We demonstrate in the soft x-ray regime a novel technique for high-resolution lensless imaging based on differential holographic encoding. We have achieved superior resolution over x-ray Fourier ...transform holography while maintaining the signal-to-noise ratio and algorithmic simplicity. We obtain a resolution of 16 nm by synthesizing images in the Fourier domain from a single diffraction pattern, which allows resolution improvement beyond the reference fabrication limit. Direct comparisons with iterative phase retrieval and images from state-of-the-art zone-plate microscopes are presented.
Measurement and understanding of the microscopic pathways materials follow as they transform is crucial for the design and synthesis of new metastable phases of matter. Here we employ femtosecond ...single-shot X-ray diffraction techniques to measure the pathways underlying solid–solid phase transitions in cadmium sulfide nanorods, a model system for a general class of martensitic transformations. Using picosecond rise-time laser-generated shocks to trigger the transformation, we directly observe the transition state dynamics associated with the wurtzite-to-rocksalt structural phase transformation in cadmium sulfide with atomic-scale resolution. A stress-dependent transition path is observed. At high peak stresses, the majority of the sample is converted directly into the rocksalt phase with no evidence of an intermediate prior to rocksalt formation. At lower peak stresses, a transient five-coordinated intermediate structure is observed consistent with previous first principles modeling.
Abstract
Electronic instabilities drive ordering transitions in condensed matter. Despite many advances in the microscopic understanding of the ordered states, a more nuanced and profound question ...often remains unanswered: how do the collective excitations influence the electronic order formation? Here, we experimentally show that a phonon affects the spin density wave (SDW) formation after an SDW-quench by femtosecond laser pulses. In a thin film, the temperature-dependent SDW period is quantized, allowing us to track the out-of-equilibrium formation path of the SDW precisely. By exploiting its persistent coupling to the lattice, we probe the SDW through the transient lattice distortion, measured by femtosecond X-ray diffraction. We find that within 500 femtoseconds after a complete quench, the SDW forms with the low-temperature period, directly bypassing a thermal state with the high-temperature period. We argue that a wavevector-matched phonon launched by the quench changes the formation path of the SDW through the dynamic pinning of the order parameter.
Split-pulse x-ray photon correlation spectroscopy has been proposed as one of the unique capabilities made possible with x-ray free electron lasers. It enables characterization of atomic-scale ...structural dynamics that dictates the macroscopic properties of various disordered material systems. Central to the experimental concept are x-ray optics that are capable of splitting an individual coherent femtosecond x-ray pulse into two distinct pulses, introduce an adjustable time delay between them, and then recombine the two pulses at the sample position such that they generate two coherent scattering patterns in rapid succession. Recent developments in such optics showed that, while true “amplitude-splitting” optics at hard x-ray wavelengths remains a technical challenge, wavefront and wavelength splitting are both feasible, able to deliver two micron-sized focused beams to the sample with sufficient relative stability. Here we show, however, that the conventional approach to speckle visibility spectroscopy using these beam-splitting techniques can be problematic, even leading to a decoupling of speckle visibility and material dynamics. In response, we discuss the details of the experimental approaches and data analysis protocols for addressing issues caused by subtle beam dissimilarities for both wavefront- and wavelength-splitting setups. We also show that in some scattering geometries, the Q-space mismatch can be resolved by using two beams of slightly different incidence angles and slightly different wavelengths at the same time. Instead of measuring the visibility of weak speckle patterns, the time correlation in sample structure is encoded in the “side band” of the spatial autocorrelation of the summed speckle patterns and can be retrieved straightforwardly from the experimental data. We demonstrate this with a numerical simulation.
X‐ray free‐electron lasers (FELs) have opened unprecedented possibilities to study the structure and dynamics of matter at an atomic level and ultra‐fast timescale. Many of the techniques routinely ...used at storage ring facilities are being adapted for experiments conducted at FELs. In order to take full advantage of these new sources several challenges have to be overcome. They are related to the very different source characteristics and its resulting impact on sample delivery, X‐ray optics, X‐ray detection and data acquisition. Here it is described how photon‐in photon‐out hard X‐ray spectroscopy techniques can be applied to study the electronic structure and its dynamics of transition metal systems with ultra‐bright and ultra‐short FEL X‐ray pulses. In particular, some of the experimental details that are different compared with synchrotron‐based setups are discussed and illustrated by recent measurements performed at the Linac Coherent Light Source.
X-ray free electron lasers, with their ultrashort highly coherent pulses, opened up the opportunity of probing ultrafast nano- and atomic-scale dynamics in amorphous and disordered material systems ...via speckle visibility spectroscopy. However, the anticipated count rate in a typical experiment is usually low. Therefore, visibility needs to be extracted via photon statistics analysis, i.e., by estimating the probabilities of multiple photons per pixel events using pixelated detectors. Considering the realistic X-ray detector responses including charge cloud sharing between pixels, pixel readout noise, and gain non-uniformity, speckle visibility extraction relying on photon assignment algorithms are often computationally demanding and suffer from systematic errors. In this paper, we present a systematic study of the commonly-used algorithms by applying them to an experimental data set containing small-angle coherent scattering with visibility levels ranging from below 1% to ∼60%. We also propose a contrast calibration protocol and show that a computationally lightweight algorithm can be implemented for high-speed correlation evaluation.
Beam splitters and delay lines are among the key building blocks of modern-day optical laser technology. Progress in x-ray free electron laser source development and applications over the past decade ...is calling for their counterpart operating at the Angstrom wavelength regime. Recent efforts in x-ray optics development demonstrate relatively stable delay lines that most often adopt the division-of-wavefront approach for the beam splitting and recombination. However, the two exit beams in such configurations struggle to achieve sufficient mutual coherence to enable applications such as interferometry, correlation spectroscopy, and nonlinear spectroscopy. We present an experimental realization of the generation of highly mutually coherent pulse pairs using an amplitude-split delay line design based on transmission grating beam splitters and channel-cut crystals. The performance of the prototype system was analyzed in the context of x-ray coherent scattering and correlation spectroscopy, where nearly identical high-contrast speckle patterns from both branches were observed. We show in addition the high level of dynamical stability during continuous delay scans, a capability essential for high sensitivity ultrafast measurements.
X‐ray focus optimization and characterization based on coherent scattering and quantitative speckle size measurements was demonstrated at the Linac Coherent Light Source. Its performance as a ...single‐pulse free‐electron laser beam diagnostic was tested for two typical focusing configurations. The results derived from the speckle size/shape analysis show the effectiveness of this technique in finding the focus' location, size and shape. In addition, its single‐pulse compatibility enables users to capture pulse‐to‐pulse fluctuations in focus properties compared with other techniques that require scanning and averaging.
We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of Fe(bpy)2(CN)2, where bpy=2,2′-bipyridine, initiated by ...metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2′-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of Fe(bpy)2(CN)2 undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of Fe(bpy)2(CN)2 complement prior measurement performed on Fe(bpy)32+ and Fe(bpy)(CN)42− in dimethylsulfoxide solution and help complete the chemical series Fe(bpy)N(CN)6–2N2N-4, where N = 1–3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.
PDF
