Extreme focusing of hard X-ray free-electron laser pulses enables 7 nm focus width and 1022 W cm−2 intensity

Abstract

By illuminating matter with bright and intense light, we gain profound insights into its composition and property. In the regime of extremely short wavelengths, X-ray free-electron lasers (XFELs) with exceptional peak brilliance have unveiled crucial details about the structures, dynamics, and physics of various materials. Although X-ray focusing optics to enhance the intensity have progressed, achieving a singlenanometre focusing spot that fully exploits the source performance has remained elusive. Aberrations arising from reflective optical schemes noticeably degrade the focus, in conjunction with the inevitable angular transition and pointing errors. Here, we present an innovative approach that directly forms a source image into a tiny focusing spot, achieving 7 nm focusing of 9.1-keV XFELs with the unprecedented intensity of 1.45 × 10 22 W/cm 2 . This breakthrough was made possible by a scheme combining concave and convex X-ray mirrors with suppressed aberrations and high angular tolerances. The attained highly intense XFELs, surpassing the previous intensity by a hundred-fold, induced the vigorous ionization of chromium, suggesting the creation of solid-density heavy bare atomic nuclei. Our results, which realize stable ultraintense XFEL beams by forming demagnified source images, hold immediate significance in wide research fields, including atomic, molecular and optical physics and high-energy-density sciences.Since the discovery of X-rays by W. C. Röntgen, the development of X-ray sources has continually progressed, driven by the increasing significance of X-rays in diverse scientific fields. The advent of bright synchrotron radiation has marked a momentous milestone 1-3 , and more recently, the emergence of X-ray free-electron lasers (XFELs) 4-8 based on self-amplified spontaneous emission (SASE) 9,10 has revolutionized the growth of peak brilliance. X-rays emitted from these bright sources have been focused into small spots, thereby enhancing the resolution and sensitivity of X-ray analyses and contributing to a wide range of studies 11,12 . The state-of-the-art Xray sources possessing an ultra-low emittance, that is, small source size and low angular divergence, offer distinct advantages for generating high-intensity focusing spots. Specifically, the small source size facilitates achieving a