We photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition (molecular phase) for different states of the ion. We observe a $0.9\ensuremath{\pi}$ shift for the electrons produced in the ionization channels leading to the $X\text{ }{^{2}\ensuremath{\Sigma}}_{g}^{+}$, ${v}^{\ensuremath{'}}=1$, and ${v}^{\ensuremath{'}}=2$ states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short-lived states, which is often not accessible otherwise.

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.

Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the C-C bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results.

A method for ultrafast ellipticity modulation of femtosecond intense lasers is introduced and demonstrated. The method is based on the coherent superimposition of two linearly polarized visible/ infrared (Vis-IR) laser beams with orthogonal polarizations. Tuning their delay by a quarter of the wavelength, i.e., a few hundred nanometers, achieves the same function as the rotation of a quarter-wave plate by 45 degrees, switching the polarization from linear to circular. We demonstrate the portability of this method to high-intensity processes by upconverting a femtosecond Vis-IR laser beam to the extreme ultraviolet (EUV) spectral range through high-harmonic generation. These results open the way to lock-in detection of small absorption chiroptical signals in the EUV spectral range, including pump probe signals. (C) 2018 Optical Society of America

The dynamics of quantum systems are encoded in the amplitude and phase of wave packets. However, the rapidity of electron dynamics on the attosecond scale has precluded the complete characterization of electron wave packets in the time domain. Using spectrally resolved electron interferometry, we were able to measure the amplitude and phase of a photoelectron wave packet created through a Fano autoionizing resonance in helium. In our setup, replicas obtained by two-photon transitions interfere with reference wave packets that are formed through smooth continua, allowing the full temporal reconstruction, purely from experimental data, of the resonant wave packet released in the continuum. In turn, this resolves the buildup of the autoionizing resonance on an attosecond time scale. Our results, in excellent agreement with ab initio time-dependent calculations, raise prospects for detailed investigations of ultrafast photoemission dynamics governed by electron correlation, as well as coherent control over structured electron wave packets.

We describe the versatile features of the attosecond beamline recently installed at CEA-Saclay on the PLFA kHz laser. It combines a fine and very complete set of diagnostics enabling high harmonic spectroscopy (HHS) through the advanced characterization of the amplitude, phase, and polarization of the harmonic emission. It also allows a variety of photo-ionization experiments using magnetic bottle and COLTRIMS (COLd Target Recoil Ion Momentum Microscopy) electron spectrometers that may be used simultaneously, thanks to a two-foci configuration. Using both passive and active stabilization, special care was paid to the long term stability of the system to allow, using both experimental approaches, time resolved studies with attosecond precision, typically over several hours of acquisition times. As an illustration, applications to multi-orbital HHS and electron-ion coincidence time resolved spectroscopy are presented.

Molecular polarimetry, based on the determination of molecular frame photoemission in dissociative photoionization (DPI) studied by electron-ion coincident momentum spectroscopy, allows us to measure the complete state of elliptically polarized light. Here, we demonstrate the first complete characterization of high-order harmonic polarization state for three benchmark generation processes with inherent symmetry breaking based on the polarization of the generating beam (elliptical or counter-rotating fields) or on anisotropic targets e.g. aligned molecules. The ability to disentangle circular and unpolarized components of the XUV pulses is of particular interest to qualify the HHG sources.

We present procedures to develop and characterise chirped multilayer mirrors for attosecond pulses. The design procedure involves a simulated annealing optimisation algorithm to obtain a multilayer structure with the desired performances. The characterisation step requires the use of well-calibrated attosecond and synchrotron beam lines, allowing one to measure the amplitude and phase response of the mirror. We illustrate these approaches with a set of mirrors designed to have zero or negative dispersion. The remarkable agreement between theoretical and experimental performances validates this overall process, and demonstrates the capability of such mirrors to control the temporal profile of attosecond pulses. These tools and techniques for developing attosecond chirped mirrors will facilitate the generalisation of such components on attosecond experiments.

We have analyzed the angular distributions of the photoelectrons emitted upon photoionization of rare gases by a comb of harmonics in the extreme ultraviolet range, in the presence of a "dressing" infrared (IR) field with controlled delay tau, stabilized down to about +/-60 as. The measurements have been performed with the help of the coincidence momentum imaging technique. We evidence marked differences in the measured angular distributions of the photoelectrons, depending on the number of IR photons exchanged. Joined to a theoretical interpretation, these observations bring new insights into the dynamics of this class of two-color photoionization processes that are a key step towards studying photoionization in the time domain, with attosecond time resolution.

We report on high-order harmonic generation (HHG) using a Ti:sapphire laser beam phase shaped with a binary diffractive optical element (DOE) to create two spatially separated synchronized HHG sources at the focus of a lens. Using full three-dimensional computations, we show numerically that the harmonic dipole phase is imprinted in the resulting far-field fringe pattern. Using the corresponding experimental arrangement, we measure HHG phase in aligned carbon dioxide. This arrangement is robust, extremely stable, simple to use, and gives highly resolved fringes. It thus opens new perspectives for combined and refined HHG phase measurements in excited samples.

We present single shot nanoscale imaging using a table-top femtosecond soft X-ray laser harmonic source at a wavelength of 32 nm. We show that the phase retrieval process in coherent diffractive imaging critically depends on beam quality. Coherence and image fidelity are measured from single-shot coherent diffraction patterns of isolated nano-patterned slits. Impact of flux, wave front and coherence of the soft X-ray beam on the phase retrieval process and the image quality are discussed. After beam improvements, a final image reconstruction is presented with a spatial resolution of 78 nm (half period) in a single 20 fs laser harmonic shot.

