Ultrashort X-ray Sources Research Articles

A fully dynamical description of time-resolved resonant inelastic X-ray scattering of pyrazine.

Recent advancements in ultrashort and intense X-ray sources have enabled the utilisation of resonant inelastic X-ray scattering (RIXS) as a probing technique for monitoring photoinduced dynamics in molecular systems. To account for dynamic phenomena like non-adiabatic transitions across the relevant electronic state manifold, a time-dependent framework is crucial. Here, we introduce a fully time-dependent approach for calculating transient RIXS spectra using wavepacket dynamics simulations, alongside an explicit treatment of the X-ray probe pulse that surpasses Kramers-Heisenberg-Dirac constraints. Our analysis of pyrazine at the nitrogen K-edge underscores the importance of considering nuclear motion effects in all electronic states involved in the transient RIXS process. As a result, we propose a numerically exact approach to computationally support and predict cutting-edge time-resolved RIXS experiments.

A Highly Stable-Output Kilohertz Femtosecond Hard X-ray Pulse Source for Ultrafast X-ray Diffraction

Femtosecond hard X-ray pulses generated by laser-driven plasma sources are eminently suitable to probe structural dynamics due to the angstrom spatial resolution and sub-picosecond time resolution. However, the insufficient flux of X-ray photons and high pulse-to-pulse instability compared with the large-scale ultrashort X-ray source, such as X-ray free-electron laser and synchrotrons, largely restricts its applications. In this work, we have optimized automation control and mechanical designs to significantly enhance the reliability and photon flux in our femtosecond laser plasma-induced X-ray source. Specifically, the optimized source provides a reliable pulse-to-pulse stability with a fluctuation of less than 1% (root-mean-square) and a total flux of Cu-Kα X-ray photons above 1011 photons/s. To confirm its functionality, ultrafast X-ray diffraction experiments are conducted on two different samples and the high consistency with previous results verifies the system’s superior performance.

Generation of sub-100fs electron pulses for time-resolved electron diffraction using a direct synchronization method.

To investigate photoinduced phenomena in various materials and molecules, ultrashort pulsed x-ray and electron sources with high brightness and high repetition rates are required. The x-ray and electron's typical and de Broglie wavelengths are shorter than lattice constants of materials and molecules. Therefore, photoinduced structural dynamics on the femtosecond to picosecond timescales can be directly observed in a diffraction manner by using these pulses. This research created a tabletop ultrashort pulsed electron diffraction setup that used a femtosecond laser and electron pulse compression cavity that was directly synchronized to the microwave master oscillator (∼3GHz). A compressed electron pulse with a 1kHz repetition rate contained 228 000 electrons. The electron pulse duration was estimated to be less than 100fs at the sample position by using photoinduced immediate lattice changes in an ultrathin silicon film (50nm). The newly developed time-resolved electron diffraction setup has a pulse duration that is comparable to femtosecond laser pulse widths (35-100fs). The pulse duration, in particular, fits within the timescale of photoinduced phenomena in quantum materials. Our developed ultrafast time-resolved electron diffraction setup with a sub-100fs temporal resolution would be a powerful tool in material science with a combination of optical pump-probe, time-resolved photoemission spectroscopic, and pulsed x-ray measurements.

<i>In situ</i> observation of phase transition in polycrystalline under high-pressure high-strain-rate shock compression by X-ray diffraction

