Short period, high field undulators can enable short wavelength free electron lasers (FELs) at low beam energy. A research group led by Professor J. Rosenzweig (
Recently in Lightsources Category
A Brazilian research group recently discussed the thermal influence of soft X-ray free-electron-laser (FEL) pulses on silicon substrate. Such analysis is important, because the peak power of a single FEL pulse is roughly four orders of magnitude higher than that in conventional synchrotron light facilities. Their detailed time-evolution analysis indicates that in a worst case scenario, the second pulse could be adversely affected by dynamic thermal distortion induced by the preceding pulse. For more information, see the paper, "Thermoelastic analysis of a silicon surface under x-ray free-electron-laser irradiation", A. R. B. de Castro et al., Rev. Sci. Instrum. 81, 073102 (2010).
What happens when an atom is excited by extremely strong X-ray photons such as an X-ray laser? A Stanford research group recently published a very exciting report on the ionization of neon (Z=10) by X-ray laser at the Linac Coherent Light Source (LCLS) housed at the SLAC National Accelerator Laboratory in
A Chinese group led by Professor J. Zhang (President of Shanghai Jiao Tong University) recently published a report on the generation of X-ray pulses of around 3 keV by using an Ar clustering gas jet target (~3mm dia.) and a Ti:sapphire laser (power 800 mJ, pulse width 28 fs, wavelength 800 nm, frequency 10 Hz). The intensity of the Ar K-shell emissions in the forward direction was found to be around 104 photons/mrad2/pulse. The group emphasized the significance of laser contrast, which is a ratio of the main pulse and pre-pulse, and found that X-ray flux is reduced by 2 orders of magnitude if the laser pulse contrast decreases from 109 to 107 with constant laser pulse energy. For more information, see the paper, "Intense High-Contrast Femtosecond K-Shell X-Ray Source from Laser-Driven Ar Clusters", L. M. Chen et al., Phys. Rev. Lett. 104, 215004 (2010).
The Advanced Photon Source has received approval from the U.S. Department of Energy (DOE) for the first stage of an upgrade to the facility. Details of the upgrade program can be found in a downloadable movie, http://www.aps.anl.gov/Upgrade/CDR
In the May 2010 issue of Nature Materials, Joerg Heber interviewed Professor G. Materlik, CEO of the Diamond Light Source,
Professor S. Techert (Max-Planck-Institute for Biophysical Chemistry, Goettingen , Germany ) and his colleagues have reported on Bragg diffraction experiments with a soft X-ray laser (wavelength 8 nm, pulse width 30 fs, power 4×1011 photons/pulse) from the free electron laser at FLASH, Deutsche Elektronen-Synchrotron (DESY) in Hamburg
With linac-based light sources, the electron beam has a high peak current and small energy spread, and this can be used to drive a seeded single pass free electron laser. On the other hand, the beams in a storage ring usually have a relatively low current and large energy spread. To generate ultrashort coherent radiation, the coherent harmonic generation (CHG) technique is a promising candidate. Dr. D. Xiang (SLAC National Accelerator Laboratory,
Laser sources in the hard X-ray region have already become a reality at some free electron laser (FEL) facilities. However, typical hard X-ray FELs use an accelerator that is several km long to generate a ~10 GeV electron beam with ~kA peak current to drive the FEL. Dr. D. Xiang (SLAC National Accelerator Laboratory, USA
X-ray Photon Correlation Spectroscopy (XPCS) is a novel technique which reveals the slow dynamics of equilibrium and non-equilibrium processes in condensed matter systems. A group led by Professor N. P. Balsara (
Nanometer scale dipole moments in the polarization clusters in BaTiO3 are believed to be thermally excited and thermally relaxed within a picosecond time scale. However, so far, there have been no reports on the direct observation of the dynamics of these dipole moments in such a very short time scale. The limitation here is mainly due to the low spatial coherence of the X-ray beam, in particular when synchrotron radiation is used as a light source. Professor K. Namikawa (Tokyo Gakugei Univ, Japan
Professors T. Narayanan (ESRF, Grenoble, France), M. Giglio (XFEL, Hamburg, Germany) and their collaborators have recently published an interesting paper on a novel method to map the two-dimensional transverse coherence of an X-ray beam. The technique uses the dynamical near-field speckles formed by scattering from colloidal particles, which are executing Brownian motions. It is possible to measure the change of the interference fringes, and consequently the fluctuation of speckles. It was found that the coherence properties of synchrotron radiation from an undulator source are obtained with high accuracy. For more information, see the paper, "Probing the transverse coherence of an undulator X-ray beam using Brownian particles", M. D. Alaimo et al., Phys. Rev. Lett., 103, 194805 (2009).
