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We have observed evidence of the emission of energetic He-and H-like ions of fluorine more than 1 MeV produced via the optical field ionization (OFI) from a solid target irradiated by an intense I=(2-4)x10(18) W/cm(2) (60 fs, lambda=800 nm), obliquely incident p-polarized pulse laser. The measured blue wing of He(alpha), He(beta), and Ly(alpha) lines of fluorine shows a feature of the Doppler-shifted spectrum due to the self-similar ion expansion dominated by superthermal electrons with the temperature T(h) approximately 100 keV. Using a collisional particle-in-cell simulation, which incorporates the nonlocal-thermodynamic-equilibrium ionization including OFI, we have obtained the plasma temperature, line shape, and maximal energy of accelerated ions, which agree well with those determined from the experimental spectra. The red wing of ion spectra gives the temperature of bulk plasma electrons.

We present the characteristics of a high-density pulsed gas jet developed to study the interaction of a short-pulse, high-intensity laser with a large scale underdense plasma. The jet was characterized by laser interferometry. Measurements of density profile were performed for a 5 mm diam cylindrical nozzle and for a 20×1 mm slit. In this latter case, a systematic study of the output density as a function of the backing pressure and of the valve opening duration is presented. At 1 mm from its outlet, the cylindrical nozzle provides densities up to 6.9×1019 cm−3 for an 8 bar backing pressure with a 3 mm full width at half maximum Gaussian-shaped profile. Under the same conditions, the slit produces a 20 mm long flat-top profile with a maximum density of 2.1×1019 cm−3.

For the first time registration of high-resolution soft x-ray emission and atomic data calculations of hollow-atom dielectronic satellite spectra of highly charged nitrogen have been performed. Double-electron charge-exchange processes from excited states are proposed for the formation of autoionizing levels in high-intensity laser-produced plasmas, when field-ionized ions penetrate into the residual gas. Good agreement is found between theory and experiment. Plasma spectroscopy with hollow ions is proposed and a temperature diagnostic for laser-produced plasmas in the long-lasting recombining regime is developed.

A hydrodynamic model of the interaction of a short, intense laser pulse with moderate-density gases is constructed. The model systematically takes into account both tunneling-ionization processes and the relativistic motion of the electrons that are produced by the ionization of the gas in the relativistically strong field of the pulse. X-ray spectroscopy data agree well with the proposed theory.

International audience; The principal objective of Apollon-10P project is the generation of 10 PW peak power pulses of 15 fs duration at 1 shot/minute. The technological challenges and the current status of the Apollon laser system are discussed in detail.

A 65-fs, 800-nm, 2-TW laser pulse propagating through a nitrogen gas jet has been experimentally studied by 90 degrees Thomson scattering. Time-integrated spectra of scattered light show unprecedented broadening towards the blue which exceeds 300 nm. Images of the scattering region provide for the first time a space-and time-resolved description of the process leading quite regularly to such a large upshift. The mean shifting rate was as high as delta lambda/delta t approximate to 3 angstrom/fs, never observed before. Interferometry shows that it occurs after partial laser defocusing. Numerical simulations prove that such an upshift is consistent with a laser-gas late interaction, when laser intensity has decreased well below relativistic values (a(0) << 1) and ionization process involves most of the laser pulse. This kind of interaction makes spectral tuning of ultrashort intense laser pulses possible in a large spectral range. (C) 2013 AIP Publishing LLC.

This paper reports on experimental investigations on relativistic self-focusing and self-channeling of a terawatt laser pulse (0.7 TW/spl les/P/spl les/15 TW) in an underdense plasma. We present results obtained with picosecond (/spl tau/=1 ps) and subpicosecond (/spl tau/=0.4 ps) pulses. In the "long pulse" regime, modifications in the laser propagation are observed for P

We present an experimental investigation of the propagation of a 1-ps-terawatt laser pulse in a hydrogen gas jet. A study of the output beam profile shows that beam divergence is significantly reduced when laser power is close to---but lower than---the critical power for relativistic self-focusing. It is shown that this effect depends dramatically on the gas used.

