Coherent X-ray Sources Research Articles

Holographic soft X-ray omni-microscopy of biological specimens

Mask-based digital soft X-ray Fourier transform holography using coherent synchrotron radiation is applied to image diatoms and freeze-dried 3T3 fibroblast cells. At soft X-ray wavelengths between 4 nm and 8 nm we achieve spatial resolutions down to 450 nm over a 33.5 microm field of view. As holography records amplitude and phase of the scattered wavefield, images with different contrast mechanisms can be generated a posteriori by numerical processing of the holographic data ("omni-microscopy"). We present Zernike phase contrast, Schlieren phase contrast, differential interference and gradient contrast images based on amplitude and phase information. We discuss the potential of this approach for destructive single-shot imaging beyond dose limitations of biological samples at coherent femtosecond-pulsed X-ray sources.

「高輝度コヒーレントX線光源」特集号によせて

「高輝度コヒーレントX線光源」特集号によせて

NONDESTRUCTIVE THREE-DIMENSIONAL X-RAY DIFFRACTION IMAGING OF NANOSCALE PARTICLES

We report a novel approach to X-ray diffraction data analysis for nondestructive determination of the shape of nanoscale particles and clusters in three dimensions with a spatial resolution of a few nanometers. The advantage of the proposed approach is that it does not require a coherent X-ray source and therefore is suitable for almost any synchrotron radiation beamline and many laboratory sources. The technique is insensitive to the coherence of the X-rays, and 3D reconstruction of a modal image is possible without tomographic synthesis, rendering the approach suitable for laboratory facilities. Successful application of the technique to the characterization of nanoparticulate in a range of dispersed-phase nanocomposite structures illustrates this novel approach.

Wavelength Dispersive X-ray Absorption Fine Structure Imaging by Parametric X-ray Radiation

The parametric X-ray radiation (PXR) generator system at Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University is a monochromatic and coherent X-ray source with horizontal wavelength dispersion. The energy definition of the X-rays, which depends on the horizontal size of the incident electron beam on the generator target crystal, has been investigated experimentally by measuring the X-ray absorption near edge structure (XANES) spectra on Cu and CuO associated with conventional X-ray absorption imaging technique. The result demonstrated the controllability of the spectrum resolution of XANES by adjusting of the horizontal electron beam size on the target crystal. The XANES spectra were obtained with energy resolution of several eV at the narrowest case, which is in qualitative agreement with the energy definition of the PXR X-rays evaluated from geometrical consideration. The result also suggested that the wavelength dispersive X-ray absorption fine structure measurement associated with imaging technique is one of the promising applications of PXR.

Phase-contrast imaging of nanostructures by soft x rays from a femtosecond-laser plasma

The possibility of phase-contrast imaging of nanostructures has been analyzed with the use of a femtosecond-laser plasma as a spatially coherent soft x-ray source and a LiF crystal as an x-ray detector having both the submicron spatial resolution in a wide field of view and a high contrast. It is demonstrated that the spatial coherence length of radiation in the wavelength range 1–13 nm at a distance of 30 cm from the femtosecond-laser plasma source is ≃1.5 μm. The achieved spatial coherence of the source is sufficient to obtain high-quality phase-contrast x-ray images of foils with various chemical compositions and a thickness of ≃100 nm.

Two-dimensional multiwire gas proportional detector for X-ray photon correlation spectroscopy of condensed matter

Details of a two-dimensional (2-D) multiwire gas proportional detector for X-ray photon correlation spectroscopy (XPCS) of condensed matter are described. The characteristics of the gas proportional detector at 8 keV, 0.3 pC anode charge, and 3 bar (absolute) of Xe/10%CO 2 are as follows: 8.5×10 −7 counts/s (100×100 μm 2) dark count rate, ∼μs time resolution, ∼48 and 73 μm position resolution (FWHM) along and across the anode wire direction, respectively, and ∼80% quantum efficiency. The effects of incident photon energy, anode charge (i.e., gain), gas drift depth, and gas pressure on position resolution are discussed. Static and dynamic speckle patterns, measured from disordered aerogel and polystyrene/polybutadiene blends by a partially coherent synchrotron X-ray source, demonstrate that a 2-D multiwire gas proportional detector is very suitable for the dynamic study of condensed matter with relaxation times in the order of μs to 10 3 s and atomic length scale.

