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Warm dense matter (WDM) is a ubiquitous state of matter encountered in various high energy density environments, at the frontier between a plasma and a condensed phase. It is key to astrophysics, planetary science and inertial confinement fusion research, but its electronic and ionic structure and dynamics remain poorly understood. Here, an intense and ultrafast X-ray free electron laser (XFEL) pulse is used to simultaneously create and characterize warm dense copper via L-edge X-ray absorption spectroscopy. The rich electron dynamics occurring within the 15-femtosecond pulse duration are revealed over a large irradiation intensity range. Below 1015 W cm-2, an absorption peak below the L-edge appears, originating from the depletion of the 3d band, and shows a distinct energy redshift with increasing intensity. This shift is a possible signature of ionization potential depression and electronic bond hardening. At higher intensities, massive ionization and collisions lead to the transition from reverse saturable absorption (opening absorption channels) to saturable absorption (material bleaching) of the XFEL pulse, an effect that holds promise for X-ray pulse shaping. The presented method is applicable to a wide range of materials and constitutes a robust benchmark for non-equilibrium models of electron dynamics in WDM.

A real-time and accurate characterization of the X-ray beam size is essential to enable a large variety of different experiments at free-electron laser facilities. Typically, ablative imprints are employed to determine shape and size of µm-focused X-ray beams. The high accuracy of this state-of-the-art method comes at the expense of the time required to perform an ex-situ image analysis. In contrast, diffraction at a curved grating with suitably varying period and orientation forms a magnified image of the X-ray beam, which can be recorded by a 2D pixelated detector providing beam size and pointing jitter in real time. In this manuscript, we compare results obtained with both techniques, address their advantages and limitations, and demonstrate their excellent agreement. We present an extensive characterization of the FEL beam focused to ≈1 µm by two Kirkpatrick-Baez (KB) mirrors, along with optical metrology slope profiles demonstrating their exceptionally high quality. This work provides a systematic and comprehensive study of the accuracy provided by curved gratings in real-time imaging of X-ray beams at a free-electron laser facility. It is applied here to soft X-rays and can be extended to the hard X-ray range. Furthermore, curved gratings, in combination with a suitable detector, can provide spatial properties of µm-focused X-ray beams at MHz repetition rate.

Symmetry is a powerful concept in physics, but its applicability to far-from-equilibrium states is still being understood. Recent attention has focused on how far-from-equilibrium states lead to spontaneous symmetry breaking. Conversely, ultrafast optical pumping can be used to drastically change the energy landscape and quench the magnetic order parameter in magnetic systems. Here, we find a distinct symmetry-dependent ultrafast behaviour by use of ultrafast x-ray scattering from magnetic patterns with varying degrees of isotropic and anisotropic symmetry. After pumping with an optical laser, the scattered intensity reveals a radial shift exclusive to the isotropic component and exhibits a faster recovery time from quenching for the anisotropic component. These features arise even when both symmetry components are concurrently measured, suggesting a correspondence between the excitation and the magnetic order symmetry. Our results underline the importance of symmetry as a critical variable to manipulate the magnetic order in the ultrafast regime.

Ultrashort and intense extreme ultraviolet (XUV) and X-ray pulses readily available at free-electron lasers (FELs) enable studying non-linear light−matter interactions on femtosecond timescales. Here, we report on the non-linear fluence dependence of magnetic scattering of Co/Pt multilayers, using FERMI FEL’s 70-fs-long single and double XUV pulses, the latter with a temporal separation of 200 fs, with a photon energy slightly detuned to the Co M2,3 absorption edge. We observe a quenching in magnetic scattering that sets-in already in the non-destructive fluence regime of a few mJ/cm² typically used for FEL-probe experiments on magnetic materials. Calculations of the transient electronic structure in tandem with a phenomenological modeling of the experimental data by means of ultrafast demagnetization unambiguously show that XUV-radiation-induced demagnetization is the dominant mechanism for the quenching in the investigated fluence regime of

X-ray absorption spectroscopy (XAS) was used to elucidate changes in the electronic structure caused by the pressure-induced structural collapse in EuCo

The goal is to nondestructively reconstruct the structure of a fabricated integrated circuit using x-ray diffraction measurement. In particular, to detect any deviation from the specification of the circuit design. Determining phase is critical. However, additional information exists as the phase of the design specification is known. Further phase information is determined by oversampling and a better estimate of the correct phase is obtained through convex optimization. The authors illustrate this technique with a simulated one dimensional example and a simple two dimensional experimental sample.

Single femtosecond optical laser pulses, of sufficient intensity, are demonstrated to reverse magnetization in a process known as all-optical switching. Gold two-wire antennas are placed on the all-optical switching film TbFeCo. These structures are resonant with the optical field, and they create a field enhancement in the near-field which confines the area where optical switching can occur. The magnetic switching that occurs around and below the antenna is imaged using resonant X-ray holography and magnetic circular dichroism. The results not only show the feasibility of controllable switching with antenna assistance but also demonstrate the highly inhomogeneous nature of the switching process, which is attributed to the process depending on the material's heterogeneity.

