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

Nonlinear Thomson scattering from relativistic laser plasma interaction

International audienceAs an electron oscillates in an intense laser field, it acquires a highly nonlinear motion and emits X-ray radiation by nonlinear Thomson scattering. In this paper we present a numerical and experimental analysis of this radiation. We show that this radiative process becomes dominant at relativistic laser intensities (a0>1), for which the laser light scatters off MeV electrons accelerated in the plasma

Microsynchrotron

International audienceGrâce aux progrès réalisés sur les chaînes laser ultra intenses, il est maintenant possible de produire des faisceaux de rayons X par interaction laser-plasma en utilisant un laser ultra bref. De plus, cette source est polychromatique et de durée femtoseconde, ce qui ouvre la voie vers de nombreuses applications. Lorsqu'un laser intense (50TW, 30fs) est focalisé sur le front avant d'un jet d'hélium, des électrons sont piégés dans le sillage de l'impulsion laser et ensuite accélérés pour enfin osciller dans le plasma lui même. Comme dans un synchrotron, un faisceau X de faible divergence est produit par ces oscillations électroniques. Après avoir présenté le principe de cette source, nous montrons les derniers résultats obtenus pour sa caractérisation spatiale et spectrale, ainsi que la corrélation entre le faisceau d'électrons et de rayons X produits par ce procédé

Adaptive wavefront correction on a 100-TW/10-Hz chirped pulse amplification laser and effect of residual wavefront on beam propagation

International audienceWe demonstrate the wavefront correction of a 100-TW (25 fs, 2.5 J) high repetition rate Ti:sapphire laser. A 36-actuator bimorph deformable mirror was used under vacuum and the adaptive optic system is working at 10 Hz in a closed-loop configuration with a Shack–Hartmann wavefront sensor. We also numerically studied the effects of the deformable mirror actuators on laser beam free propagation to determine the ideal position of the deformable mirror in the laser chain. Our ability to focus the beam was improved to near-diffraction-limited quality and intensity as high as 1.4 × 10^20 W/cm^2 was reached with a 500-mm focal length off-axis parabolic mirror

Polychromatic X-ray Beam from the Acceleration of Energetic Electrons in Ultrafast Laser-Produced Plasmas

International audiencePolychromatic beams of hard X-rays from ultrafast laser plasma interaction are studied. Just as in a conventional synchrotron, electrons are accelerated and wiggled, but on a much shorter scale of a few millimeters. By focusing a 50 TW CPA laser system (30 fs duration) onto a helium gas jet, we obtained a polychromatic collimated beam (50 mrad) of X-ray radiation in the keV range. In addition, its perfect synchronization with the laser system, its ultrafast duration (≃30 fs) and its brightness (up to 10^8 photons/shot/solid angle at 0.1% BW) will make it applicable to both X-ray science and backlighting to address laboratory astrophysics research issues

Comparison of two spatial characterisation methods of XUV high-order harmonics for spatio-temporal control of attosecond pulses

International audienceExtreme ultraviolet (XUV) sources (10–100 nm) based upon high harmonic generation (HHG) in gases emit attosecond pulses and exhibit a high spatial and temporal coherence useful for a great range of applications. To further develop applications, it is very important to control the focusing of XUV beams as well as the evolution of the attosecond pulse duration during propagation. It has been demonstrated that the spatial profile of high harmonics strongly depends on the harmonic order [1] which consequently introduces chromatic aberrations at XUV focus and subsequent inhomogeneities in the attosecond temporal profile [2]. Nevertheless, the spatial properties of the XUV beam can be controlled to some extend [3], [4] in order to make it more homogeneous thus improving the spatio-temporal properties. To that respect retrieving the front phase of the XUV beam becomes essential. We will show two techniques to experimentally extract the front-phase

Combined high-harmonic interferometries for vectorial spectroscopy

We present a new method to characterize transverse vectorial light produced by high-harmonic generation (HHG). The incoherent sum of the two components of the electric field is measured using a bi-dimensional transient grating while one of the components is simultaneously characterized using two-source interferometry. The combination of these two interferometric setups enables the amplitude and phase measurement of the two vectorial components of the extreme ultraviolet radiation. We demonstrate the potential of this technique in the case of HHG in aligned nitrogen, revealing the vectorial properties of harmonics 9–17 of a Ti:sapphire laser

DEVELOPMENT OF LASER BASED SYNCHROTRON X-RAY SOURCE

International audienceBy focusing an ultraintense laser onto a helium gaz jet, a collimated beam of ultrafast broadband X-ray radiation can now be generated. The X-ray radiation results from the betatron oscillations of relativistic electrons in the laser created plasma channel. Thus, just as in a synchrotron, the spectral and flux properties of the X-ray beam can be linked to the electron beam through the plasma wiggler strength. The radiation has been observed within 1-10 keV with filters and presents a divergence of as low as 20 mrad. In addition, this source possesses the unique properties to be ultrafast and perfectly synchronized with the laser system, which opens the way toward new types of pump probe experiments

Imaging Electron Trajectories in a Laser-Wakefield Cavity Using Betatron X-Ray Radiation

International audienceWe demonstrate that betatron x-ray radiation accurately provides direct imaging of electrons trajectories accelerated in laser wakefields. Experimental far field x-ray beam profiles reveal that electrons can follow similar transverse trajectories with typical excursions of 1.5  μm±0.5  μm in the plane of laser polarization and 0.7  μm±0.2  μm in the plane perpendicular

Contrôle spatial de la structure temporelle d’impulsions attosecondes

International audienceNous avons demontré expérimentalement la sélection spectrale d’un groupe d’harmoniques d’ordre elevé XUV au moyen d’une modification des propriétés spatiales du rayonnement XUV. Des simulations ont également montré l’amélioration du profil spatio-temporel d’un train d’impulsions attosecondes après propagation