The knowledge of phase transition of material under dynamic loading is an important area of research in inertial confinement fusion and material science. Though the shock-induced phase transitions of various materials over a broad pressure range have become a field of study for decades, the loading strain rates in most of these experiments is not more than <inline-formula><tex-math id="M2">\begin{document}$ {10^{6}}\;{{\rm{s}}^{ - 1}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20200929_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20200929_M2.png"/></alternatives></inline-formula>. However, in contrast with the strain rate range where the phase diagram is a good predictor of the crystal structure of a material, at higher strain rate (<inline-formula><tex-math id="M3">\begin{document}$ > {10^{6}}\;{{\rm{s}}^{ - 1}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20200929_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20200929_M3.png"/></alternatives></inline-formula>) the phase diagram measured can be quite different not only in shifting the boundary line between various phases, but also in giving a different sequence of crystal structure. High-power laser facility can drive shock wave and simultaneously provide a precisely synchronized ultra-short and ultra-intense X-ray source. Here, based on the Prototype laser facility, an <i>in situ</i> X-ray diffraction platform for diagnosing shock-induced phase transition of polycrystalline material is established. The <i>in situ</i> observation of material phase transition under high-strain-rate shock loading is carried out with typical metals of vanadium and iron. Diffraction results are consistent with vanadium remaining in the body-centered-cubic structure up to 69 GPa, while iron transforms from the body-centered-cubic structure into hexagonal-close-packed structure at 159 GPa. The compressive properties of vanadium and iron obtained in <i>in situ</i> X-ray diffraction experiment are in good agreement with their macroscopic Hugonoit curves. The decrease in the lattice volume over the pressure step period yields a strain rate on the order of <inline-formula><tex-math id="M4">\begin{document}$ {10^{8}} - {10^{9}}\;{{\rm{s}}^{ - 1}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20200929_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20200929_M4.png"/></alternatives></inline-formula>. The available of the presented <i>in situ</i> X-ray diffraction plateform offers the potential to extend our understanding of the kinetics of phase transition in polycrystalline under high-pressure high-strain-rate shock compression.

Improvement of harmonic spectra from superposition of initial state driven by homogeneous and inhomogeneous combined field

A larger harmonic cutoff with a higher harmonic yield is a necessary requirement for producing intense ultrashort extreme ultraviolet (XUV) and X-ray sources. In this paper, we present a potential method to improve the harmonic cutoff and harmonic yield from the superposition of initial states in the He atom driven by the low-intensity homogeneous and inhomogeneous combined field. The results can be separated into two parts, that is, (i) the extension of harmonic cutoff via the optimization of three-color homogeneous and inhomogeneous field (covering the carrier envelope phase, delay time, intensity ratio, and inhomogeneous effect); and (ii) the enhancement of harmonic yield via the superposition of ground state and excited state as the initial state. Consequently, a water window harmonic plateau with an intensity enhancement of six orders of magnitude can be obtained. Moreover, the harmonic plateau is contributed by a single harmonic emission peak. Finally, through the Fourier transformation of some selected harmonics on the spectral continuum, a number of sub-50 as water window attosecond pulses with the intensity enhancement of six orders of magnitude can be obtained.

OA083] Toward 4d micro-CT and radiotherapy of small animals using all-optical, ultrashort X-ray sources: feasibility study and preliminary experimental results

Purpose All-optical, laser-driven X/gamma-ray sources, based on the primary electron acceleration via the Laser WakeField Acceleration (LWFA) process, have now entered a mature phase in terms of beam parameters, shot-to-shot stability, and reliability, so as to be considered as promising tools in medical physics. The high electron acceleration gradient results in the possibility of accelerating electrons up to the 100s MeV range over very short distances and thus opening the way to the generation of high energy photons using “table-top” machines. The PRELUDE (PREclinical Laser-based Ultrafast Diagnostics and therapy) project is ongoing, as a collaboration among different Italian Institutions aimed at studying the application of a laser-driven X-ray source for 4D micro-CT and radiotherapy of small animals. Methods As a first step, the exploitation of a Bremsstrahlung source driven by a LWFA, plasma-based accelerator is investigated, both theoretically and experimentally. The preliminary experimental results focused on characterizing the photon source, based on a 10TW ultrashort laser system for micro-CT of small animals is discussed. Issues related to both the primary electron acceleration and the generation of secondary photons via the Bremsstrahlung process will be discussed, including both numerical and experimental results. Results The X-ray spectrum, beam intensity, e- contamination and photon dosimetry have been simulated using the GEANT4 toolkit on top of experimental measurements of the e- spectrum obtained using our setup and numerical estimations of the accelerated charge per pulse. Proof-of-principle radiographic images of physical phantoms obtained with photostimulable phosphor image plates are presented. Conclusions We presented a system for small animal 4D-CT and radiotherapy based on a laser-driven X-ray source. The results of a theoretical and numerical study of a 4D micro-CT device based on a more advanced all-optical X-ray source based on Thomson scattering will be presented.

