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L’acquacoltura ha sempre rappresentato un’importante attività di allevamento animale e mai come oggi ha assunto un’importanza crescente dovuta ad una sempre maggior domanda di consumo annuo pro capite di pesce che, tuttavia, ha comportato una contestuale diminuzione dell’apporto della pesca. La movimentazione di pesce vivo è parte integrante della attività di piscicoltura in tutta Europa. Negli ultimi decenni in Italia l’allevamento del pesce e di altre specie acquatiche è stato in costante aumento, stabilizzandosi solo negli ultimi anni. Si è registrato un sempre maggiore interesse alle pratiche di acquacoltura utilizzate e ai relativi problemi di benessere dei pesci da parte del legislatore, nel campo della ricerca e da parte dei consumatori. I dati dimostrano che l’acquacoltura è un settore in crescita: nell’anno 2016 sono stati trasportati e introdotti in Italia oltre 72 milioni di Kg. di pesce vivo (35.5% trote – 14.6% spigole e orate – 4% pesce gatto e anguille) e si prevede che a livello mondiale nel 2030 il 60% del pesce consumato sarà di allevamento. In Italia le specie più trasportate sono le orate, le spigole nelle fasi giovanili e le trote. La maggior parte delle specie di pesce che sono allevate vengono spostate almeno una volta durante il loro ciclo produttivo, mentre alcuni animali vengono movimentati più volte. In acquacoltura sono allevate e trasportate oltre 60 specie diverse di pesci (oltre 30 specie in Europa). Se ne deduce che quantità di animali trasportati e la e la lunghezza durata del viaggio possono variare considerevolmente, in funzione del ciclo produttivo e delle necessità commerciali, che talvolta possono cambiare anche nel corso dello stesso anno. Il trasporto dei pesci è forse il più difficile e delicato rispetto alle altre specie di vertebrati, per cui ne deriva che un piccolissimo errore nelle varie fasi di trasporto ne comprometterebbe il benessere degli animali nonché una perdita economica nel settore. L’azione di protezione e di mantenimento delle condizioni di benessere del pesce vivo durante le operazioni di trasporto, che dovrebbe essere un dovere morale dell’allevatore e/o autotrasportatore, è un obbligo sancito dalla legislazione vigente e comporta una responsabilità legale sia degli operatori del settore, sia delle autorità competenti preposte ai controlli, le quali devono avere una alta professionalità e competenza nel corso delle operazioni ispettive e di vigilanza. Nelle attività di trasporto del pesce, è necessario eseguire le operazioni secondo specifici protocolli operativi, allo scopo di non pregiudicare lo stato di salute e indirettamente anche il valore economico degli animali oggetto di movimentazione. E’ rilevante quindi che gli operatori e il personale addetto siano formati e preparati per garantire ai pesci un trasporto senza stress. A tal proposito è importante sottolineare l’importanza fondamentale della figura del veterinario, il quale rappresenta oggi la sola figura professionale e con competenze scientifiche a cui la legge attribuisce il compito-dovere di verifica e di controllo delle condizioni degli animali e dei loro prodotti, nello specifico anche del pesce, ivi compresi i provvedimenti a tutela della protezione del benessere dell’animale durante il trasporto e che comportano, inoltre, anche la valutazione delle condizioni di dolore, stress, o sofferenze evitabili nel corso delle operazioni. Con il presente ‘Manuale’, vengono riportate una serie di misure specifiche da applicare nella gestione del trasporto su strada del pesce vivo, al fine di definire ed individuare le condizioni ottimali di benessere nel corso delle movimentazioni.

