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

By illuminating matter with bright and intense light, researchers gain insights into material composition and properties. 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 single-nanometre focal spot that fully exploits the source performance remains elusive. Aberrations arising from reflective optical schemes noticeably degrade the focal spot, even in the presence of inevitably slight angular transition and pointing errors. Here we present an approach that directly forms a source image in an extremely small focal spot, achieving 7 nm focusing, in both transverse dimensions, of 9.1 keV XFELs with the extremely high intensity of 1.45 x 10(22) W cm(-)(2). This was made possible by a scheme combining concave and convex X-ray mirrors with suppressed aberrations and high angular tolerances. The attained highly intense X-rays, 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 demonstrate the realization of stable ultraintense XFEL beams by forming demagnified source images, hold immediate significance to a wide range of research fields, including atomic, molecular and optical physics and high-energy-density sciences.