High-order harmonic transient grating spectroscopy of SF6 molecular vibrations

special issue : Ultrafast electron and molecular dynamicsInternational audienceStrong field transient grating spectroscopy has shown to be a very versatile tool in time-resolved molecular spectroscopy. Here we use this technique to investigate the high-order harmonic generation from SF6 molecules vibrationally excited by impulsive stimulated Raman scattering. Transient grating spectroscopy enables us to reveal clear modulations of the harmonic emission. This heterodyne detection shows that the harmonic emission generated between 14 to 26 eV is mainly sensitive to two among the three active Raman modes in SF6, i.e. the strongest and fully symmetric nu 1-A1g mode (774 cm-1, 43 fs) and the slowest mode nu5-T2g (524 cm-1, 63 fs). A time-frequency analysis of the harmonic emission reveals additional dynamics: the strength and central frequency of the nu 1 mode oscillate with a frequency of 52 cm-1 (640 fs). This could be a signature of the vibration of dimers in the generating medium. Harmonic 11 shows a remarkable behavior, oscillating in opposite phase, both on the fast (774 cm-1) and slow (52 cm-1) timescales, which indicates a strong modulation of the recombination matrix element as a function of the nuclear geometry. These results demonstrate that the high sensitivity of high-order harmonic generation to molecularvibrations, associated to the high sensitivity of transient grating spectroscopy, make their combination a unique tool to probe vibrational dynamics

Role of Spin-Orbit Coupling in High-order Harmonic Generation Revealed by Super-Cycle Rydberg Trajectories

High-harmonic generation is typically thought of as a sub-laser-cycle process, with the electron's excursion in the continuum lasting a fraction of the optical cycle. However, it was recently suggested that long-lived Rydberg states can play a particularly important role in atoms driven by the combination of the counter-rotating circularly polarized fundamental light field and its second harmonic. Here we report direct experimental evidence of long and stable Rydberg trajectories contributing to high-harmonic generation. We confirm their effect on the harmonic emission via Time-Dependent Schr{\"o}dinger Equation simulations and track their dynamics inside the laser pulse using the spin-orbit evolution in the ionic core, utilizing the spin-orbit Larmor clock. Our observations contrast sharply with the general view that long-lived Rydberg orbits should generate negligible contribution to the macroscopic far-field high harmonic response of the medium. Indeed, we show how and why radiation from such states can lead to well collimated macroscopic signal in the far field

Amplitude and phase control of attosecond light pulses

We report the generation, compression, and delivery on target of ultrashort extreme-ultraviolet light pulses using external amplitude and phase control. Broadband harmonic radiation is first generated by focusing an infrared laser with a carefully chosen intensity into a gas cell containing argon atoms. The emitted light then goes through a hard aperture and a thin aluminum filter that selects a 30-eV bandwidth around a 30-eV photon energy and synchronizes all of the components, thereby enabling the formation of a train of almost Fourier-transform-limited single-cycle 170 attosecond pulses. Our experiment demonstrates a practical method for synthesizing and controlling attosecond waveforms. © 2005 The American Physical Society

Multidimensional high harmonic spectroscopy of polyatomic molecules: detecting sub-cycle laser-driven hole dynamics upon ionization in strong mid-IR laser fields

High harmonic generation (HHG) spectroscopy has opened up a new frontier in ultrafast science, where electronic dynamics can be measured on an attosecond time scale. The strong laser field that triggers the high harmonic response also opens multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. Using combined experimental and theoretical approaches, we show how multi-dimensional HHG spectroscopy can be used to detect and follow electronic dynamics of core rearrangement on sub-laser cycle time scales. We detect the signatures of laser-driven hole dynamics upon ionization and reconstruct the relative phases and amplitudes for relevant ionization channels in a CO2 molecule on a sub-cycle time scale. Reconstruction of channel-resolved complex ionization amplitudes on attosecond time scales has been a long-standing goal of high harmonic spectroscopy. Our study brings us one step closer to fulfilling this initial promise and developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems

