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LASERIX est une plateforme de l’IJClab sur le campus de l’Université Paris Saclay. Le laser est une chaîne CPA de niveau 50 TW à 10 Hz. Des thématiques de recherche en rapport avec la physique des deux infinis sont menées en parallèle avec une vocation affirmée d’ouverture à la formation. Nous présentons un panorama sur les sources XUV de lumière structurée, l’accélération laser plasma et des études prospectives sur l’étude des propriétés du vide soumis à un champ électromagnétique ultra-intense.

We demonstrate the production of EUV vortex beams, and vector-vortex beams merging the helical phase of a vortex and the spatially variant polarization of a vector beam, both carrying large orbital angular momentum per photon.

[EN]Structured light in the short-wavelength regime opens exciting avenues for the study of ultrafast spin and electronic dynamics. Here, we demonstrate theoretically and experimentally the generation of vector-vortex beams (VVB) in the extreme ultraviolet through high-order harmonic generation (HHG). The up-conversion of VVB, which are spatially tailored in their spin and orbital angular momentum, is ruled by the conservation of the topological Pancharatnam charge in HHG. Despite the complex propagation of the driving beam, high-harmonic VVB are robustly generated with smooth propagation properties. Remarkably, we find out that the conversion efficiency of high-harmonic VVB increases with the driving topological charge. Our work opens the possibility to synthesize attosecond helical structures with spatially varying polarization, a unique tool to probe spatiotemporal dynamics in inhomogeneous media or polarization-dependent systems., European Research Council (851201); Ministerio de Ciencia de Innovación y Universidades, Agencia Estatal de Investigación and European Social Fund (PID2019-106910GB-I00, RYC-2017-22745); Junta de Castilla y León and FEDER Funds (SA287P18); Université Paris-Saclay (2012-0333T-OASIS, 50110000724-OPTX, PhOM REC-2019-074-MAOHAm); Conseil Régional, Île-de-France (501100003990); Barcelona Supercomputing Center (FI-2020-3-0013).

We report temporal coherence measurement of solid-target plasma-based soft X-ray laser (XRL) in amplified spontaneous emission (ASE) mode. By changing the XRL pumping angle, we generate lasing at two-times higher electron density than the routine condition. A relatively shorter coherence time at a higher pumping angle indicates a clear spectral signature of higher electron density in the gain region. We probe the amplification dynamics of XRL in routine, and high electron density conditions to confirm gain-duration reduction resulting from ionization gating in the latter case. We also present recent results on the seeding of a vortex beam carrying orbital angular momentum (OAM) in XRL plasma. A small part of the high topological charge extreme ultraviolet (EUV) vortex is injected in XRL. These preliminary results suggest that the vortex seed indeed can be efficiently amplified. In the end, we propose a pathway towards the seeding of the complete vortex beam and wavefront characterization of the amplified beam.

With widespread applications in optical trapping, super-resolution microscopy, quantum communications & information, and material processing, there is an ever-mounting interest in the generation and characterization of light beams carrying topological singularities [1] . An emblematic situation of this singular-optics is the Laguerre-Gauss laser modes presenting an azimuthal phase dependence: φ(r,θ)=l θ [2] . The integer l also called the topological charge of the singularity, is the number of 2π phase shifts along the azimuthal coordinate of the beam. This phase structure corresponds to a helicoidal wavefront and is often named an optical vortex. These helically phased beams are known to carry Orbital Angular Momentum (OAM). Besides, high-harmonic generation (HHG) in rare gases has proven to be an effective way to extend singular light beams to the extreme-ultraviolet (EUV) spectral range [3] , [4] . During HHG with a driver of charge l 1 , the momentum conservation results in a linear upscaling of the topological charge with harmonic order [2] : l q = ql 1 , where l q is the topological charge of q tℎ harmonic. In this work, we will present our recent results on the generation, and amplitude, phase, and modal content characterization of EUV vortices until l =100. Furthermore, we will support our experimental findings with simulation results [5] .

