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Reaching the milestone of X-ray lasers is something that X-ray physicists have dreamed of for many years. Recently developed X-ray free-electron lasers (XFEL) based on self-amplified spontaneous emission (SASE) constitute a very promising tool for future X-ray laser technologies. A team led by Professor H. Yoneda (University of Electro-Communications, Tokyo, Japan) has recently carried out very impressive experiments at Japanese XFEL facility, SACLA, on the same campus as SPring-8. Similar to an X-ray tube, the researchers employed a solid copper target to generate X-rays. However, their experiment is very unique in that two colors were employed by tuning undulator gaps; one energy is around 9 keV, i.e., above the Cu-K absorption edge, and the other energy is almost the same energy as Cu Kα₁ or Kα₂. While 9 keV photons ionize copper atoms and generate Cu K X-ray fluorescence spectra, the lower energy photons can amplify the X-ray fluorescence because of their temporal coherence. Their Cu Kα spectra are impressive, because Kα₁/Kα₂ can be controlled by tuning the second energy of XFEL pulses. The reason for the limited amplification is probably due to the energy band width of incoming temporary coherent X-ray photons. The researchers did not use any monochromators, but controlled only the undulator gaps. They have two different X-ray energies, but unfortunately monochromaticy has some limits and the band width is still quite wide. The present work could be a very important step toward achieving X-ray lasers by using atomic energy levels. For more information, see the paper, "Atomic inner-shell laser at 1.5-angstrom wavelength pumped by an X-ray free-electron laser", H. Yoneda et al., Nature 524, 446 (2015).

Dr. F. Dorchies (Universite Bordeaux, CNRS-CELIA, France) and his colleagues have recently developed a laser-base X-ray absorption spectrometer covering 0.5-4.0 keV with a time resolution of around 3.3 pico second. The spectrometer uses bremsstrahlung caused by the extremely high impact of laser pulses on the metallic target. To perform time-resolved X-ray spectroscopic studies, there have been quite a few challenges. For most research, it is crucial to avoid damaging/destroying samples, and the measuring time should not be very long. In addition, scientists don't like to compromise the signal-to-background ratio of spectral data taken at each time point, even though the quality is not the same as that of ordinary X-ray absorption spectra. The authors seem to believe that they are getting some breakthroughs. Their setup is a combination of a table-top laser (Ti: Sapphire, 800nm, 150mJ, 10Hz) and a Johan spectrometer equipped with a CCD camera. A set of polycapillary optics were employed as a beamline transport between the X-ray source and the sample (1 m distance) to maintain a clean, independent and flexible environment for the sample. The X-ray intensity near the Al K edge and Cu L edges is 1.3 × 10⁶ photons/eV/pulse. For more information, see the paper, "Experimental station for laser-based picosecond time-resolved x-ray absorption near-edge spectroscopy", F. Dorchies et al., Rev. Sci. Instrum. 86, 073106 (2015).

A team led by Dr. M. Minitti (SLAC National Accelerator Laboratory, USA) has recently succeeded in recording the time evolution of a structural change of ring-type 1,3-cyclohexadiene gas molecule to linear 1,3,5-hexatriene. The employment of the X-ray free-electron laser at LCLS (Linac coherent light source), Stanford allowed them to do ultra fast snapshots of X-ray scattering in several tens of fs (femtosecond) scale. The study is based on pump-and-probe measurement; i.e., X-ray data were collected as a function of the controlled delay time between the UV pump pulse (267 nm, 65 fs, 4-8 μJ, 100 μm size) and X-ray probe pulse (8.3 keV, around 30 fs, 10¹² photons/pulse, 30 μm square size). The team established that some signals caused by structural change are found as early as 30 fs, and the reaction finishes at 200 fs. For more information, see the paper, "Imaging Molecular Motion: Femtosecond X-Ray Scattering of an Electrocyclic Chemical Reaction", M. P. Minitti et al., Phys. Rev. Lett. 114, 255501 (2015).

A research group led by Professor Jorg Evers (Max Planck Institute for Nuclear Physics, Heidelberg, Germany) has recently reported a method for narrowing the spectral width of X-ray pulses by the use of subluminal light propagation. So far, in visible light, slow group velocity such as 17 m/sec has been observed in low temperature sodium gas at 435 nK (see, L. V. Hau et al., Nature, 397, 594 (1999)). The authors intend a similar effect in X-ray wavelength photons by manipulating the optical response of the 14.4 keV Mössbauer resonance of ⁵⁷Fe nuclei. The method combines coherent control, as well as cooperative and cavity enhancements of light-matter interaction in a single setup. It was found that the reduced group velocity of the obtained X-ray pulses is lower than 10^-4 of the speed of the light. For more information, see the paper, "Tunable Subluminal Propagation of Narrow-band X-Ray Pulses", K. P. Heeg et al., Phys. Rev. Lett. 114, 203601 (2015).

National Synchrotron Light Source II of Brookhaven National Laboratory will officially start user runs from the 3^rd cycle in 2015. Seven beamlines will be commissioned in September, 2015, and a further 21 beamlines will be designed and constructed in the coming years. The facility provides the world's smallest electron beam emittance, resulting in the brightest X-ray source. For more information, visit the Web page, http://www0.bnl.gov/ps/nsls2/about-NSLS-II.php

The following YouTube video also gives useful information.

https://www.youtube.com/watch?v=AzP8EGHw4BA

At the soft X-ray free-electron laser (XFEL) facility, FLASH, in Hamburg, Germany, all-optical synchronization has finally been achieved. Scientists are reporting that the timing is better than 30 fs rms for 90 fs X-ray photon pulses. As one of the most promising experiments using XFEL is time-resolved analysis based on the pump & probe scheme, it is crucial to synchronize all independent components, including all accelerator modules and all external optical lasers, to better than the delivered free-electron laser pulse duration such as shorter than 100 fs. For more information, see the paper, "Femtosecond all-optical synchronization of an X-ray free-electron laser", S. Schulz et al., Nature Communications, 6, 5895 (2015).