Laser-Induced Electron Diffraction in Chiral Molecules

Strong laser pulses enable probing molecules with their own electrons. The oscillating electric field tears electrons off a molecule, accelerates them, and drives them back toward their parent ion within a few femtoseconds. The electrons are then diffracted by the molecular potential, encoding its structure and dynamics with angstrom and attosecond resolutions. Using elliptically polarized laser pulses, we show that laser-induced electron diffraction is sensitive to the chirality of the target. The field selectively ionizes molecules of a given orientation and drives the electrons along different sets of trajectories, leading them to recollide from different directions. Depending on the handedness of the molecule, the electrons are preferentially diffracted forward or backward along the light propagation axis. This asymmetry, reaching several percent, can be reversed for electrons recolliding from two ends of the molecule. The chiral sensitivity of laser-induced electron diffraction opens a new path to resolve ultrafast chiral dynamics

Roadmap on photonic, electronic and atomic collision physics: I. Light-matter interaction

We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. In Roadmap I, we focus on light-matter interaction. In this area, studies of ultrafast electronic and molecular dynamics have been rapidly growing, with the advent of new light sources such as attosecond lasers and X-ray free electron lasers. In parallel, experiments with established synchrotron radiation sources and femtosecond lasers using cutting- edge detection schemes are revealing new scientific insights that have never been exploited. Relevant theories are also being rapidly developed. Target samples for photon-impact experiments are expanding from atoms and small molecules to complex systems such as biomolecules, fullerene, clusters and solids. This Roadmap aims at looking back along the road, explaining the development of these fields, and looking forward, collecting contributions from twenty leading groups from the field

Génération et caractérisation d'impulsions attosecondes

Attosecond pulse trains in the extreme ultraviolet range can be produced by high-order harmonic generation, by focusing an intense femtosecond pulse in a rare gas jet. In this thesis, we present a temporal characterization of this radiation on the femtosecond and attosecond timescales. By transposing a spectral interferometry technique commonly used in the infrared range (SPIDER), we make a complete single-shot characterization of the temporal profile of individual harmonics, on the femtosecond timescale. In a second part, we study experimentally the attosecond structure of the harmonic radiation, and demonstrate a temporal drift in the emission: the lowest harmonics are emitted before the highest ones. This chirp, which is directly related to the electron dynamics in the generation process, imposes a lower limit to the duration that can be achieved by increasing the spectral range. We show how generating conditions can be optimized in order to enhance the synchronization in the emission, and how attosecond pulses can be recompressed. Last, we propose a new technique for the complete characterization of arbitrary attosecond pulses: FROG CRAB. This method would allow simultaneous measurements of the femtosecond and attosecond structures of the radiation, and thus a complete knowledge of the attosecond light source in the perspective of applications.La génération d'harmoniques d'ordre élevé par focalisation d'un laser intense femtoseconde dans un jet de gaz rare permet d'obtenir des trains d'impulsions attosecondes dans l'extrême ultraviolet. Dans cette thèse, nous présentons une caractérisation temporelle de ce rayonnement sur deux échelles de temps, femtoseconde et attoseconde. En transposant une technique d'interférométrie spectrale couramment utilisée pour la caractérisation complète d'impulsions infrarouges (SPIDER), nous effectuons une caractérisation complète monocoup du profil temporel d'harmoniques individuelles, à l'échelle femtoseconde.Ensuite, nous étudions expérimentalement la structure attoseconde du rayonnement harmonique, et mettons en évidence une dérive temporelle dans l'émission : les harmoniques les plus faibles sont émises avant les plus élevées. Cette dérive, qui est directement liée à la dynamique électronique microscopique dans le processus de génération, limite la durée d'impulsion que l'on peut obtenir en augmentant la largeur spectrale. Nous présentons les résultats de l'optimisation des conditions de génération afin d'améliorer la synchronisation dans l'émission. Nous montrons également la possibilité de recomprimer les impulsions attosecondes.Enfin, nous proposons une nouvelle technique pour la caractérisation complète d'impulsions attosecondes arbitraires : FROGCRAB. Elle permettrait une mesure simultanée des caractéristiques femtoseconde et attoseconde du rayonnement, et ainsi une connaissance complète de la source lumineuse attoseconde en vue de son utilisation dans des expériences d'applications