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International audience; Technological advances in femtosecond laser sources call for the development of increasingly refined characterization tools implying to enrich the existing panel of operable nonlinear interactions. Toward that end, we have recently proposed a variant of SPIDER (spectral phase interferometry for direct electric field reconstruction) based on a nonstandard effect for producing the frequency shear, the so-called rotational Doppler effect. The method called DEER-SPIDER, for Doppler effect E-field replication, has the advantage of producing a spectral shearing at/near the fundamental wavelength, thus allowing operation in the ultraviolet spectral range. The present paper provides a deeper study of this approach. The method is tested under two different challenging conditions, and a thorough theoretical analysis is proposed. Possible improvements and an outlook are also discussed.

Additional experimental data and setup description

In this letter we demonstrate agile femtosecond pulse generation with a widely tunable repetition rate (10–100 MHz) from a continuous wave laser diode optically gated by a Mach–Zehnder electro-optic intensity modulator. Initial sub-50 ps pulses are strongly spectral broadened (>5 nm) by self-phase modulation in a polarization maintaining single-mode fiber. A tunable optical pulse train with pulse durations of a few hundred femtoseconds is obtained using a simple fixed grating compressor, thanks to spectral broadening saturation phenomena. The source is easily synchronized with low timing jitter using an external clock signal.

The recent availability of intense and ultrashort extreme ultraviolet sources opens the possibility to investigate ultrafast electronic relaxation processes in matter in an unprecedented regime. In this work we report on the observation of two-photon excitation of interatomic Coulombic decay (ICD) in neon dimers using the tunable intense pulses delivered by the free electron laser FERMI@Elettra. The unique characteristics of FERMI (narrow bandwidth, spectral stability, and tunability) allow one to resonantly excite specific ionization pathways and to observe a clear signature of the ICD mechanism in the ratio of the ion yield created by Coulomb explosion. The present experimental results are explained by \emph{ab initio} electronic structure and nuclear dynamics calculations.

We observe a new regime of coherent XUV radiation generation in noble gases induced by femtosecond pulses at very high intensities. This XUV emission has both a reduced divergence and spectral width as compared to high-order harmonic generation (HHG). It is not emitted at a moderate intensity of the driving pulses where only high-order harmonics are generated. At high driving intensities, the additional XUV comb appears near all harmonic orders and even exceeds the HHG signal on the axis. The peaks are observed in several gases and their frequencies do not depend on the driving intensity or gas pressure. We analyze the divergence, spectral width and spectral shift of this XUV emission. We show that these specific features are well explained by high-order parametric generation (HPG) involving multiphoton absorption and combined emission of an idler THz radiation and an XUV beam with remarkably smooth spatial and spectral characteristics., Comment: 18 pages, 12 figures. To be published in a peer-reviewed scientific journal

We report experimental observation of a new regime of XUV emission in conditions close to high harmonic generation in gases. It appears as an additional comb of spectral peaks that are stronger than harmonics.

A simple and compact single-shot autocorrelator is presented and analyzed in detail. The autocorrelator is composed of two elements only: a Fresnel biprism used to create two temporally delayed replicas of the pulse to characterize and a camera in which two-photon absorption takes place. The two-photon absorption signal obtained in the camera can be used to retrieve the pulse duration, the frequency chirp, and the pulse spectrum, provided that a Gaussian temporal shape is assumed. Thanks to its extreme simplicity, the autocorrelator is robust and easy to align. The presented design can theoretically characterize the pulse duration from about 25 fs to 1.5 ps in the two-photon spectral range of the camera (1200–2400 nm). Finally, a proof-of-principle demonstration is also performed at 3.1 μm by using an InGaAs camera, whose two-photon spectral range is located further in the infrared (1800–3400 nm).A simple and compact single-shot autocorrelator is presented and analyzed in detail. The autocorrelator is composed of two elements only: a Fresnel biprism used to create two temporally delayed replicas of the pulse to characterize and a camera in which two-photon absorption takes place. The two-photon absorption signal obtained in the camera can be used to retrieve the pulse duration, the frequency chirp, and the pulse spectrum, provided that a Gaussian temporal shape is assumed. Thanks to its extreme simplicity, the autocorrelator is robust and easy to align. The presented design can theoretically characterize the pulse duration from about 25 fs to 1.5 ps in the two-photon spectral range of the camera (1200–2400 nm). Finally, a proof-of-principle demonstration is also performed at 3.1 μm by using an InGaAs camera, whose two-photon spectral range is located further in the infrared (1800–3400 nm).