A recent edition of Nature News featured the international race to build X-ray free electron laser facilities. At the Linac Coherent Light Source (LCLS),
Some readers might remember the news article, "A compact synchrotron light source driven by pulse laser", in X-ray Spectrometry, Vol. 37, No.2 (2008). The essential point is that a table top pulse laser can be used as an alternative to a linear or circular electron accelerator. The article above reported the first successful synchrotron radiation generation from laser-plasma-accelerated electrons, but the wavelength was only in the visible to infrared region. Recently, an international team led by Professors S. Karsch and F. Grunera achieved a new breakthrough. The team belongs to Munich 's Cluster of Excellence "Munich Centre for Advanced Photonics" (MAP), in the Laboratory for Attosecond Physics (LAP) of Ludwig-Maximilians-Universitat (LMU) in Munich
At Stanford's linac coherent light source (LCLS), a great deal of effort has been devoted since April this year to initial scientific tests of an X-ray laser. In September, scientists attempted to strip all ten electrons from an atom of neon. They were able to adjust the proportion of different neon species, from non-ionized Ne (no missing electrons) to Ne10+ (lacking all 10 electrons), by fine-tuning the powerful LCLS X-ray beam. For more information, visit the Web page, http://today.slac.stanford.edu/
Nearly $19 million in funding through the American Recovery and Reinvestment Act is supporting the Cornell High Energy Synchrotron Source (CHESS), Cornell Electron Storage Ring (CESR) and ongoing efforts to plan and build a new linear accelerator, the Energy Recovery Linac (ERL). So far, Cornell has received more than 90 ARRA grants, totally about $76 million. For more information, visit the Web page, http://www.news.cornell.edu/
In the film Star Trek IV (1986), transparent aluminum is used for the exterior portals and windows of spacecraft. Now transparent aluminum has become a hot topic for real, rather than in science fiction. An international team, led by Oxford University scientists, has recently reported that a short pulse from the FLASH laser (wavelength 13.5 nm) knocks out a core L-shell electron from every aluminium atom in a 50 nm Al thin film without destroying the metal's crystalline structure. This rendered the aluminium almost invisible for this wavelength. This phenomenon is called saturable absorption. The transient state of aluminium produced in this way is as dense as ordinary matter but can only exist for an extremely short period of time of 40 femtoseconds. For more information, see the paper, "Turning solid aluminium transparent by intense soft X-ray photoionization", B. Nagler et al., Nature Physics 5, 693 (2009).
When a strong laser beam hits the surface of a material, plasma is produced there, subsequently leading to the emission of a short burst of X-rays. It is believed that the electrons in the surface plasma are accelerated by the strong electric field of the laser and then penetrate the solid behind. There, they knock out electrons from inner electronic shells, which subsequently undergo inner-shell recombination, leading to characteristic line emissions such as Kα and Kβ spectra. A research group led by Professor U. Teubner (
Since 1984, laboratory-scale X-ray lasers have been extensively studied. The shortest wavelength achieved so far is 3.6 nm, with a weak intensity. On the other hand, X-ray free-electron lasers (XFEL) based on self-amplified spontaneous emission (SASE) from a long undulator in the linear electron accelerator will be available in near future. The next idea is the use of XFEL to pump a photoionization inner-shell X-ray laser in an atomic gas. Dr. R. London (Lawrence Livermore National Lab) and a colleague have recently published their theoretical calculations. For more information, see the paper, "Atomic inner-shell X-ray laser pumped by an x-ray free-electron laser", N. Rohringer et al., Phys. Rev. A 80, 013809 (2009).
