Yuichi Inubushi

Understanding ultrafast ionization dynamics in high-density plasmas is essential for applications such as particle acceleration, short-pulse x-ray generation, and laser fusion. However, resolving ionization states inside optically opaque plasmas with femtosecond resolution remains challenging. Using femtosecond-resolved x-ray absorption spectroscopy, we directly measured inner-shell vacancies, overall charge states, and the nonthermal electron population responsible for ionization in solid-density copper plasma. In copper plasma generated by a 40-fs relativistic laser pulse, ionization proceeds within 0.5 ps. M- and outer-shell ionization persist for over 10 ps, whereas L-shell vacancies decay within 4 ps. The population of electrons capable of L-shell ionization decreases in picoseconds, in agreement with particle-in-cell simulations incorporating atomic processes. Our spectroscopic approach provides direct experimental insights into ultrafast ionization dynamics in high-density plasmas, supporting improvements in the modeling of laser-driven plasmas.

Extreme pressures and temperatures create conditions that allow even hard and brittle materials to flow plastically. Despite extensive research, the limits of flow strength under such conditions remain uncertain, and the mechanisms driving deformation at the relevant stresses are a subject of debate. Using femtosecond in situ x-ray diffraction experiments and large-scale molecular dynamics simulations, we demonstrate that stacking fault-mediated strengthening enables shock-compressed nanopolycrystalline diamond to achieve a peak flow strength of 92 ± 3 GPa at a stress of 212 ± 6 GPa. Our findings show that extreme conditions can unlock ultrahigh strength via a complex array of competing deformation mechanisms and thermodynamic effects.

A background-free intensity autocorrelation measurement for 10-keV femtosecond x-ray pulses is demonstrated using x-ray second harmonic generation. The narrow phase-matching condition of x-ray second harmonic generation allows for the selective generation of either the autocorrelation or background signals in a quasicollinear geometry with a crossing angle of ∼300 μrad, achieving a geometric time resolution of ∼1 attosecond. The measured autocorrelation traces exhibit zero background and clearly reveal temporal modifications through an Si(311) monochromator, verifying x-ray temporal shaping with perfect crystals. The applicability of this method to attosecond x-ray pulses is also discussed.

We have developed a compact tape drive (CoT) with on-demand sample delivery for time-resolved serial femtosecond crystallography (SFX) experiments, which can deliver sample droplets and/or initiate reactions with a drop-on-drop strategy. Two disposable piezoelectric injectors are positioned in tandem along the tape to produce a queue of nanolitre-scale droplets. X-ray free-electron laser pulses arrive perpendicular to and pass through the broad face of the tape. The pulse is synchronized and aligned to the droplets, thereby enabling highly efficient SFX data collection. The tape transport speed and the delivery distance can be varied to control the mixing time from approximately 130 ms to tens of seconds. We conducted time-resolved SFX experiments utilizing a basic enzymatic reaction model of hen egg white lysozyme (HEWL) and N -acetyl- D -glucosamine (GlcNAc) to demonstrate the drop-on-drop capabilities of the CoT, and the full binding process of GlcNAc to HEWL was observed at 1.3–9.7 s.

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.

Nanowire arrays are excellent nanostructured target materials for high-energy density (HED) plasma studies and applications because of their enhanced energy absorption properties. However, investigations of the spatiotemporal dynamics of laser-irradiated nanowire arrays remain limited, since conventional time-resolved diagnostics such as X-ray streak cameras and flash radiography cannot readily capture the rapid plasma-state transitions within these structures. This study reports spatiotemporally resolved measurements of laser energy absorption and electron transport in laser-irradiated nanowire arrays using an X-ray free-electron laser (XFEL). Pump–probe shadowgraph measurements were performed with an XFEL, achieving a temporal resolution of 100 fs and spatial resolution of 3 µm. Instantaneous heating of the nanowire array was observed at the onset of laser irradiation, followed by a significant temperature increase due to wire collapse. The X-ray shadowgraph results confirmed that further enlargement of the heated area by laser irradiation was suppressed by the restricted electron transport in the array. These observations advance our understanding of HED plasma formation and evolution within the laser-irradiated nanowire arrays, laying a foundation for various applications that employ such arrays.

The martensitic transformation, which influences the mechanical properties of metals, is essential for ensuring high strength in steel. Workability is also improved by the formation of ultrafine grains using this reverse transformation. This study evaluated the reverse transformation kinetics during cyclic ultrafast heating and cooling using femtosecond X-ray diffraction to measure dislocation densities in 50% rolled Fe–0.1 mass% C–2.0 mass% Mn martensitic steel. We also developed a unique cyclic ultrafast heating and cooling system to determine the reverse transformation mechanism from martensite (α΄) to austenite (γ). The maximum heating and cooling rates achieved were 1.2 × 10⁴ °C s⁻¹ and 4.0 × 10³ °C s⁻¹, respectively, which were sufficient to avoid diffusive reversion and bainitic transformation. We then measured the dislocation density and reverse transformation during a two-cycle ultrafast heating and cooling process. The reverse transformation under ultrafast heating was massive in the first cycle, while a displacive transformation was exhibited in the second cycle. Additionally, we found that cyclic ultrafast heating and cooling results in a finer microstructure, irrespective of the martensitic transformation mode. These findings represent an advance in our understanding of functional steels.