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X-ray free-electron lasers (XFELs)1,2 are widely operated on the basis of self-amplified spontaneous emission (SASE)3,4, where spontaneous radiation from the electron beam is amplified along the magnetic field in undulators. Despite their high intensities, SASE-XFELs have a broad spectrum due to the stochastic starting-up process5. To narrow the bandwidth, self-seeding has been proposed6,7 and recently demonstrated8,9, where the seed pulse produced by monochromatizing the SASE-XFELs from the first section of undulators using a thin crystal in transmission geometry is amplified in the remaining undulators. Here, we present an efficient self-seeding scheme using the Bragg reflection to produce a seed pulse. We applied this scheme to SPring-8 Angstrom Compact free-electron LAser (SACLA)10, and produced nearly Fourier-transform-limited XFEL pulses that correspond to an increase in spectral brightness by a factor of six compared with SASE-XFELs. This achievement will not only enhance the throughput of present XFEL experiments but also should open new opportunities for X-ray science.A nearly Fourier-limited X-ray free-electron laser beam is generated by a self-seeding scheme. The beam in the first half of the undulators is monochromatized via Bragg reflection, and is subsequently amplified in the remaining undulators.

This study proposes and demonstrates a simple method for improving the energy resolution in a single‐shot X‐ray spectrometer, which consists of a focusing mirror and a single‐crystal analyzer. Two Si(220) channel‐cut crystals arranged in a non‐dispersive geometry are employed as the analyzer. The angular width of diffraction for the multi‐crystal analyzer is reduced by detuning one of the crystals, thereby enhancing the energy resolution of the spectrometer while maintaining the energy range. A proof‐of‐principle experiment with 10.4 keV X‐rays clearly shows a resolution enhancement by a factor of two. It was found that X‐ray penetration within the crystals broadened the point‐spread function on the detector, significantly impacting the energy resolution under highly detuned conditions. A long detector distance of greater than 14 m is expected to achieve a high energy resolution of 100 meV and a range of 80 eV, enabling full spectral characterization of X‐ray free‐electron laser radiation as well as advanced spectroscopy techniques. Resolution enhancement on a single‐shot X‐ray spectrometer with a detuned non‐dispersive multi‐crystal analyzer is proposed and demonstrated, indicating the promising potential for capturing the full spectral information, including the fine‐spike structure in self‐amplified spontaneous emission X‐ray free‐electron laser radiation.

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 × 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. Researchers focused hard X-rays from a free-electron laser down to transverse dimensions of ~7 nm × 7 nm, enabling a two-order increase in intensity of photons and yielding access to the elusive 10 22  W cm −2 regime. Such intense, short-wavelength electromagnetic radiation may probe atomic, molecular and optical physics with extremely high resolution.

An automated alignment procedure, based on wavefront measurement with a single‐grating interferometer, has been developed for precise tuning of Kirkpatrick–Baez nanofocusing mirrors for X‐ray free‐electron lasers (XFELs). This approach optimizes focus size and maximizes peak intensity while minimizing aberrations. Wavefront errors are quantitatively correlated with alignment deviations – incidence angle, perpendicularity and astigmatism – via Legendre polynomial analysis. These errors are subsequently corrected through a straightforward optimization process. Implemented at the SPring‐8 Angstrom Compact Free‐Electron Laser (SACLA), the system consistently achieves a reproducible XFEL focus below 150 nm × 200 nm within 10 min. Routine operation at SACLA demonstrates the reliability and efficacy of this method, enabling rapid restoration of optimal nanofocusing conditions. An automated alignment system for nanofocusing Kirkpatrick–Baez mirrors has been developed at SACLA. A wavefront optimization technique routinely provides nanofocused XFEL beams.

High sensitivity of the Kβ fluorescence spectrum to electronic state is widely used to investigate spin and oxidation state of first-row transition-metal compounds. However, the complex electronic structure results in overlapping spectral features, and the interpretation may be hampered by ambiguity in resolving the spectrum into components representing different electronic states. Here, we tackle this difficulty with a nonlinear resonant inelastic X-ray scattering (RIXS) scheme, where we leverage sequential two-photon absorption to realize an inverse process of the Kβ emission, and measure the successive Kα emission. The nonlinear RIXS reveals two-dimensional (2D) Kβ-Kα fluorescence spectrum of copper metal, leading to better understanding of the spectral feature. We isolate 3 d -related satellite peaks in the 2D spectrum, and find good agreement with our multiplet ligand field calculation. Our work not only advances the fluorescence spectroscopy, but opens the door to extend RIXS into the nonlinear regime. X-ray fluorescence spectroscopy is a powerful tool to investigate atomic properties. Here the authors report a two-dimensional fluorescence spectrum of copper metal using X-ray nonlinear scattering and find two-hole satellite feature resulting from atomic transitions.