High numerical aperture tabletop soft x-ray diffraction microscopy with 70-nm resolution

Light microscopy has greatly advanced our understanding of nature. The achievable resolution, however, is limited by optical wavelengths to approximately 200 nm. By using imaging and labeling technologies, resolutions beyond the diffraction limit can be achieved for specialized specimens with techniques such as near-field scanning optical microscopy, stimulated emission depletion microscopy, and photoactivated localization microscopy. Here, we report a versatile soft x-ray diffraction microscope with 70- to 90-nm resolution by using two different tabletop coherent soft x-ray sources-a soft x-ray laser and a high-harmonic source. We also use field curvature correction that allows high numerical aperture imaging and near-diffraction-limited resolution of 1.5lambda. A tabletop soft x-ray diffraction microscope should find broad applications in biology, nanoscience, and materials science because of its simple optical design, high resolution, large depth of field, 3D imaging capability, scalability to shorter wavelengths, and ultrafast temporal resolution.

Grating-Assisted Phase Matching in Extreme Nonlinear Optics

We propose a new technique for phase matching high harmonic generation that can be used for generating bright, tabletop, tunable, and coherent x-ray sources at keV photon energies. A weak quasi-cw counterpropagating field induces a sinusoidal modulation in the phase of the emitted harmonics that can be used for correcting the large plasma-induced phase mismatch. We develop an analytical model that describes this grating-assisted x-ray phase matching and predicts that very modest intensities (<10(10) W/cm2) of quasi-cw counterpropagating fields are required for implementation.

Polychromatic phase‐contrast computed tomography

Polychromatic phase-contrast radiography differs from traditional (absorption-only) radiography in that the method requires at least a partially coherent x-ray source and the resulting images contain information about the phase shifts of x-rays in addition to the traditional absorption information. In a typical embodiment, this effect results in a measurable enhancement in image contrast at the edges of objects. In this study, a phase-contrast imaging system was adapted to allow an object to be imaged at multiple projections, and these projections were used to generate phase-contrast computed tomography images. The images obtained with this technique show edge enhancements surrounding the objects within the image.

Instability of Solution of Phase Retrieval in DirectDiffraction Phase-Contrast Imaging with Partially Coherent X-Ray Source

The theoretical model of direct diffraction phase-contrast imaging withpartially coherent x-ray source is expressed by an operator of multipleintegral. It is presented that the integral operator is linear. Theproblem of its phase retrieval is described by solving an operatorequation of multiple integral. It is demonstrated that the solution ofthe phase retrieval is unstable. The numerical simulation is performedand the result validates that the solution of the phase retrieval isunstable.

Diffractive imaging of highly focused X-ray fields

The rapid development of new sources of coherent X-rays, such as third-generation synchrotrons, high-harmonic-generation lasers1 and X-ray free-electron lasers2, has led to the emergence of the new field of X-ray coherent science. The extension of coherent methods to the X-ray regime makes possible methods such as coherent diffraction, X-ray photon-correlation spectroscopy, speckle interferometry and ultrafast probing at atomic resolution and femtosecond timescales. Despite rapid improvements in the resolution that conventional X-ray optics can achieve, new methods for manipulating X-rays are required to push this to the atomic scale3. Here we demonstrate a coherent imaging technique that enables us to image the complex field at the focus of an X-ray zone plate without the need for conventional X-ray lenses. There are no fundamental limits on the resolution of this lensless imaging technique other than the wavelength of the X-rays themselves. The ability to characterize the beam with one measurement makes the method ideally suited to characterizing the fields generated by pulsed coherent X-ray sources.