Controlling generation of XUV high-order harmonic beams provides focusing of attosecond pulses without any XUV optics., By experimentally studying high-order harmonic beams generated in gases, we show how the spatial characteristics of these ultrashort extreme-ultraviolet (XUV) beams can be finely controlled when a single fundamental beam generates harmonics in a thin gas medium. We demonstrate that these XUV beams can be emitted as converging beams and thereby get focused after generation. We study this optics-less focusing using a spatially chirped beam that acts as a probe located inside the harmonic generation medium. We analyze the XUV beam evolution with an analytical model and obtain very good agreement with experimental measurements. The XUV foci sizes and positions vary strongly with the harmonic order, and the XUV waist can be located at arbitrarily large distances from the generating medium. We discuss how intense XUV fields can be obtained with optics-less focusing and how the order-dependent XUV beam characteristics are compatible with broadband XUV irradiation and attosecond science.

Autoionizing hole states with electron configuration nsnp5 mp are studied in Ar+ and Kr+. Total and partial photoionization cross sections, photoelectron spectra and photoelectron angular distributions in the region of the resonances are obtained theoretically in extensive R-matrix calculations. The states of Ar+ are observed by means of excitation by a free-electron laser operating in the vacuum- and extreme-ultraviolet wavelength regime combined with angle-resolved photoelectron spectroscopy. Fine tuning of the photon energy allows scanning of the resonances and the observation of the shape of the partial cross section ratio, as well as the asymmetry parameter of the angular distribution of the photoelectrons. The calculations are in good agreement with the experimental data.

To be complete, the characterization of the photoionization process of atoms and molecules requires the extraction of all quantum-mechanical phases and amplitudes. So far, complete experiments have accessed only the ionization process of neutral atoms and molecules. Here we report the quantum-mechanically complete characterization of the single and double ionization of neon to yield doubly charged ions. The first ionization step by intense, polarized extreme ultraviolet light from a free-electron laser leaves the ion in a polarized state (that is, one in which the angular momentum of the ion is aligned in space). By controlling the polarization of the light, we determine the bound and continuum components of the system in the first and second ionization steps leading to the formation of doubly charged neon ions. We test the validity of our approach by characterizing the influence of autoionizing ionic states on the two-photon double-ionization mechanism. Our results are important for understanding the physics of the interaction of extreme ultraviolet radiation with ions. Photoionization is one of the most important photophysical events. This process can now be characterized in a quantum-mechanically complete manner by use of polarization-controlled extreme-ultraviolet light derived from a free-electron laser

The interaction of intense extreme ultraviolet (XUV) pulses with an atom can lead to multi-photon absorption and multiple ionization of the target. By using intense, linearly and circularly polarized XUV pulses of the Free Electron Laser (FEL) FERMI [1], we realized the first experimental demonstration of a quantum mechanically complete experiment (CE) in an ionic system. The quest for a CE, as the strongest test of theory and as minimal source of information from which any observable can be predicted, was first formulated at the end of the 1960s, [2], for electron-atom scattering, and then extended to atomic and molecular photoionization (PI). The complete information cannot be obtained by measuring only the photo-electron angular distribution (PAD) and the cross sections, but additional variables are required. Thus, CEs in atomic PI were realized by measuring the angle-resolved PAD, combined with the observation of other variables or, in case of atoms with open shells, by controlling the initial polarization of the target. All the CEs on PI so far have been performed with neutral targets. As atomic PI leads to polarized residual ions, CE in positive ions can be realized within the two photon double sequential ionization (TPDI) process, now available thanks to the intense XUV pulses of FELs.

Recent results in laser beam delivery and ultra-short pulse (USP) compression using hypocycloid-core Kagome hollow-core photonic crystal fibers (HC-PCFs) [1] proved that this type of optical fiber is an excellent candidate as a photonic component for these applications. For example, it has been demonstrated that the fiber guides up to 1 mJ of600 fs long pulses with no damage, and by a simple choice of gas and fiber dispersion, the authors achieved both USP guidance with no pulse broadening nor narrowing, guidance with self-phase modulation (SPM) spectral broadening, and finally guidance with over 10-fold self-compression using solitonic dynamics. However, these compression results necessitated both fiber gas-loading-system and tailored fiber-fabrication, which are not necessarily accessible to the broader research community. Here, we report on a set of results of self-compression of a USP laser based on a commercially available Kagome fiber (PMC-C-YB-7C from GLOphotonics [2]) and with no need of gas loading management. The compression relies on pulse dynamics near the photoionization threshold, which shows a strong and abrupt self-compression via the formation of a soliton at a well-defined pulse energy value and then its breakup at higher energy values [3]. By simply adjusting the fiber length from 10 cm to 4 m, we achieved compression of an initial 600 fs from Yb-doped USP-laser down to ∼20 fs (a compression ratio ∼30) over an energy span of 10–500 μJ. Figure 1(a) shows a summary of calculated (dotted curves) and measured (solid curves) pulse duration evolution with input energy for different fiber lengths. All curves show a “step shape” corresponding to this sudden compression, and the input energy value at which the self-compression occurs increases with shortening fiber length. A typical experimental FROG [4] evolution of such dynamic is shown in Figure 1 (c) for the case of 2 m fiber length resulting in a spectral broadening, a soliton red-shift and a strong pulse compression as the energy is increased. For this conditions, the maximum compression occurs at an energy around 60 μΐ. For higher energy, the pulse breaks up via soliton fission.

