Nanofocusing of X-ray free-electron laser using wavefront-corrected multilayer focusing mirrors

S Matsuyama,T Koyama,H Ohashi,J Kim,J Yamada,H Yumoto,K Tono,T Inoue,T Ishikawa,Y Inubushi,I Inoue,T Osaka,M Yabashi,K Yamauchi

doi:10.1038/s41598-018-35611-0

Abstract

A method of fabricating multilayer focusing mirrors that can focus X-rays down to 10 nm or less was established in this study. The wavefront aberration induced by multilayer Kirkpatrick–Baez mirror optics was measured using a single grating interferometer at a photon energy of 9.1 keV at SPring-8 Angstrom Compact Free Electron Laser (SACLA), and the mirror shape was then directly corrected by employing a differential deposition method. The accuracies of these processes were carefully investigated, considering the accuracy required for diffraction-limited focusing. The wavefront produced by the corrected multilayer focusing mirrors was characterized again in the same manner, revealing that the root mean square of the wavefront aberration was improved from 2.7 (3.3) rad to 0.52 (0.82) rad in the vertical (horizontal) direction. A wave-optical simulator indicated that these wavefront-corrected multilayer focusing mirrors are capable of achieving sub-10-nm X-ray focusing.

Highlights

Since the discovery of X-rays, X-ray sources have evolved from X-ray tubes to synchrotron radiation sources
Focusing X-rays down to 10 nm or less using conventional total reflection mirrors is impossible, because mirrors with large numerical apertures (NAs) are necessary to focus beams so narrowly and large grazing incidence angles are required to increase the NAs of the mirrors
Object positioning with an accuracy smaller than the beam size is required. This approach is not suitable for the combination of large-NA focusing optics and X-ray free-electron lasers (XFELs), where the XFEL spot position varies shot by shot

Summary

Introduction

Since the discovery of X-rays, X-ray sources have evolved from X-ray tubes to synchrotron radiation sources. Two types of total-reflection Kirkpatrick–Baez (KB) mirrors were developed to focus XFELs at SACLA in the early days of its operation: a simple 1 μm focusing system[18] and a 50 nm focusing system[19] that utilizes a two-stage focusing scheme for tightly focusing X-rays The latter focusing system could reach a peak power density of 1020 W/cm[2]. Focusing X-rays down to 10 nm or less using conventional total reflection mirrors is impossible, because mirrors with large numerical apertures (NAs) are necessary to focus beams so narrowly and large grazing incidence angles are required to increase the NAs of the mirrors This approach is limited by the critical angle, which depends on the electron density of the surface material employed. The phase retrieval and X-ray pencil beam methods are incompatible with XFEL nanofocusing because they require precise beam profiling very near the focus, where the peak power density is sufficiently high to break an inserted object

Methods

Conclusion