We review the main results concerning high-order generation processes from the point of view of a potential user of this new source of XUV radiation. The perspectives for optimizing the source, both in efficiency and in spectral range, its characteristics and in particular, its coherence properties, are discussed. Finally, we describe two experiments, which demonstrate the usefulness of the harmonics as a short-pulse, coherent source in the XUV domain.

The propagation of intense, picosecond laser pulses in tenuous plasmas is studied for conditions close to those required for relativistic self‐focusing. Solutions of the steady‐state wave‐envelope equations are used to obtain experimentally measurable quantities such as the Thomson‐scattered light at 90°, and the far‐field radiation pattern. It is demonstrated that in the presence of electron cavitation, the Thomson signal has a longitudinal structure which is generally the converse of the intensity pattern. Furthermore, beam collimation in the far‐field depends on where the laser is focused in the plasma, and may actually be degraded if self‐channeling occurs.

Reported is on the propagation and on the focusing of a 1064 nm 1 ps multiterawatt laser pulse in a H 2 gas jet. The focusing is diagnosed by viewing the second harmonic emission. It is shown that high intensities can be maintained in the presence of a high density plasma.

Cross polarized wave generation (XPW) is a well established technique for contrast enhancement of ultrashort pulses in high energy laser systems [1]. Efficient conversion with XPW only occurs at high intensities. Weaker, unconverted pre and post pulses are thus rejected by the second polarizer, thereby improving the temporal contrast of the pulse. XPW has several drawbacks especially in terms of simultaneously achieving high conversion efficiencies and output energies. Limitations in seeding the nonlinear filter with high energies arise from the upper intensity limit of white light generation while high conversion efficiencies require excellent beam quality.

We present a compact and energy-scalable crossed polarized wave generation setup with waveguide filtering. A 1.4 mJ, 14.5 fs FTL, 10−4 contrast ratio enhanced XPW pulse is produced with 11 mJ, 40 fs input pulse.

International audience; The generation of high contrast and ultrashort laser pulses via a compact and energy-scalable cross polarized wave filter is presented. The setup incorporates a waveguide spatial filter into a single crystal XPW configuration, enabling high energy and high intensity transmission, efficient contrast enhancement and pulse shortening at the multi-mJ level. Excellent XPW conversion of up to 33% (global efficiency: 20%, intensity transmission: 40%) led to an output energy of 650 ?J for an input of 3.3 mJ. Additionally, efficient conversion under specific input phase conditions, allowed pulse shortening from 25 fs to 9.6 fs, indicating the prospective application of this setup as a high energy, ultrabroad laser source. © 2010 Optical Society of America.

We present a compact and energy-scalable ultrashort laser setup based on waveguide filtering and cross polarized wave generation. A 650 µJ, 15.5 fs, 10-10 contrast ratio XPW pulse is produced with 3.3 mJ, 25 fs input pulses

We report a new experimental situation where the main contribution to the energy of electrons submitted to an ultrastrong laser field is given by the ponderomotive force. Electrons released from a low-density gas medium with energies up to 5 keV have been detected, which is far above the energy acquired during the ionization process. Numerical simulations of the electron trajectories are in good agreement with the experimental results

Cette contribution donne une rapide description de quelques phenomenes physiques lies a l'ionisation multiphotonique d'atomes en champs laser relativement intenses, de 10 12 a 10 15 W/cm 2 . Cette ionisation conduit a la production d'ions multicharges (etude de la dynamique de formation des ions), ionisation au-dessus du seuil (etude de l'energie des electrons crees) et a la generation d'harmoniques d'ordre tres eleve. Un atome place dans un champs laser intense, va emettre des radiations qui sont les harmoniques impaires du champs laser excitateur, avec une distribution en amplitude presque constante

Cette contribution donne une rapide description de quelques phenomenes physiques lies a l'ionisation multiphotonique d'atomes en champs laser relativement intenses, de 10 12 a 10 15 W/cm 2 . Cette ionisation conduit a la production d'ions multicharges (etude de la dynamique de formation des ions), ionisation au-dessus du seuil (etude de l'energie des electrons crees) et a la generation d'harmoniques d'ordre tres eleve. Un atome place dans un champs laser intense, va emettre des radiations qui sont les harmoniques impaires du champs laser excitateur, avec une distribution en amplitude presque constante