Les sources attosecondes sont maintenant une realite. Elles ont ouvert le domaine de l’attoscience et permettent l’etude de dynamiques ultrarapides a des echelles de temps attoseconde et, en particulier, l’etude de dynamiques electroniques. Ces sources XUV ultracourtes sont aussi coherentes et ont de nombreuses caracteristiques similaires au laser. Cet article presente le principe physique de leurs generations, leurs controles et leurs utilisations et montre ainsi comment la physique non-lineaire induite par laser femtoseconde intense permet aujourd’hui le developpement de l’attophysique.

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.

International audience

We present a comprehensive analysis of autoionization processes in Ne clusters (~5000 atoms) after multiple valence excitations by free electron laser radiation. The evolution from 2-body interatomic Coulombic decay (ICD) to 3-body ICD is demonstrated when changing from surface to bulk Frenkel exciton excitation. Super Coster-Kronig type 2-body ICD is observed at Wannier exciton which quenches the main ICD channel.

The extreme ultraviolet absorption spectrum of an atom is strongly modified in the presence of a synchronized intense infrared field. In this work we demonstrate control of the absorption properties of helium atoms dressed by an infrared pulse by changing the relative polarization of the infrared and extreme ultraviolet fields. Light-induced features associated with the dressed $1s2s, 1s3s$, and $1s3d$ states, referred to as $2{s}^{+}, 3{s}^{\ifmmode\pm\else\textpm\fi{}}$, and $3{d}^{\ifmmode\pm\else\textpm\fi{}}$ light-induced states, are shown to be strongly modified or even eliminated when the relative polarization is rotated. The experimental results agree well with calculations based on the solution of the time-dependent Schr\"odinger equation using a restricted excitation model that allows efficient treatment of the three-dimensional problem. We also present an analysis of the light-induced states based on Floquet theory, which allows for a simple explanation of their properties. Our results open a new route to creating controllable superpositions of dipole allowed and nondipole allowed states in atoms and molecules.

We present a comprehensive analysis of autoionization processes in Ne clusters (similar to 5000 atoms) after multiple valence excitations by free electron laser radiation. The evolution from 2-body interatomic Coulombic decay (ICD) to 3-body ICD is demonstrated when changing from surface to bulk Frenkel exciton excitation. Super Coster-Kronig type 2-body ICD is observed at Wannier exciton which quenches the main ICD channel.

Pump-probe experiments are commonly used at Free Electron Lasers (FEL) to elucidate the femtosecond dynamics of atoms, molecules, clusters, liquids and solids. Maximizing the signal-to-noise ratio of the measurements is often a primary need of the experiment, and the aggregation of repeated, rapid, scans of the pump-probe delay is preferable to a single long-lasting scan. The limited availability of beamtime makes it impractical to repeat measurements indiscriminately, and the large, rapid flow of single-shot data that need to be processed and aggregated into a dataset, makes it difficult to assess the quality of a measurement in real time. In post-analysis it is then necessary to devise unbiased criteria to select or reject datasets, and to assign the weight with which they enter the analysis. One such case was the measurement of the lifetime of Intermolecular Coulombic Decay in the weakly-bound neon dimer. We report on the method we used to accomplish this goal for the pump-probe delay scans that constitute the core of the measurement; namely we report on the use of simple auto- and cross-correlation techniques based on the general concept of “matched filter”. We are able to unambiguously assess the signal-to-noise ratio (SNR) of each scan, which then becomes the weight with which a scan enters the average of multiple scans. We also observe a clear gap in the values of SNR, and we discard all the scans below a SNR of 0.45. We are able to generate an average delay scan profile, suitable for further analysis: in our previous work we used it for comparison with theory. Here we argue that the method is sufficiently simple and devoid of human action to be applicable not only in post-analysis, but also for the real-time assessment of the quality of a dataset.