The COMIX polarimeter: a compact device for XUV polarization analysis.

We report on the characterization of a novel extreme-ultraviolet polarimeter based on conical mirrors to simultaneously detect all the components of the electric field vector for extreme-ultraviolet radiation in the 45-90 eV energy range. The device has been characterized using a variable polarization source at the Elettra synchrotron, showing good performance in the ability to determine the radiation polarization. Furthermore, as a possible application of the device, Faraday spectroscopy and time-resolved experiments have been performed at the Fe M2,3-edge on an FeGd ferrimagnetic thin film using the FERMI free-electron laser source. The instrument is shown to be able to detect the small angular variation induced by an optical external stimulus on the polarization state of the light after interaction with magnetic thin film, making the device an appealing tool for magnetization dynamics research.

Observation of Magnetic Helicoidal Dichroism with Extreme Ultraviolet Light Vortices.

We report on the experimental evidence of magnetic helicoidal dichroism, observed in the interaction of an extreme ultraviolet vortex beam carrying orbital angular momentum with a magnetic vortex. Numerical simulations based on classical electromagnetic theory show that this dichroism is based on the interference of light modes with different orbital angular momenta, which are populated after the interaction between light and the magnetic topology. This observation gives insight into the interplay between orbital angular momentum and magnetism and sets the framework for the development of new analytical tools to investigate ultrafast magnetization dynamics.

Backside-illuminated scientific CMOS detector for soft X-ray resonant scattering and ptychography.

The impressive progress in the performance of synchrotron radiation sources is nowadays driven by the so-called `ultimate storage ring' projects which promise an unprecedented improvement in brightness. Progress on the detector side has not always been at the same pace, especially as far as soft X-ray 2D detectors are concerned. While the most commonly used detectors are still based on microchannel plates or CCD technology, recent developments of CMOS (complementary metal oxide semiconductor)-type detectors will play an ever more important role as 2D detectors in the soft X-ray range. This paper describes the capabilities and performance of a camera equipped with a newly commercialized backside-illuminated scientific CMOS (sCMOS-BSI) sensor, integrated in a vacuum environment, for soft X-ray experiments at synchrotron sources. The 4 Mpixel sensor reaches a frame rate of up to 48 frames s-1 while matching the requirements for X-ray experiments in terms of high-intensity linearity (>98%), good spatial homogeneity (<1%), high charge capacity (up to 80 ke-), and low readout noise (down to 2 e- r.m.s.) and dark current (3 e- per second per pixel). Performance evaluations in the soft X-ray range have been carried out at the METROLOGIE beamline of the SOLEIL synchrotron. The quantum efficiency, spatial resolution (24 line-pairs mm-1), energy resolution (<100 eV) and radiation damage versus the X-ray dose (<600 Gy) have been measured in the energy range from 40 to 2000 eV. In order to illustrate the capabilities of this new sCMOS-BSI sensor, several experiments have been performed at the SEXTANTS and HERMES soft X-ray beamlines of the SOLEIL synchrotron: acquisition of a coherent diffraction pattern from a pinhole at 186 eV, a scattering experiment from a nanostructured Co/Cu multilayer at 767 eV and ptychographic imaging in transmission at 706 eV.

Ultrafast Structural Dynamics along the ß-γ Phase Transition Path in MnAs.

We investigate the orthorhombic distortion and the structural dynamics of epitaxial MnAs layers on GaAs(001) using static and time-resolved x-ray diffraction. Laser-induced intensity oscillations of Bragg reflections allow us to identify the optical phonon associated with orthorhombic distortion and to follow its softening along the path towards an undistorted phase of hexagonal symmetry. The frequency of this mode falls in the THz range, in agreement with recent calculations. Incomplete softening suggests that the ß-γ transformation deviates from a purely second-order displacive transition.

Pump-probe experiments at the TEMPO beamline using the low-α operation mode of Synchrotron SOLEIL.

The SOLEIL synchrotron radiation source is regularly operated in special filling modes dedicated to pump-probe experiments. Among others, the low-α mode operation is characterized by shorter pulse duration and represents the natural bridge between 50 ps synchrotron pulses and femtosecond experiments. Here, the capabilities in low-α mode of the experimental set-ups developed at the TEMPO beamline to perform pump-probe experiments with soft X-rays based on photoelectron or photon detection are presented. A 282 kHz repetition-rate femtosecond laser is synchronized with the synchrotron radiation time structure to induce fast electronic and/or magnetic excitations. Detection is performed using a two-dimensional space resolution plus time resolution detector based on microchannel plates equipped with a delay line. Results of time-resolved photoelectron spectroscopy, circular dichroism and magnetic scattering experiments are reported, and their respective advantages and limitations in the framework of high-time-resolution pump-probe experiments compared and discussed.