In addition to large-scale X-ray facilities such as synchrotrons and X-ray FELs, there have been increasing demands for much more compact X-ray sources with high brilliance, ultra short pulse properties and coherence. Dr. W. S. Graves (Massachusetts Institute of Technology, USA) and his colleagues have proposed a design for a compact X-ray source based on inverse Compton scattering. The source consists of a 1m linuc and an ultra short pulse laser. The whole size of the source including X-ray experiment space is nearly 4m. The colliding laser is a Yb:YAG solid-state amplifier producing 1030 nm, 100 mJ pulses at 1 kHz repetition rate. The calculation shows that X-ray intensity at 12.4 keV is 5×10¹¹ photons/second in a 5% bandwidth. For more information, see the paper, "Compact x-ray source based on burst-mode inverse Compton scattering at 100 kHz", W. S. Graves et al., Phys. Rev. STAB, 17, 120701 (2014).

A German and Austrian group has recently developed a table-top X-ray source based on ultra-short laser pulses. Generation of X-ray pulses by lasers may not be a big surprise for readers (See, for example, "Ultrafast X-ray Pulses from Laser-Produced Plasmas" by M. M. Murnane, Science, 251, 531 (1991), "Microfocus Cu Kα source for femtosecond x-ray science" by N. Zhavoronkov, Opt. Letter, 30, 1737 (2005)). However, so far, the X-ray intensity has not been sufficient for use in practical measurements such as pump-and-probe time resolved X-ray analysis. This time, scientists employed a mid infrared wavelength (3.9 micron) to accelerate electrons from the copper tape target to very high kinetic energy by making use of its comparably long optical period. The pulse width of the laser employed is 80 femto second. It was found that the system gives 10⁹ copper Kα photons per pulse generated with pulses of a peak intensity of 6×10¹⁶ W/cm². This is about 25 times higher than that generated by 800 nm wavelength laser pulses. For more information, see the paper, "High-brightness table-top hard X-ray source driven by sub-100-femtosecond mid-infrared pulses", J. Weisshaupt et al., Nature Photonics, 8, 927 (2014).

It is now known that X-ray free-electron lasers can produce ultrafast X-ray pulses as short as 3 fs in FWHM. Scientists at the Linac Coherent Light Source (LCLS), Stanford are trying to reduce delay time errors in optical-pump & X-ray probe measurements to the 1 fs level, by 2D spectrogram measurement of the relative X-ray/optical delay. For more information, see the paper, "Sub-femtosecond precision measurement of relative X-ray arrival time for free-electron lasers", N. Hartmann et al., Nature Photonics, 8, 706 (2014).

The Ultrafast X-ray Summer Seminar (UXSS) 2014 took place from June 15 to 19 at SLAC National Accelerator Laboratory, California, United States. The program is organized specifically to train students and post-docs on new opportunities in ultrafast science, particularly using X-ray Free Electron Lasers. Almost all the lectures presented by expert scientists are now available as videos on YouTube. The lecture by Dr. Pieter Glatzel (ESRF) on "Hard X-ray Spectroscopy" (https://www.youtube.com/watch?v=0sMD8lZzuTE) is surely useful for young X-ray spectroscopists. Other exciting lectures are available from Dr. Oleg Shpyrko (UCSD) on "Coherent X-ray Scattering at Ultrafast Timescales" (https://www.youtube.com/watch?v=OIR_ltSOl2U), Dr. Michael Odelius (Stockholm University) on "Electronic Structure & Ultrafast Solution Dynamics in Xray vision w/ theoretical spectacles" (https://www.youtube.com/watch?v=ITIzAmYuyWA), Dr. Alexander Fohlisch (Helmholtz Zentrum Berlin) on "Soft X-ray General and Solid State Aspects" (https://www.youtube.com/watch?v=xTz1oCV5cWI), Dr. Philippe Wernet (Helmholtz Zentrum Berlin) on "Ultrafast Molecular Spectroscopy with X-rays: Experiment", and Prof. Claudio Pellegrini (UCLA) on "X-ray Free Electron Lasers" (https://www.youtube.com/watch?v=5v68nuOTwns). For more information on this summer seminar, visit the following Web site, https://conf-slac.stanford.edu/uxss-2014/

So far, laser combs in visible light wavelength have been known as an extremely precise measure of dimensions. What would happen if they move into the X-ray region? The advent of an X-ray free electron laser (XFEL) may realize an X-ray frequency comb in the near future. Dr. S. M. Cavalettobe (Max-Planck-Institut fur Kernphysik, Heidelberg, Germany) is proposing such an ambitious experiment. The research could open up wide-ranging applications; ultraprecise X-ray atomic clocks, determination of many X-ray fundamental parameters, quantitative understanding of astrophysical models and quantum electrodynamics etc. For more information, see the paper, "Broadband high-resolution X-ray frequency combs", S. M. Cavaletto et al, Nature Photonics, June 2014 (DOI: 10.1038/nphoton.2014.113).

