Spatiotemporal Response of Crystals in X-ray Bragg Diffraction

The spatiotemporal response of crystals in x-ray Bragg diffraction resulting from excitation by an ultra-short, laterally confined x-ray pulse is studied theoretically. The theory presents an extension of the analysis in symmetric reflection geometry [1] to the generic case, which includes Bragg diffraction both in reflection (Bragg) and transmission (Laue) asymmetric scattering geometries. The spatiotemporal response is presented as a product of a crystal-intrinsic plane wave spatiotemporal response function and an envelope function defined by the crystal-independent transverse profile of the incident beam and the scattering geometry. The diffracted wavefields exhibit amplitude modulation perpendicular to the propagation direction due to both angular dispersion and the dispersion due to Bragg's law. The characteristic measure of the spatiotemporal response is expressed in terms of a few parameters: the extinction length, crystal thickness, Bragg angle, asymmetry angle, and the speed of light. Applications to self-seeding of hard x-ray free electron lasers are discussed, with particular emphasis on the relative advantages of using either the Bragg or Laue scattering geometries. Intensity front inclination in asymmetric diffraction can be used to make snapshots of ultra-fast processes with femtosecond resolution

Tunable optical cavity for an x-ray free-electron-laser oscillator

An x-ray free-electron laser oscillator proposed recently for hard x rays [K. Kim, Y. Shvyd’ko, and S. Reiche, Phys. Rev. Lett. 100, 244802 (2008)PRLTAO0031-900710.1103/PhysRevLett.100.244802] can be made tunable by using an x-ray cavity composed of four crystals, instead of two. The tunability of x-ray energy will significantly enhance the usefulness of an x-ray free-electron laser oscillator. We present a detailed analysis of the four-crystal optical cavity and choice of crystals for several applications: inelastic x-ray scattering, nuclear resonant scattering, bulk-sensitive hard x-ray photoemission spectroscopy, other high-energy-resolution (≲1  meV) spectroscopic probes, and for imaging with hard x rays at near-atomic resolution (≃1  nm)

Hard-X-Ray Spectroscopy with a Spectrographic Approach

Hard-x-ray spectroscopy relies on a suite of modern techniques for studies of vibrational, electronic, andmagnetic excitations in condensed matter. At present, the energy resolution of these techniques can beimproved only by decreasing the spectral window of the involved optics—monochromators and analyzers—thereby sacrificing the intensity. Here,we demonstrate hard-x-ray spectroscopy with greatly improved energyresolution without narrowing the spectral window by adapting principles of spectrographic imaging tothe hard-x-ray regime. Similar to Newton’s classical prism, the hard-x-ray spectrograph disperses different“colors”—i.e., energies—of x-ray photons in space. Then, selecting each energy component with a slit ensureshigh energy resolution, whereas measuring x-ray spectra with all components of a broad spectral windowkeeps the intensity. We employ the principles of spectrographic imaging for phonon spectroscopy. Here thenew approach revealed anomalous soft atomic dynamics in α-iron, a phenomenon which was not previouslyreported in the literature.We argue that hard-x-ray spectrographic imaging also could be a path to discoveringnew physics in studies of electronic and magnetic excitations

High-contrast sub-millivolt inelastic X-ray scattering for nano- and mesoscale science

Photon and neutron inelastic scattering spectrometers are microscopes for imaging condensed matter dynamics on very small length and time scales. Inelastic X-ray scattering permitted the first quantitative studies of picosecond nanoscale dynamics in disordered systems almost 20 years ago. However, the nature of the liquid-glass transition still remains one of the great unsolved problems in condensed matter physics. It calls for studies at hitherto inaccessible time and length scales, and therefore for substantial improvements in the spectral and momentum resolution of the inelastic X-ray scattering spectrometers along with a major enhancement in spectral contrast. Here we report a conceptually new spectrometer featuring a spectral resolution function with steep, almost Gaussian tails, sub-meV (≃620 μeV) bandwidth and improved momentum resolution. The spectrometer opens up uncharted space on the dynamics landscape. New results are presented on the dynamics of liquid glycerol, in the regime that has become accessible with the novel spectrometer