Emiliano Principi

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.

Plasma-acceleration technology offers a route to drastically reduce the size and cost of x-ray free-electron lasers (FELs), potentially enabling a broad diffusion of FEL facilities worldwide and a scientific impact comparable to, or even exceeding, that associated with the advent of third-generation synchrotron radiation sources three decades ago. However, in order to compete with conventional FELs based on high-quality relativistic electron bunches with low energy spread and high repetition rates, substantial advances in plasma-acceleration technique are still required. This Perspective analyzes the classes of experiments that could best exploit the characteristics of the forthcoming EuPRAXIA@SPARC_LAB facility, set to be the first plasma wakefield acceleration soft x-ray FEL, during its initial operational phase. The transformative potential of this new light source as well as its role in the photon science community are strictly related to the adopted experimental strategy.

Materials with a vanishing dielectric constant provide an ideal platform for achieving plasmon-enhanced light-matter interactions and are widely employed in various cutting-edge nonlinear photonics applications. In this study, we present the first experimental demonstration of extreme ultraviolet (XUV) plasmon-enhanced self-driven spectral modification using a submicrometric foil of aluminium. This is achieved through the excitation of widely tunable Ferrell-Berreman epsilon-near-zero resonances with extremely low absorption. Our angle-dependent measurements of spectral modulation enhancement, supported by theoretical analysis, reveal efficient spectral modification at peak intensities as low as 380 GW/cm ² , which we attribute to ultrafast heating and saturation effects. These findings mark a breakthrough in the enhancement of typically weak nonlinearities in the XUV regime through nonlinear plasmonics, potentially paving the way for unprecedented tools for the manipulation and control of XUV radiation.

We have observed amplification of weak fluorescence from He atoms in the 3a P o 1 autoionizing state, which decay by spontaneous emission of 40.74 eV photons in one out of 1600 cases. On average, a 4.1 ± 1.6 % conversion of 63.67 eV photons was detected in the forward direction at 20 μ J pump pulse energy and 40 mbar He gas pressure. A comparison with state-of-the-art theoretical simulations shows an unexpected spectral line shift at high gas pressure that is ascribed to transient molecular and electron scattering effects.

Thermite reactions –highly energetic redox processes between a metal and an oxide—are used in welding, propulsion, and the fabrication of advanced materials. When reduced to the nanoscale, these reactions exhibit enhanced energetic performance, but their ultrafast dynamics remain poorly understood. Gaining insight into charge transfer during these processes is essential for advancing applications in energy conversion and materials design. Here we show that the reaction between aluminium and hematite, a common iron oxide, can be tracked with femtosecond resolution using extreme ultraviolet (EUV) time-resolved absorption spectroscopy at the Fe M_2,3 and Al L_2,3 edges. By exciting the system with an ultrashort optical pulse and probing element-specific absorption changes, we observe an early spectral shift that reveals the formation of localized charge carriers (polarons). Comparing samples with different supporting substrates highlights ultrafast electron transfer from aluminium to hematite. These results demonstrate an approach to investigating charge flow in energetic materials and provide a basis for studying fast chemical reactions with chemical specificity.

We present a technique based on the optical force of a femtosecond laser acting on liquid micro-droplets for their precise manipulation in a vacuum, enabling an efficient sample delivery system for soft X-ray experiments. Conventional liquid jet methods, which are utilized in soft X-ray experiments, consume large sample volumes and offer limited control over droplet trajectories, leading to significant sample waste. Our approach uses optical forces from a femtosecond-pulsed focused laser to deflect free-falling droplets, guiding them with high precision toward the interaction region. This significantly reduces sample waste while enabling real-time control over droplet positioning. To understand the behavior of droplets in vacuum and their interaction with the focused laser beam, we employ theoretical analysis and numerical simulations. Hertz–Knudsen equations describe the thermodynamics of free-falling and deflected droplets, allowing estimation of their temperature and size as a function of time and position. The optical force acting on the droplets is determined using the transfer matrix method and Lorenz–Mie theory. The proposed technique provides fine tuning over delivery time and thermodynamic properties of the liquid sample, offering a promising platform for investigating supercooled liquid micro-droplets and phase transitions. It is a particularly well suited liquid sample delivery method for ultrafast X-ray experiments using tabletop sources, as well as current and future free-electron laser and high harmonic generation facilities.

The EuPRAXIA EU project is at the forefront of advancing particle accelerator research and the development of photon sources through innovative plasma acceleration approaches. Within this framework, the EuAPS project aims to exploit laser wakefield acceleration to build and operate a betatron radiation source at the INFN Frascati National Laboratory. The EuAPS source will provide femtosecond X-ray pulses in the spectral region between about 1 and 10 keV, unlocking a realm of experimental ultrafast methodologies encompassing diverse imaging and X-ray spectroscopy techniques. This paper presents a description of the EuAPS betatron source, including simulations of the photon beam parameters, outlines the preliminary design of the dedicated photon beamline, and provides an insightful overview of its photon science applications.

X-ray sources have undeniably revolutionized science; however, paradoxically, their potential has only been partially exploited thus far. Recent technological a X-ray sources have undeniably revolutionized science; however, paradoxically, their potential has only been partially exploited thus far. Recent technological a

Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown.

We study the Thomson scattering from highly oriented pyrolitic graphite excited by the extreme ultraviolet, coherent pulses of FERMI free electron laser (FEL). An apparent nonlinear behavior is observed and fully described in terms of the coherent nature of both exciting FEL beam and scattered radiation, producing an intensity-dependent enhancement of the Thomson scattering cross-section. The process resembles Dicke’s superradiant phenomenon and is thus interpreted as the observation of superradiant Thomson scattering. The process also triggers the creation of coherent, low- q ( < 0.3 Å − 1 ), low energy phonons. The experimental data and analysis provide quantitative information on the sample characteristics, absorption, scattering factor, and coherent phonon energies and populations and open the route for the investigation of the deep nature of complex materials.