Imaging Nanometer Phase Coexistence at Defects During the Insulator-Metal Phase Transformation in VO2 Thin Films by Resonant Soft X-ray Holography.

We use resonant soft X-ray holography to image the insulator-metal phase transition in vanadium dioxide with element and polarization specificity and nanometer spatial resolution. We observe that nanoscale inhomogeneity in the film results in spatial-dependent transition pathways between the insulating and metallic states. Additional nanoscale phases form in the vicinity of defects which are not apparent in the initial or final states of the system, which would be missed in area-integrated X-ray absorption measurements. These intermediate phases are vital to understand the phase transition in VO2, and our results demonstrate how resonant imaging can be used to understand the electronic properties of phase-separated correlated materials obtained by X-ray absorption.

Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-ray Methods.

We review the tremendous advances in ultrafast X-ray science, over the past 15 years, making the best use of new ultrashort X-ray sources including table-top or large-scale facilities. Different complementary X-ray-based techniques, including spectroscopy, scattering, and diffraction, are presented. The broad and expanding spectrum of these techniques in the ultrafast time domain is delivering new insight into the dynamics of molecular systems, of solutions, of solids, and of biosystems. Probing the time evolution of the electronic and structural degrees of freedom of these systems on the time scales of femtosecond to picoseconds delivers new insight into our understanding of dynamical matter.

Experimental and numerical study of ultra-short laser-produced collimated Cu Kα X-ray source

AbstractKα X-ray sources generated from the interaction of ultra-short laser pulses with solids are compact and low-cost source of ultra-short quasi-monochromatic X-rays compared with synchrotron radiation source. Development of collimated ultra-short Kα X-ray source by the interaction of 45 fs Ti:sapphire laser pulse with Cu wire target is presented in this paper. A study of the Kα source with laser parameters such as energy and pulse duration was carried out. The observed Kα X-ray photon flux was ~2.7 × 108 photons/shot at the laser intensity of ~2.8 × 1017 W cm−2. A model was developed to analyze the observed results. The Kα radiation was coupled to a polycapillary collimator to generate a collimated low divergence (0.8 mrad) X-ray beam. Such sources are useful for time-resolved X-ray diffraction and imaging studies.

Extreme ultraviolet resonant inelastic X-ray scattering (RIXS) at a seeded free-electron laser.

In the past few years, we have been witnessing an increased interest for studying materials properties under non-equilibrium conditions. Several well established spectroscopies for experiments in the energy domain have been successfully adapted to the time domain with sub-picosecond time resolution. Here we show the realization of high resolution resonant inelastic X-ray scattering (RIXS) with a stable ultrashort X-ray source such as an externally seeded free electron laser (FEL). We have designed and constructed a RIXS experimental endstation that allowed us to successfully measure the d-d excitations in KCoF3 single crystals at the cobalt M2,3-edge at FERMI FEL (Elettra-Sincrotrone Trieste, Italy). The FEL-RIXS spectra show an excellent agreement with the ones obtained from the same samples at the MERIXS endstation of the MERLIN beamline at the Advanced Light Source storage ring (Berkeley, USA). We established experimental protocols for performing time resolved RIXS experiments at a FEL source to avoid X ray-induced sample damage, while retaining comparable acquisition time to the synchrotron based measurements. Finally, we measured and modelled the influence of the FEL mixed electromagnetic modes, also present in externally seeded FELs, and the beam transport with ~120 meV experimental resolution achieved in the presented RIXS setup.