Circularly polarized light is a fundamental tool in magnetic studies, notably for magnetization dynamics. It is less common in magneto-optics to exploit the orbital angular momentum (OAM) of value ℓ carried by light beams possessing a helical wavefront. After finding many applications in the visible range, recently OAM pulses with ultra-short duration and XUV wavelengths became available, widening the range of experiments that can be envisaged. We modelled the interaction of an XUV OAM beam with non-uniform magnetic structures, showing that the far field scattered intensity profile encodes the symmetry of the magnetic structure in a way that depends on the sign and value of ℓ. In analogy with magnetic circular dichroism, this effect, named magnetic helicoidal dichroism (MHD), can be observed by inverting the sign of either the orbital momentum or of the magnetization. We obtained experimental evidence of MHD by measuring ℓ-dependent resonant scattering from a magnetic vortex. The results of recent complementary experiments match well the theoretical predictions, confirming the potential of the new toolset provided by MHD for studying the laser-triggered ultrafast dynamics of complex magnetic materials.

The optimal performance of high-brightness free-electron lasers (FELs) is limited by the microbunching instability, which can cause variations in both the slice energy spread and longitudinal profile of electron beams. In this paper, we perform 2D Fourier analysis of the full bunch longitudinal phase space, such that modulations in both planes can be studied simultaneously. Unlike the standard 1D analysis, this method is able to reveal modulations in a folded phase space, which would otherwise remain uncovered. Additionally, the plasma oscillation between energy and density modulations is also revealed by this method. The damping of the microbunching instability, through the use of a laser heater, is also analysed with this technique. We confirm a mitigation of the amplitude of modulation and a red-shift of the microbunching frequency as the energy spread added increases. As an outcome of this work, a systematic experimental comparison of the development of the instability in the presence of different compression schemes is here presented for the first time.

The journal of chemical physics 157(16), 164705 (2022). doi:10.1063/5.0114399, We report on carbon monoxide desorption and oxidation induced by 400 nm femtosecond laser excitation on the O/Ru(0001) surface probed by time-resolved x-ray absorption spectroscopy (TR-XAS) at the carbon K-edge. The experiments were performed under constant background pressures of CO (6 × 10$^{−8}$ Torr) and O$_2$ (3 × 10$^{−8}$ Torr). Under these conditions, we detect two transient CO species with narrow 2$π^*$ peaks, suggesting little 2π* interaction with the surface. Based on polarization measurements, we find that these two species have opposing orientations: (1) CO favoring a more perpendicular orientation and (2) CO favoring a more parallel orientation with respect to the surface. We also directly detect gas-phase CO2 using a mass spectrometer and observe weak signatures of bent adsorbed CO2 at slightly higher x-ray energies than the 2$π^*$ region. These results are compared to previously reported TR-XAS results at the O K-edge, where the CO background pressure was three times lower (2 × 10$^{−8}$ Torr) while maintaining the same O$_2$ pressure. At the lower CO pressure, in the CO 2$π^*$ region, we observed adsorbed CO and a distribution of OC–O bond lengths close to the CO oxidation transition state, with little indication of gas-like CO. The shift toward “gas-like” CO species may be explained by the higher CO exposure, which blocks O adsorption, decreasing O coverage and increasing CO coverage. These effects decrease the CO desorption barrier through dipole–dipole interaction while simultaneously increasing the CO oxidation barrier., Published by American Institute of Physics, Melville, NY

Physical review accelerators and beams 24(8), 080702 (2021). doi:10.1103/PhysRevAccelBeams.24.080702, Echo-enabled harmonic generation free-electron lasers (EEHG FELs)are promising candidates to produce fully coherent soft x-ray pulses by virtue of efficient high harmonic frequency up-conversion from UV lasers.The ultimate spectral limit of EEHG, however, remains unclear, because of the broadening and distortions induced in the output spectrum by residual broadband energy modulations in the electron beam. We present a mathematical description of the impact of incoherent(broadband) energy modulations on the bunching spectrum produced by the microbunching instability through both the accelerator and the EEHG line. The model is in agreementwith a systematic experimental characterization of the FERMI EEHGFEL in the photon energy range $130-210$ eV. We find that amplification of electron beam energy distortions primarily in the EEHG dispersive sections explains an observed reductionof the FEL spectral brightness that is proportional to the EEHGharmonic number. Local maxima of the FEL spectral brightness and of the spectralstability are found for a suitable balance of the dispersive sections'strength and the first seed laser pulse energy. Such characterizationprovides a benchmark for user experiments and future EEHG implementations designed to reach shorter wavelengths., Published by American Physical Society, College Park, MD