The intensities within the focal spots of the output of recently developed X-ray sources based on free-electron-laser (FEL) technology are so great that atoms within the focal region can potentially absorb several photons during the few tens of femtosecond X-ray pulse. Furthermore, the duration of the FEL X-ray pulse is comparable to the Auger decay times of inner-shell holes created by the X-rays themselves. We report here how such a scenario can lead to opacity broadening of the fluorescence radiation emitted by the hot, dense plasma, which is created as a result of the X-radiation focussed onto a solid target, and in particular present calculations of the broadening of the Kα emission in a solid-density aluminium target, produced when the FEL photon energy is tuned below the Al K-edge, but is resonant with the Kα transition. © 2014 Elsevier B.V.

Nature materials 12(10), 882 - 886 (2013). doi:10.1038/nmat3718, As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown1, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator–metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase9. Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics, Published by Nature Publishing Group, Basingstoke

Nature materials 12, 293 - 298 (2013). doi:10.1038/nmat3597, Ultrafast laser techniques have revealed extraordinary spin dynamics in magnetic materials that equilibrium descriptions of magnetism cannot explain. Particularly important for future applications is understanding non-equilibrium spin dynamics following laser excitation on the nanoscale, yet the limited spatial resolution of optical laser techniques has impeded such nanoscale studies. Here we present ultrafast diffraction experiments with an X-ray laser that probes the nanoscale spin dynamics following optical laser excitation in the ferrimagnetic alloy GdFeCo, which exhibits macroscopic all-optical switching. Our study reveals that GdFeCo displays nanoscale chemical and magnetic inhomogeneities that affect the spin dynamics. In particular, we observe Gd spin reversal in Gd-rich nanoregions within the first picosecond driven by the non-local transfer of angular momentum from larger adjacent Fe-rich nanoregions. These results suggest that a magnetic material’s microstructure can be engineered to control transient laser-excited spins, potentially allowing faster (~ 1 ps) spin reversal than in present technologies, Published by Nature Publishing Group, Basingstoke

Physical review letters 108, 267403 (2012). doi:10.1103/PhysRevLett.108.267403, Published by APS, College Park, Md.

We present the x-ray optical design of the soft x-ray materials science instrument at the Linac Coherent Light Source, consisting of a varied line-spaced grating monochromator and Kirkpatrick-Baez refocusing optics. Results from the commissioning of the monochromator are shown. A resolving power of 3000 was achieved, which is within a factor of two of the design goal.

High Resolution X-ray Lensless Imaging by Differential Holographic Encoding Diling Z h u , Manuel Guizar-Sicairos, Benny W u , Andreas Scherz. Yves Acremann, Tolek Tylisczcak, Peter Fischer, Nina Friedenberger, Katharina Ollefs, Michael Farle, James R. Fienup, Joachim S t o h r Department of Applied Physics, Stanford University, Stanford, California, U.S.A. Stanford Institute for Material and Energy Science, U.S.A. SLAC National Accelerator Laboratory, Menlo Park, California, The Institute of Optics, University of Rochester, Rochester, New York, U.S.A. PULSE Center for Energy Science, SLAC National U.S.A. Accelerator Laboratory, Menlo Park, California, Chemical Sciences Division, Advanced Light Source, U.S.A. U.S.A. Lawrance Berkeley National Laboratory, Berkeley, California, Center for X-ray Optics, Lawrance Berkeley National Laboratory, Berkeley, California, ''Fachbereich Physik and Center for Nanointegration Universitat Duisburg-Essen, s Duisburg-Ess en Germany. Accelerator (CeNIDE), Duisburg, Linac Coherent Light Source, SLAC National Laboratory, Menlo Park, California, * Corresponding author: U.S.A. and dlzhu@stanford.edu X - r a y free e l e c t r o n lasers ( X - F E L s ) [ 1 , 2] w i l l soon offer f e m t o s e c o n d pulses o f l a t e r a l l y c o h e r e n t x - r a y s w i t h sufficient i n t e n s i t y t o r e c o r d single-shot coherent s c a t t e r i n g p a t t e r n s for nanoscale i m a g i n g [ 3 - 6 ] . Pulse t r a i n s c r e a t e d b y s p l i t - a n d - d e l a y t e c h n i q u e s even o p e n t h e d o o r for c i n e m a t o g r a p h y o n u n p r e c e d e n t e d n a n o m e t e r l e n g t h a n d f e m t o s e c o n d t i m e scales [7, 8], A key t o real space u l - t r a f a s t m o t i o n p i c t u r e s is fast a n d r e l i a b l e i n v e r s i o n o f t h e r e c o r d e d reciprocal x-ray space s c a t t e r i n g p a t t e r n s . H e r e we for t h e first t i m e d e m o n s t r a t e i n t h e r e g i m e t h e p o w e r o f a novel t e c h n i q u e for lensless h i g h r e s o l u t i o n i m a g i n g , p r e ­ v i o u s l y suggested b y G u i z a r - S i c a i r o s a n d F i e n u p [9] t e r m e d h o l o g r a p h y with