Partially coherent ultrafast spectrography.

Modern ultrafast metrology relies on the postulate that the pulse to be measured is fully coherent, that is, that it can be completely described by its spectrum and spectral phase. However, synthesizing fully coherent pulses is not always possible in practice, especially in the domain of emerging ultrashort X-ray sources where temporal metrology is strongly needed. Here we demonstrate how frequency-resolved optical gating (FROG), the first and one of the most widespread techniques for pulse characterization, can be adapted to measure partially coherent pulses even down to the attosecond timescale. No modification of experimental apparatuses is required; only the processing of the measurement changes. To do so, we take our inspiration from other branches of physics where partial coherence is routinely dealt with, such as quantum optics and coherent diffractive imaging. This will have important and immediate applications, such as enabling the measurement of X-ray free-electron laser pulses despite timing jitter.

Electronic Structure Investigation of Highly Compressed Aluminum withKEdge Absorption Spectroscopy

The electronic structure evolution of highly compressed aluminum has been investigated using time resolved K edge x-ray absorption spectroscopy. A long laser pulse (500 ps, I(L)≈8×10(13) W/cm(2)) was used to create a uniform shock. A second ps pulse (I(L)≈10(17) W/cm(2)) generated an ultrashort broadband x-ray source near the Al K edge. The main target was designed to probe aluminum at reshocked conditions up to now unexplored (3 times the solid density and temperatures around 8 eV). The hydrodynamical conditions were obtained using rear side visible diagnostics. Data were compared to ab initio and dense plasma calculations, indicating potential improvements in either description. This comparison shows that x-ray-absorption near-edge structure measurements provide a unique capability to probe matter at these extreme conditions and severally constrains theoretical approaches currently used.

A soft X-ray approach to electron–phonon interactions beyond the Born–Oppenheimer approximation

With modern soft X-ray methods, the whole field of electron–phonon interactions becomes accessible directly in the ultrafast time domain with ultrashort pulsed X-ray sources, as well as in the energy domain through modern highly resolving spectrometers. The well-known core-hole clock approach plays an intermediate role, resolving energetic and temporal features at the same time. In this perspective paper, we review several experiments to illustrate the modern advances in the selective study of electron–phonon interactions as fundamentally determining ingredients for materials properties. We present the different complementary approaches that can be taken with soft X-ray methods to conquer this field beyond the Born–Oppenheimer approximation.

Driver linac design for a femtosecond head-on inverse Compton scatteringX-ray source

The feasibility of generating an ultra-short relativistic electron beam with a thermionic cathode rf gun driver linac for a femtosecond head-on inverse Compton scattering (ICS) X-ray source has been examined by space charge tracking throughout the entire beamline, from the cathode to the interaction point. It has been determined that GHz-repetition-rate electron pulses as short as 49 fs can be produced by compressing the energy-chirped beam from a 2998-MHz, 1.5-cell rf gun with an alpha magnet and an rf linac operating at the injection phase near the zero crossing. These electron pulses, with a 42-pC bunch charge, are accelerated to 27 MeV with an S-band traveling-wave constant-gradient linac structure for the ICS in an interaction chamber located downstream. The driver linac design that allows the operation of an ultra-short ICS X-ray source at a 0.7-Å wavelength, with a peak photon flux of 9.08×10 17 photons/s, is presented in this paper.

Quasi-monoenergetic electron beam generation by laser wakefield acceleration for all-optical ultrashort X-ray sources

Quasi-monoenergetic electron beam generation by laser wakefield acceleration for all-optical ultrashort X-ray sources

Single-Shot Diffractive Imaging with a Table-Top Femtosecond Soft X-Ray Laser-Harmonics Source

Coherent x-ray diffractive imaging is a powerful method for studies on nonperiodic structures on the nanoscale. Access to femtosecond dynamics in major physical, chemical, and biological processes requires single-shot diffraction data. Up to now, this has been limited to intense coherent pulses from a free electron laser. Here we show that laser-driven ultrashort x-ray sources offer a comparatively inexpensive alternative. We present measurements of single-shot diffraction patterns from isolated nano-objects with a single 20 fs pulse from a table-top high-harmonic x-ray laser. Images were reconstructed with a resolution of 119 nm from the single shot and 62 nm from multiple shots.

