Simone Laterza

Externally seeded free-electron lasers (FELs) provide radiation with full longitudinal and transverse coherence in the extreme ultraviolet and soft x-ray spectral ranges, inaccessible to conventional laser sources. Until now, externally seeded operation has been limited to the extreme ultraviolet and soft x-ray regime, while shorter wavelengths have been accessed primarily through stochastic self-amplified spontaneous emission processes (or derivatives thereof, one based on self-seeding) or through nonlinear harmonic generation at strongly reduced pulse energy. At the FERMI FEL facility, we have demonstrated stable operation in high-gain harmonic generation (HGHG) covering the nitrogen K-edge, most of the water-window spectral range, and approaching the oxygen K-edge by combining short-wavelength ultraviolet seeding (200 nm), increased electron beam energy, and preservation of an ultralow relative slice energy spread at the 10 − 5 level. The emitted radiation at such short wavelengths still exhibits the typical narrow bandwidth and wavelength stability of the HGHG scheme enabling, for example, time-resolved x-ray absorption spectroscopy measurements. Longitudinally coherent emission at 1 nm is obtained as the third harmonic of the fundamental FEL emission. Despite the lower intensity typical of nonlinear harmonic emission, these results demonstrate that external seeded FELs can be extended to nanometer wavelengths, establishing deterministic, fully coherent soft x-ray generation and surpassing the previous wavelength operation limits. This advance opens new opportunities for coherent control and nonlinear x-ray experiments that require both high photon energy and longitudinal coherence, and demonstrates a promising pathway toward further extension of externally seeded FELs to shorter wavelengths.

Ultrafast photoexcitation can drive solids far from equilibrium by dynamically renormalizing their electronic structure, yet direct time-domain access to bulk band-structure reshaping with orbital and momentum selectivity has remained limited. Here, we show in photoexcited graphite that femtosecond carbon K -edge resonant inelastic x-ray scattering (RIXS), used in the time domain, provides a momentum- and orbital-selective spectroscopic fingerprint of transient valence-band reshaping. Momentum- and orbital-selective measurements along the K → M contour disentangle π and σ contributions and reveal a structured redistribution of spectral weight emerging within ∼70 femtoseconds and evolving on picosecond timescales. Nonequilibrium first-principles RIXS calculations performed within a frozen-lattice approximation show that the earliest response is dominated by electronic correlations and transient screening. At longer delays, the dynamics become strongly momentum dependent: Near K , the signal reflects the decay of the electronic contribution to the renormalized band structure under a hot-phonon bottleneck, whereas at the M -point van Hove singularity, it reveals a characteristic spectroscopic signature consistent with electron-phonon–driven band red shift and lifetime broadening induced by strongly coupled optical phonons. A three-temperature model constrained by the orbital-resolved tr-RIXS data captures the ensuing energy cascade through a hot-phonon reservoir dominated by the E 2 g and A 1 ′ modes. These results establish time-domain RIXS as a complementary probe of transient band-structure renormalization and mode-selective energy flow in quantum materials.

We investigate the thermoelastic dynamics of an 80 nm titanium thin film supported by a Si3N4 membrane by extreme-ultraviolet transient-grating (EUV-TG) spectroscopy across varying excitation fluences. A dominant oscillatory response at 143.8–150.6 GHz from time-domain fits is identified as a guided Lamb wave of the Ti/Si3N4 bilayer membrane with an in-plane phase velocity of approximately 6.3 km s−1. The near-coincidence of the Ti longitudinal and Si3N4 transverse acoustic velocities (∼6.1 km s−1) means that the dominant guided mode phase velocity simultaneously approximates both—a direct fingerprint of strong mechanical coupling between the layers. With increasing fluence, a systematic phase velocity increase of 4.7% is observed. A lower-frequency feature near 20–25 GHz is also detected, consistent with a breathing-type thickness response driven by transient uniform stress. Thermal relaxation analysis yields effective in-plane thermal diffusivities of (1.3–2.7) × 10−6 m2 s−1 and thermal conductivities of approximately 3–7 W m−1 K−1, significantly reduced relative to bulk Ti and consistent with boundary-limited heat transport in the confined membrane geometry. By combining nanometer-scale spatial selectivity with picosecond temporal resolution, EUV-TG enables simultaneous access to guided elastic wave propagation and nanoscale in-plane thermal transport in metallic membranes under non-equilibrium excitation. These results demonstrate how nanoscale confinement and multilayer coupling modify thermoelastic dynamics relative to bulk materials and highlight EUV-TG as a powerful tool for probing ultrafast thermoelastic dynamics in thin metallic films.

Time-resolved resonant inelastic X-ray scattering (tr-RIXS) is a powerful technique for probing quasiparticle interactions in quantum materials under nonequilibrium conditions. Here, we implement tr-RIXS at the carbon K-edge to investigate the ultrafast dynamics of core excitons coupled to vibrational modes in graphite. Using femtosecond X-ray pulses from a free-electron laser, we monitor the temporal evolution of vibronically dressed excitons and their interaction with symmetry-selective optical phonons. By tuning the incident photon energy across the 1s → σ^* resonance and analyzing the integrated inelastic sideband intensity, we reveal a detuning-controlled crossover between two complementary dynamical regimes. Phenomenological modeling and first-principles calculations reproduce both the magnitude and detuning dependence of the spectral-weight changes. In this work, enabled by the unique capabilities of X-ray free-electron lasers, we demonstrate how tr-RIXS can access coupled electronic and lattice dynamics with elemental and symmetry specificity, opening new routes to control vibronic interactions in light-element and low-dimensional quantum materials.

Light manipulation at the nanoscale is essential both for fundamental science and modern technology. The quest to shorter lengthscales, however, requires the use of light wavelengths beyond the visible. In particular, in the extreme ultraviolet regime these manipulation capabilities are hampered by the lack of efficient optics, especially for polarization control. Here, we present a method to create periodic, polarization modulations at the nanoscale using a tailored configuration of the FERMI free electron laser and demonstrate its capabilities by comparing the dynamics induced by this polarization transient grating with those driven by a conventional intensity grating on a thin ferrimagnetic alloy. While the intensity grating signal is dominated by the thermoelastic response, the polarization grating excitation minimizes it, uncovering helicity-dependent responses previously undetected. We anticipate nanoscale polarization transient gratings to become useful for the study of physical, chemical and biological systems possessing chiral symmetry.

Understanding how a spin current flows across metal-semiconductor interfaces at pico- and femtosecond time scales is of paramount importance for ultrafast spintronics, data processing, and storage applications. However, the possibility to directly access the propagation of spin currents, within such time scales, has been hampered by the simultaneous lack of both ultrafast element-specific magnetic sensitive probes and tailored well-built and characterized metal-semiconductor interfaces. Here, by means of a novel free-electron laser-based element-sensitive ultrafast time-resolved Kerr spectroscopy, we reveal different magnetodynamics for the Ni M 2 , 3 and Si L 2 , 3 absorption edges. These results are assumed to be the experimental evidence of photoinduced spin currents propagating at a speed of ∼ 0.2 nm/fs across the Ni/Si interface.

Here, we report on the conceptual design, the hardware realization, and the first experimental results of a novel and compact x-ray polarimeter capable of a single-pulse linear polarization angle detection in the extreme ultraviolet photon energy range. The polarimeter is tested by performing time resolved pump–probe experiments on a Ni₈₀Fe₂₀ Permalloy film at the M_2,3 Ni edge at an externally seeded free-electron laser source. Comparison with similar experiments reported in the literature shows the advantages of our approach also in view of future experiments.