The use of X-ray free-electrons has enabled plenty of fascinating science, such as watching non-equilibrium excited-state dynamics in complexes of 3d transition metals. Scientists at LCLS, Stanford have performed femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, elucidating the spin crossover dynamics of [Fe(2, 2ˈ-bipyridine)₃]²⁺ on photoinduced metal-to-ligand charge transfer excitation. For more information, see the paper, "Tracking excited-state charge and spin dynamics in iron coordination complexes", W. Zhang et al., Nature, 509, 345 (2014).

Professor Y. Takahashi (Osaka University, Japan) and his colleagues have recently reported that coherent X-ray imaging using Bragg diffraction can aid the observation of nanoscale dislocation strain fields in a silicon single crystal. The experiments were done with 11.8 keV micro-focused X-ray photons, around 1 μm in both directions, using KB mirrors at BL-29XUL, SPring-8, Japan. In this research, a 1 μm thick silicon (100) single crystal was placed in the X-ray path so that X-rays could pass through it and the 220 Bragg reflection spot was observed by a CCD camera 2 m behind the sample. The sample was scanned in XY directions as well. The research team found phase singularities, i.e., two pairs of vortices with opposite directions in the phase map, that corresponded to the locally dark positions in the intensity map. It was concluded that this corresponded to the projection of the {111} dislocation loops. For more information, see the paper, "Bragg x-ray ptychography of a silicon crystal: Visualization of the dislocation strain field and the production of a vortex beam", Y. Takahashi et al., Phys. Rev. B87, 121201(R) (2013).

The construction of Brookhaven's National Synchrotron Light Source II is approaching its final stage. Recently the last of 150 magnet girders was installed in the storage ring. Magnets traveled from across the globe, supplied by ring magnet vendors based in six countries: Buckley Systems Ltd (New Zealand), Budker Institute of Nuclear Physics (Russia), Danfysik (Denmark), Everson Tesla Incorporated (U.S.), Institute of High Energy Physics (China), and Tesla Engineering (U.K.). In the experimental hall, meanwhile, 17 hutches have been delivered and constructed for seven beamlines; CSX1 and CSX2 (two branches of Coherent Soft X-ray Scattering and Polarization), CHX (Coherent Hard X-ray Scattering), IXS (Inelastic X-ray Scattering), HXN (Hard X-ray Nanoprobe), SRX (Submicron Resolution X-ray Spectroscopy) and XPD (X-ray Powder Diffraction). For further information, visit the Web page, http://www.bnl.gov/ps/news/news.php?a=23725

An explanation of the CSX beamline construction can be viewed on You Tube.

http://www.youtube.com/watch?feature=player_embedded&v=rximpW0aR9A

The extremely high peak power of an X-ray free electron laser pulse can be an attractive tool for clarifying the core-level excitation and relaxation process. Recently, Dr. B. Rudek and his colleagues have reported their time-of-flight ion spectroscopy studies on sequential inner-shell multiple ionization of krypton at photon energies at 2 keV and 1.5 keV, which are higher than the LI (~1.92 keV) and lower than the LIII (~1.67 keV) edges for ordinary neutral krypton, respectively. The experiments were done with two X-ray pulse widths (5 and 80 fs) and various pulse energies (from 0.07 to 2.6 mJ), at the Linac Coherent Light Source (LCLS), Stanford, USA. The highest charge state observed at 1.5 keV photon energy (below the LI edge) is Kr¹⁷⁺; at 2 keV photon energy (above the LIII edge), it is Kr²¹⁺. It was found that theoretical calculations based on a rate-equation model can explain the obtained experimental data for 1.5 keV, but fails to do so at 2 keV, where the experimental spectrum shows higher charge states. They discussed that this enhancement is due to a resonance-enhanced X-ray multiple ionization mechanism, i.e., resonant excitations followed by autoionization at charge states higher than Kr¹²⁺, where direct L-shell photoionization at 2 keV is energetically closed. For more information, see the paper, "Resonance-enhanced multiple ionization of krypton at an x-ray free-electron laser", B. I. Cho et al., Phys. Rev. A87, 023413 (2013).

In spite of the recent advent of few fs pulse X-ray free-electron laser sources, so far, synchronization between optical lasers and X-ray pulses has been challenging, and the jitter, typically, 100~200 fs r.m.s., has limited the time-resolution of the measurement. At the Linac Coherent Light Source (LCLS), Stanford, scientists have recently solved this problem by introducing a "measure-and-sort" approach, which records all single-shot data with time information to ensure resorting of the data. In the beamline, the same optical laser beam is split into three beams: with the first, the relative delay between laser and X-ray is encoded into wavelength by using a broadband chirped supercontinuum; in the second, the temporal delay is spatially encoded; in the third, pump-probe experiments are performed with time-sorting tools. It was concluded that the error in the delay time between optical and X-ray pulses can be substantially improved to 6 fs r.m.s., leading to time-resolved measurement with only a few fs resolution. For more information, see the paper, "Achieving few-femtosecond time-sorting at hard X-ray free-electron lasers", M. Harmand et al., Nature Photonics, doi:10.1038/nphoton.2013.11; published online, February 17, 2013.