Widely tunable two-colour seeded free-electron laser source for resonant-pump resonant-probe magnetic scattering.

The advent of free-electron laser (FEL) sources delivering two synchronized pulses of different wavelengths (or colours) has made available a whole range of novel pump-probe experiments. This communication describes a major step forward using a new configuration of the FERMI FEL-seeded source to deliver two pulses with different wavelengths, each tunable independently over a broad spectral range with adjustable time delay. The FEL scheme makes use of two seed laser beams of different wavelengths and of a split radiator section to generate two extreme ultraviolet pulses from distinct portions of the same electron bunch. The tunability range of this new two-colour source meets the requirements of double-resonant FEL pump/FEL probe time-resolved studies. We demonstrate its performance in a proof-of-principle magnetic scattering experiment in Fe-Ni compounds, by tuning the FEL wavelengths to the Fe and Ni 3p resonances.

Thermally induced magnetization switching in Fe/MnAs/GaAs(001): selectable magnetic configurations by temperature and field control.

Spintronic devices currently rely on magnetization control by external magnetic fields or spin-polarized currents. Developing temperature-driven magnetization control has potential for achieving enhanced device functionalities. Recently, there has been much interest in thermally induced magnetisation switching (TIMS), where the temperature control of intrinsic material properties drives a deterministic switching without applying external fields. TIMS, mainly investigated in rare-earth-transition-metal ferrimagnets, has also been observed in epitaxial Fe/MnAs/GaAs(001), where it stems from a completely different physical mechanism. In Fe/MnAs temperature actually modifies the surface dipolar fields associated with the MnAs magnetic microstructure. This in turn determines the effective magnetic field acting on the Fe overlayer. In this way one can reverse the Fe magnetization direction by performing thermal cycles at ambient temperatures. Here we use element selective magnetization measurements to demonstrate that various magnetic configurations of the Fe/MnAs/GaAs(001) system are stabilized predictably by acting on the thermal cycle parameters and on the presence of a bias field. We show in particular that the maximum temperature reached during the cycle affects the final magnetic configuration. Our findings show that applications are possible for fast magnetization switching, where local temperature changes are induced by laser excitations.

Design and performance of AERHA, a high acceptance high resolution soft x-ray spectrometer.

A soft x-ray spectrometer based on the use of an elliptical focusing mirror and a plane varied line spacing grating is described. It achieves both high resolution and high overall efficiency while remaining relatively compact. The instrument is dedicated to resonant inelastic x-ray scattering studies. We set out how this optical arrangement was judged best able to guarantee performance for the 50 - 1000 eV range within achievable fabrication targets. The AERHA (adjustable energy resolution high acceptance) spectrometer operates with an effective angular acceptance between 100 and 250 µsr (energy dependent) and a resolving power well in excess of 5000 according to the Rayleigh criterion. The high angular acceptance is obtained by means of a collecting pre-mirror. Three scattering geometries are available to enable momentum dependent measurements with 135°, 90°, and 50° scattering angles. The instrument operates on the Synchrotron SOLEIL SEXTANTS beamline which serves as a high photon flux 2 × 200 µm(2) focal spot source with full polarization control.

Femtosecond single-shot imaging of nanoscale ferromagnetic order in Co/Pd multilayers using resonant x-ray holography.

We present the first single-shot images of ferromagnetic, nanoscale spin order taken with femtosecond x-ray pulses. X-ray-induced electron and spin dynamics can be outrun with pulses shorter than 80 fs in the investigated fluence regime, and no permanent aftereffects in the samples are observed below a fluence of 25 mJ/cm(2). Employing resonant spatially muliplexed x-ray holography results in a low imaging threshold of 5 mJ/cm(2). Our results open new ways to combine ultrafast laser spectroscopy with sequential snapshot imaging on a single sample, generating a movie of excited state dynamics.

Magnetic imaging by Fourier transform holography using linearly polarized x-rays.

We present a method for imaging magnetic domains via x-ray Fourier transform holography at linearly polarized sources. Our approach is based on the separation of holographic mask and sample and on the Faraday rotation induced on the reference wave. We compare images of perpendicular magnetic domains obtained with either linearly or circularly polarized x-rays and discuss the relevance of this method to future experiments at free-electron laser and high-harmonic-generation sources.

Experimental setup for lensless imaging via soft x-ray resonant scattering.

We have developed a setup for measuring holographically formed interference patterns using an integrated sample-mask design. The direct space image of the sample is obtained via a two-dimensional Fourier transform of the X-ray diffraction pattern. We present the details of our setup, commenting on the influence of geometrical parameters on the imaging capabilities. As an example, we present and discuss the results of test experiments on a patterned Co film.