Imaging individual objects of several nanometer resolution in space and several femtosecond resolution in time, is now one of the most exciting experiments in X-ray physics. Over the past decade, coherent X-ray diffraction has overcome a lot of limits in imaging noncrystalline objects at a resolution in the order of X-ray wavelength. So far, X-ray free electron lasers (or, in the mean time, 3rd generation synchrotron sources) have been considered as a promising source, but the table-top source is no doubt extremely important for many new sciences. Recently, Dr. H. Merdji (CEA Saclay, France) and his colleagues reported the feasibility of a laser-driven soft X-ray source, which uses the 25th harmonics (32 nm wavelength, 20 fs pulse width) of a Ti:sapphire laser. They succeeded in observing diffraction patterns from isolated nano-objects with a single 20 fs pulse. Images were reconstructed with a spatial resolution of 119 nm from the single shot and 62 nm from multiple shots. For more information, see the paper, "Single-Shot Diffractive Imaging with a Table-Top Femtosecond Soft X-Ray Laser-Harmonics Source", A. Ravasio et al., Phys. Rev. Lett. 103, 028104 (2009).
As reported here previously, in April this year, the first 1.5 Å wavelength laser light was generated at
At the U.S. Department of Energy's SLAC National Accelerator Laboratory, scientists have observed the first beam generated by the hard X-ray laser. The Linac Coherent Light Source (LCLS) now supplies 1.5 Å wavelength coherent hard X-ray ultra short pulses with 100 femtosecond duration. Unlike conventional lasers, which use mirrored cavities to amplify light, the LCLS is a free-electron laser, creating light using free-flying electrons in a vacuum. The LCLS uses the final third of SLAC's two-mile linear accelerator to drive electrons to high energy and through an array of undulator magnets that steer the electrons rapidly back and forth, generating a brilliant beam of coordinated X-rays. LCLS scientists used only 12 of an eventual 33 undulator magnets to generate the facility's first laser light. It is the first time that an X-ray laser has operated at such short wavelengths in the truly hard X-ray region, with such brightness and short pulses. The laser paves the way to a new way of looking at not only the structure of matter but also its dynamics. By using laser pulses of less than 100 femtosecond duration, the dynamics of chemical reactions can be caught in process, and even single molecules can be imaged. For further information, see the facility's Web page, http://home.slac.stanford.edu/pressreleases/2009/20090421.htm In Science Now Daily News, Adrian Cho wrote a comprehensive article, http://sciencenow.sciencemag.org/cgi/content/full/2009/421/2
Laser generation in the X-ray region has become realistic because of the construction of free electron laser facilities, which will be available in the near future (Linac Coherent Light Source (LCLS) at Stanford in 2009; European XFEL in 2014). Another significant route is the extension of existing laser technologies such as high-order harmonic generation (HOHG), particularly from relativistically oscillating plasma mirror-like surfaces. Professor M. Zepf (Queens University Belfast, UK) and his colleagues recently published an interesting paper showing that it is possible to achieve a near-diffraction-limited focal spot size that is also controllable. For more information, see the paper, "Diffraction-limited performance and focusing of high harmonics from relativistic plasmas", B. Drome et al., Nature Physics, advanced online publication doi:10.1038/nphys1158
Lyncean Technologies, Inc., which was founded in
Professor S. Putterman (University of California, Los Angeles, USA) and his colleagues recently demonstrated that simply peeling ordinary sticky tape in a moderate vacuum can generate sufficient X-rays to take an image of a human finger. The phenomenon has long been known as tribo-luminescence (or mechano-luminescence), but their report (including online video accessible from the Nature News page) has impressed many. Nanosecond, 100-mW X-ray pulses as well as radio and visible light have been clearly confirmed to be correlated with stick-slip peeling events. They observed a 15-keV peak in X-ray energy spectra, and attempted to explain it by various models. For more information, see the paper, "Correlation between nanosecond X-ray flashes and stick-slip friction in peeling tape", C. G. Camara et al., Nature, 455, 1089-1092 (2008), and the news article, "Sticky tape generates X-rays - How weird is that?", Katharine Sanderson, Nature News, http://www.nature.com/news/2008/081022/full/news.2008.1185.html as well as readers' comments thereon. A very old and pioneering report describing how peeling tape can be a source of X-rays is "Investigation of electron emission on tearing away highpolymer film from glass in vacuum", V. Karasev et al., Doklady Akademii Nauk SSSR, 88, 777-780 (1953).