Development of CMR Manganite Sensors for Energy Measurement of the Linear Coherent Light Source Free Electron Laser Xray Pulses

ABSTRACTWe are developing Colossal Magnetoresistive (CMR) manganite thin film bolometric sensors to be employed as total energy detectors for beam diagnostics of the Linear Coherent Light Source (LCLS) Free Electron Laser (FEL) (at the Stanford Linear Accelerator). LCLS is an ultra bright, ultra short coherent x-ray source whose peak brightness will exceed that of third generation x-ray sources by about ten orders of magnitude and average brightness by three orders of magnitudes. It is expected to produce 1012 x-rays per 200 fs pulse with a repeat frequency of 120 Hz through self-amplified stimulated emission. In characterizing the beam, it will be necessary to measure the total energy of the FEL pulse. The Advanced Detector Group at Lawrence Livermore Laboratory has developed a scheme for FEL total energy measurements based on bolometric detection and are collaborating with Towson University to implement such a detector using CMR manganite thin films. Here we discuss the basic scheme, results of simulations of the thermal response and the materials development efforts towards fabricating the thin film detectors.

Thin crystal layers in superstrong laser fields: Dynamics and coherent x-ray generation

Coherent x-ray generation is investigated for a thin solid layer or any other periodic atomic structure with limited dimensions exposed to a short, superintense laser pulse. We single out a selective range of parameters at which the impact of nonlinear collective processes on ion dynamics is reduced. This allows us to consider an ionic structure that is quasiregular during the brief interaction, which we prove also by particle-in-cell simulations. In the relativistic regimes of the interaction, when the transversal oscillation amplitude of the electron in the laser field exceeds the layer transversal width, the multiphoton scattering of electrons dominates over recombination to bound states. Electrons accelerated by the laser field penetrate through the periodic ionic structure, emitting coherent bremsstrahlung. Hard x rays are shown to amplify along the interaction length due to stimulated coherent bremsstrahlung. The high gain of the scheme compensates for the short interaction length, yielding a gain-length product competitive with other coherent x-ray sources. Limitations on the gain due to the energy spread of the electrons and the short time of the interaction are estimated.

Toward time-resolved soft X-ray microscopy using pulsed fs-high-harmonic radiation

Coherent soft X-ray sources open the way to new capabilities in high-resolution imaging, site- and element-specific spectroscopy and biomicroscopy. In this paper we demonstrate imaging with a table-top soft X-ray microscope. By combining a laser driven high-harmonic light source, optimized for having the maximum brightness at around 100 eV, a pair of multilayer mirrors to select a narrow spectral band and acting simultaneously as a condenser and a Fresnel zone plate as microscope objective, we were able to resolve 200 nm structures of a diatom sample. Further, the pulsed nature of our X-ray source offers the possibility of time-resolved spectromicroscopy with a temporal resolution in the order of a few femtoseconds.

Contrast and resolution in direct Fresnel diffraction phase-contrast imaging with partially coherent x-ray source

A general treatment of x-ray image formation by direct Fresnel diffraction with partially coherent hard x rays is presented. Contrast and resolution are the criteria used to specify the visibility of an image, which depend primarily on the spatial coherence of the illumination and the distance from object to the image, with chromatic coherence of lesser importance. The dependence of the quality of phase-contrast images on the parameters of in-line imaging configuration is described quantitatively. The influence of spatial coherence of hard x-ray source on the imaging quality is also discussed based on the partially coherent direct Fresnel diffraction phase-contrast imaging theory. Experimental results are also presented for phase-contrast x-ray images with partially coherent hard x rays.

Diffractive imaging of nonperiodic materials with future coherent X-ray sources.

Coherent diffractive imaging using a coherent X-ray source promises to be a useful microscopic method for imaging noncrystalline objects at high spatial resolution. In this article a simple method to estimate the coherently scattered signal as a function of resolution is presented, and it is shown that the required X-ray flux or dose scales as the inverse third power of resolution for a specimen of constant volume and density. A simulated case study using the proposed energy-recovery linac source is also presented, which confirms the estimated flux requirement.

Low-divergence coherent soft x-ray source at 13 nm by high-order harmonics

We have generated a few tens of nanojoules of coherent soft x-ray pulses at a wavelength around 13 nm with high-order harmonics. Output energy was estimated to be 25 nJ/pulse at the 59th harmonic (13.5 nm), and the beam divergence was measured to be 0.35 mrad full width at half maximum. Since a Mo/Si mirror covers two harmonics in the high reflectivity region, we can use ∼50 nJ harmonic energy per pulse in the 13 nm wavelength. This high-coherent low-emittance x-ray source is useful for the characterization and inspection of an optical system for extreme-ultraviolet lithography.