Optical vortex and vector beams are two paradigmatic examples of structured light. While the first ones carry orbital angular momentum (OAM), and thus their wavefront spirals along the propagation direction, the second ones present spatially inhomogeneous distributions of polarization. Harnessing structured light is opening new perspectives in communications, quantum information, processing, high-resolution imaging, or particle trapping. Among them, the generation of coherent structured beams in the extreme-ultraviolet (EUV)/soft x-ray regime is of high interest due to their unique ability to interact with matter at the nanometer and sub-femtosecond scales. During the last years, high-order harmonic generation (HHG) has opened a new paradigm to up-convert infrared vortex [1] , [2] and vector beams [3] into the EUV, circumventing the use of inefficient optical converters. Previous results proved that a synchronous control over the polarization and the OAM is possible through HHG [4] , [5] .

The synchronous control of spin and orbital angular momentum in high-harmonic generation allows us to introduce experimentally and theoretically a novel XUV structured beam with spatially-varying polarization and phase, high topological charge, and robust propagation.

Quantum electrodynamics predicts that the vacuum must behave as a nonlinear optical medium: the speed of light should be modified when the vacuum is stressed by intense electromagnetic fields. This optical phenomenon has not yet been observed. The DeLLight (deflection of light by light) experiment aims to observe the optically induced index change of vacuum, a nonlinear effect which has never been explored. The experiment is installed in the LASERIX facility at IJCLab, which delivers ultrashort intense laser pulses (2.5 J per pulse, each of 30 fs duration, with a 10 Hz repetition rate). The proposal is to measure the refraction of a probe laser pulse when crossing a transverse vacuum index gradient, produced by a very intense pump pulse. The refraction induces a transverse shift in the intensity profile of the probe, whose signal is amplified by a Sagnac interferometer. In this article we describe the experimental method and setup, and present the complete theoretical calculations for the expected signal. With a minimum waist at focus of $5\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ (corresponding to a maximum intensity of $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$), and with the nonlinear vacuum index derived from QED, the expected refraction angle is 0.13 prad. First results of the interferometer prototype are presented. It is shown that an extinction factor $\mathcal{F}=0.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ (corresponding to a signal amplification factor of 250) and a spatial resolution ${\ensuremath{\sigma}}_{y}=10\phantom{\rule{4pt}{0ex}}\mathrm{nm}$ are achievable. The expected signal is then about 15 pm, and could be observed at a 5-sigma confidence level with about one month of collected data.

International audience; Optische Strahlen mit Orbital Angular Momentum (OAM) sind ein sehr aktives Forschungsfeld für ihre prospektiven Anwendungen, insbesondere bei kurzen Wellenlängen. Wir betrachten hier solche Strahlen, die durch hochharmonische Erzeugung (HHG) in einem seltenen Gas erzeugt werden, und analysieren die Charakterisierung ihrer Hochladungs-Wirbelstruktur durch einen EUV (Extreme UltraViolet) Hartmann-Wellenfrontsensor. Wir zeigen, dass solche HHG-Strahlen in der Regel aus einer Reihe von zahlreichen Vortex-Modi bestehen. Die Empfindlichkeit der Intensität und Phase des HHG-Strahls auf die infraroten (IR) Laseraberrationen wird mit einem deformierbaren Spiegel untersucht.; Optical beams carrying Orbital Angular Momentum (OAM) are a very active field of research for their prospective applications, especially at short wavelengths. We consider here such beams produced through high-harmonic generation (HHG) in a rare gas and analyze the characterization of their high charge vortex structure by an EUV (Extreme UltraViolet) Hartmann wavefront sensor. We show that such HHG beams are generally composed of a set of numerous vortex modes. The sensitivity of the intensity and phase of the HHG beam to the infrared (IR) laser aberrations is investigated using a deformable mirror.; Les faisceaux optiques portant un Moment Angulaire Orbital (OAM) font l'objet de recherches très actives,en raison de l'intérêt de leurs applications potentielles, particulièrement à courtes longueurs d'onde. Nous nous intéressons ici à des faisceaux produits par Génération d'Harmoniques d'Ordre Elevé (HOE) dans un gaz rare, et analysons la structure du vortex de haute charge grâce à la caractérisation complète obtenue par un capteur de front d'onde de type Hartmann dans l'Extrême UltraViolet (EUV). Nous montrons que ces faisceaux HOE sont généralement constitués d'un ensemble de nombreux modes de vortex. Un miroir déformable nous permet d'étudier la sensibilité de l'éclairement et de la phase du faisceau HOE vis à vis des aberrations du laser infrarouge (IR).

