Design of soft x-ray fluorescence microscopy beyond 100-nm spatial resolution with ultrashort Kirkpatrick-Baez mirror

A Kirkpatrick-Baez (KB) mirror is a reflective focusing device that sequentially positions a pair of crossed mirrors in a grazing-incidence setup. Typically, this type of device offers a relatively long working distance of 20 mm or longer, which allows specimens to be installed in various configurations. However, there is a tradeoff between the long working distance and both the numerical aperture and demagnification factor, resulting in drawbacks for KB mirrors for a sub-micron focus size in a soft-X-ray region. This research explores an ultrashort KB mirror composed of 2- and 8-mm-long mirrors with focal lengths of 2 and 8 mm, respectively. Its demagnification is designed to be up to 10000 and its focus size can theoretically be below 50 nm at a photon energy of 1 keV. To demonstrate an X-ray nanoprobe based on this focusing device for X-ray microscopy, the low-energy X-ray fluorescence (LEXRF) technique is incorporated into the focusing system. The aim of this LEXRF system is to observe light elements in biological specimens. After X-ray fluorescence yields were estimated, the experimental setup was examined for a forward- and side-scattering configuration. Preliminary studies examined the fluorescence detector performance and the fluorescence detection of biological and pharmaceutical specimens in the traditional backscattering configuration. Compared with diffractive focusing devices, which condense approximately 10% of the incident X-rays, the reflective ultrashort KB mirror can benefit LEXRF by more efficiently collecting X-rays to its nanoprobe, thus enhancing the fluorescence signals from specimens.