A research group at Lawrence Livermore National Laboratory recently reported an interesting application of ultrafast X-ray spectrometry to studies on the compression and heating of shocked matter. Here, the sample is 300 μm thick LiH, which is heated by a 450 J nsec laser, and the X-ray used is Ti Kα X-ray fluorescence (4.51 keV) from Ti foil heated by another pulse laser of 5 psec. X-ray photons produced at the Ti foil are estimated as a 2 × 1013/pulse. The energy spectra of X-ray scattering by the LiH sample during compression were taken by a spectrometer consisting of a large curved graphite (HOPG) crystal in van Hamos geometry and an Imaging Plate (IP) detector. It was found that the X-ray scattering spectrum from shocked LiH shows elastic Rayleigh scattering and inelastic plasmon scattering features. Whereas earlier in time only elastic scattering was observed, at 7 nsec, a plasmon energy shift of 24 eV was detected. This indicates the transition to metallic free electron plasma in the solid phase. For more information, see the paper, "Ultrafast X-ray Thomson Scattering of Shock-Compressed Matter", A. L. Kritcher et al., Science, 322, 69 -71 (2008).
Aerogel is a form of nanofoam, an engineered material designed for its high strength-to-weight ratio for application wherever lightness and strength are needed. Now, the internal structure is within the scope of X-ray analysis. Lawrence Livermore and Lawrence Berkeley scientists have successfully applied the coherent X-ray diffraction technique to Ta2O5 nanofoam, the density of which is 1.2 % to the bulk, and have reconstructed 3D images to determine its strength and potential new applications. Combining the obtained structural information with detailed simulations, the research team showed that the blob-and-beam network structure explains why the materials are weaker than expected. For more information, see the paper, "Three-Dimensional Coherent X-Ray Diffraction Imaging of a Ceramic Nanofoam: Determination of Structural Deformation Mechanisms", A. Barty et al., Phys. Rev. Lett., 101, 055501 (2008).
Scanning diffraction microscopy, or ptychography, was first developed for the scanning transmission electron microscope (STEM). In the same way, by using an X-ray nano beam, one can use a STXM. The X-ray beam is focused onto the sample via a lens, and the transmission is measured. The image is obtained by plotting the transmission as a function of the sample position, as it is rastered across the beam. The analysis is straightforward, but its resolution is limited by the beam size. On the other hand, coherent diffractive imaging (CDI) now reaches resolutions below 10 nm, but the reconstruction procedures are not always easy due to the influences of data quality, sample conditions etc. A Swiss research group led by Drs. C. David and F. Pfeiffer (Paul Scherrer Institut) recently demonstrated a ptychographic imaging method that bridges the gap between STXM and CDI by measuring complete diffraction patterns at each point of a STXM scan. The group employed an advanced large-area pixel detector, Pilatus, to obtain the diffraction pattern efficiently. These diffraction data were then treated with an image reconstruction algorithm developed by the team. Several tens of thousands of diffraction images were processed to obtain one super-resolution X-ray image. The algorithm not only reconstructs the sample but also the exact shape of the light probe resulting from the X-ray beam. The 6.8 keV X-ray beam was focused using a zone plate, and the beam size was 300 nm. The spatial resolution achieved was about five times higher. For more information, see the paper, "High-Resolution Scanning X-ray Diffraction Microscopy", P. Thibault et al., Science, 321, 379 - 382 (2008).