We present in this paper ongoing studies on seeded plasma-based soft X-ray lasers using the LASERIX facility. After an overview of the source setup and characteristics, we will discuss two prospects that might increase the interest of these coherent soft X-ray sources. We have successfully generated harmonic beams carrying Orbital Angular momentum. We briefly present this experiment and discuss the remaining challenges for amplifying these kinds of beams in a plasma-based soft X-ray laser. In the second part, we present numerical results on harmonic seed amplification in conditions leading to Rabi oscillations. This regime is known to be a promising way to reach femtosecond pulses in the present plasma amplifiers. We will discuss the possibility of increasing the stability of such a regime by the introduction of a continuous refractive loss in the gain region.

In this paper, we present the characterization of high charge vortex structure of an OAM beam produced through high-harmonic generation in a rare gas using an XUV Hartmann wavefront sensor. We show that the phase-matched absorption limited high-harmonic generation is able to retain the high-charge vortex structure of the XUV beam even in a rather long (1 cm) generation medium. Additionally, our recent results on the influence of infrared driver wavefront quality on the characteristics of XUV vortex beam are presented.

We demonstrate that high harmonic generation when driven by vortex driver contaminated with various orbital angular momentum (OAM) modes, the upconverted EUV harmonic deviates from perturbative conversion law to exhibit a spectrum of OAM orders.

We demonstrate for the first time, to the best of our knowledge, the ability of extreme ultraviolet (XUV) Hartmann wavefront sensors to characterize high charge vortex beams produced by high-order harmonic generation up to the order of 25. We also show that phase matched absorption limited high harmonic generation is able to maintain the high charge vortex structure of the XUV beam even in a rather long (1 cm) generation medium.

10.18429/JACoW-IPAC2018-TUPML079; International audience; We present a start to end (s2e) simulation of the Laser-plasma Wakefield Accelerator (LPWA) foreseen as the ESCULAP project. We use a photo injector to produce a 5MeV 10pC electron bunch with a duration of ~ 1 ps RMS, it is boosted to 10MeV by a S-band cavity and then compressed to 74 fs RMS (30 fs FWHM) by a magnetic compression chicane (dogleg). After the dogleg, a quadrupole doublet and a triplet are utilized to match the Twiss parameters before injecting into the subsequent plasma wakefield. A 40 TW laser is used to excite plasma wakefield in the 10 cm plasma cell. An optimized configuration has been determined yielding at the plasma exit an electron beam at 180 MeV with energy spread of ~ 4.2%, an angular divergence of 0.6 mrad and a duration of 4 fs.

We present theoretical and numerical studies of longitudinal compression and transverse matching of electron bunch before injecting into the Laser–plasma Wake Field Accelerator (LWFA) foreseen at the ESCULAP project in ORSAY. Longitudinal compression is performed with a dogleg chicane, the chicane is designed based on theory of beam optics, beam dynamics in dogleg is studied with ImpactT (Martin Dohlus and Torsten Limberg, 2004) and cross checked with CSRtrack (Qiang and Steve Lidia, 2006), both 3D space charge (SC) and coherent synchrotron radiation (CSR) effects are included. Simulation results show that the energy chirp at the dogleg entrance should be smaller than the nominal optic design value, in order to compensate the negative energy chirp increase caused by longitudinal SC, while CSR can be ignored in our case. With an optimized configuration, the electron bunch ( ∼ 10 MeV, 10 pC) is compressed from 0.9 ps RMS to 70 fs RMS (53 fs FWHM), with a peak current of 152 A. Transverse matching is realized with a doublet and a triplet, they are matched with Madx and the electron bunch is tracked with ImpactT, simulation results show little difference with the nominal design values, that is due to the SC effect. Finally, by simply adjusting the quadrupole strength, a preliminary optimized configuration has been achieved, that matches the Courant–Snyder (C–S) parameters to α x = 0 . 01 , α y = − 0 . 02 , β x = 0 . 014 m , β y = 0 . 012 m at the plasma entrance.