Coherent X-ray diffractive imaging has made remarkable progress over the past 15 years. The technique basically reconstructs real space microscopic images with the spatial resolution of nm without the use of lenses, mainly because of the ability to retrieve phases. However, it relies on the degree of high coherence of the available X-ray photon beam, and, until now, almost all experimental studies have been subject to some limits. It is not very easy to satisfy the ideal conditions, mainly because of the partial coherence of the beam itself and some decoherence caused by imperfect detection as well as the dynamic motions of the sample. Dr. P. Thibaut (Technische Universität München, Germany) and his colleague have recently reported their analytical studies into extending ptychography by formulating it as low-rank mixed states. The procedure is closely related to quantum state tomography and is equally applicable to high-resolution microscopy, wave sensing and fluctuation measurements. They concluded that some of the most stringent experimental conditions in ptychography can be relaxed, and susceptibility to imaging artifacts is reduced even when the coherence conditions are not ideal. For more information, see the paper, "Reconstructing state mixtures from diffraction measurements", P. Thibault et al., Nature, 494, 68 (2013).

Perhaps some readers already know Dr. Ken Lea's synchrotron song, but now it is available on YouTube. The song is about synchrotron radiation and many scientific studies, which have been done at The Synchrotron Radiation Source (SRS), Daresbury Laboratory in UK, from 1980 to 2008. As so many scientific terms (such as wavelength, beamline, monochromator, polarization, collimation, surface acoustic wave, sample chamber etc) are included in the lyrics, it may not be easy for ordinary people to sing this song. Visit the following You Tube site and have fun!

http://www.youtube.com/watch?v=-Rc5OfEcSZk

In Japan, a research team led by Professor K. Yamauchi (Osaka University) and Professor T. Ishikawa (Riken, Harima, Japan) has recently succeeded in focusing ultra short X-ray laser pulses from the SPring-8 Angstrom Compact free electron LAser (SACLA). With reflective optics comprising elliptically figured mirrors with nm accuracy to preserve a coherent wavefront, they have obtained a focused small beam of 0.95 × 1.20 μm² at 10 keV. The estimated achievable power density at the sample position is 6 × 10¹⁷ W/cm². For more information, see the paper, "Focusing of X-ray free-electron laser pulses with reflective optics", H. Yumoto et al., Nature Photonics, 7, 43 (2013).

At Linac Coherent Light Source (LCLS), Stanford, USA, a series of experimental works has been carried out based on the core-level excitation and relaxation process. One recently published paper from Stanford reports the resonant generation of Kα emission from aluminum foil (1μm thick) in a solid-plasma state created by irradiating very strong X-ray free-electron laser pulses (less than 80 fs time width, 1.6×10¹² photons/pulse). In the experiment, quasimonochromatic (0.5% bandwidth) X-ray pulses in the energy range of 1480-1580 eV (below and slightly above the K edge of ground state Al) were focused onto a 3μm diameter spot on the sample, with a corresponding peak intensity in excess of 10¹⁷ W/cm². To analyze the X-ray spectra, the research group employed a wavelength-dispersive X-ray spectrometer with a flat ADP (101) crystal and an X-ray CCD camera. Since the same atom can absorb multiple photons contained in the single pulse time width, with L-shell holes being created and leading to the excitation of a K-shell electron into one of these L-holes, the Kα X-rays are produced. The research group studied many such emission spectra produced by tuning the XFEL energy to the K-L transitions of those highly charged ions that have transition energies below the K edge of the cold material. It was also found that resonance emission peaks broaden significantly, and this was explained as opacity effects. Because of the intensity-dependent optical depth, the transparent sample at low intensity thickens optically with an intense XFEL pulse. For more information, see the paper, "Resonant Kα Spectroscopy of Solid-Density Aluminum Plasmas", B. I. Cho et al., Phys. Rev. Lett., 109, 245003 (2012).

The recent advent of the X-ray free-electron laser (XFEL) based on self-amplified spontaneous emission (SASE) has brought new opportunities in X-ray physics and many scientific applications. On the other hand, the shot-noise start-up in the SASE mechanism lends an inherent stochastic character to X-ray pulses, leading to rather large variations both in wavelength and intensity. One strategy to solve the problem is to use an XFEL pulse to create a population inversion in a medium which then lases in the X-ray region (See, N. Rohringer et al., Nature, 481, 488 (2012)). Alternatively, resonant core excitation can be used as well. Recently, a theoretical chemistry group led by Professor F. Gel'mukhanov (Royal Institute of Technology, Sweden) has published a prediction of X-ray lasing based on resonant core excitation of a molecule to a state which is subject to ultrafast dissociation, i.e., a state in which dissociation precedes the femtosecond core hole decay. As an example, Cl 2p_1/2 → 6σ excitation of the HCl molecule by an XFEL pulse and the subsequent ultrafast dissociation were studied. For more information, see the paper, "Dissociative X-ray Lasing", Q. Miao et al., Phys. Rev. Lett., 109, 233905 (2012).

Coherent X-ray diffraction imaging is a promising new technique to observe samples in material science and biology with a spatial resolution of around 10 nm. However, the range of applications is still not very wide, because the method requires that the X-ray source be highly coherent both laterally and longitudinally. Thus, one of the most important questions for users is the feasibility of the technique when only a partially coherent source is available. A research group led by Professor K. Nugent (University of Melbourne, Australia) has recently reported some quite good news on this issue. So far, it has been often said that the lateral coherence length should be at least twice the greatest spatial extent of the object. The longitudinal coherence length is determined by the bandwidth of the monochromatic X-ray beam. According to the present study, one could relax the minimal criteria by a factor of 2 for both lateral coherence length and longitudinal coherence length, if the coherence properties are known either a priori or through experiment. In other words, more flux could be made available at the sample position for the coherent X-ray diffraction imaging experiments with the use of a partially coherent X-ray source. For more information, see the paper, "Diffraction imaging: The limits of partial coherence", B. Chen et al., Phys. Rev. B86, 235401 (2012).

