FERMI | Machine, DiProl, MagneDyn Elliptically polarized free-electron laser pulses enable L-edge dichroism in magnetic metals The ability to control light polarization is central to many advanced spectroscopic techniques used to investigate the electronic and magnetic properties of matter. Among these, X-ray Magnetic Circular Dichroism (XMCD) relies on circularly polarized light to detect element-specific magnetic signatures. However, generating intense, circularly or elliptically polarized light in the soft x-ray energy range remains a technical challenge for Free-Electron Laser (FEL) sources, particularly in seeded configurations, which are known for their high coherence. In this work, we report the successful generation of elliptically polarized femtosecond pulses in the 700–800 eV spectral range using a seeded FEL at the FERMI facility in Trieste, Italy. This was achieved by exploiting nonlinear harmonic generation (NHG), specifically the third harmonic of the FEL's fundamental wavelength. Unlike the fundamental emission, which typically provides limited access to high photon energies in seeded FELs, the third harmonic allows for extension into the L-edge region of transition metals such as Fe, Co, and Ni. A key innovation—previously explored using the storage-ring FEL at the Elettra synchrotron—consists in the use of an APPLE-type variable-polarization undulator operated in a finely tuned hybrid configuration, capable of generating on-axis harmonic radiation with a substantial degree of circular polarization. This overcomes a known limitation in FELs: planar undulators emit linearly polarized light on-axis, while helical undulators can produce circular polarization only off-axis and at much reduced flux—typically several orders of magnitude lower, making them unsuitable for photon-demanding techniques, like XMCD. The experiment was conducted at FERMI, operating at a fundamental wavelength of 5.2 nm and its third harmonic at 1.75 nm (708 eV). The process begins with the modulation of the electron beam by an external seed laser, inducing microbunching at the fundamental wavelength. This bunching is further amplified in the radiator, a chain of six variable-polarization (APPLE-type) undulators separated by drift sections, see Fig. 1 (a). APPLE-type undulators offer the ability to tune the magnetic field configuration by shifting magnet arrays longitudinally, see Fig. 1 (b). When the arrays are aligned (zero shift), the field lies in the vertical plane, producing linear horizontal polarization— this is the planar mode. A quarter-period shift between the arrays produces equal magnetic field components in both transverse directions, resulting in a helical field and circular polarization. Intermediate shifts generate elliptical polarization. To produce on-axis harmonic radiation with a significant degree of circular polarization, the radiator was configured with the first four undulators in right circular polarization and the final two in a hybrid configuration. This approach maintains strong electron-field coupling in the first part of the radiator while enabling polarization tuning in the final part. The degree of circular polarization depends on the ratio of the horizontal to vertical magnetic field components (Bx/By). A theoretical and experimental analysis shows that an optimal balance—yielding 30–60% circular polarization with less than 30% reduction in harmonic intensity—is achieved for Bx/By between 0.2 and 0.4. This configuration enables efficient NHG while preserving sufficient intensity for dichroism experiments. To demonstrate the feasibility of this scheme, two proof-of-principle experiments were performed. The first, conducted at the MagneDyn beamline, involved static X-ray Magnetic Circular Dichroism (XMCD) measurements at the Fe L₃ absorption edge on a permalloy (NiFe alloy) sample, see Fig. 2 (a, b). The XMCD signal, directly quantifying the sample magnetization, was obtained looking at the asymmetry ratio (I+ − I- )/( I+ + I- ), where I+ and I- are the transmitted intensities of the third harmonic signal (red lines in Fig. 2 a) for opposite values of the magnetic field applied to the sample. After setting FEL photon energy in correspondence of the peak of the symmetry ratio, we mapped the latter as a function of Bx/By, normalized to the FEL harmonic intensity. This resulted in the figure of merit displayed in Fig. 2 (b). The data indicate that the optimal Bx/By ratio is ~0.29, in good agreement with the qualitative estimate derived from the method outlined above. The second experiment, conducted at FERMI’s DiProi beamline, used soft X-ray Resonant Magnetic Scattering (XRMS) at the Fe L₃ edge to investigate both static and ultrafast spin dynamics in a Synthetic Antiferromagnet (SAFM) Multilayer. A 50 fs infrared pump pulse triggered the dynamics, while elliptically polarized FEL harmonic pulses served as the probe, see Fig. 2 (c). At the Bragg condition for specular reflectivity from the multilayer structure, chiral and magnetic scattering signals were detected using a CCD camera. The dichroic contrast reached a maximum of ~6.5%, corresponding to a polarization degree of ~43%. Fig. 2 (d) shows the azimuthal dependence of the chiral intensity for various pumpprobe delays. While the chiral contrast is clearly reduced after excitation (0.5 ps), the overall angular profile remains largely unchanged over time, indicating preservation of the Néel-type spin-spiral texture despite transient demagnetization. The reported results demonstrate that harmonics from a seeded FEL can produce highly circularly polarized soft x-ray pulses with excellent spatial and temporal coherence, enabling advanced static and time-resolved experiments at the L edges of transition metals and opening new avenues for high-resolution imaging and spin-resolved spectroscopy at short wavelengths. Elettra Highlights 2024–2025 152
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