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High-intensity, short-pulse lasers are crucial for generating energetic electrons that produce high-energy-density (HED) states in matter, offering potential applications in igniting dense fusion fuels for fast ignition laser fusion. High-density targets heated by these electrons exhibit spatially non-uniform and highly transient conditions, which have been challenging to characterize due to limitations in diagnostics that provide simultaneous high spatial and temporal resolution. Here, we employ an X-ray Free Electron Laser (XFEL) to achieve spatiotemporally resolved measurements at sub-micron and femtosecond scales on a solid-density copper foil heated by laser-driven fast electrons. Our X-ray transmission imaging reveals the formation of a solid-density hot plasma localized to the laser spot size, surrounded by Fermi degenerate, warm dense matter within a picosecond, and the energy relaxation occurring within the hot plasma over tens of picoseconds. These results validate 2D particle-in-cell simulations incorporating atomic processes and provide insights into the energy transfer mechanisms beyond current simulation capabilities. This work significantly advances our understanding of rapid fast electron heating and energy relaxation in solid-density matter, serving as a key stepping stone towards efficient high-density plasma heating and furthering the fields of HED science and inertial fusion energy research using intense, short-pulse lasers. Intense, short-pulse laser irradiation generates energetic electrons that heat targets to extreme conditions relevant to laser fusion. Here, authors used an X-ray Free Electron Laser to perform spatiotemporal measurements in solid-density copper foil with sub-micron and femtosecond resolutions.

Two-photon Rabi oscillations are observed in He on a timescale of 10 fs by utilizing the strong-field phenomenon known as Freeman resonance. The importance of ponderomotive shifts of both the Rydberg states and the ionization limit is highlighted. Coherent light–matter interaction provides powerful methods for manipulating quantum systems 1 , 2 , 3 . Rabi oscillation is one such process. As it enables complete population transfer to a target state, it is thus routinely exploited in a variety of applications in photonics, notably quantum information processing 4 , 5 . The extension of coherent control techniques to the multiphoton regime offers wider applicability, and access to highly excited or dipole-forbidden transition states. However, the multiphoton Rabi process is often disrupted by other competing nonlinear effects such as the a.c. Stark shift, especially at the high laser-field intensities necessary to achieve ultrafast Rabi oscillations 6 . Here we demonstrate a new route to drive two-photon Rabi oscillations on timescales as short as tens of femtoseconds, by utilizing the strong-field phenomenon known as Freeman resonance 7 . The scenario is not specific to atomic helium as investigated in the present study, but broadly applicable to other systems, thus opening new prospects for the ultrafast manipulation of Rydberg states 8 .

Turbulence is ubiquitous in the universe and in fluid dynamics. It influences a wide range of high energy density systems, from inertial confinement fusion to astrophysical-object evolution. Understanding this phenomenon is crucial, however, due to limitations in experimental and numerical methods in plasma systems, a complete description of the turbulent spectrum is still lacking. Here, we present the measurement of a turbulent spectrum down to micron scale in a laser-plasma experiment. We use an experimental platform, which couples a high power optical laser, an x-ray free-electron laser and a lithium fluoride crystal, to study the dynamics of a plasma flow with micrometric resolution (~1μm) over a large field of view (>1 mm 2 ). After the evolution of a Rayleigh–Taylor unstable system, we obtain spectra, which are overall consistent with existing turbulent theory, but present unexpected features. This work paves the way towards a better understanding of numerous systems, as it allows the direct comparison of experimental results, theory and numerical simulations. Turbulence effects explored use macroscale systems in general. Here the authors generate a turbulent plasma using laser irradiation of a solid target and study the dynamics of the plasma flow at the micron-scale by using scattering of an XFEL beam.

High resolution X-ray imaging is crucial for many high energy density physics (HEDP) experiments. Recently developed techniques to improve resolution have, however, come at the cost of a decreased field of view. In this paper, an innovative experimental detector for X-ray imaging in the context of HEDP experiments with high spatial resolution, as well as a large field of view, is presented. The platform is based on coupling an X-ray backligther source with a Lithium Fluoride detector, characterized by its large dynamic range. A spatial resolution of 2 µm over a field of view greater than 2 mm 2 is reported. The platform was benchmarked with both an X-ray free electron laser (XFEL) and an X-ray source produced by a short pulse laser. First, using a non-coherent short pulse laser-produced backlighter, reduced penumbra blurring, as a result of the large size of the X-ray source, is shown. Secondly, we demonstrate phase contrast imaging with a fully coherent monochromatic XFEL beam. Modeling of the absorption and phase contrast transmission of X-ray radiation passing through various targets is presented.

The dynamics of the magnetic correlations after photo-doping Sr 2 IrO 4 are studied through ultrafast time-resolved resonant inelastic X-ray spectroscopy. The timescales are found to depend on the dimensionality of the correlations. Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity 1 , 2 , 3 , 4 . Recently, photo-excitation has been used to induce similarly exotic states transiently 5 , 6 , 7 . However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr 2 IrO 4 . We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.