The initial deactivation pathways of gaseous 2-nitrophenol excited at 268 nm were investigated by time-resolved photoelectron spectroscopy (TRPES) with femtosecond-VUV light, produced by a monochromatized high harmonic generation source. TRPES allowed us to obtain new, valuable experimental information about the ultrafast excited-state dynamics of 2-nitrophenol in the gas phase. In accord with recent ab initio on-The-fly nonadiabatic molecular dynamic simulations, our results validate the occurrence of an ultrafast intersystem crossing leading to an intermediate state that decays on a subpicosecond time scale with a branched mechanisms. Two decay pathways are experimentally observed. One probably involves proton transfer, leading to the most stable triplet aci-form of 2-nitrophenol; the second pathway may involve OH rotation. We propose that following intersystem crossing, an ultrafast fragmentation channel leading to OH or HONO loss could also be operative.

Self-action nonlinearity is a key aspect -- either as a foundational element or a detrimental factor -- of several optical spectroscopies and photonic devices. Supercontinuum generation, wavelength converters and chirped pulse amplification are just a few examples. The recent advent of Free Electron Lasers (FEL) fostered building on nonlinearity to propose new concepts and extend optical wavelengths paradigms for extreme ultraviolet (EUV) and X-ray regimes. No evidence for intrapulse dynamics, however, has been reported at such short wavelengths, where the light-matter interactions are ruled by the sharp absorption edges of core-electrons. Here, we provide experimental evidence for self-phase modulation of femtosecond FEL pulses, which we exploit for fine self-driven spectral tunability by interaction with sub-micrometric foils of selected monoatomic materials. Moving the pulse wavelength across the absorption edge, the spectral profile changes from a non-linear spectral blue-shift to a red-shifted broadening. These findings are rationalized accounting for ultrafast ionization and delayed thermal response of highly excited electrons above and below threshold, respectively., 20 pages, 6 figures

Physical review letters 127(1), 016802 (2021). doi:10.1103/PhysRevLett.127.016802, We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ���100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes., Published by APS, College Park, Md.

We report on the observation of time-resolved quantum beats in the helium fluorescence from the transition 1s3p -> 1s2s, where the initial state is excited by XUV free-electron laser radiation. The quantum beats originate from the Zeeman splitting of the magnetic substates due to an external magnetic field. We perform a systematic study of this effect and discuss the possibilities of studying this phenomenon in the XUV and x-ray regime., 7 pages, 4 figures

We present and discuss the role of nanoparticles size and stoichiometry over the local atomic environment of nanostructured VOx films. The samples have been characterized in situ using X-ray absorption near-edge structure (XANES) spectroscopy identifying the stoichiometry-dependent fingerprints of disordered atomic arrangement. In vanadium oxides, the ligand atoms arrange according to a distorted octahedral geometry depending on the oxidation state, e.g. trigonal distortion in V2O3 and tetragonal distortion in bulk VO2. We demonstrate, taking VO2 as a case study, that as a consequence of the nanometric size of the nanoparticles, the original ligands symmetry of the bulk is broken resulting in the coexistence of a continuum of distorted atomic conformations. The resulting modulation of the electronic structure of the nanostructured VOx as a function of the oxygen content reveals a stoichiometry-dependent increase of disorder in the ligands matrix. This work shows the possibility to produce VOx nanostructured films accessing new disordered phases and provides a unique tool to investigate the complex matter.