Recently, Professor K.-J. Kim (Argonne National Lab., USA) and his colleagues published a very interesting proposal for the world's brightest X-ray source. In most currently on-going X-ray free electron laser (FEL) projects, self-amplified spontaneous emission (SASE) is employed. It is known that SASE-FEL creates extremely brilliant, coherent X-ray pulses of 0.1 ps duration. Due to the low repetition rate, the average brightness is only about 10,000 times compared with existing 3rd generation synchrotron sources. On the other hand, future X-ray sciences will require other types of X-ray laser source, with an even smaller number of photons in one pulse (to reduce radiation damage to the sample) and with much greater average intensity via a high repetition rate. In Professor Kim's X-ray source based on a FEL oscillator (X-FELO), a pulse of electrons is carried into an undulator as ordinary FEL, but in order to reflect back the generated X-rays into the undulator entrance, there is an optical cavity consisting of two or more Bragg reflectors with low-Z atoms and with low Debye temperature, such as diamond, beryllium oxide and sapphire crystals. In the next step, the X-ray photons connect with the next electron bunch and again travel back along the undulator. This pattern is repeated indefinitely with the X-ray intensity growing each time until equilibrium is reached. As the spectral bandwidth is extremely narrow, at three to four orders of magnitude finer than those produced by SASE-FEL, the intensity of an individual X-ray pulse from an X-FELO is rather low. But the average X-ray intensity is higher than that of SASE-FEL. Over the past 5 years, highly advanced electron beam technologies, which can be used, for example, for a multi-GeV class energy recovery linac (ERL), have become available. One of the key elements of Professor Kim's idea is combination with ERL. This is predicted to produce X-ray pulses with 109 photons at a repetition rate of 1-100 MHz. The pulses are temporarily and transversely coherent, with a rms bandwidth of about 2 meV, and rms pulse length of about 1 ps. To gain an understanding of the original concept of X-FELO, see the paper, "Proposal for a free electron laser in the X-ray region", R. Colella and A. Luccio, Optical Commun., 50, 41-44 (1984). For more information on the proposed X-ray source, see the paper, "A Proposal for an X-Ray Free-Electron Laser Oscillator with an Energy-Recovery Linac", K.-J. Kim et al., Phys. Rev. Lett., 100, 244802 (2008).
As an X-ray free-electron laser (X-FEL) provides extremely strong pulses, it is necessary to understand the photon-induced damage processes for biological samples. A research group led by Dr. Chapman (DESY, Germany and Lawrence Livermore National Lab, USA) has discussed how several aspects of existing continuum damage models can be tested during early operation of X-FEL at lower X-ray energies in the range of 0.8-5 keV and low fluences, focusing particularly on macroscopic collective effects such as particle charging, expansion, and average ionization of nanospheres. For more information, see the paper, "Modeling of the damage dynamics of nanospheres exposed to x-ray free-electron-laser radiation", S. P. Hau-Riege et al., Phys. Rev. E77, 041902 (2008).
Electromagnetically induced transparency (EIT) is a coherent optical nonlinearity, and brings dramatic changes in optical properties such as absorption, emission, refraction etc. The phenomena relate to the quantum mechanical overlapping state created by two different wavelengths of coherent light. Recently, EIT for X-rays has been theoretically predicted. According to the theory, it is possible to make Ne gas, which is normally opaque, transparent by exposing it to laser light of 800 nm with extremely high flux of 1012 W/cm2. The scheme could be used for producing ultra-short X-ray pulses. For more information, see the paper, "Electromagnetically Induced Transparency for X Rays ", C. Buth et al., Phys. Rev. Lett., 98, 253001 (2007). For more about general EIT, see, for example, "Electromagnetically Induced Transparency.", S. Harris, Physics Today, 50, 36-42 (1997).
A research group at the Japan Atomic Energy Agency (Kizugawa, Japan) has recently developed a novel table-top pulsed X-ray source. The source employs a Ti:sapphire laser, emitting 70 fs duration 2 TW pulses of 800 nm wavelength at 10 Hz. The laser beam is focused to the flow of high-density Ar gas. The source was applied to perform phase contrast imaging. For more information, see the paper, "Phase-contrast x-ray imaging with intense Ar Ka radiation from femtosecond-laser-driven gas target", L. M. Chen et al., Appl. Phys. Lett. 90, 211501 (2007).