A copper target is used to achieve an atomic laser in the hard-X-ray regime with strong amplified spontaneous coherent emission at a wavelength ten times shorter than previous lasers have achieved. Copper boost to an atomic laser Generating coherent X-rays with short-wavelength lasers has been a long-standing goal in X-ray science. Previously, an atomic laser based on neon atoms and pumped by an X-ray free-electron laser had been developed for soft X-rays. Hitoki Yoneda et al . use a solid copper target to achieve an atomic laser in the hard X-ray regime, at 1.54 Å. The target is ionized by SACLA, the SPring-8 Angstrom Compact Free Electron Laser, to achieve strong amplified spontaneous emission. The resulting atomic laser generates an X-ray beam that is superior to the pumping X-ray free-electron laser pulse. Its wavelength is almost ten times shorter than previously reported and will open many opportunities for ultrafast X-ray spectroscopy and quantum optics. Since the invention of the first lasers in the visible-light region, research has aimed to produce short-wavelength lasers that generate coherent X-rays 1 , 2 ; the shorter the wavelength, the better the imaging resolution of the laser and the shorter the pulse duration, leading to better temporal resolution in probe measurements. Recently, free-electron lasers based on self-amplified spontaneous emission 3 , 4 have made it possible to generate a hard-X-ray laser (that is, the photon energy is of the order of ten kiloelectronvolts) in an ångström-wavelength regime 5 , 6 , enabling advances in fields from ultrafast X-ray spectrosopy to X-ray quantum optics. An atomic laser based on neon atoms and pumped by a soft-X-ray (that is, a photon energy of less than one kiloelectronvolt) free-electron laser has been achieved at a wavelength of 14 nanometres 7 . Here, we use a copper target and report a hard-X-ray inner-shell atomic laser operating at a wavelength of 1.5 ångströms. X-ray free-electron laser pulses with an intensity of about 10 19 watts per square centimetre 7 , 8 tuned to the copper K-absorption edge produced sufficient population inversion to generate strong amplified spontaneous emission on the copper Kα lines. Furthermore, we operated the X-ray free-electron laser source in a two-colour mode 9 , with one colour tuned for pumping and the other for the seed (starting) light for the laser.

In 1913, Maurice de Broglie discovered the presence of X-ray absorption bands of silver and bromine in photographic emulsion. Over the following century, X-ray absorption spectroscopy was established as a standard basis for element analysis, and further applied to advanced investigation of the structures and electronic states of complex materials. Here we show the first observation of an X-ray-induced change of absorption spectra of the iron K-edge for 7.1-keV ultra-brilliant X-ray free-electron laser pulses with an extreme intensity of 10 20  W cm −2 . The highly excited state yields a shift of the absorption edge and an increase of transparency by a factor of 10 with an improvement of the phase front of the transmitted X-rays. This finding, the saturable absorption of hard X-rays, opens a promising path for future innovations of X-ray science by enabling novel attosecond active optics, such as lasing and dynamical spatiotemporal control of X-rays. Saturable absorption is a widely used process in optical-wavelength laser technologies that arises when the transmittance of a material increases upon high-intensity light illumination. Here, Yoneda et al. tightly focus free-electron laser light and demonstrate hard X-ray saturable absorption in iron.

The success 1 , 2 of X-ray free-electron lasers (XFELs) has extended the frontier of nonlinear optics into the hard X-ray region. Recently, sum-frequency generation 3 has been reported, as well as parametric downconversion 4 , 5 , 6 . These are of the lowest (second) order, and higher-order processes remain unexplored. Here, we report the first observation of a third-order process: two-photon absorption of a 5.6 keV XFEL beam by germanium. We find that two-photon absorption competes with single and sequential multiphoton processes 7 , 8 , but we successfully determine the intrinsic cross-section by analysing the pulse-energy dependence. We also discuss the two-photon absorption cross-section by comparing a new mechanism unique to X-rays with the conventional mechanism and show that the latter is consistent with the present result. The experimental determination and understanding of the cross-section would allow two-photon absorption spectroscopy. Our result indicates that X-ray analogues of other third-order nonlinear optical processes 9 , such as nonlinear Raman and optical Kerr effects, are available for XFEL applications in spectroscopy, imaging and beam control. The first observation of a third-order process induced by an X-ray beam from a free-electron laser is realized in germanium using a 5.6-keV X-ray beam. Two-photon absorption is confirmed, suggesting that X-ray analogues of other third-order nonlinear processes may be available for exploitation in X-ray experiments.

This study evaluates phase transformation kinetics under ultrafast cooling using femtosecond X-ray diffraction for the operand measurements of the dislocation densities in Fe–0.1 mass% C–2.0 mass% Mn martensitic steel. To identify the phase transformation mechanism from austenite (γ) to martensite (α′), we used an X-ray free-electron laser and ultrafast heating and cooling techniques. A maximum cooling rate of 4.0 × 10 3  °C s –1 was achieved using a gas spraying technique, which is applied immediately after ultrafast heating of the sample to 1200 °C at a rate of 1.2 × 10 4  °C s –1 . The cooling rate was sufficient to avoid bainitic transformation, and the transformation during ultrafast cooling was successfully observed. Our results showed that the cooling rate affected the dislocation density of the γ phase at high temperatures, resulting in the formation of a retained γ owing to ultrafast cooling. It was discovered that Fe–0.1 mass% C–2.0 mass% Mn martensitic steels may be in an intermediate phase during the phase transformation from face-centered-cubic γ to body-centered-cubic α′ during ultrafast cooling and that lattice softening occurred in carbon steel immediately above the martensitic-transformation starting temperature. These findings will be beneficial in the study, development, and industrial utilization of functional steels.