We introduce a noniterative image-reconstruction technique for coherent diffractive imaging. Through the application of differential and integral operators, an extended reference can be used to recover the complex-valued transmissivity of an object, in closed form, from a measurement of its far-field (Fraunhofer) diffraction intensity. We demonstrate the feasibility of this approach, using a reference of a pair of crossed wires and slits, through numerical simulations and a soft x-ray coherent diffractive imaging experiment.

The methodology of multiple-wavelength anomalous diffraction, widely used for macromolecular structure determination, is extended to the imaging of nonperiodic nanostructures. We demonstrate the solution of the phase problem by a combination of two resonantly recorded coherent scattering patterns at the carbon K edge (285 eV). Our approach merges iterative phase retrieval and x-ray holography approaches, yielding unique and rapid reconstructions. The element, chemical, and magnetic state specificity of our method further renders it widely applicable to a broad range of nanostructures, providing a spatial resolution that is limited, in principle, by wavelength only.

We demonstrate phase imaging by means of resonant soft x-ray holography. Our holographic phase-contrast method utilizes the strong energy-dependence of the refractive index at a characteristic x-ray absorption resonance. The general concept is shown by using a $\mathrm{Co}∕\mathrm{Pd}$ multilayer sample which exhibits random nanosized magnetic domains. By tuning below the Co $L$-edge resonance, our quantitative and spectroscopic phase method allows high-contrast imaging of nanoscale electronic and magnetic order while increasing the probing depth and decreasing the radiation dose by an order of magnitude. The complex refractive index is quantitatively obtained through the interference between resonant and nonresonant scattering.

We analyze the magnetic cluster size (MCS) and magnetic cluster size distribution (MCSD) in a variety of perpendicular magnetic recording (PMR) media designs using resonant small angle x-ray scattering at the Co L3 absorption edge. The different PMR media flavors considered here vary in grain size between 7.5 and 9.5 nm as well as in lateral inter-granular exchange strength, which is controlled via the segregant amount. While for high inter-granular exchange, the MCS increases rapidly for grain sizes below 8.5 nm, we show that for increased amount of segregant with less exchange the MCS remains relatively small, even for grain sizes of 7.5 and 8 nm. However, the MCSD still increases sharply when shrinking grains from 8 to 7.5 nm. We show evidence that recording performance such as signal-to-noise-ratio on the spin stand correlates well with the product of magnetic cluster size and magnetic cluster size distribution.

We analyze the magnetic design for different generations of perpendicular magnetic recording (PMR) media using resonant soft x-ray small angle x-ray scattering. This technique allows us to simultaneously extract in a single experiment the key structural and magnetic parameters, i.e., lateral structural grain and magnetic cluster sizes as well as their distributions. We find that earlier PMR media generations relied on an initial reduction in the magnetic cluster size down to the grain level of the high anisotropy granular base layer, while very recent media designs introduce more exchange decoupling also within the softer laterally continuous cap layer. We highlight that this recent development allows optimizing magnetic cluster size and magnetic cluster size distribution within the composite media system for maximum achievable area density, while keeping the structural grain size roughly constant.

We measured the relative arrival time between an optical pulse and a soft x-ray pulse from a free-electron laser. This femtosecond cross-correlation measurement was achieved by observing the change in optical reflectivity induced through the absorption of a fraction of the x-ray pulse. The main x-ray pulse energy remained available for an independent pump-probe experiment where the sample may be opaque to soft x-rays. The method was employed to correct the two-pulse delay data from a canonical pump-probe experiment and demonstrate 130 ± 20 fs (FWHM) temporal resolution. We further analyze possible timing jitter sources and point to future improvements.

We show the ability to determine the relative phase between the object and a reference scatterer by tuning the overall intensity and phase of the reference wave. The proposed reference-guided phase retrieval algorithm uses the relative phase as a constraint to iteratively reconstruct the object and the reference simultaneously, and thus does not require precisely defined reference structures. The algorithm also features rapid and reliable convergence and overcomes the uniqueness problem. The method is demonstrated by a soft-x-ray coherent imaging experiment that utilizes a large micrometer-sized reference structure that can be turned on and off, yielding an object image with resolution close to the reconstruction pixel size of 21 nm.

Panoramic full-field imaging is demonstrated by applying spatial multiplexing to Fourier transform holography. Multiple object and reference waves extend the effective field of view for lensless imaging without compromising the spatial resolution. In this way, local regions of interest distributed throughout a sample can be simultaneously imaged with high spatial resolution. A method is proposed for capturing multiple ultrafast images of a sample with a single x-ray pulse.