X-ray mirrors play a crucial role in transporting and focusing X-rays in synchrotron radiation beamlines due to their high reflectivity and low chromatic aberration properties. Recent advancements in ultraprecise surface finishing and testing techniques1–3 have enabled the fabrication of mirrors with accuracy down to the 1-nm level. Consequently, X-ray mirrors capable of focusing down to 50~30 nm with Kirkpatrick–Baez (KB) geometry have been established. However, achieving sub-10 nm focusing at an X-ray free-electron laser (XFEL) source remains a significant challenge. The inherent comatic aberration of KB geometry and unavoidable pointing/angular jitter of the source have noticeably degraded the focusing condition. Although previous demonstrations of X-ray nanofocusing utilized a secondary source slit to precisely define the source position, this approach led to a significant loss of photons, thereby compromising the high-peak-brilliance properties of XFELs. In this study, we utilized advanced KB (AKB) mirrors for achieving sub-10 nm focusing of XFEL beams. The AKB mirrors, which comprise one-dimensional Wolter mirrors, reduce the comatic aberration by satisfying Abbe’s sine condition, ensuring highly stable nanofocusing with wide tolerance to incident angle errors. Based upon the know-how of the mirror characteristics and mirror-tuning strategies, amassed in the past development of full-field imaging 4,5, we designed multilayer-coated advanced KB nanofocusing mirrors based on Wolter-type III geometry6. The mirrors have been fabricated by a wavefront correction method 7,8 and achieved an accuracy of less than λ/15 in root-mean-square. Consequently, the ultra-intense XFEL sub-10 nm focusing system without the secondary source slits has been established at SACLA. The achieved focus, evaluated by the ptychography, indicated the spot size of 7 × 7 nm², corresponding to an XFEL intensity of 1.45 × 1022 W/cm², representing the highest XFEL intensity ever recorded9. This high-intensity XFEL has been recently zapplied to research in atomic, molecular, and optical (AMO) physics, as well as nonlinear X-ray science. In the presentation, we will show the results of AKB mirror developments, especially the design, fabrication, and implementation of the sub-10 nm XFEL focusing mirrors, along with showcasing the latest experimental results. Refrences 1. Yamauchi, K. et al., (2003). Figuring with subnanometer level accuracy by numerically controlled elastic emission machining. Rev. Sci. Instrum., 73 4028–40332. Yamauchi, K. et al., (2003). Microstitching interferometry for X-ray reflective optics. Rev. Sci. Instrum., 74 2894– 28983. Mimura, H. et al., (2005). Relative angle determinable stitching interferometry for hard x-ray reflective optics. Rev. Sci. Instrum., 76 0451024. Matsuyama, S. et al., (2017). 50-nm-resolution full-field X-ray microscope without chromatic aberration using total-reflection imaging mirrors. Sci. Rep., 7 463585. Yamada, J. et al., (2020). Compact full-field hard x-ray microscope based on advanced Kirkpatrick–Baez mirrors. Optica, 7 367-3706. Yamada, J. et al., (2019). Compact reflective imaging optics in hard X-ray region based on concave and convex mirrors. Opt. Express, 27 3429-343787. Matsuyama, S. et al., (2018). Nanofocusing of X-ray free-electron laser using wavefront-corrected multilayer focusing mirrors. Sci. Rep., 8 174408. Yamada, J. et al., (2020). X-Ray Single-Grating Interferometry for Wavefront Measurement and Correction of Hard X-Ray Nanofocusing Mirrors. Sensors, 20 73569. Yamada, J. et al., (2024). Extreme focusing of hard X-ray free-electron laser pulses enables 7 nm focus width and 1022 W cm−2 intensity. Nat. Photon. in-press (published online)
Jumpei Yamada (Tue,) studied this question.