Multi-scale model of HHG in gases

International audienceThe High-order harmonic generation (HHG) is a workhorse tool of the whole attosecond science. Our intention is to have a comprehensive computational picture of HHG in a gas phase. Such a picture requires to couple two scales: 1) The macroscopic model of the driving pulse strongly affected by a non-linear propagation. 2) A single microscopic system – an atom or a molecule interacting with the pulse – described naturally by quantum physics.The primary goal is to provide an ab-initio fully-numerical solver of the process. The microscopic aspect is covered by solving the Time-dependent Schrödinger equation (TDSE) and the resulting harmonic field is computed by the means of the diffraction integral based on the Hankel transform. Finally, the non-linear propagation of the driving pulse is computed by the unidirectional solver [1].Coupling these solvers already provided a detailed insight in generating mechanisms. We used the multi-scale approach to model the control of the focusing properties of the generated field in thin targets [2] without a need of an XUV optics. Next, we modelled the optimisation of HHG in long media controlling the initial degree of the ionisation of the generating medium [3]. Both these schemes were investigated in close collaboration of experimentalists and theorists. The numerical model proved its indispensable role toextract key mechanisms and to sort complex interplays of microscopic and macroscopic effects within the process. The multi-scale approach reached a good quantitative agreement with experiments.Finally, the fully numerical model is computationally expensive. Our palette is then enriched by simpler tools (modelling both IR- and harmonic beams by Gaussian optics, using simplified models of the microscopic response, etc.). A large parametric space may be searched by these simple models and promising configuration pivot refinements and detailed studies by the full model

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

Absorption-limited XUV generation in noble gases

International audienceWe demonstrate absorption-limited high-order harmonic generation by an 800 nm femtosecond laser in argon and krypton. Phase-matching is achieved by pre-ionizing the gas using a weak capillary discharge

Phase-matched absorption-limited HHG using preionization

International audienceWe have discovered that ionization-induced defocusing is the main cause of inefficient high-order harmonic generation in long absorption-limited media. A novel method utilizing preionization to overcome this bottleneck was validated both experimentally and with numerical simulations

Absorption-limited XUV generation in noble gases

International audienceWe demonstrate absorption-limited high-order harmonic generation by an 800 nm femtosecond laser in argon and krypton. Phase-matching is achieved by pre-ionizing the gas using a weak capillary discharge

Spatiotemporal control of attosecond XUV beams

International audienceSpectral and spatial control of XUV beams is demonstrated combining divergence control at the generating plane with intermediate-field spatial filtering. This control is reproduced by simulations and shows a net improvement of attosecond beam homogeneity

Optics-less focusing of XUV high-order harmonics

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