The X-ray free electron laser (XFEL) is a new type of light source, which can provide coherent, high-flux, ultra-short photon pulses in the soft and hard X-ray energy region. Until now, a long linear accelerator as well as long linear undulators have been thought indispensable, because the principle is based on the self amplified spontaneous emission (SASE). Indeed, both FLASH at DESY and LCLS at SLAC, which are the world's first X-FEL facilities in soft and hard X-rays, respectively, are facilities on a huge scale. Recently, Dr. Z. Huang (SLAC National Accelerator Laboratory, USA) and his colleagues have published a very interesting idea for a compact XFEL facility that uses an ultra-short pulse laser instead of an ordinary linear accelerator. It is known that laser-plasma accelerators can produce high energy electron beams with low emittance, high peak current but a rather large energy spread, which makes it difficult to consider XFEL applications. Their main strategy is the introduction of a transverse field variation into the FEL undulator. In their calculation, such a transverse gradient undulator together with a properly dispersed beam can greatly reduce the effects of electron energy spread and jitter on the performance of XFEL generation. For more information, see the paper, "Compact X-ray Free-Electron Laser from a Laser-Plasma Accelerator Using a Transverse-Gradient Undulator", Z. Huang et al., Phys. Rev. Lett. 109, 204801 (2012).

Readers might recall several previous news articles on X-ray spectra of neon excited by ultra-short, high-intensity pulses from an X-ray free electron laser source at LCLS, Stanford ("Observation of non-linear resonances of inner-shell electrons by X-ray free electron laser", No.1, Vol. 41 (2012), "Calculation of X-ray emission from doubly ionized neon", No.1, Vol. 40 (2011), ""Hollow" neon atom created by X-ray laser excitation", No.5, Vol. 39 (2010) and "Removing all electrons from neon by X-ray laser", No.6, Vol. 38 (2009) ). Recently, a research team led by Professor C. H. Keitel (Max-Planck-Institut für Kernphysik, Germany) has published its calculation of the resonance X-ray fluorescence spectra of neon, based on a so-called two-level model, which is used to study the transition of 1s2p_z^-1→1s^-12p_z in Ne⁺ at an energy of 848 eV. As X-rays induce Rabi oscillations so fast, they compete with Ne 1s-hole decay. The research group discusses resonance X-ray fluorescence spectra for two different cases; the first is chaotic pulses, which are most likely based on the SASE principle employed in the present XFEL facilities, and the second is Gaussian pulses available from the more ideal types of X-ray lasers expected in the future. For more information, see the paper, "Resonance fluorescence in ultrafast and intense x-ray free-electron-laser pulses", S. M. Cavaletto et al., Phys. Rev. A86, 033402 (2012).

Extremely strong pulses from X-ray free electron laser (XFEL) can change the material structure. Recently, scientists at LCLS (Linac Coherent Light Source), Stanford, USA, have reported the amorphous to crystalline phase transition of carbon by femtosecond 830 eV XFEL beam. The research group employed atomic force microscopy, photoelectron microscopy, and micro-Raman spectroscopy to discuss the change of the sp²/sp³ ratio (graphitization), as well as the change of local order of the irradiated sample area. It was found that the phase transition threshold fluence is 282 ± 11 mJ/cm², and also the transition is mainly due to thermal activation rather than a non-thermal mechanism such as ionization etc. For more information, see the paper, "Amorphous to crystalline phase transition in carbon induced by intense femtosecond x-ray free-electron laser pulses", J. Gaudin et al., Phys. Rev. B86, 024103 (2012).

One very interesting outcome at LCLS (Linac Coherent Light Source), Stanford, USA has recently been published. The experiment was single-shot imaging of ferromagnetic, nanoscale spin order taken with femtosecond X-ray free electron laser pulses. For more information, see the paper, "Femtosecond Single-Shot Imaging of Nanoscale Ferromagnetic Order in Co/Pd Multilayers Using Resonant X-Ray Holography", T. Wang et al., Phys. Rev. Lett. 108, 267403 (2012).

A new X-ray free electron laser facility at the SPring-8 campus in Harima, Japan, has started its user run. This is the world's second XFEL facility in the hard X-ray region after the LCLS at Stanford, USA. One of the most important properties of this new Japanese facility is the short wavelength of the X-ray photon; the shortest wavelength attained is 0.634 Å (63.4 pm), which is almost half that achieved at Stanford. The facility uses a 400m linear accelerator as well as a short-gap and very long undulator (periodic length 18mm, minimum gap 3.5 mm, total number of periods 4,986). The maximum power exceeds 10 GW with a pulse duration of 10-14 s. For more information, see the paper, "A compact X-ray free-electron laser emitting in the sub-angstrom region", T. Ishikawa et al., Nature Photonics, 6, 540 (2012). Also visit the Web page, http://xfel.riken.jp/eng/

High-harmonic generation (HHG) is a universal response of atoms and molecules in strong femtosecond laser fields, and can be used to generate coherent photons in the soft X-ray region. Simply speaking, HHG is the coherent version of an X-ray tube; instead of accelerating thermal electrons emitted from the filament and generating incoherent X-rays by hitting a metallic target, HHG begins with tunnel ionization of an atom in a strong laser field. The portion of the electron wave function that escapes the atom is accelerated by the laser electric field and, when driven back to its parent ion by the laser, can coherently convert its kinetic energy into a high-harmonic photon. So far, for many cases, around 100 near-infrared laser photons have been combined to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms is added constructively. Recently, a team led by Professor H. C. Kapteyn and Professor M. M. Murnane (University of Colorado at Boulder, USA) have employed a mid-infrared femtosecond laser in a high-pressure gas, and succeeded in getting ultrahigh harmonics up to orders greater than 5000, resulting in a bright continuum spectra ranging from 0.2 to around 1.6 keV. The energy has still not yet reached the hard X-ray regime, but this would be a very attractive coherent ultra short pulse source for soft X-rays. For more information, see the paper, "Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers", T. Popmintchev et al., Science, 336, 1287 (2012).