International audience; The study of the interaction between ultrashort laser pulses at relativistic intensities and solid matter has promoted, in recent years, a promising way of investigating ultrafast phenomena and has suggested the possibility of efficiently producing accelerated particle beams. As laser technology has been improving and increasing intensities and energies, it has become clear that the temporal contrast of a laser pulse is a key parameter in defining the conditions of interaction and the involved phenomena. In this paper we present the results that have been obtained in laser ion acceleration experiments from recent past to present days, using the most recent temporal cleaning methods. Different technologies have been adopted to improve the temporal contrast: the obtained effect on the proton acceleration is presented and discussed.

The conditions for achieving the laser acceleration of quasi‐monochromatic ion bunches with present‐day, fs laser systems are theoretically discussed. The study suggests the possibility of achieving quasi‐monochromaticity via irradiation of double‐layer, nano‐structured foils and the conjecture is numerically confirmed by means of two dimensional, Particle‐In‐Cell (PIC) simulations. A feasible setup in order to experimentally validate this approach is thus proposed.

In a recent experiment [1] a high efficiency regime of stable electron acceleration to kinetic energies ranging from 10 to 40 MeV has been achieved. The main parameters of the electron bunches are comparable with those of bunches provided by commercial Radio‐Frequency based Linacs currently used in Hospitals for Intra‐Operative Radiation Therapy (IORT). IORT is an emerging technique applied in operating theaters during the surgical treatment of tumors. Performances and structure of a potential laser‐driven Hospital accelerator are compared in detail with the ones of several commercial devices. A number of possible advantages of the laser based technique are also discussed.

High-energy electrons can be produced in interactions of intense, ultra-short laser pulses with plasmas. Experiments conducted in the regime of moderate laser power (a few terawatts [TW]) are attracting increasing attention for their possibility of optimizing the acceleration process. Here we report the successful production of several-MeV electron bunches in interactions of femtosecond laser pulses from a 10 TW tabletop laser with supersonic gas-jets. The laser–plasma interaction and the obtained electron bunches have been characterized in detail, and conditions for stable and reproducible acceleration have been found. The accelerated electron bunches have been characterized by means of the measurement of the induced photo-activation of a gold sample via bremsstrahlung-generation of photons with suitable energy. The obtained result opens up a wide range of possible applications of the compact electron source for the concerns of nuclear physics studies. Some of them are briefly considered in this paper.

Multiply charged ion production in He, Ne, Ar and Xe has been investigated using a 1 ps Nd-glass laser pulse at lambda =1053 nm with intensities in the 1013-1018 W cm-2 range. All the outer-shell electrons are removed except in Ne gas in which ion charge states up to Ne7+ are observed. The influence of the polarization of the laser radiation is also investigated. Experimental ion production rates are compared with those predicted by tunnelling and barrier-suppression ionization models.

International audience; We report on an innovative two-dimensional imaging extreme ultraviolet (XUV) interferometer operating at 32 nm based on the mutual coherence of two laser high order harmonics (HOH) sources, separately generated in gas. We give the first evidence that the two mutually coherent HOH sources can be produced in two independent spatially separated gas jets, allowing for probing centimeter-sized objects. A magnification factor of 10 leads to a micron resolution associated with a subpicosecond temporal resolution. Single shot interferograms with a fringe visibility better than 30% are routinely produced. As a test of the XUV interferometer, we measure a maximum electronic density of 3×10^20 cm^−3 1.1 ns after the creation of a plasma on aluminum target.

International audience; We discuss the two mechanisms involved in high-order harmonic generation from plasma mirrors, and show that they can be clearly identified experimentally. The very different phase properties of the corresponding harmonics lead to light beams with different divergences. This can be exploited to select a particular type of harmonic by spatial filtering in the far-field.