We present numerical simulations results on the injection and acceleration of a 10 MeV, 10 pC electrons beam in a plasma wave generated in a gas cell by a 2J, 45 fs laser beam . This modeling is related to the ESCULAP project in which the electrons accelerated by the PHIL photo-injector is injected in a gas cell irradiated by the laser beam of the LASERIX system. Extensive modeling of the experiment was performed in order to determine optimal parameters of the laser plasma configurations. This was done with the newly developed numerical code WakeTraj . We propose a configuration that benefits of a highly compressed electron bunch and for which the injected electron beam can be efficiently coupled to the plasma wave and accelerated up to 140 MeV, with an energy spread lower than 5%.

International audience; We present the status of optical field ionization soft X-ray lasers. The amplifying medium is generated by focusing a high-energy circularly polarized 30-fs 10-Hz Ti: sapphire laser system in a gaseous medium. Using xenon or krypton, strong laser emission at 41.8 and 32.8 nm, respectively, has been observed. After presenting the basis of the physics, we present recent characterization of the sources as well as dramatic improvement of their performances using the waveguiding technique.

We have investigated high harmonic generation in an argon gas cell driven by two femtosecond laser pulses separated by a variable delay ranging from 500 fs to 100 ps. Experiments were performed at the LASERIX IR-EUV facility using an amplified 10 Hz CPA Ti:Sapphire laser system on a beamline delivering 50 fs pulses at 800 nm with energy up to 25 mJ. In the case of a non-zero time delay we studied the optimal conditions for equilibrated double pulse generation despite perturbation of the generating medium induced by the first pulse. We showed how high harmonic double pulse generation varies with the gas pressure, the excitation energy, the delay and the relative polarization between the two laser pulses.

LASERIX is a high-power laser facility leading to High-repetition-rate XUV laser pumped by Titanium:Sapphire laser. The aim of this laser facility is to offer Soft XRLs in the 30–7 nm range and auxiliary IR beam, which could also be used to produce synchronized XUV sources. In this contribution, the main results concerning both the development of XUV sources and their use for applications (irradiation of DNA samples) are presented, as well the present status and some perspectives for LASERIX.

OPT2X gathers researchers of the newly formed Paris-Saclay University with the goal of building an instrumental platform dedicated to the control, characterization and transport of ultra-short coherent XUV light pulses, in view of cross disciplinary applications in ultrafast science

We review development of multimillijoule X-ray lasers and of applications of these new laboratory sources carried out recently at the PALS facility. A backbone of this development is the neon-like zinc laser providing saturated output at 21.2 nm, with up to 10 mJ of energy per pulse. This represents currently the most energetic soft X-ray laboratory source. Recent improvements in its operation include better control of the beam shape, and more complete understanding of the prepulse pumping. The laser at 21.2 nm has been employed for a number of application experiments reviewed in this paper. They include transmission measurements of intense soft X-ray radiation, studies of fundamental processes of soft X-ray ablation, ablation micropatterning, feasibility study of soft X-ray Thomson scattering from dense plasmas, visualization of nanometric transient perturbation of optical surfaces, measurements of ablation rates of foils heated by IR pulses, and studies of 2D plasma hydrodynamics in the regime of sequen...

Intense and stable laser operation with Ni-like Zr and Ag was demonstrated at pump energies between 2 J and 5 J energy from the PHELIX pre-amplifier section. A novel single mirror focusing scheme for the TCE x-ray laser (XRL) has been successfully implemented by the LIXAM/MBI/GSI collaboration under different pump geometries. This shows potential for an extension to shorter XRL wavelength. Generation of high quality XRL beams for XRL spectroscopy of highly charged ions is an important issue within the scientific program of PHELIX. Long range perspective is the study of nuclear properties of radioactive isotopes within the FAIR project.