Recently, a very stimulating paper has been published discussing experimentally the fundamental processes of photo-absorption and excitation of electrons by using extremely high-fluence, ultra-short X-ray pulses. The research was done for the electron system in inert Ne gas at LCLS (Linac Coherent Light Source), Stanford, USA, which is the world's first hard X-ray free-electron laser facility. The scheme is as follows: an intense single X-ray pulse of sub-10-fs duration at 848 eV first strips a 2p electron from Ne and, at this stage, since the X-ray energy is below the binding energy of a 1s electron in neutral neon, 870 eV, a 1s hole cannot be produced, but because of the above 2p hole, the next pulse can excite the 1s electron, leading to 1s-2p resonance in the Ne⁺ ion and, finally, stimulated emission (2p-1s) competes with Auger decay to refill the 1s hole. The results have indicated that intense X-ray pulses of sub-10-fs duration can modify and even control the Auger decay process. For more information, see the paper, "Unveiling and Driving Hidden Resonances with High-Fluence, High-Intensity X-Ray Pulses", E. P. Kanter et al., Phys. Rev. Lett. 107, 233001 (2011).

Scientists at Argonne National Laboratory, USA have recently reported a novel set of optics for X-ray monochromators, which combine the effect of angular dispersion and anomalous transmission of X-rays in Bragg reflection from asymmetrically cut crystals. The optics employ a five-reflection, three-crystal arrangement, and it was found that the spectral contrast, the bandwidth and the angular acceptance are approximately 500, 0.5 meV, and 0.1 mrad, respectively, for 9.1 keV X-rays. The new optics could be a foundation for next-generation inelastic X-ray scattering spectrometers. For more information, see the paper, "Using angular dispersion and anomalous transmission to shape ultramonochromatic x rays", Y. Shvyd'ko et al., Phys. Rev. A84, 053823 (2011).

A U-shaped design for rotating anode X-ray sources is one method for enabling high brilliance, and was first proposed by Professor N. Sakabe (KEK, Tsukuba, Japan) in 1995. Unlike ordinary rotating anode X-ray sources, the electron beam goes beyond the outside surface of the rotating anode and then reverses its direction so that it can hit the inside surface. In this case, because of the centrifugal force of the rotating anode, the surface can be much smoother than usual even near the melting point, enabling the production of more X-ray photons. A KEK research group has published a report on recent progress with this type of X-ray source. According to their simulation, by optimizing both the bending and the steering magnets, the beam size can be 0.45 mm (horizontal) × 0.05 mm (vertical) for a 120 keV/75 mA beam. The effective brilliance is about 500 kW/mm². For more information, see the paper, "Research and development of an electron beam focusing system for a high-brightness X-ray generator", T. Sakai et al., J. Synchrotron Rad. 18, (2011) (Published online, DOI:10.1107/S0909049510029948).

Professor J. Kawai (Kyoto University, Japan; Associate editor of X-ray Spectrometry) and his colleagues recently developed a novel tiny X-ray instrument equipped with a pyroelectric LiTaO₃ crystal as an electron source, a sample stage and an X-ray detector. The research group found that adequate X-ray fluorescence spectra can be measured for 0.03 mm² titanium, iron, and nickel wires. For more information, see the paper, "Development of Miniaturized Electron Probe X-ray Microanalyzer", S. Imashuku et al., Anal. Chem., 83, 8363 (2011).

European XFEL and the Spanish Center for Ultrashort Ultraintense Pulsed Lasers (CLPU) in Salamanca will pool their efforts to promote research into high-energy density science and to develop new ultrafast lasers to analyze physical and chemical processes in conjunction with the X-ray beams of the European XFEL. Both research institutions signed a memorandum of understanding at the European XFEL headquarters in Hamburg. In the framework of this cooperation, an optical laser expert from CLPU has now joined the European XFEL Optical Lasers Group for an initial period of six months. For further information, visit the web page, http://www.xfel.eu/

A Korean group led by Professor J. H. Je (Pohang University of Science and Technology) has recently reported some interesting experiments on water vaporization by X-ray photons. The experiments were done at the undulator beamline, XSD 32-ID, Advanced Photon Source in Argonne, USA. It was found that water is vaporized at a rate of 5.5 pL/sec at a 100 msec exposure of 1-Å-wavelength (~13 keV) X-ray irradiation of around 10⁷ photons/μm² (0.1 photons/Ų), which corresponds to a dose rate of ~50 kGy/sec. They also confirmed that water vapor is reversibly condensed during pauses in irradiation. This result suggests that photoionization induces vaporization through the reduction of the surface tension of water. For more information, see the paper, "X-ray-induced water vaporization", B. M. Weon et al., Phys. Rev. E84, 032601 (2011).

