XRM2024 - Tue05K - "Ultracompact Kirkpatrick-Baez mirror for forming 20-nm achromatic soft-X-ray focus toward multimodal and multicolor analyses"

Ultracompact Kirkpatrick-Baez mirror for forming 20-nm achromatic soft-X-ray focus toward multimodal and multicolor analyses

Nanoscale soft-X-ray techniques often leverage nanoprobes formed by X-ray focusing devices. Zone plates, which are commonly used, can produce sub-10-nm spot size using monochromatic soft X-rays. However, the focus position is strongly chromatic. Energy scanning thus requires samples to be repositioned throughout or between the spectroscopy procedures. Chromatic aberration also imposes a monochromaticity requirement on photon-hungry methods that otherwise could use polychromatic X-rays. Low-energy X-ray fluorescence (LEXRF), for instance, increases the fluorescence count by expanding the illuminated area to the sub-micrometer level or extending the
measurement time rather than by enhancing the photon flux using polychromatic or broadband-energy nanoprobes. To enhance soft-X-ray microscopy toward multimodal and multi-energy nanoscale analyses, chromatic aberration in the soft-X-ray region must be tackled.

Soft-X-ray focusing mirrors based on total reflection are inherently achromatic. Although their fabrication technology has been improved to achieve surface perfection towards the diffraction limit, the best manufacturing methods still fall short. A new strategy is necessary to realize achromatic and diffraction-limited nanofocusing over the soft-X-ray range.

Besides straightforwardly developing a fabrication technique, we adopted a short-focal-length strategy to reduce the effect of surface figure errors on the focus size . We fabricated an ultracompact Kirkpatrick-Baez (ucKB) mirror, whose component focal lengths were 2 mm and 8 mm, as shown in Figure 1. We achieved a focus size of 20.4 nm at 2 keV, which represents a significant improvement in achromatic soft-X-ray focusing. The ucKB mirror extends LEXRF by producing a two-color nanoprobe with photon energies of 1 and 2 keV. We propose a subcellular chemical mapping method that allows a comprehensive analysis of specimen morphology and the distribution of light elements and metal elements.

References:
[1] Rösner, B. et al., (2020). Optica 7 (1602–1608)
[2] Shimamura, T. et al., (2023). Rev. Sci. Instrum. 94 (7821)
[3] Shimamura, T. et al., (2024). Nat. Commun. 15 (665)