Emission coherente de sillage et dynamique plasma C. Thaury1, F. Quere1, H. George1, R.A. Loch2, J.-P. Geindre3, G. Bonnaud1, P. Monot1 et Ph. Martin1 1Service des Photons, Atomes et Molecules, IRAMIS/DSM, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France 2Laser Physics and Nonlinear Optics Group, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, The Netherlands 3Laboratoire pour l’Utilisation des Lasers Intenses, CNRS, Ecole Polytechnique, 91128 Palaiseau, France

We use optical interferometry to study the propagation of femtosecond laser pulses in gases. We show the measurements of propagation in a nitrogen gas jet and we compare the results with propagation in He under the same irradiation conditions. We find that in the case of nitrogen, the detailed temporal structure of the laser pulse can be tracked and visualized by measuring the phase and the resulting electron-density map. A dramatically different behavior occurs in He gas jets, where no details of the temporal structure of the laser pulse are visible. These observations are explained in terms of the ionization dynamics of nitrogen compared to helium. These circumstances make ${\text{N}}_{2}$ gas sensitive to variations in the electric field and, therefore, allow the laser-pulse temporal and spatial structures to be visualized in detail.

We discuss the two mechanisms involved in high-order harmonic generation from plasma mirrors, which can be clearly identified in numerical simulations: the coherent wake emission (CWE) and the relativistic oscillating mirror (ROM) processes. Using high-contrast multi-TeraWatt ultrashort laser pulses, harmonics originating from these two processes can also be distinguished experimentally.

We will discuss the two mechanisms involved in high-order harmonic generation from plasma mirrors, and show that they can be clearly identified experimentally. The phase and coherence properties of these harmonics will be analyzed.

We used X-ray spectroscopy as a diagnostic tool for investigating the properties of laser-cluster interactions at the stage in which non-adiabatic cluster expansion takes place and a quasi-homogeneous plasma is produced. The experiment was carried out with a 10 TW, 65 fs Ti:Sa laser focused on CO2 cluster jets. The effect of different laser-pulse contrast ratios and cluster concentrations was investigated. The X-ray emission associated to the Rydberg transitions allowed us to retrieve, through the density and temperature of the emitting plasma, the time after the beginning of the interaction at which the emission occurred. The comparison of this value with the estimated time for the “homogeneous” plasma formation shows that the degree of adiabaticity depends on both the cluster concentration and the pulse contrast. Interferometric measurements support the X-ray data concerning the plasma electron density.

A gamma-ray source with an intense component around the giant dipole resonance for photonuclear absorption has been obtained via bremsstrahlung of electron bunches driven by a 10-TW tabletop laser. 3D particle-in-cell simulation proves the achievement of a nonlinear regime leading to efficient acceleration of several sequential electron bunches per each laser pulse. The rate of the gamma-ray yield in the giant dipole resonance region (8

We report on simultaneous measurements of backward- and forward-accelerated protons spectra when an ultrahigh intensity ($\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$), ultrahigh contrast ($g{10}^{10}$) laser pulse interacts with foils of thickness ranging from 0.08 to $105\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Under such conditions, free of preplasma originating from ionization of the laser-irradiated surface, we show that the maximum proton energies are proportional to the $p$ component of the laser electric field only and not to the ponderomotive force and that the characteristics of the proton beams originating from both target sides are almost identical. All these points have been corroborated by extensive 1D and 2D particle-in-cell simulations showing a very good agreement with the experimental data.

International audience; As 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.

High harmonics from ultra-intense laser-matter interaction can be generated by both linear means and by relativistic means. In experiments up to a few 1019 Wcm-2 , we show the distinctions and means to control each.

Specular reflection is one of the most fundamental processes of optics. At moderate light intensities generated by conventional light sources this process is well understood. But at those capable of being produced by modern ultrahigh-intensity lasers, many new and potentially useful phenomena arise. When a pulse from such a laser hits an optically polished surface, it generates a dense plasma that itself acts as a mirror, known as a plasma mirror (PM). PMs do not just reflect the remainder of the incident beam, but can act as active optical elements. Using a set of three consecutive PMs in different regimes, we significantly improve the temporal contrast of femtosecond pulses, and demonstrate that high-order harmonics of the laser frequency can be generated through two distinct mechanisms. A better understanding of these processes should aid the development of laser-driven attosecond sources for use in fields from materials science to molecular biology.