Physical review accelerators and beams 23(12), 120704 (1-8) (2020). doi:10.1103/PhysRevAccelBeams.23.120704, FERMI is a seeded FEL based on the High Gain Harmonic Generation (HGHG) scheme and has been generating intense and fully coherent extreme ultra-violet and soft x-ray pulses for several years. The high-degree performance of the FEL leans on a high brightness electron beam, with small transverse emittance ( ∼1$μm$) and a peak current of about 700-800 A. The main constraint on lasing at high harmonics of the seed is low electron time-slice energy spread. The optimization of the photoinjector and of some linac parameters has allowed a reduction of the relative slice energy spread to the level of few times of 10$^{−5}$. With these new conditions, the FEL can be operated without the need of a laser heater to suppress micro bunching instabilities and this "cold’’ beam has allowed generation of extreme UV pulses with pulse energy exceeding a mJ, and with peak power of about 10 GW. We describe the electron beam characterisation and the FEL performance improvement, including the extension of the range of harmonics of the seed which can be amplified, up to the twenty-fifth harmonic, i.e. 10 nm., Published by American Physical Society, College Park, MD

High-brightness electron bunches, such as those generated and accelerated in free-electron lasers (FELs), can develop small-scale structure in the longitudinal phase space. This causes variations in the slice energy spread and current profile of the bunch which then undergo amplification, in an effect known as the microbunching instability. By imposing energy spread modulations on the bunch in the low-energy section of an accelerator, using an undulator and a modulated laser pulse in the centre of a dispersive chicane, it is possible tomanipulate the bunch longitudinal phase space. This allows for the control and study of the instability in unprecedented detail. We report measurements and analysis of such modulated electron bunches in the 2Dspectro-temporal domain at the FERMI FEL, for three different bunch compression schemes. We also perform corresponding simulations of these experiments and show that the codes are indeed able to reproduce the measurements across a wide spectral range. This detailed experimental verification of the ability of codes to capture the essential beam dynamics of the microbunching instability will benefit the design and performance of future FELs., 12 pages, 14 figures

We combined a spin-resolved photoemission spectrometer with a high-harmonic generation (HHG) laser source in order to perform spin-, time- and angle-resolved photoemission spectroscopy (STARPES) experiments on the transition metal dichalcogenide bulk WTe$_2$, a possible Weyl type-II semimetal. Measurements at different femtosecond pump-probe delays and comparison with spin-resolved one-step photoemission calculations provide insight into the spin polarization of electrons above the Fermi level in the region where Weyl points of WTe$_2$ are expected. We observe a spin accumulation above the Weyl points region, that is consistent with a spin-selective bottleneck effect due to the presence of spin polarized cone-like electronic structure. Our results support the feasibility of STARPES with HHG, which despite being experimentally challenging provides a unique way to study spin dynamics in photoemission., 6 pages, 4 figures

The recent development of ultrafast extreme ultraviolet (XUV) coherent light sources bears great potential for a better understanding of the structure and dynamics of matter. Promising routes are advanced coherent control and nonlinear spectroscopy schemes in the XUV energy range, yielding unprecedented spatial and temporal resolution. However, their implementation has been hampered by the experimental challenge of generating XUV pulse sequences with precisely controlled timing and phase properties. In particular, direct control and manipulation of the phase of individual pulses within an XUV pulse sequence opens exciting possibilities for coherent control and multidimensional spectroscopy, but has not been accomplished. Here, we overcome these constraints in a highly time-stabilized and phase-modulated XUV-pump, XUV-probe experiment, which directly probes the evolution and dephasing of an inner subshell electronic coherence. This approach, avoiding any XUV optics for direct pulse manipulation, opens up extensive applications of advanced nonlinear optics and spectroscopy at XUV wavelengths., Light pulses with controllable parameters are desired for studying the fundamental properties of matter. Here the authors generate and use phase-manipulated and highly time-stable XUV pulse pairs to probe the coherent evolution and dephasing of XUV electronic coherences in helium and argon.

