Comment on “Transition to the Relativistic Regime in High Order Harmonic Generation”

International audienceIn [Phys. Rev. Lett. 98, 103902 (2007)], Tarasevitch et al. demonstrate the existence of two generation mechanisms for laser high-order harmonicsfrom overdense plasmas. One of these mechanisms leads to harmonics with frequencies up to the maximum plasmafrequency of the target and occurs even at nonrelativistic laser intensities. We show that the mechanism responsiblefor these harmonics is coherent wake emission (CWE), a process that significantly differs from thequalitative model proposed by these authors, and it leads toa different interpretation of several essential features of this emission

Phase Properties of Laser High-Order Harmonics Generated on Plasma Mirrors

International audienceAs a high-intensity laser-pulse reflects on a plasma mirror, high-order harmonics of the incident frequency can be generated in the reflected beam. We present a numerical study of the phase properties of these individual harmonics, and demonstrate experimentally that they can be coherently controlled through the phase of the driving laser field. The harmonic intrinsic phase, resulting from the generation process, is directly related to the coherent sub-laser-cycle dynamics of plasma electrons, and thus constitutes a new experimental probe of these dynamics

Coherent Wake Emission of High-Order Harmonics from Overdense Plasmas

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Femtosecond Laser-Produced Plasma X-Rays from Periodically Modulated Surface Targets

We have studied theoretically and experimentally the x-ray production above 1 keV from femtosecond laser plasmas generated on periodically modulated surface targets. Laser energy coupling to plasma surface waves has been modeled using a numerical differential method. Almost total absorption of incident laser radiation is predicted for optimized interaction conditions. Silicon gratings have been irradiated by a 120fs Ti:sapphire laser at irradiances in excess of 1016 W/cm2. X-ray intensities above 1.5 keV (K-shell lines) have been measured as a function of the incidence angle. Results show a distinct x-ray emission maximum for the first order diffraction angle and are in good qualitative agreement with our theoretical predictions

Role of Histone Acetylation in the Stimulatory Effect of Valproic Acid on Vascular Endothelial Tissue-Type Plasminogen Activator Expression

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Laser-induced gas-breakdown model

A one-dimensional model oriented toward a thorough understanding of line and continuum emission from a CO2 laser-induced helium plasma is presented. The model combines a time-dependent calculation of the ionization and radiation dynamics with a lagrangian hydrodynamic simulation code. Detailed collisional radiative models involving many neutral and ion excited levels are used to predict the plasma ionization state. A radiation transport algorithm including self-trapping effects and Stark broadening is developed which directly gives the space- and time-resolved spectrum of the breakdown plasma. Calculated plasma expansion, time- and space-resolved absolute light intensities and line and continuum spectral shapes compare quite favorably with experimental data.Nous présentons un modèle unidimensionnel destiné à l'interprétation détaillée du spectre continu et discret émis par un plasma d'hélium créé par un laser à CO2. Ce modèle tient compte simultanément de la dynamique d'ionisation, du transfert de rayonnement, et de l'hydrodynamique (traitée par une simulation lagrangienne) du plasma. L'état d'ionisation du plasma est évalué par des modèles collisionnels radiatifs mettant en jeu de nombreux états excités pour chaque type d'ions. Le spectre émis par le plasma de claquage est calculé en fonction de la position et du temps au moyen d'un algorithme de transfert qui tient compte de la réabsorption et de l'élargissement Stark. Les résultats de la simulation qui concernent l'expansion du plasma, l'intensité absolue de l'émission lumineuse et le spectre continu et discret sont en accord favorable avec les résultats expérimentaux obtenus au laboratoire

X ray spectroscopy of dense plasmas produced by laser irradiation at the GRECO ILM

In this paper we present in the first part the new experimental techniques used for the spectroscopic studies of the plasma obtained by laser irradiation of planar target at the GRECO ILM laser facilities. In a second part, we resume the methods used for the measurements of electronic temperature and density of these plasma, and finally we describe the hydrodynamique code FILM which describes atomic physics and plasma dynamic in a coherent simulation