The European Molecular Biology Laboratory (EMBL) and the European XFEL have signed a Memorandum of Understanding, thereby laying the foundation for close future collaboration in deciphering the structure and dynamics of biomolecules. For further information, visit the Web page, http://www.xfel.eu/

The research team led by Professors V. Holý (Charles University, Czech Republic) and T. Baumbach (ANKA-Institute for Synchrotron radiation, Germany) have recently performed some extension of coherent X-ray diffractive imaging for high-resolution strain analysis in crystalline nanostructured devices such as layered nanowires and/or dots. Their research successfully determined the strain distribution in (Ga,Mn)As/GaAs nanowires. The key was their improvement of the phase-retrieval algorithm, i.e., separation of diffraction signals in reciprocal spaces. It was found that individual parts of the device can be reconstructed independently by this inversion procedure. The method is effective even for strongly inhomogeneously strained objects. For more information, see the paper, "Selective coherent x-ray diffractive imaging of displacement fields in (Ga,Mn)As/GaAs periodic wires", A. A. Minkevich et al., Phys. Rev. B84, 054113 (2011).

An innovative X-ray camera, designed to record bursts of images at an unprecedented speed of 4.5million frames per second, is being built with the help of the UK's Science and Technology Facilities Council (STFC) and will be delivered to the European XFEL (X-ray Free-Electron Laser) in 2012. For further information, visit the Web page, http://www.stfc.ac.uk/About%20STFC/36221.aspx

Parametric down-conversion is a quantum-optical process in which a 'pump' photon splits spontaneously into two (the 'signal' and 'idler') in a nonlinear optical medium. Recently, Professor T. Ishikawa (RIKEN, Harima, Japan) and his colleagues reported their experiments with X-ray photons. They have visualized three-dimensionally the local optical response of diamond at wavelengths between 103 and 206 Å with a resolution as fine as 0.54 Å. This corresponds to a resolution from λ/190 to λ/380, an order of magnitude that is the best ever achieved. For more information, see the paper, "Visualizing the local optical response to extreme-ultraviolet radiation with a resolution of λ/380", K. Tamasaku et al., Nature Physics 7, 705 (2011).

The European XFEL under construction at Hamburg in Germany aims to have the first beam ready in 2015. Very recently, it was found that the design parameters can be further improved. The first revision is to the energy range. This will now be 260 eV - 25 keV, while the 2006 design was 800 eV - 12.4 keV. The second is X-ray pulse duration. This will become of variable duration from a few femtoseconds (fs) to about 100 fs, instead of about 100 fs only. Such upgrades will be realized by improving electron beam quality by building on the experience with the X-ray free-electron laser at Stanford. For further information, visit the Web page, http://www.xfel.eu/news/2011/x_ray_flashes_revised/

When laser light hits thin solid foil, one can obtain soft X-rays, and this is sometimes called a laser plasma X-ray source. When the peak power of the laser becomes extremely high by shortening the pulse duration, it is also possible to observe hard X-ray spectra including Kα and Kβ emission. A team at Sandia National Laboratory has recently reported some calculations on the efficiency of Kα emission. The conversion efficiency of laser energy into Kα X-ray energy is clearly a critical parameter for designing an X-ray source. Basically the value is fairly small, but the team's simulations indicate that an enhancement of efficiency greater than tenfold over conventional single targets may be possible by introducing a two-phase target concept. For more information, see the paper, "Efficiency Enhancement for Kα X-Ray Yields from Laser-Driven Relativistic Electrons in Solids", A. B. Sefkow et al., Phys. Rev. Lett. 106, 235002 (2011).

As reported in the previous news article, "Influence of the M9 class earthquake on synchrotron facilities in Japan", No.3, Vol. 40 (2011)), the Photon Factory, located to the north of Tsukuba city in Ibaraki prefecture, had to cancel all beamtime allocated in the term from May to September 2011. However, scientists have devoted a great deal of time and effort to recovery work, and on May 16, the ring became capable of storing electron beams, and generating synchrotron radiation. Recovery commissioning at each beamline started in the 4^th week of May. Many users are involved in test experiments with their own samples. Some readers may be interested in the status of BL-4A, which is the beamline for X-ray fluorescence spectroscopic analysis. Recovery at the beamline appears more or less complete. Some data taken on March 10, one day before the earthquake, were reproduced almost perfectly. Commissioning will continue until early July. For further information, visit the Web page, http://www.kek.jp/ja/news/highlights/2011/PF_recovery.html (only in Japanese language).

A research team led by Professor J. Stohr (SLAC National Accelerator Laboratory, Stanford, USA) has recently performed time-resolved scanning transmission X-ray microscopy measurements to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. Because of both the short-time (70 ps) and high-spatial (25 nm) resolutions of this imaging technique, the detailed mechanism has become clear as follows; after an incubation time of ~1.3 ns, a 100×300nm² ellipsoidal device switches in ~1 ns via a central domain nucleation and opposite propagation of two domain walls toward the edges. For further understanding, micromagnetic simulations were done and shown as being in good agreement with experiments. For more information, see the paper, "Nonuniform switching of the perpendicular magnetization in a spin-torque-driven magnetic nanopillar", D. P. Bernstein et al., Phys. Rev. B83, 180410(R) (2011).