We demonstrate a novel timing tool for Free-Electron Lasers to determine the delay between an attosecond pulse train and infrared pulse with sub-optical-cycle resolu-. tion.

Attosecond pulses are fundamental for the investigation of valence and core-electron dynamics on their natural timescale. At present the reproducible generation and characterisation of attosecond waveforms has been demonstrated only through the process of high-order harmonic generation. Several methods for the shaping of attosecond waveforms have been proposed, including metallic filters, multilayer mirrors and manipulation of the driving field. However, none of these approaches allow for the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free Electron Lasers, on the contrary, deliver femtosecond, extreme ultraviolet and X-ray pulses with energies ranging from tens of $\mathrm{\mu}$J to a few mJ. Recent experiments have shown that they can generate sub-fs spikes, but with temporal characteristics that change shot-to-shot. Here we show the first demonstration of reproducible generation of high energy ($\mathrm{\mu}$J level) attosecond waveforms using a seeded Free Electron Laser. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with a novel approach for its temporal reconstruction. The results presented here open the way to perform attosecond time-resolved experiments with Free Electron Lasers.

The transient dynamics of carbon monoxide (CO) molecules on a Ru(0001) surface following femtosecond optical laser pump excitation has been studied by monitoring changes in the unoccupied electronic structure using an ultrafast X-ray free-electron laser (FEL) probe. The particular symmetry of perpendicularly chemisorbed CO on the surface is exploited to investigate how the molecular orientation changes with time by varying the polarization of the FEL pulses. The time evolution of spectral features corresponding to the desorption precursor state was well distinguished due to the narrow line-width of the C K-edge in the X-ray absorption (XA) spectrum, illustrating that CO molecules in the precursor state rotated freely and resided on the surface for several picoseconds. Most of the CO molecules trapped in the precursor state ultimately cooled back down to the chemisorbed state, while we estimate that similar to 14.5 +/- 4.9% of the molecules in the precursor state desorbed into the gas phase. It was also observed that chemisorbed CO molecules diffused over the metal surface from on-top sites toward highly coordinated sites. In addition, a new "vibrationally hot precursor" state was identified in the polarization-dependent XA spectra.

International audience; We exploit echo-enabled harmonic generation (EEHG) to produce fully coherent free-electron laser (FEL) pulses at soft-x-ray wavelengths and shape their spectrotemporal content. In an EEHG FEL, the longitudinal phase space of the relativistic electron beam that amplifies light is precisely tailored using two external seed lasers and two magnetic chicanes. We show that the spectrotemporal properties of the emitted radiation can be controlled by tuning the bandwidth, linear frequency chirp, and intensity of one of the seed lasers. The experimental data are supported by analytical and numerical models. Our results open a pathway toward coherent control of quantum processes at short wavelengths in the fields of applied physics, chemistry and biology, where manipulating the radiation spectrum is essential. The ability to precisely control the spectrotemporal content of intense, short-wavelength FEL pulses and the low sensitivity of the radiation to electron-beam imperfections make the technique an ideal candidate for use in chirped-pulse amplification schemes.

Laser-slicing at a diffraction-limited storage ring light source in the soft X-ray region is investigated with theoretical and numerical modelling. It turns out that the slicing efficiency is favoured by the ultra-low beam emittance, and that slicing can be implemented without interference to the standard multi-bunch operation. Spatial and spectral separation of the sub-picosecond radiation pulse from a hundreds of picosecond-long background is achieved by virtue of 1:1 imaging of the radiation source. The spectral separation is enhanced when the radiator is a transverse gradient undulator. The proposed configuration applied to the Elettra 2.0 six-bend achromatic lattice envisages total slicing efficiency as high as 10−7, one order of magnitude larger than the demonstrated state-of-the-art, at the expense of pulse durations as long as 0.4 ps FWHM and average laser power as high as ∼40 W.

