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We report the simultaneous lasing of two distinct K α emissions at photon energies of 7.48 keV (Ni K α 1 ) and 8.05 keV (Cu K α 1 ). This was achieved by a population inversion induced through intense X-ray free-electron laser (XFEL) irradiation of a Cu–Ni alloy foil. This demonstration of multi-color X-ray lasing using a single XFEL source is expected to contribute significantly to the future development of X-ray lasers and their applications.

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.

We present the development of a portable and compact nanofocusing system utilizing Kirkpatrick–Baez optics for X-ray free-electron lasers (XFELs). The system has a total length of merely 340 mm from the initial elliptical mirror to the focal point. With this setup, we achieve focusing capabilities reaching dimensions as small as 150 nm horizontally and 220 nm vertically, resulting in an intense beam with an intensity of 2.5 × 10 19  W cm −2 for a 10 keV XFEL with a pulse energy of 110 µJ. As a demonstration of its applicability in X-ray nonlinear optics, we successfully observed an 8.64 keV Zn K α laser by irradiating a Zn foil with the focused XFEL beam. The attained photon energy is currently the highest photon energy for amplified spontaneous emission via bound–bound transitions of electrons in the world. We anticipate that this portable and compact nanofocusing system will pave the way for new frontiers in X-ray nonlinear optics and high-energy density science, owing to its adaptability to various experimental conditions.

Since the invention of the laser, nonlinear effects such as filamentation 1 , Rabi cycling 2 , 3 and collective emission 4 have been explored in the optical regime, leading to a wide range of scientific and industrial applications 5 , 6 , 7 – 8 . X-ray free-electron lasers (XFELs) have extended many optical techniques to X-rays for their advantages of ångström-scale spatial resolution and elemental specificity 9 . An example is XFEL-driven inner-shell Kα 1 (2 p 3/2  → 1 s 1/2 ) X-ray lasing in elements ranging from neon to copper, which has been used for nonlinear spectroscopy and development of new X-ray laser sources 10 , 11 , 12 , 13 , 14 , 15 – 16 . Here we show that strong lasing effects similar to those in the optical regime can occur at 1.5–2.1 Å wavelengths during high-intensity (>10 19  W cm −2 ) XFEL-driven Kα 1 lasing of copper and manganese. Depending on the temporal XFEL pump pulse substructure, the resulting X-ray pulses (about 10 6 −10 8 photons) can exhibit strong spatial inhomogeneities and spectral splitting, inhomogeneities and broadening. Three-dimensional Maxwell–Bloch calculations 17 show that the observed spatial inhomogeneities result from X-ray filamentation and that the broad spectral features are driven by sub-femtosecond Rabi cycling. Our simulations indicate that these X-ray pulses can have pulse lengths of less than 100 attoseconds and coherence properties that provide opportunities for quantum X-ray optics applications. Strong lasing effects similar to those in the optical regime can occur at 1.5–2.1 Å wavelengths during high-intensity XFEL-driven Kα 1 lasing of copper and manganese.

X-ray free electron laser (XFEL) experiments using self-amplified spontaneous emission (SASE) pulses typically achieve temporal resolutions of order several femtoseconds, as the pulse duration puts a practical limit to pump-probe or probe-probe schemes. Even with the emerging capabilities to generate pulses with attosecond durations with new single-spike SASE schemes, direct access to attosecond electron dynamics remains an experimental challenge. Here we show how X-ray coherent attosecond pulse-pair spectroscopy (X-CAPPS) provides a powerful new approach to access the ultrashort time-delay window. Coherent attosecond pulse pairs with time delays varying from ~500 as to ~5 fs are generated with Cu K\\(_1\\) stimulated X-ray emission from a gain medium pumped by intense SASE XFEL pulses. These pulse pairs are analyzed with two subsequent Bragg crystal spectrometers, and the resulting interference spectrum is captured on two sequential 2D image detectors encoding their time separation, relative amplitudes, and phases with high precision. X-CAPPS requires no split-and-delay X-ray optics, nor XFEL pulse modifications, making it broadly implementable across existing facilities. This technique enables the investigation of attosecond processes with \\(A\\)ngstr\"om resolution, providing a new tool for probing ultrafast dynamics across a wide range of atomic, molecular, and solid systems.

We have developed X-ray telescope mirrors using an original electroforming replication technique established through the fabrication of millimeter-aperture, ultra-short-focal-length nanofocusing mirrors for synchrotron X-ray microscopy. This paper presents detailed results of X-ray illumination tests of a 60-mm-diameter, full-circumference, double-reflection monolithic electroformed nickel mirror and its Mirror Module Assembly (MMA). The experiments were conducted at the 1-km beamline BL29XUL at SPring-8. To simulate a parallel X-ray beam from celestial sources, we constructed a dedicated evaluation system, the High-Brilliance X-ray Kilometer-long Large-Area Expanded-beam Evaluation System (HBX-KLAEES). Owing to the high photon flux and the quasi-point-like source with a small divergence provided by HBX-KLAEES, the imaging performance was evaluated with high fidelity, resolving both the sharp core and large-angle components of the Point Spread Function (PSF). The results show an extremely sharp core with a Full Width at Half Maximum (FWHM) of 0.7 arcsec and a Half Power Diameter (HPD) of 14 arcsec, even after integration into the MMA. In addition, a positive correlation was found between angular resolution and axial figure error in both the primary and secondary mirror sections, indicating that axial figure errors contribute to image degradation. Based on these results, the MMA was selected as one of the hard X-ray optics for the FOXSI-4 sounding rocket experiment, which performs high-resolution soft and hard X-ray imaging spectroscopy of solar flares and was successfully launched. These results demonstrate the potential for further improvements in angular resolution and the development of high-resolution, ultra-short focal length X-ray optics for small satellites, including CubeSats.