In the presence of German President Christian Wulff and Brazilian President Dilma Rousseff, the three directors of DESY, the European XFEL, and LNLS have signed a cooperation agreement in Brasilia. For further information, visit the Web page, http://www.xfel.eu/news/2011/brasilian_german_agreement/

RIKEN and the Japan Synchrotron Radiation Research Institute (JASRI) have announced the start-up of the X-ray Free Electron Laser (XFEL) facility in Harima, named "SACLA" (SPring-8 Angstrom Compact Free Electron Laser). For further information, visit the Web page, http://xfel.riken.jp/eng/index.html

In 2009, the U.S. Department of Energy's Brookhaven National Laboratory started construction of the National Synchrotron Light Source II (NSLS-II), which is a new advanced synchrotron X-ray source with a 3 GeV storage ring and around 30 beamlines. Construction has now passed the halfway stage, and magnet installation has just started. The completion of the facility is expected in 2015. For further information, visit the web page, http://www.bnl.gov/nsls2/

It is extremely important to develop new X-ray sources for future X-ray spectrometry. One promising direction is a table-top synchrotron X-ray source, which consists of a high-power pulse laser and an undulator. The method uses acceleration of electrons by pulse laser photons. The idea becomes realistic once the energy reaches GeV and other properties such as stability, emittance etc are improved sufficiently. For such development, it is indispensable to establish the method for quantitatively investigating the structure of the electron beam in time and space. Recently, a German group succeeded in taking snapshots of the magnetic field generated by an accelerated electron bunch and simultaneously of a plasma wave by a combination of two techniques: time-resolved polarimetry and plasma shadowgraphy. For more information, see the paper, "Real-time observation of laser-driven electron acceleration", A. Buck et al., Nature Physics (Published online, March 13, 2011 DOI:10.1038/nphys1942).

As a result of the Tohoku Region Pacific Coast Earthquake in Japan, which took place on March 11, 2011, nearly 30,000 people were killed or are still missing. As can be clearly seen from the map of the magnitude of shaking intensity (see, for example, http://www.scientificamerican.com/article.cfm?id=fast-facts-japan), several research facilities were affected by this disaster. Very strong quakes took place in Tsukuba, Ibaraki prefecture, where the Photon Factory, a synchrotron source, is located. However, first of all, the map does not correspond very well to the loss of lives and damage to buildings, roads, railways and other infrastructure. While the coastal areas of Miyagi, Iwate and Fukushima prefectures were destroyed by the tsunami, many cities and towns in the inland area were quite safe. In spite of the largest earthquake since scientific surveys started, damage was minimal. No lives were lost, and no buildings were completely destroyed in the campus of the Photon Factory. The detailed status of the facility is available in the following Web page, http://pfwww.kek.jp/whats_new/earthquakeinfo/announce_e.html.

All beamtime allocated in the term from May to September has been cancelled. On the other hand, another Japanese synchrotron radiation facility, SPring-8 had no damage, because the location is far from the source of the earthquake. The SPring-8 plans to accept some users of the Photon Factory for experiments. For further information, visit the Web page, http://www.spring8.or.jp/en/urgentnews/110401

One of the hottest topics in X-ray crystallography in the early 21^st century is coherent X-ray diffraction imaging and its application to the determination of atomic structures of non-crystalline materials - the ultimate goal can be a single molecule. The technique appears to require non-ordinary coherent photon sources, such as X-ray free-electron lasers (XFEL), which are now in operation at Stanford. On the other hand, there are several challenging questions basically concerning sample damage, Coulomb explosion, and the role of nonlinearity. Recently, Dr. A. Fratalocchi and his colleague published their calculations showing that XFEL-based single-molecule imaging will only be possible with a few-hundred long attosecond pulses, due to significant radiation damage and the formation of preferred multisoliton clusters which reshape the overall electronic density of the molecular system at the femtosecond scale. For more information, see the papers, "Single-Molecule Imaging with X-Ray Free-Electron Lasers: Dream or Reality?", A. Fratalocchi et al., Phys. Rev. Lett. 106, 105504 (2011).

Two very exiting experimental reports have been published on the application of an X-ray free electron laser (XFEL) at Linac Coherent Light Source (LCLS, Stanford, USA). An international research team led by Dr. H. Chapman (DESY, Hamburg, Germany) and Professor J. Hajdu (Uppsala University, Sweden) has demonstrated a new advanced stage of protein crystallography, which uses only tiny proteins instead of preparing large-size crystals. This could open up new possibilities for the analysis of proteins that have been difficult or even impossible to prepare so far. The technique has been basically known as coherent X-ray diffraction imaging. The present research is the first experimental application of extremely brilliant femtosecond XFEL pulses. In addition to the demonstration of snapshots of nano-crystalline proteins, they have reported the first single-shot images of intact viruses. For more information, see the papers, "Femtosecond X-ray protein nanocrystallography", H. N. Chapman et al., Nature, 470, 73 (2011) and "Single mimivirus particles intercepted and imaged with an X-ray laser", M. M. Seibert et al., Nature, 470, 78 (2011).

One of hottest topics related to the application of an X-ray free electron laser (XFEL) is how to determine the structure of non-crystalline membrane proteins. There has been a clear conflict between the incident brightness required to achieve diffraction-limited atomic resolution and the electronic and structural damage induced by such illumination. Professors K. A. Nugent and H. M. Quiney (ARC Centre of Excellence for Coherent X-ray Science, University of Melbourne, Victoria, Australia) have recently published their theoretical research on this problem. They have improved the imaging model by using optical coherence theory and quantum electrodynamics, and concluded that the analysis is far more tolerant of electronic damage than believed so far. For more information, see the paper, "Biomolecular imaging and electronic damage using X-ray free-electron lasers", H. M. Quiney et al., Nature Physics, 7, 142 (2011).