International audience; We investigate the orthorhombic distortion and the structural dynamics of epitaxial MnAs layers on GaAs(001) using static and time-resolved x-ray diffraction. Laser-induced intensity oscillations of Bragg reflections allow us to identify the optical phonon associated with orthorhombic distortion and to follow its softening along the path towards an undistorted phase of hexagonal symmetry. The frequency of this mode falls in the THz range, in agreement with recent calculations. Incomplete softening suggests that the $\beta-\gamma$ transformation deviates from a purely second-order displacive transition.

In this paper, we will present the measurements of diffraction efficiency of commercially available replica reflection gratings which are applied to the realization of monochromators for soft X-rays in the classical and off-plane geometry. The efficiency curves for different blaze wavelengths has been measured at the Circular Polarization beamline of the ELETTRA Synchrotron (Italy) and simulated with GSolver, which is a full vector implementation of a class of algorithms known as Rigorous Coupled Wave Analysis (RCWA). Aim of the measurements is the comparison between the two diffraction geometries when used for the realization of instruments with ultrafast response. The advantages and drawbacks of both configurations when applied to the spectral selection of ultrafast pulses below 10 nm are discussed.

Quantum mechanically, photoionization can be fully described by the complex photoionization amplitudes that describe the transition between the ground state and the continuum state. Knowledge of the value of the phase of these amplitudes has been a central interest in photoionization studies and newly developing attosecond science, since the phase can reveal important information about phenomena such as electron correlation. We present a new attosecond-precision interferometric method of angle-resolved measurement for the phase of the photoionization amplitudes, using two phase-locked Extreme Ultraviolet pulses of frequency $\omega$ and $2\omega$, from a Free-Electron Laser. Phase differences $\Delta \tilde \eta$ between one- and two-photon ionization channels, averaged over multiple wave packets, are extracted for neon $2p$ electrons as a function of emission angle at photoelectron energies 7.9, 10.2, and 16.6 eV. $\Delta \tilde \eta$ is nearly constant for emission parallel to the electric vector but increases at 10.2 eV for emission perpendicular to the electric vector. We model our observations with both perturbation and \textit{ab initio} theory, and find excellent agreement. In the existing method for attosecond measurement, Reconstruction of Attosecond Beating By Interference of Two-photon Transitions (RABBITT), a phase difference between two-photon pathways involving absorption and emission of an infrared photon is extracted. Our method can be used for extraction of a phase difference between single-photon and two-photon pathways and provides a new tool for attosecond science, which is complementary to RABBITT.

Spatially patterned extreme ultraviolet light pulses can be used for probing multiple dynamical processes at the nanoscale., Advances in developing ultrafast coherent sources operating at extreme ultraviolet (EUV) and x-ray wavelengths allow the extension of nonlinear optical techniques to shorter wavelengths. Here, we describe EUV transient grating spectroscopy, in which two crossed femtosecond EUV pulses produce spatially periodic nanoscale excitations in the sample and their dynamics is probed via diffraction of a third time-delayed EUV pulse. The use of radiation with wavelengths down to 13.3 nm allowed us to produce transient gratings with periods as short as 28 nm and observe thermal and coherent phonon dynamics in crystalline silicon and amorphous silicon nitride. This approach allows measurements of thermal transport on the ~10-nm scale, where the two samples show different heat transport regimes, and can be applied to study other phenomena showing nontrivial behaviors at the nanoscale, such as structural relaxations in complex liquids and ultrafast magnetic dynamics.

A plane-grating monochromator for the extreme-ultraviolet region that uses a plane grating operated at variable included angle and illuminated in converging light has been realized. The configuration consists of three optical elements: the focusing mirror, the plane grating and the plane mirror. In the proposed design, the spectral focusing is kept on the exit slit plane by changing the grating included angle through the use of an additional plane mirror after the grating. The coma aberration given by the grating is compensated by an opposite coma introduced by the focusing mirror, that is used in a very asymmetrical configuration. The feasibility of the configuration has been demonstrated on a monochromator for the 10-80 eV region, with sub-10-meV output bandwidth. The correction of defocusing and coma at the proper included angle has been clearly demonstrated. The monochromator efficiency has been measured to be in the 0.08-0.18 range.