Recently, some very interesting research on magnetic noise from antiferromagnets has been published. Unlike ferromagnets, the characteristics of which have been studied for many years, antiferromagnets have remained a mystery because their internal structure was too fine to be measured. Their internal order is on the same scale as the wavelength of X-rays, and therefore, X-ray photon correlation spectroscopy, which measures 'speckle' patterns, can give a unique 'fingerprint' of a particular magnetic domain configuration. It was found that the domain wall motion is thermally activated at temperatures above 100 K, but not so at lower temperatures. For more information, see the paper, "Direct measurement of antiferromagnetic domain fluctuations", O. G. Shpyrko, et al., Nature 447, 68 (2007).
Atoms become ions when exposed to extremely intense light. The process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Professor F. Krausz (Max Planck Institute of Quantum Optics in Garching, Germany) and his collaborators recently reported the real-time observation of this most elementary step in strong-field interactions, i.e., light-induced electron tunnelling. The team used 250-attosecond pulses of UV radiation, and confirmed theoretical predictions about the tunneling process. It was also found that the process lasted for several hundred attoseconds, depleting atomic-bound states. This would suggest that the use of tunneling itself is feasible for probing short-lived, transient states of atoms or molecules, e.g., multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes etc. For more information, see the paper, "Attosecond real-time observation of electron tunnelling in atoms", M. Uiberacker et al., Nature, 446, 627 (2007).
The Sub-Picosecond Pulse Source (SPPS) is a prototype X-ray free electron laser built using the 2-mile-long linear accelerator at Stanford Linear Accelerator Center (SLAC), California, United States. To date, ultrafast phenomena have been mainly studied with femtosecond lasers operating at ultraviolet to infrared wavelengths; however, these wavelengths are not short enough for structural studies on atomic distances. Therefore, the emergence of short pulse laser in the hard X-ray region represents a significant challenge. Recently, at Stanford, an international collaborative team from 20 different institutions succeeded in observing the atomic motion of Bismuth crystal, which, although cubic, has a slight elongation along the diagonal called a Peierls distortion. The measurements have brought new fundamental insights into the dynamics of the material, which shows very strong coupling between the electronic and ionic structures. The results could also be used to screen many theoretical calculations made so far. For more information, see the paper, "Ultrafast Bond Softening in Bismuth: Mapping a Solid's Interatomic Potential with X-rays ", D. M. Fritz et al., Science 315, 633 (2007).
At Brookhaven National Laboratory, United States, researchers have recently found a novel way to generate a very short controllable free electron laser (FEL) pulse, which usually depends on the length of the electron pulse. The main idea is the use of a Ti:Sapphire laser that combines a 150 femtosecond (FWHM) pulse of light with the much longer electron beam. This leads to a femtosecond FEL pulse that keeps growing in intensity and shortening in time duration, which is attributed to a phenomenon called superradiance (for details, see, R. H. Dicke, Phys. Rev. 93, 99 (1954)). The present research is the first to experimentally observe the effects of superradiance in a FEL setup. The output FEL pulse duration was measured to be as short as 81 femtoseconds, a roughly 50% reduction compared to the input seed laser. Understanding how to produce these intense, ultrafast pulses of light could help scientists around the world as they begin to construct the next generation of light source facilities. For more information, see the paper, "Experimental Characterization of Superradiance in a Single-Pass High-Gain Laser-Seeded Free-Electron Laser Amplifier ", T. Watanabe et al., Phys. Rev. Lett. 98, 034802 (2007).