Quasi-monoenergetic Electron Acceleration for All-optical Ultrashort X-ray Source

In laser-driven plasma acceleration, which is charged particle acceleration via the interaction of an intense laser pulse with a plasma, the accelerating electric field is a thousand times higher than that of conventional radio-frequency accelerators. Such high accelerating field enables a compact accelerator. Recently, the generation of quasi-monoenergetic electron beams has been demonstrated and the way toward a practical, compact electron accelerator is opened. Furthermore, the electron pulse duration is extremely short, of the order of femtosecond in laser-driven plasma acceleration. This set of unique characteristics opens the way toward a novel, compact, all-optical, ultrashort X-ray source based on such as laser Compton scattering scheme. The present status of the quasi-monoenergetic electron acceleration and prospect for all-optical ultrashort X-ray sources are reviewed.

Physical properties of laser-electron Compton scattering

The energy properties and the differential cross-sections of laser-electron-Compton scattering are quantitatively investigated. The calculations show simple structures of the scattered photon energy and the angular distribution of the differential cross-section. At high incident electron energy, Compton scattering X-ray source has features of easily tunable photon energy and small forward emission angle. Although the dispersion of Compton scattering X-ray source is large, narrow bandwidth X-ray emission can be obtained experimentally by using specific monochromatic filter. The total cross-section and the cross-section within the forward emission angle （γ, where γ= E/m0c2 is the total electron energy （E） in the unit of its static energy （m0c2）） change slightly for different laser wave lengths and different incident electron energies in a broad range from 1 MeV to 10 GeV. They have values of 0067 mb and 0033 mb, respectively. The total cross-section of the visible lights is very small. These results may be useful for building new generation ultra-short X-ray source based on Compton scattering technique.

Ways to produce and measure atto- and femtosecond soft X-ray pulses

The ways to produce and measure atto-and femtosecond soft X-ray pulses are reported. The laser phase relation of high-order harmonic generation (HHG) shows two different radiation energy distributions in time (or laser phase) domain. These energy-phase relations are helpful for realizing the dynamic processes of HHG. Two presented parameterized formulas can be used to calculate the durations of the energy distributions with a bandwidth of the pulse. These formulas are useful in calculating and simulating pulses transports and interactions with mediums. The time structures of atto-and femtosecond soft X-ray pulses can be directly measured with photoelectron spectrum transfer equations and the related laser phase determination methods without any previous pulse shape and the instantaneous frequency assumptions. These equations and methods can be used to evaluate and improve the technical parameters of the ultra-short X-ray sources. They have wide measurement ranges and high time resolutions, which may enable ultra-fast measurements to reach metrological precisions, and lead to a new tide of scientific researches in physics, chemistry, biochemistry, etc. The application of atto-and femtosecond X-rays as well as the theoretical and technical problems in measurements are briefly discussed.

1 kHz tabletop ultrashort hard x-ray source for time-resolved x-ray protein crystallography

We describe a compact, reliable, and high-average-power femtosecond x-ray source and its first application to diffraction on protein crystal. The setup relies on a homemade Ti: sapphire system delivering 12 mJ at a 1 kHz repetition rate, associated with a small vacuum chamber especially designed for laser-plasma interaction and x-ray applications. This device allows the generation of 5 x 10(9) photons/s/sr at 8 keV and optimized x-ray irradiation of the studied sample, which can be placed close to the source. We present the diffraction pattern of a protein crystal in a divergent beam geometry, which is a first step to a subpicosecond x-ray diffraction experiment.