X-ray free-electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths. Echo-enabled harmonic generation in a free-electron laser enables 45th harmonic pulses from a 264 nm wavelength seed, yielding 5.9 nm wavelength coherent output.

International audience; Manganese arsenide layers epitaxially grown on GaAs(001) are known to feature a temperature dependent self-assembled microstructure of ordered stripes, alternating the ferromagnetic α and paramagnetic β phases. The surface dipolar fields generated by the α/β stripes have been used for achieving temperature controlled magnetization switching of a ferromagnetic overlayer. For this kind of application, it is advantageous to minimize the MnAs layer thickness. In this work we investigate, using x-ray scattering techniques, the presence of the ordered microstructure as a function of the MnAs layer thickness and we identify a minimum value of ~40 nm for the formation of ordered α/β stripes in MnAs/GaAs(001). These results have an impact for envisaging magnetization-switching applications that rely on the control of the temperature-or laser-driven surface dipolar fields in MnAs-based devices.

Synopsis We report recent results of photoelectron angular distributions (PADs) for photoionization (PI) of atomic and molecular targets, resolved in the time domain at the attosecond scale, combining electron-ion coincidence momentum spectroscopy and the RABBITT scheme. We propose a unified formalism synthetizing the PAD(θ, τ) results in terms of a reduced set of coefficients.

The design and realization of a plane-grating monochromator mainly intended for high-energy resolution in the extreme-ultraviolet and soft-X-ray spectral regions is presented. The principal application is the spectral selection of high-order laser harmonics generated in gas combined with the possibility to achieve sub-20-meV bandwidth, approaching synchrotron-like beamlines resolution performances. This geometry has three optical elements: a uniform-line-spaced plane grating illuminated by the converging light coming from a focusing cylindrical mirror and an additional plane mirror that is needed to change the grating subtended angle to keep the system in focus on a fixed slit. The parameters of the focusing mirror are determined to introduce a coma that compensates for the coma given by the grating. A monochromator for the 12-50 eV region has been realized to show the feasibility and the performances of the configuration.

We demonstrate that emission of coherent transition radiation by a ∼1 GeV energy-electron beam passing through an Al foil is enhanced in intensity and extended in frequency spectral range, by the energy correlation established along the beam by coherent synchrotron radiation wakefield, in the presence of a proper electron optics in the beam delivery system. Analytical and numerical models, based on experimental electron beam parameters collected at the FERMI free electron laser (FEL), predict transition radiation with two intensity peaks at ∼0.3 THz and ∼1.5 THz, and extending up to 8.5 THz with intensity above 20 dB w.r.t. the main peak. Up to 80-µJ pulse energy integrated over the full bandwidth is expected at the source, and in agreement with experimental pulse energy measurements. By virtue of its implementation in an FEL beam dump line, this work promises dissemination of user-oriented multi-THz beamlines parasitic and self-synchronized to EUV and x-ray FELs.

International audience; We demonstrate experimentally the control of the polarization (ellipticity and polarization axis) of femtosecond high-order harmonics. The method relies on a two-color (omega - 2 omega) configuration, where both omega and 2 omega generating fields have linear polarizations with a variable crossing angle. We correlate our measurements with the conservation rules of energy and linear momentum accounting for harmonic generation in a two-color field. We evidence that the generation process, and especially the number of omega and 2 omega photons absorbed for generating a given harmonic, strongly depends on the crossing angle. Finally, relying on a simple formalism, we derive analytical formulae for calculating both polarization axis and ellipticity of the separate harmonics. The model corroborates our results and represents a base for future investigations.