We present the first direct experimental confirmation of attosecond pulse generation in the hard X-ray regime with a free-electron laser. Our experiment is based on measurements of a nonlinear optical phenomenon known as amplified spontaneous emission (ASE) from 3d transition metals. By analyzing the yield of the collective X-ray fluorescence induced by ultrashort pulses at the Linac Coherent Light Source, we identify the generation of attosecond pulses and shot-to-shot fluctuations in their duration, ranging from 100 as to 400 as. The observed product of bandwidth and pulse duration for 100 as pulses is approximately 2 fs\\(\\)eV, indicating the generation of nearly transform-limited pulses. Our results extend the photon energy reach of attosecond techniques by one order of magnitude, providing the ability to simultaneously probe matter on the time-scales of electronic phenomena and with atomic spatial resolution. Furthermore, attosecond hard X-ray pulses can outrun the fastest radiation damage processes, paving the way to single-shot damage-free X-ray measurements.

Since the invention of the laser nonlinear effects such as filamentation, Rabi-cycling and collective emission have been explored in the optical regime leading to a wide range of scientific and industrial applications. X-ray free electron lasers (XFELs) have led to the extension of many optical techniques to X-rays for their advantages of angstrom scale spatial resolution and elemental specificity. One such example is XFEL driven population inversion of 1s core hole states resulting in inner-shell K\\(\\) (2p to 1s) X-ray lasing in elements ranging from neon to copper, which has been utilized for nonlinear spectroscopy and development of next generation X-ray laser sources. Here we show that strong lasing effects, similar to those observed in the optical regime, can occur at 1.5 to 2.1 angstrom wavelengths during high intensity (> \\(10^19\\) W/cm\\(^2\\)) XFEL driven inner-shell lasing and superfluorescence of copper and manganese. Depending on the temporal substructure of the XFEL pump pulses(containing \\(~10^6\\) - \\(10^8\\) photons) i, the resulting inner-shell X-ray laser pulses can exhibit strong spatial inhomogeneities as well as spectral splitting, inhomogeneities and broadening. Through 3D Maxwell Bloch theory we show that the observed spatial inhomogeneities result from X-ray filamentation, and that the spectral splitting and broadening is driven by Rabi cycling with sub-femtosecond periods. Our simulations indicate that these X-ray pulses can have pulse lengths of less than 100 attoseconds and coherence properties that open the door for quantum X-ray optics applications.

Since the invention of the laser nonlinear effects such as filamentation, Rabi-cycling and collective emission have been explored in the optical regime leading to a wide range of scientific and industrial applications. X-ray free electron lasers (XFELs) have led to the extension of many optical techniques to X-rays for their advantages of angstrom scale spatial resolution and elemental specificity. One such example is XFEL driven population inversion of 1s core hole states resulting in inner-shell K\\(\\) (2p to 1s) X-ray lasing in elements ranging from neon to copper, which has been utilized for nonlinear spectroscopy and development of next generation X-ray laser sources. Here we show that strong lasing effects, similar to those observed in the optical regime, can occur at 1.5 to 2.1 angstrom wavelengths during high intensity (> \\(10^19\\) W/cm\\(^2\\)) XFEL driven inner-shell lasing and superfluorescence of copper and manganese. Depending on the temporal substructure of the XFEL pump pulses, the resulting inner-shell X-ray laser pulses can exhibit strong spatial inhomogeneities as well as spectral inhomogeneities and broadening. Through 3D Maxwell Bloch theory we show that the observed spatial inhomogeneities result from X-ray filamentation, and that the spectral broadening is driven by Rabi cycling with sub-femtosecond periods. These findings indicate that we have generated Angstrom-wavelength x-ray pulses (containing \\(10^6\\) - \\(10^8\\) photons) in the strong lasing regime, some of them with pulse lengths of less than 100 attoseconds.

Thermal transpiration flow, a flow from cold to hot, driven by a temperature gradient along a wall under a high Knudsen number condition, was studied using the molecular dynamics method with a two-dimensional channel consisting of infinite parallel plates with nanoscale clearance based on our previous study. To accelerate the numerical analysis, a dense gas was employed in our previous study. In this study, the influence of the number density of gas was investigated by varying the height of the channel while keeping the number of molecules to achieve the flow ranging from dense to dilute gas while maintaining a constant Knudsen number. From the flow velocity profile compared to the number density profile, the thermal transpiration flow was observed for all number density conditions from dense to dilute gas. A similar flow structure was exhibited regardless of the number density. Thus, the numerical analysis in a dense gas condition is considered to be valid and useful for analyzing the thermal transpiration flow.