We present and characterize a very efficient optical device that employs the plasma mirror technique to increase the contrast of high-power laser systems. Contrast improvements higher than 10(4) with 50% transmission are shown to be routinely achieved on a typical 10 TW laser system when the pulse is reflected on two consecutive plasma mirrors. Used at the end of the laser system, this double plasma mirror preserves the spatial profile of the initial beam, is unaffected by shot-to-shot fluctuations, and is suitable for most high peak power laser systems. We use the generation of high-order harmonics as an effective test for the contrast improvement produced by the double plasma mirrors.

We report and discuss experimental results on the propagation of CPA pulses of moderately relativistic intensity in gas: they evidence the effects of the precursor pedestals of the main pulse. Details of great interest were observed for the first time with high quality femtosecond 90-degree interferometry. The interferometric data are also correlated with imaging and spectroscopy data of laser pulse transmitted through the gas. The most relevant physical features are confirmed by a numerical code which simulates the laser pulse propagation self-consistently with the ionization of the gas. We found that in this regime, the propagation of the intense femtosecond pulse is basically stable apart from very weak refractive effects. In order to allow propagation at fixed intensity along an optical path larger than the Rayleigh range, we performed a first successful attempt at producing hollow plasma channels able to guide the pulse.

ARC‐EN‐CIEL (Accelerator‐Radiation Complex for Enhanced Coherent Intense Extended Light) proposal is based on a CW 1 GeV superconducting linear accelerator delivering high charge, subpicosecond, low emittance electron bunches with a high repetition rate (1 kHz). The FEL uses High Harmonics Generation in gases in a High Gain Harmonic Generation scheme, leading to a rather compact solution. The radiation extends down to 0.8 nm with the non‐linear harmonics and reproduces the good longitudinal and transverse coherence of the harmonics generated in gas. Optional beam loops, foreseen to increase the beam current or the energy, will accommodate infrared CSR source, femtosecond undulator sources in the VUV and X‐ray ranges, and a FEL oscillator in the 10 nm range. An important synergy is expected between accelerator and laser communities, in particular for electron plasma acceleration tests.

We present a new mechanism for high-order harmonic generation by reflection of a laser beam from an overdense plasma, efficient even at moderate laser intensities (down to $I{\ensuremath{\lambda}}^{2}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }\ensuremath{\mu}{\mathrm{m}}^{2}$). In this mechanism, a transient phase matching between the electromagnetic field and plasma oscillations within a density gradient leads to the emission of harmonics up to the plasma frequency. These plasma oscillations are periodically excited in the wake of attosecond electron bunches which sweep across the density gradient. This process leads to a train of unevenly spaced chirped attosecond pulses and, hence, to broadened and chirped harmonics. This last effect is confirmed experimentally.

By focusing an intense femtosecond, high temporal contrast, laser on ultra-thin foils (100 nm) in the 1018W/cm2 intensities range, we demonstrate that we create instantaneously a hot solid-density plasma. The use of highorder harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate in such media, enables to study the dynamics of this plasma on the picosecond time-scale. The comparison of the transmission of two successive harmonics permits to determine the electronic density and the temperature with an accuracy better than 15% never achieved up to date in relativistic regimes.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper, we detail the new scheme that precompensates 30-fs, 10-TW laser beam aberrations in order to get the best laser focal spot on target. The adaptive optics loop consists of two four-wave lateral shearing interferometer as the wave front sensor. Wave front correction is achieved dielectric coated bimorph deformable mirror.

We use optical interferometry to investigate ultrafast ionization induced by an intense, ultrashort laser pulse propagating in a helium gas. Besides standard phase shift information, our interferograms show a localized region of fringe visibility depletion (FVD) that moves along the laser propagation axis at luminal velocity. We find that such a loss of visibility can be quantitatively explained by the ultrafast change of refractive index due to the field ionization of the gas in the laser pulse width. We demonstrate that by combining the post facto phase shift distribution with the probe pulse transit effect in the ionizing region, the analysis of the observed FVD yields significant information on the ultrafast dynamics of propagation of the ionization front in the gas.