An understanding of collective effects is of fundamental importance for the design and optimisation of the performance of modern accelerators. In particular, the design of an accelerator with strict requirements on the beam quality, such as a free electron laser (FEL), is highly dependent on a correspondence between simulation, theory and experiments in order to correctly account for the effect of coherent synchrotron radiation (CSR), and other collective effects. A traditional approach in accelerator simulation codes is to utilise an analytic one-dimensional approximation to the CSR force. We present an extension of the 1D CSR theory in order to correctly account for the CSR force at the entrance and exit of a bending magnet. A limited range of applicability to this solution, in particular in bunches with a large transverse spot size or offset from the nominal axis, is recognised. More recently developed codes calculate the CSR effect in dispersive regions directly from the Lienard-Wiechert potentials, albeit with approximations to improve the computational time. A new module of the General Particle Tracer (GPT) code was developed for simulating the effects of CSR, and benchmarked against other codes. We experimentally demonstrate departure from the commonly used 1D CSR theory for more extreme bunch length compression scenarios at the FERMI FEL facility. Better agreement is found between experimental data and the codes which account for the transverse extent of the bunch, particularly in more extreme compression scenarios., 33 pages, 19 figures, submitted to New J. Phys

We have investigated by time-resolved X-ray diffraction the manganese arsenide structure dynamics in MnAs/GaAs (001) epitaxial films, when the $\alpha \to \beta $ magnetostructural phase transition is triggered by an ultra-fast optical laser pulse. Understanding the photo-induced phase transition dynamics is relevant for applications of MnAs/GaAs (001) as a magnetically active template in laser-induced magnetization switching processes. The main goal of our experiment is to determine the timescales of the structural changes that are likely to play a role in laser-driven magnetization reversal.

It was shown recently that the Fe magnetization reversal in the Fe/MnAs/GaAs(001) epitaxial system, attained by temperature control of the regular stripe pattern of the MnAs ?- and ?-phases, can also be driven by an ultrashort optical laser pulse. In the present time-resolved scattering experiment, we address the dynamics of the MnAs ?/? self-organized stripe pattern induced by a 100 fs optical laser pulse, using as a probe the XUV radiation from the FERMI free-electron laser. We observe a loss in the diffraction intensity from the ordered ?/? stripes that occurs at two characteristic timescales in the range of ~10-12 and ~10-10 s. We associate the first intensity drop with ultrafast electron-lattice energy exchange processes within the laser-MnAs interaction volume and the second with thermal diffusion towards the MnAs/GaAs interface. With the support of model calculations, the observed dynamics are interpreted in terms of the formation of a laterally homogeneous MnAs overlayer, the thickness of which evolves in time, correlating the MnAs microstructure dynamics with the Fe magnetization response.

Spintronic devices are most often controlled by applied magnetic fields or by the injection of spin-polarized currents.

The potential of the two-color mode implemented at the FERMI free-electron laser (FEL) source for pumping and probing selectively different atomic species has been demonstrated by time-resolved scattering experiments with permalloy (FeNi alloy) and NiFe 2 O 4 samples. We monitored the ultra-fast demagnetization of Ni induced by the pump FEL pulse, by tuning the linearly-polarized FEL probe pulse to the Ni-3p resonance and measuring the scattered intensity in the transverse magneto-optical Kerr effect geometry. The measurements were performed by varying the intensity of the FEL pump pulse, tuning its wavelength to and off of the Fe-3p resonance, and by spanning the FEL probe pulse delays across the 300–900 fs range. The obtained results have Photonics 2017, 4, 6 2 of 10 evidenced that for the case of NiFe 2 O 4 , there is a sensible difference in the magnetic response at the Ni site when the pump pulse causes electronic excitations at the Fe site.

The hitherto unexplored two-photon doubly excited states [Ne∗(2p-13s)]2 were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne2+ ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light–matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon's angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms., The controlled generation of extreme-ultraviolet beams with controllable topological charge has not been demonstrated. Here, Gauthier et al. report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of orbital angular momentum.