Time-resolved hard x-ray photoelectron spectroscopy (trHAXPES) is established using the x-ray free-electron laser SACLA. The technique extends time-resolved photoemission into the hard x-ray regime and, as a core-level spectroscopy, combines element and atomic-site specificity and sensitivity to the chemical environment with femtosecond time resolution and bulk (sub-surface) sensitivity. The viability of trHAXPES using 8 keV x-ray free-electron-laser radiation is demonstrated by a systematic investigation of probe and pump pulse-induced vacuum space-charge effects on the V 1s emission of VO2 and the Ti 1s emission of SrTiO3. The time and excitation energy dependencies of the measured spectral shifts and broadenings are compared to the results of N-body numerical simulations and simple analytic (mean-field) models. Good agreement between the experimental and calculated results is obtained. In particular, the characteristic temporal evolution of the pump pulse-induced spectral shift is shown to provide an effective means to determine the temporal overlap of pump and probe pulses. trHAXPES opens a new avenue in the study of ultrafast atomic-site specific electron and chemical dynamics in materials and at buried interfaces.

The origin of superconductivity in the iron pnictides has been attributed to antiferromagnetic spin ordering that occurs in close combination with a structural transition, but there are also proposals that link superconductivity to orbital ordering. We used bulk-sensitive laser angle—resolved photoemission spectroscopy on BaFe₂(As 0.65 P 0.35 )₂ and Ba 0.6 K 0.4 Fe₂As₂ to elucidate the role of orbital degrees of freedom on the electron-pairing mechanism. In strong contrast to previous studies, an orbital-independent superconducting gap magnitude was found for the hole Fermi surfaces. Our result is not expected from the superconductivity associated with spin fluctuations and nesting, but it could be better explained invoking magnetism-induced interorbital pairing, orbital fluctuations, or a combination of orbital and spin fluctuations. Regardless of the interpretation, our results impose severe constraints on theories of iron pnictides.

Investigation of the non-equilibrium dynamics after an impulsive impact provides insights into couplings among various excitations. A two-temperature model (TTM) is often a starting point to understand the coupled dynamics of electrons and lattice vibrations: the optical pulse primarily raises the electronic temperature T el while leaving the lattice temperature T l low; subsequently the hot electrons heat up the lattice until T el = T l is reached. This temporal hierarchy owes to the assumption that the electron-electron scattering rate is much larger than the electron-phonon scattering rate. We report herein that the TTM scheme is seriously invalidated in semimetal graphite. Time-resolved photoemission spectroscopy (TrPES) of graphite reveals that fingerprints of coupled optical phonons (COPs) occur from the initial moments where T el is still not definable. Our study shows that ultrafast-and-efficient phonon generations occur beyond the TTM scheme, presumably associated to the long duration of the non-thermal electrons in graphite.

We report thermomagnetic properties in magnetically frustrated A-site spinel magnets of CoAl2O4, CoRh2O4, and cluster glass CoGa2O4 to comprehensively show the proximity effects of the Néel-to-spin-spiral (NSS) transition, which is predicted theoretically to occur in a magnetically frustrated A-site spinel antiferromagnet. The low-temperature magnetic phase remains controversial since the magnetic state of CoAl2O4 is in the vicinity of the NSS transition derived by the inherent magnetic frustrated interaction for an A-site spinel magnet and is also considerably sensitive to crystallographic disorder. The antiferromagnetic and spin-glass transitions were detected by measuring the direct-current and alternating-current susceptibilities and also thermoremanent magnetization (TRM) developing below the magnetic transitions at the Néel point (TN) and spin-glass transition temperature (TSG). The relaxation rate and the temperature derivative of TRM were both enhanced at TN and TSG, and they decayed rapidly above and below the transitions. We succeeded in extracting the relaxation time τ and other characteristic parameters from the isothermal relaxation of TRM, which was well fitted with a non-exponential relaxation form formulated by Weron based on purely stochastic theory in order to describe the originally dielectric relaxation. For the typical CoRh2O4 normal spinel (inversion-free A site) antiferromagnet and the CoGa2O4 random spinel spin-glass, the temperature variations of these parameters can distinguish the magnetic states. In contrast to the cases for CoRh2O4 and CoGa2O4, an enhancement of the relaxation rate of TRM for CoAl2O4 is indicated at low temperatures, which is probably related to the suppression of long-range antiferromagnetic order revealed by neutron diffraction studies.

The quantum self-organization is introduced as one of the major underlying mechanisms of the quantum many-body systems. In the case of atomic nuclei as an example, two types of the motion of nucleons, single-particle states and collective modes, dominate the structure of the nucleus. The outcome of the collective mode is determined basically by the balance between the effect of the mode-driving force (e.g., quadrupole force for the ellipsoidal deformation) and the resistance power against it. The single-particle energies are one of the sources to produce such resistance power: a coherent collective motion is more hindered by larger gaps between relevant single particle states. Thus, the single-particle state and the collective mode are \"enemies\" each other. However, the nuclear forces are demonstrated to be rich enough so as to enhance relevant collective mode by reducing the resistance power by changing singleparticle energies for each eigenstate through monopole interactions. This will be verified with the concrete example taken from Zr isotopes. Thus, when the quantum self-organization occurs, single-particle energies can be self-organized, being enhanced by (i) two quantum liquids, e.g., protons and neutrons, (ii) two major force components, e.g., quadrupole interaction (to drive collective mode) and monopole interaction (to control resistance). In other words, atomic nuclei are not necessarily like simple rigid vases containing almost free nucleons, in contrast to the naïve Fermi liquid picture. Type II shell evolution is considered to be a simple visible case involving excitations across a (sub)magic gap. The quantum self-organization becomes more important in heavier nuclei where the number of active orbits and the number of active nucleons are larger. The quantum self-organization is a general phenomenon, and is expected to be found in other quantum systems.