In January 2006, the Stardust spacecraft brought back a number of tiny particles from comet Wild 2, which is believed to have originated within a cloud of comets just beyond the orbit of Neptune called the Kuiper Belt. The particles have been analyzed by X-rays at six synchrotron radiation facilities around the world, ESRF (France), APS (Argonne, USA), SSRL(Stanford, USA), ALS (Berkeley, USA), NSLS (Brookhaven, USA) and SPring-8 (Japan). The particles from this comet are important because they are believed to be close to the starting material of the solar system, which is now about 4.5 billion years old. The particles were found to contain a wide variety of minerals and organic materials that look similar to those seen in primitive meteorites found on earth, but the samples also revealed the presence of new materials not previously found in meteorites. It was also discovered that the samples contained minerals similar to Calcium Aluminum-rich inclusions (CAIs), which can be formed at high temperatures, i.e., in the innermost part of the solar nebula, well inside the orbit of Mercury. For more information on the Stardust mission, visit http://stardust.jpl.nasa.gov/home/index.html. Some interesting results have been published as part of a special series of papers in the Dec. 15, 2006, edition of the journal Science.
By combining coherent X-ray scattering with a method of direct phase recovery called over-sampling, lens-free microscopy in the X-ray region becomes a realistic technique. The latest hot topic is the extension of the technique from two to three dimensions. One of the most promising ways of applying this technique is the recently reported combination of (i) ab initio phase retrieval of 2D coherent diffraction patterns with a guided hybrid input-output algorithm and (ii) 3D image reconstruction with equally sloped tomography. The scheme was applied to quantitative 3D imaging of a heat-treated GaN particle with each voxel corresponding to 17×17×17 nm3. The internal GaN-Ga2O3 core shell structure was successfully captured in three dimensions. For more information about the analysis, see the paper, "Three-Dimensional GaN-Ga2O3 Core Shell Structure Revealed by X-Ray Diffraction Microscopy", J. Miao et al., Phys. Rev. Lett. 97, 215503 (2006).
At the FLASH free-electron laser facility at DESY in Hamburg, an international team of scientists recently published the first data on diffraction imaging of a non-crystalline sample. Theoretically, a single X-ray pulse, if it is extremely bright and perfectly coherent, can produce a diffraction pattern from a large macromolecule, a virus or a cell (for example, see, "Potential for biomolecular imaging with femtosecond X-ray pulses", R. Neutze et al., Nature, 406, 752-757 (2000)). In the present experiment, the team tested a laser pulse with 25 fs, 41013 W/cm2/pulse, containing 1012 photons at 32 nm wavelength, and obtained a coherent diffraction pattern from a nanostructured non-periodic object before this exploded into a plasma at ca. 60,000 K. They employed a novel X-ray camera assured of single-photon detection sensitivity by filtering out parasitic scattering and plasma radiation. For more information, see the paper, "Femtosecond diffractive imaging with a soft-X-ray free-electron laser", H. N. Chapman et al., Nature Physics, published online 12 November 2006.
Scientists at the Japan Atomic Energy Agency (JAEA) led by Dr W. Utsumi have proved that the formation of bulk metallic glass of elemental Zr and Ti, which was recently reported (see for example, Zhang and Zhao, Nature 430, 332 (2004) and Y. Wang et al., Phys. Rev. Lett. 95, 155501 (2005)) was some sort of phantom. The experiment basically took the form of X-ray diffraction in high-temperature and high-pressure conditions, but in addition to the normal energy-dispersive detector, the research group employed an in situ angular-dispersive X-ray diffractometer equipped with a 2D detector and X-ray transparent anvils. The disappearance of all the Bragg peaks in the one-dimensional energy-dispersive data could be taken as evidence of amorphization. However, the research group found several intense Bragg spots in their angular-dispersive data, even in the exact same conditions where amorphization was reported. This indicates that Zr and Ti do not form glass, but that the grains grow rapidly. The experiments were carried out at BL14B1 and BL22XU, SPring-8, Japan. For more information, see the paper, "Does Bulk Metallic Glass of Elemental Zr and Ti Exist?", T. Hattori et al., Phys. Rev. Lett., 96, 255504 (2006).
