, where the structure size is below 90 nm. A first linear nickel test zone plate with an outermost zone width of 75 nm and 500 nm high zones showed a diffraction efficiency of 1% at 3.4 nm wavelength in the sixth order of diffraction, which is significantly above the value predicted by the theory of thin gratings. These results open the way towards zone plate condensers with numerical apertures well beyond those of first order diffractive elements.