Nous avons démontré la faisabilité de mesures de corrélations vectorielles dans la photoionisation dissociative de H2 auprès d'une source XUV femtoseconde d'harmonique d'ordre élevé cadencée à 1 kHz [1]. Dans ce contexte, nous présentons les miroirs multicouches conçus, réalisés et caractérisés par la Centrale d'Elaboration et de Métrologie d'Optiques X pour sélectionner cette harmonique 21 à ∼32 eV (∼40 nm). Nous avons mesuré une série d'observables, de la distribution en coïncidence des énergies cinétiques de l'ion fragment et de l'électron aux distributions angulaires du photoélectron émis dans le référentiel moléculaire (MFPADs), ainsi que leur évolution avec l'énergie cinétique de l'ion fragment. Ces résultats ouvrent la voie à des études résolues en temps, basées sur la mesure des MFPADs, de la dynamique femtoseconde des états doublements excités Q1 et Q2 de H2.

LUNEX5 (free electron Laser Using a New accelerator for the Exploitation of X-ray radiation of 5th generation) aims at investigating the production of short, intense, and coherent pulses in the soft X-ray region. The project consists of a Free Electron Laser (FEL) line enabling the most advanced seeding configurations: High order Harmonic in Gas (HHG) seeding and Echo Enable Harmonic Generation (EEHG) with in-vacuum (potentially cryogenic) undulators of 15 and 30 mm period. Two accelerator types feed this FEL line : a 400 MeV Conventional Linear Accelerator (CLA) using superconducting cavities compatible with a future upgrade towards high repetition rate, for the investigations of the advanced FEL schemes; and a 0.4 1 GeV Laser Wake Field Accelerator (LWFA), to be qualified in view of FEL application, in the single spike or seeded regime. Two pilot user experiments for timeresolved studies of isolated species and solid state matter dynamics will take benefit of LUNEX5 FEL radiation and provide feedback of the performance of the different schemes under real user conditions.

LUNEX5 (free electron Laser Using a New accelerator for the Exploitation of X-ray radiation of 5th generation) aims at investigating the production of short, intense, and coherent pulses in the soft x-ray region. The single Free Electron Laser (FEL) line fed either by a superconducting Linear Accelerator of 400MeV or a 0.4-1GeV Laser Wake Field Accelerator (LWFA) enables seeding (High order Harmonic in Gas seeding and Echo Enabled Harmonic Generation) for the advanced fourth generation source with the conventional accelerator, and single spike operation for the fifth generation source with the LWFA. Two pilot user experiments for time-resolved studies of isolated species and solid state matter will take benefit of LUNEX5 FEL.

We report the first results of molecular frame photoelectron emission for dissociative photoionization (DPI) of H2 and D2 molecules induced by a spectrally filtered single high harmonic of a few femtosecond duration, using coincident electron–ion velocity vector correlation techniques. For the studied photon energies around 32 eV, where the resonant excitation of the Q1 and Q2 doubly excited states occurs, autoionization and nuclear dynamics are coupled on a few femtosecond timescale, giving rise to quantum interferences. Molecular frame photoelectron angular distributions (MFPADs), traced as a function of the kinetic energy release of the atomic fragments, provide the most sensitive observables for such complex dynamics. These results compare well with recent spectrally resolved experiments using synchrotron radiation which are also reported. As a novel XUV light source running at multi-kHz repetition rate and synchronized with laser pulses, high-order harmonic generation (HHG) opens new possibilities for extending these investigations to time-resolved studies at the femtosecond scale.

We characterized the high harmonic emission of aligned N2O and CO2 molecules in intensity and spectral phase using the RABBIT technique. The behavior of the observed phase jump is discussed regarding structural and multi-orbital contributions.

We study high harmonic generation in SF6 molecules in the vicinity of a shape resonance. Close to the resonance we observe significant distortions of the harmonic plateau and the corresponding emission times.

High-order harmonic generation in aligned molecules can be used as an ultrafast probe of molecular structure and dynamics. By characterizing the emitted signal, one can retrieve information about electronic and nuclear dynamics at the attosecond timescale. In this paper, we discuss the theoretical and experimental aspects of molecular orbital tomography in N-2 and investigate the influence of multi-channel ionization on the orbital imaging. By analyzing the harmonics' spectral phase as a function of the driving laser intensity, we address two distinct cases, which in principle allow the orbital reconstruction. First, the contributions from two molecular orbitals could be disentangled in the real and imaginary parts of the measured dipole, allowing to reconstruct both orbitals. Second, by decreasing the driving laser intensity, the transition from a multi-channel to a single-channel ionization regime is shown. The latter approach paves the way towards the generalization of tomography to more complex systems. (c) 2012 Elsevier B.V. All rights reserved.

When generated in molecules, high-order harmonics can be emitted through different ionization channels. The coherent and ultrafast electron dynamics occurring in the ion during the generation process is directly imprinted in the harmonic signal, i.e. in its amplitude and spectral phase. In aligned N2 molecules, we find evidence for a fast variation of this phase as a function of the harmonic order when varying the driving laser intensity. Basing our analysis on a three-step model, we find that this phase variation is a signature of transitions from a single- to a multi-channel regime. In particular, we show that significant nuclear dynamics may occur in the ionization channels on the attosecond timescale, affecting both the amplitude and the phase of the harmonic signal.