arXiv subjects

Michele Manfredda

Publications and source records attributed to Michele Manfredda.

13 recordsLinked to original sources

arXiv2025

Dichroic Electron Emission Patterns from Oriented Helium Ions

We report a joint experimental and theoretical study using a combination of polarization-controlled free-electron-laser (FEL) and near-infra\-red (NIR) pulses in a synchronized two-color photo\-ionization scheme. Excited He$^+$ ions, created by extreme ultraviolet (XUV) circularly polarized radiation from the XUV-FEL FERMI in the oriented $3p\, (m\!=\!+1)$ state, are exposed to circularly polarized 784-nm NIR radiation with peak intensities from $10^{12}\,\rm W/cm^2$ to $\rm 10^{13}\,W/cm^2$. The angular distribution of the ejected electrons exhibit a strong dichroism depending on the NIR intensity. While the co-rotating case is defined by a single path, for the counter-rotating case, there are two dominant pathways whose relative strength and phase difference are determined.

physics.atom-ph↗

arXiv2025

Ultrafast dynamics of vibronically dressed core-excitons in graphite: a femtosecond RIXS perspective

This study demonstrates one of the first implementations of time-resolved resonant inelastic X-ray scattering (tr-RIXS), marking a seminal extension of RIXS spectroscopy into the ultrafast time domain. By investigating the ultrafast dynamics of vibronically dressed core excitons in graphite using femtosecond X-ray pulses from a Free Electron Laser, we reveal previously inaccessible insights into the transient coupling between core excitons and specific optical phonon modes. Our approach establishes tr-RIXS as a powerful, transformative tool capable of elucidating the intricate interplay between electronic and lattice dynamics, opening new avenues in ultrafast materials research.

cond-mat.mtrl-sci↗

arXiv2024

Circular Dichroism in Multiphoton Ionization of Resonantly Excited Helium Ions near Channel Closing

The circular dichroism (CD) of photoelectrons generated by near-infrared (NIR) laser pulses using multiphoton ionization of excited He$^+$ ions in the 3p(m=+1) state. The ions were prepared by circularly polarized extreme ultraviolet (XUV) pulses. For circularly polarized NIR pulses co- and counter-rotating relative to the polarization of the XUV pulse, a complex variation of the CD is observed as a result of intensity- and polarization-dependent Freeman resonances, with and without additional dichroic AC-Stark shifts. The experimental results are compared with numerical solutions of the time-dependent Schr\"odinger equation to identify and interpret the pronounced variation of the experimentally observed CD.

physics.atom-ph↗

arXiv2024

Strong-field quantum control in the extreme ultraviolet using pulse shaping

Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency, with applications ranging from quantum information to photochemistry. While strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation, comparable phenomena have been only recently observed with short wavelengths, accessing highly-excited multi-electron and inner-shell electron states. However, the quantum control of strong-field processes at short wavelengths has not been possible, so far, due to the lack of pulse shaping technologies in the extreme ultraviolet (XUV) and X-ray domain. Here, exploiting pulse shaping of the seeded free-electron laser (FEL) FERMI, we demonstrate the strong-field quantum control of ultrafast Rabi dynamics in helium atoms with high fidelity. Our approach unravels a strong dressing of the ionization continuum, otherwise elusive to experimental observables. The latter is exploited to achieve control of the total ionization rate, with prospective applications in many XUV and soft X-ray experiments. Leveraging recent advances in intense few-femtosecond to attosecond XUV to soft X-ray light sources, our results open an avenue to the efficient manipulation and selective control of core electron processes and electron correlation phenomena in real time.

physics.atom-ph↗

arXiv2023

Generation of entanglement using a short-wavelength seeded free-electron laser

Quantum entanglement between the degrees of freedom encountered in the classical world is challenging to observe due to the surrounding environment. To elucidate this issue, we investigate the entanglement generated over ultrafast timescales in a bipartite quantum system comprising two massive particles: a free-moving photoelectron, which expands to a mesoscopic length-scale, and a light-dressed atomic ion, which represents a hybrid state of light and matter. Although the photoelectron spectra are measured classically, the entanglement allows us to reveal information about the dressed-state dynamics of the ion and the femtosecond extreme ultraviolet pulses delivered by a seeded free-electron laser. The observed generation of entanglement is interpreted using the time-dependent von Neumann entropy. Our results unveil the potential for using short-wavelength coherent light pulses from free-electron lasers to generate entangled photoelectron and ion systems for studying spooky action at a distance.

quant-ph↗

arXiv2023

FEL stochastic spectroscopy revealing silicon bond softening dynamics

Time-resolved X-ray Emission/Absorption Spectroscopy (Tr-XES/XAS) is an informative experimental tool sensitive to electronic dynamics in materials, widely exploited in diverse research fields. Typically, Tr-XES/XAS requires X-ray pulses with both a narrow bandwidth and sub-picosecond pulse duration, a combination that in principle finds its optimum with Fourier transform-limited pulses. In this work, we explore an alternative xperimental approach, capable of simultaneously retrieving information about unoccupied (XAS) and occupied (XES) states from the stochastic fluctuations of broadband extreme ultraviolet pulses of a free-electron laser. We used this method, in combination with singular value decomposition and Tikhonov regularization procedures, to determine the XAS/XES response from a crystalline silicon sample at the L2,3-edge, with an energy resolution of a few tens of meV. Finally, we combined this spectroscopic method with a pump-probe approach to measure structural and electronic dynamics of a silicon membrane. Tr-XAS/XES data obtained after photoexcitation with an optical laser pulse at 390 nm allowed us to observe perturbations of the band structure, which are compatible with the formation of the predicted precursor state of a non-thermal solid-liquid phase transition associated with a bond softening phenomenon.

