June 2012 Archives

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).

The 2012 workshop on buried interface science with X-rays and neutrons was held at KEK, Tsukuba, Japan, on June 26-28, 2012. This was the latest in a series of 18 workshops held since 2001. There are increasing demands for sophisticated metrology in order to observe multilayered materials with nano-structures (dots, wires, etc), which are finding applications in electronic, magnetic, optical and other devices. X-ray and neutron analysis is known for its ability to observe in a nondestructive manner even 'buried' function interfaces as well as the surface. In addition to such inherent advantages, recent remarkable advances in micro analysis and quick time-resolved analysis in X-ray reflectometry are extremely important. The latest progress in novel quantum beam technologies, such as XFELs, ERLs, as well as many other table-top laser-like machines could push such techniques towards further sophisticated applications. The present workshop gathered together those with different research backgrounds, i.e., from semiconductor electronics to chemical bio materials, and even theoretical groups were invited to give insights into unsolved problems on buried interfaces.

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).

Dr. D. Babonneau (PhyMat, CNRS UMR 6630, Université de Poitiers, France) and his colleagues have recently analyzed morphological characteristics of nanoripple patterns prepared by broad beam-ion sputtering of Al₂O₃ and Si₃N₄ amorphous thin films as well as 2D arrays of Ag nanoparticles obtained by glancing angle deposition on Al₂O₃ nanorippled buffer layers. They employed 3D reciprocal space mapping in the grazing incidence small-angle X-ray scattering geometry. For more information, see the paper, "Quantitative analysis of nanoripple and nanoparticle patterns by grazing incidence small-angle x-ray scattering 3D mapping", D. Babonneau et al., Phys. Rev. B85, 235415 (2012).