The appearance of the ultimate X-ray microscope, with atomic-scale resolution and capable of seeing deep inside objects, has long been awaited. Professor I. Robinson (University College London, UK) and his team recently made a significant step towards realizing this dream, using the technique of coherent X-ray diffraction imaging, the possibility of which was first pointed out by Sayre (Acta Crystallogr. 5, 843 (1952)) but not demonstrated until 1999 by Miao et al (Nature 400, 342 (1999)). They observed the growth of nanometer-sized Pb crystals inside the vacuum chamber. The results showed that asymmetries in the diffraction pattern can be mapped to deformities, providing a detailed 3-D map of their location in the crystal. This new method shows that the interior structure of atomic displacements within single nanocrystals can be obtained by direct inversion of the diffraction pattern. The technique is an attractive alternative to electron microscopy because of the superior penetration of materials of interest by the electromagnetic waves, which are often less damaging to the sample than electrons. The experiments were done at beamline 34-ID-C at the Advanced Photon Source (APS) in the United States. For more information, see the paper, "Three-dimensional mapping of a deformation field inside a nanocrystal", Mark A. Pfeifer et al., Nature 442, 63 (2006).
The mineral silica (SiO2) is a common substance that is a constituent of all of the planets in our solar system. At SPring-8, Harima, Japan, Dr. K. Hirose (Tokyo Institute of Technology; Japan Agency for Marine-Earth Science and Technology) and his co-workers recently found that, above 268 GPa and 1800 K, silica exhibits a novel stable high-pressure form with a pyrite-type structure, which is much denser than other known silica phases. This form of silica could be one of the main constituents of the core of a gas-giant planet such as Uranus or Neptune. For more information, see the paper, "The Pyrite-Type High-Pressure Form of Silica", Y. Kuwayama et al., Science, 309, 923-925 (2005).
A joint research group from Russia, the Ukraine and the USA has developed a table-top microscope, consisting of a pulsed extreme ultraviolet (EUV) capillary discharge laser emitting at 46.9 nm, a Schwarzschild condenser, a zone plate objective, and a CCD camera. To reduce image-degrading effects such as speckle and interference, the team shortened the laser's capillary tube length from 36 to 18 cm to give a low-coherence beam with a pulse energy of around 0.1 mJ. The spatial resolution is currently 100 nm. Typical exposure time is 20~70 seconds. For more information, see the paper, "Reflection mode imaging with nanoscale resolution using a compact extreme ultraviolet laser", F. Brizuela et al., Optics Express, 435, 1210-1213 (2005), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-13-11-3983
An Austrian research group recently succeeded in obtaining highly collimated, spatially coherent X-rays, at a wavelength of about 1 nm and at photon energies extending to 1.3 keV, from high-order harmonic generation in an atomic gas ionized by a 720-nm, 5-fs, 0.2-TW laser pulse. The beam divergence was evaluated as 0.2 mrad for the spectral range above 200 eV from a knife-edge scan, indicating perfect coherence of the atomic dipoles within a macroscopic volume of diameters of 13μm and 4μm at photon energies of 0.3 keV and 1 keV, respectively. The beam seems to be diffraction-limited to within a factor of five. The spectrum of the generated radiation was observed by an energy-dispersive X-ray spectrometer with some filters. The results are really exciting, because they could detect the copper L-edges (~950 eV)! One would notice that the energy of photons produced by laser technologies has been increasing every year - the main idea behind this progress is the creation of time-gradient in the driving pulse, which allows some 25% of the helium atoms to be ionized within half a cycle before the pulse peak. The electrons detached within this time are pushed in the most intense half-cycle back to the atomic core. For more information, see the paper, "Source of coherent kiloelectronvolt X-rays", J. Seres et al., Nature, 433, 596 (2005). C. Streli and P. Wobrauschek (Atominstitut der Osterreichischen Universtitaten, Technische Universitat Wien) were the co-authors of this paper.
So far, it has been difficult to observe nonlinear responses to an optical field in the extreme ultraviolet (XUV) and soft X-ray regions. A research group from the University of Tokyo recently succeeded in generating intense isolated XUV pulses (photon energy 27.9 eV) that were shorter than 1 femtosecond through high-harmonic (9th) generation by using a sub-10-femtosecond blue laser (photon energy 3.1 eV) producing a large dipole moment. For more information, see the paper, "Nonlinear optics in the extreme ultraviolet", T. Sekikawa et al., Nature, 432, 605-608 (2004).
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