cond-mat.mtrl-sci↗

arXiv2022

Extreme ultraviolet wave packet interferometry of the autoionizing HeNe dimer

Femtosecond extreme ultraviolet wave packet interferometry (XUV-WPI) was applied to study resonant inter-atomic Coulombic decay (ICD) in the HeNe dimer. The high demands on phase stability and sensitivity for vibronic XUV-WPI of molecular-beam targets are met using an XUV phase-cycling scheme. The detected quantum interferences exhibit vibronic dephasing and rephasing signatures along with an ultrafast decoherence assigned to the ICD process. A Fourier analysis reveals the molecular absorption spectrum with high resolution. The demonstrated experiment shows a promising route for the real-time analysis of ultrafast ICD processes with both high temporal and spectral resolution.

physics.atm-clus↗

arXiv2022

Studying ultrafast Rabi dynamics with a short-wavelength seeded free-electron laser

Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field. They are ubiquitous in physics with applications in different areas such as photonics, nano-electronics, electron microscopy, and quantum information. While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light-matter interaction within the rotating wave approximation\cite. Although intense nanometer-wavelength light-sources have been available for more than a decade, Rabi dynamics at such short wavelengths have not been observed directly. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser can drive Rabi oscillations between the ground state and an excited state in helium atoms. The measured photoemission signal revealed an Autler-Townes doublet as well as an avoided crossing, phenomena that are both trademarks of quantum optics. Using theoretical analyses that go beyond the strong-field approximation, we found that the ultrafast build-up of the doublet structure follows from a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond and few-femtosecond extreme-ultraviolet pulses, our results offer opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.

quant-ph↗

arXiv2021

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 the light phase chirality 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.

physics.optics↗

arXiv2019

Pulse-to-pulse wavefront sensing at free-electron lasers using ptychography

The pressing need for the detailed wavefront properties of ultra-bright and ultra-short pulses produced by free-electron lasers (FELs) has spurred the development of several complementary characterization approaches. Here we present a method based on ptychography that can retrieve full high-resolution complex-valued wave functions of individual pulses. Our technique is demonstrated within experimental conditions suited for diffraction experiments in their native imaging state. This lensless technique, applicable to many other short-pulse instruments, can achieve diffraction-limited resolution.

eess.IV↗

arXiv2018

Single shot time-resolved magnetic x-ray absorption at a Free Electron Laser

Ultrafast dynamics are generally investigated using stroboscopic pump-probe measurements, which characterize the sample properties for a single, specific time delay. These measurements are then repeated for a series of discrete time delays to reconstruct the overall time trace of the process. As a consequence, this approach is limited to the investigation of fully reversible phenomena. We recently introduced an off-axis zone plate based X-ray streaking technique, which overcomes this limitation by sampling the relaxation dynamics with a single femtosecond X-ray pulse streaked over a picosecond long time window. In this article we show that the X-ray absorption cross section can be employed as the contrast mechanism in this novel technique. We show that changes of the absorption cross section on the percent level can be resolved with this method. To this end we measure the ultrafast magnetization dynamics in CoDy alloy films. Investigating different chemical compositions and infrared pump fluences, we demonstrate the routine applicability of this technique. Probing in transmission the average magnetization dynamics of the entire film, our experimental findings indicate that the demagnetization time is independent of the specific infrared laser pump fluence. These results pave the way for the investigation of irreversible phenomena in a wide variety of scientific areas.

cond-mat.mtrl-sci↗

arXiv2017

In-situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses

Free-electron lasers (FEL) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluence-dependent absorption and scattering experiments to reveal non-linear light-matter interactions at ever shorter wavelengths. Findings of such non-linear effects in the XUV and X-ray regime are met with tremendous interest, but prove difficult to understand and model due to the inherent shot-to-shot fluctuations in photon intensity and the often structured, non-Gaussian spatial intensity profile of a focused FEL beam. Presently, the focused beam spot is characterized and optimized separately from the actual experiment. Here, we present the first simultaneous measurement of diffraction signals from solid samples in tandem with the corresponding single-shot spatial fluence distribution on the actual sample. This new in-situ characterization scheme enables fast and direct monitoring and thus control of the sample illumination which ultimately is necessary for a quantitative understanding of non-linear light-matter interaction in X-ray and XUV FEL experiments.

physics.ins-det↗

arXiv2016

Soft x-rays induce femtosecond solid-to-solid phase transition

Soft x-rays were applied to induce graphitization of diamond through a non-thermal solid-to-solid phase transition. This process was observed within poly-crystalline diamond with a time-resolved experiment using ultrashort soft x-ray pulses of duration 52.5 fs and cross correlated by an optical pulse of duration 32.8 fs. This scheme enabled for the first time the measurement of a phase transition on a timescale of ~150 fs. Excellent agreement between experiment and theoretical predictions was found, using a dedicated code that followed the non-equilibrium evolution of the irradiated diamond including all transient electronic and structural changes. These observations confirm that soft x-rays can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale.

physics.atom-ph↗