Coherent soft x-ray generation in the water window with quasi-phase matching.

We demonstrate enhanced generation of coherent light in the "water window" region of the soft x-ray spectrum at 4.4 nanometers, using quasi-phase-matched frequency conversion of ultrafast laser pulses. By periodically modulating the diameter of a gas-filled hollow waveguide, the phase mismatch normally present between the laser light and the generated soft x-ray light can be partially compensated. This makes it possible to use neon gas as the nonlinear medium to coherently convert light up to the water window, illustrating that techniques of nonlinear optics can be applied effectively in the soft x-ray region of the spectrum. These results advance the prospects for compact coherent soft x-ray sources for applications in biomicroscopy and in chemical spectroscopy.

Ultrabright multikilovolt coherent tunable x-ray source at λ ∼ 2.71–2.93Å

Detailed molecular structural information of the living state is of enormoussignificance to the medical and biological communities. Since hydrated biologicallyactive structures are small delicate complex three-dimensional (3D) entities,it is essential to have molecular scale spatial resolution, high contrast,distortionless, direct 3D modalities of visualization of naturally functioningspecimens in order to faithfully reveal their full molecular architectures. Anx-ray holographic microscope equipped with an x-ray laser as the illuminatorwould be uniquely capable of providing these images. A quantitative interlockingconcordance of physical evidence, that includes (a) the observation ofstrong enhancement of selected spectral components of several Xeq+ hollow-atomtransition arrays (q = 31,32, 34, 35, 36, 37) radiated axially from confined plasma channels, (b)the measurement of line narrowing that is spectrally correlated with theamplified transitions, (c) evidence for spectral hole-burning in thespontaneous emission, a manifestation of saturated amplification, thatcorresponds spectrally with the amplified lines, and (d) the detection ofan intense narrow (δθx ∼ 0.2 mrad)directed beam of radiation, (1) experimentally demonstratesin the λ ∼ = 2.71–2.93 Årange (ℏωx ∼ = 4230–4570 eV)the operation of a new concept capable of producing the ideal conditions foramplification of multikilovolt x-rays and (2) proves the feasibility of acompact x-ray illuminator that can cost-effectively achieve the missionof biological x-ray microholography. The measurements also (α) establish theproperty of tunability in the quantum energy over a substantial fraction of the spectralregion exhibiting amplification (Δℏωx ∼ 345 eV) and(β)demonstrate the coherence of the x-ray output through the observation of acanonical spatial mode pattern. An analysis of the physical scaling revealed bythese results indicates that the capability of the x-ray source potentially includessingle-molecule microimaging, the key for the in situ structural analysis ofmembrane proteins, a cardinal class of drug targets. An estimate of the peakbrightness achieved in these initial experiments gives a value of ∼1031–1032 photons s−1 mm−2 mrad−2/(0.1% bandwidth),a magnitude that is ∼107–108-foldhigher than presently available synchrotron technology.

Overview of FERMI@ELETTRA: a proposed ultra-bright coherent X-ray source in Italy

Together with the INFM and other Italian institutes, Sincrotrone Trieste has proposed and developed FERMI@ELETTRA in response to the Italian government's call for proposals from national institutes for a multipurpose pulsed laser X-ray source. The proposed FEL source will be located at the third-generation synchrotron radiation facility ELETTRA and will utilize the fully available 1.0-GeV normal conducting linac. The project is organized along three stages of development, allowing gradual improvements for the production of 40-, 10-, and 1.2-nm radiation. The first two phases involve upgrading the linac with a high-brightness photo-injector and bunch compressor for the generation of increasingly higher quality electron beams. The third phase requires both an increase in peak current and beam energy to 3.0GeV. The requirements of the user-community necessitate reproducible radiation sources with controllable polarization and wavelength. For this APPLE-type undulators will be implemented and seeding schemes will be used where possible. An overview of the proposal is presented elucidating linac upgrades and overall layout, electron beam simulations, and laser seed schemes.