Analytic description and optimization of magneto-optical Kerr setups with photoelastic modulation.

Instruments based on the magneto-optical Kerr effect are routinely used to probe surface magnetic properties. These tools rely on the characterization of the polarization state of reflected light from the sample to collect information on its magnetization. Here, we present a theoretical optimization of common setups based on the magneto-optical Kerr effect. A detection scheme based on a simple analyzer and photodetector and one made from a polarizing beam splitter and balanced photodetectors are considered. The effect of including a photoelastic modulator (PEM) and a lock-in amplifier to detect the signal at harmonics of the modulating frequency is studied. Jones formalism is used to derive general expressions that link the intensity of the measured signal to the magneto-optical Fresnel reflection coefficients for any orientation of the polarizing optical components. Optimal configurations are then defined as those that allow measuring the Kerr rotation and ellipticity while minimizing nonmagnetic contributions from the diagonal Fresnel coefficients in order to improve the signal-to-noise ratio (SNR). The expressions show that with the PEM, setups based on polarizing beam splitters inherently offer a twofold higher signal than commonly used analyzers, and the experimental results confirm that the SNR is improved by more than 150%. Furthermore, we find that while all proposed detection schemes measure Kerr effects, only those with polarizing beam splitters allow measuring the Kerr rotation directly when no modulator is included. This accommodates, for instance, time-resolved measurements at relatively low laser pulse repetition rates. Ultrafast demagnetization measurements are presented as an example of such applications.

Single-shot experiments at the soft X-FEL FERMI using a back-side-illuminated scientific CMOS detector. Corrigendum.

The name of one of the authors in the article by Léveillé et al. [(2022), J. Synchrotron Rad. 29, 103-110] is corrected.

Single-shot experiments at the soft X-FEL FERMI using a back-side-illuminated scientific CMOS detector.

The latest Complementary Metal Oxide Semiconductor (CMOS) 2D sensors now rival the performance of state-of-the-art photon detectors for optical application, combining a high-frame-rate speed with a wide dynamic range. While the advent of high-repetition-rate hard X-ray free-electron lasers (FELs) has boosted the development of complex large-area fast CCD detectors in the extreme ultraviolet (EUV) and soft X-ray domains, scientists lacked such high-performance 2D detectors, principally due to the very poor efficiency limited by the sensor processing. Recently, a new generation of large back-side-illuminated scientific CMOS sensors (CMOS-BSI) has been developed and commercialized. One of these cost-efficient and competitive sensors, the GSENSE400BSI, has been implemented and characterized, and the proof of concept has been carried out at a synchrotron or laser-based X-ray source. In this article, we explore the feasibility of single-shot ultra-fast experiments at FEL sources operating in the EUV/soft X-ray regime with an AXIS-SXR camera equipped with the GSENSE400BSI-TVISB sensor. We illustrate the detector capabilities by performing a soft X-ray magnetic scattering experiment at the DiProi end-station of the FERMI FEL. These measurements show the possibility of integrating this camera for collecting single-shot images at the 50 Hz operation mode of FERMI with a cropped image size of 700 × 700 pixels. The efficiency of the sensor at a working photon energy of 58 eV and the linearity over the large FEL intensity have been verified. Moreover, on-the-fly time-resolved single-shot X-ray resonant magnetic scattering imaging from prototype Co/Pt multilayer films has been carried out with a time collection gain of 30 compared to the classical start-and-stop acquisition method performed with the conventional CCD-BSI detector available at the end-station.

Sub-15-fs X-ray pump and X-ray probe experiment for the study of ultrafast magnetization dynamics in ferromagnetic alloys.

In this paper, we present a new setup for the measurement of element-specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub-15-fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X-rays pulses delivered by X-ray Free Electrons Lasers (close to the attosecond range), in an X-ray pump - X-ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni80Fe20 (Permalloy) samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto-optical Kerr effect (T-MOKE) and the Faraday effect as probing mechanisms.

Toward ultrafast magnetic depth profiling using time-resolved x-ray resonant magnetic reflectivity.

During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump-probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm ( ≃ 310 eV ), we were able to probe close to the Fe L 3 edge ( 706.8 eV ) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization.

Simultaneous two-color snapshot view on ultrafast charge and spin dynamics in a Fe-Cu-Ni tri-layer.

Ultrafast phenomena on a femtosecond timescale are commonly examined by pump-probe experiments. This implies multiple measurements, where the sample under investigation is pumped with a short light pulse and then probed with a second pulse at various time delays to follow its dynamics. Recently, the principle of streaking extreme ultraviolet (XUV) pulses in the temporal domain has enabled recording the dynamics of a system within a single pulse. However, separate pump-probe experiments at different absorption edges still lack a unified timing, when comparing the dynamics in complex systems. Here, we report on an experiment using a dedicated optical element and the two-color emission of the FERMI XUV free-electron laser to follow the charge and spin dynamics in composite materials at two distinct absorption edges, simultaneously. The sample, consisting of ferromagnetic Fe and Ni layers, separated by a Cu layer, is pumped by an infrared laser and probed by a two-color XUV pulse with photon energies tuned to the M-shell resonances of these two transition metals. The experimental geometry intrinsically avoids any timing uncertainty between the two elements and unambiguously reveals an approximately 100 fs delay of the magnetic response with respect to the electronic excitation for both Fe and Ni. This delay shows that the electronic and spin degrees of freedom are decoupled during the demagnetization process. We furthermore observe that the electronic dynamics of Ni and Fe show pronounced differences when probed at their resonance, while the demagnetization dynamics are similar. These observations underline the importance of simultaneous investigation of the temporal response of both charge and spin in multi-component materials. In a more general scenario, the experimental approach can be extended to continuous energy ranges, promising the development of jitter-free transient absorption spectroscopy in the XUV and soft X-ray regimes.

Tailoring the material properties of gelatin hydrogels by high energy electron irradiation.

Natural hydrogels such as gelatin are highly desirable biomaterials for application in drug delivery, biosensors, bioactuators and extracellular matrix components due to strong biocompatibility and biodegradability. Typically, chemical crosslinkers are used to optimize material properties, often introducing toxic byproducts into the material. In this present work, electron irradiation is employed as a reagent-free crosslinking technique to precisely tailor the viscoelasticity, swelling behavior, thermal stability and structure of gelatin. With increasing electron dose, changes in swelling behavior and rheology indicate increasing amounts of random coils and dangling ends as opposed to helical content, a result confirmed through Fourier transform infrared spectroscopy. Gel fraction, rheology and swelling measurements at 37 °C were used to verify thermal stability in biological conditions. Scanning electron microscopy images of dried gelatin samples support these conclusions by revealing a loss of free volume and apparent order in the fracture patterns. The degree of crosslinking and mesh size are quantified by rubber elasticity theory and the Flory-Rehner equation. Overall, precise control of material properties is demonstrated through the interplay of concentration and irradiation dose, while providing an extensive parameter-property database suitable for optimized synthesis.