Ellipsoidal mirror for two-dimensional 100-nm focusing in hard X-ray region

Hirokatsu Yumoto,Takahisa Koyama,Yoshiki Kohmura,Kazuto Yamauchi,Tetsuya Ishikawa,Satoshi Matsuyama,Haruhiko Ohashi

doi:10.1038/s41598-017-16468-1

Abstract

Cutting-edge hard X-ray microscopy strongly depends on sophisticated focusing optics and ultrabright X-ray sources at synchrotron-radiation and X-ray free-electron laser (XFEL) facilities. These facilities typically provide two-dimensional nanofocusing X-ray beams by combining one-dimensional focusing mirrors. However, single-reflecting two-dimensional focusing mirrors with an ellipsoidal surface, which are well-known to possess high efficiency, have limited microfocusing applications. In this paper, we present an ultrahigh-precision ellipsoidal mirror for two-dimensional X-ray nanofocusing by overcoming the difficulties faced in the manufacturing process of its aspherical surface, including the surface-processing methods and surface metrology. The developed mirror has nanoscale accuracy, and it achieves focus size of 85 nm × 125 nm (full width at half maximum) using 7-keV X-rays. Two-dimensional focus was demonstrated in the same focal plane by resolving 50-nm test structures by scanning X-ray microscopy using a focusing beam. These achievements represent an important first step toward realizing two-dimensional aspherical mirrors with complex designs, in addition to ultralow loss and unprecedented small focusing property for extensive optical applications in synchrotron-radiation and XFEL facilities as well as in other scientific fields that require ultraprecision optical surfaces.

Highlights

Owing to constant developments in high-resolution microscopy, X-ray analytical techniques have become essential tools in various fields of science in which focusing optics play an important role[1,2]
The measured focus size reflected by the total area was larger than the estimated value, whereas the reflection image from the mirror was relatively clean
The measured minimum focus size reflected by the central-part area was 1.2–1.8 times larger than the diffraction-limited focus size of 70 nm × 69 nm (FWHM); this was estimated by the wave-optical simulator using the corresponding mirror aperture

Summary

Introduction

Owing to constant developments in high-resolution microscopy, X-ray analytical techniques have become essential tools in various fields of science in which focusing optics play an important role[1,2]. Ideal two-dimensional nanofocusing mirrors with an ellipsoid revolution in the soft and hard X-ray regions remain exceedingly difficult to manufacture because of their complex and aspherical shape with nanoscale accuracy and atomically controlled surface roughness[27,28,29]. Ellipsoidal focusing mirrors allow a small diffraction-limited focus size and high-density focus with the design of a large spatial aperture in the sagittal focusing direction, compared with that in the meridional focusing direction. These characteristics are advantageous and are not available www.nature.com/scientificreports/. A pilot study was conducted to examine the accuracy of the metrology with a partly manufactured mirror using hard X-rays[31,32]

Methods

Results

Conclusion