Extreme ultra-violet (EUV) laboratory lasers can be used to probe energy transport in laser irradiated solid targets. We report on a recent experiment undertaken at LASERIX whereby the heating of laser-irradiated targets containing a thin layer of iron (50 nm) encased in plastic (CH) was diagnosed using EUV laser (13.9 nm) back-lighter probing. The heating laser pulse duration was 35fs with focal irradiances of \(3 \times 10^{16}\)Wcm\(^{-2}\) and a deliberate prepulse 20 ps before the main pulse at irradiances of \(3 \times 10^{15}\)Wcm\(^{-2}\). A one dimensional hydrodynamic fluid code HYADES has been used to simulate the temporal variation in EUV transmission using IMP opacity values for the iron layer and the simulated transmissions compared to measured transmission values. When a deliberate prepulse is used to preform an expanding plastic plasma, it is found that radiation heating is dominant in the heating of the iron layer giving rise to a rapid decrease in EUV opacity and an increase in the transmission of the 13.9nm laser radiation as the iron ionizes to Fe\(^{5+}\) and above.

X-ray laser simulations based on Ehybrid code have shown that enhanced plasma x-ray laser emission can be achieved mastering the ionization dynamics and plasma temperature using one long and two short pulses (Ursescu and Ionel, J Optoelectron Adv Mat 12:48–51, 2010). In parallel, two simple methods to generate multiple short pulses for pumping x-ray lasers were reported in conjunction with x-ray laser developments. Five to ten fold enhancement in the emission of the silver x-ray laser was demonstrated using the newly developed pump methods, when compared with the traditional one, based on a long pulse followed by one short pump pulse. An overview of these recent experiments will be presented. The possible implementation of these novel x-ray laser pumping methods at Extreme Light Infrastructure—Nuclear Physics facility will be discussed.

Seeded operation of a Neon-like Titanium plasma-based soft x-ray laser is described. The plasma amplifier is pumped using a variation of the classical grazing incidence pumping technique, combining a long low energy pulse followed by a main short pulse. Because the preformed plasma is underionized, a part of the main short pulse energy is used to ionized it to the lasing stage. Consequences of this feature on seeded laser properties are discussed.

In very intense electromagnetic fields, the vacuum refractive index is expected to be modified due to nonlinear quantum electrodynamics (QED) properties. Several experimental tests using high intensity lasers have been proposed to observe electromagnetic nonlinearities in vacuum, such as the diffraction or the reflection of intense laser pulses. We propose a new approach which consists in observing the refraction, i.e. the rotation of the waveplanes of a probe laser pulse crossing a transverse vacuum index gradient. The latter is produced by the interaction of two very intense and ultra short laser pulses, used as pump pulses. At the maximum of the index gradient, the refraction angle of the probe pulse is estimated to be $0.2 \times (\frac{w_0}{10 \mathrm{\mu m}})^{-3} \times \frac{I}{1 \mathrm{J}}$~picoradians, where $I$ is the total energy of the two pump pulses and $w_0$ is the minimum waist (fwhm) at the interaction area. Assuming the most intense laser pulses attainable by the LASERIX facility ($I = 25$~J, 30~fs fwhm duration, 800~nm central wavelength) and assuming a minimum waist of $w=10 \mathrm{\mu m}$ (fwhm) (corresponding to an intensity of the order of $10^{21}$~W/cm$^2$), the expected maximum refraction angle is about 5~picoradians. An experimental setup, using a Sagnac interferometer, is proposed to perform this measurement., Comment: Accepted for publication in Eur. Phys. Journal D

We report the work done on a transient soft X-ray laser (SXRL) beam to deliver a proper extreme UV irradiation source for applications. The same optical tool was first demonstrated on a quasi stationnary state (QSS) soft X-Ray laser at the PALS Institute in Prague. The problem set by the transient soft X-Ray laser developed by the LIXAM at the LULI installation in Palaiseau is more crucial, first because the beam spatial profile is more irregular secondly because high repetition rate soft X-ray laser facilities in the future are based on this SXRL type. The spots obtained show a 20 micron average diameter and a rather homogeneous and smooth profile that make them a realistic irradiation source to interact with targets requiring relatively high fluence (near 1 J/cm2) or intensity (near 1011 W/cm2) in the extreme UV domain.

The spectral width of a Ni-like silver transient soft X-ray laser (4d-4p λ= 13.9 nm) was experimentally inferred from the measurement of its temporal coherence, performed with a novel wavefront division interferometer. The measured coherence time of 3 ps corresponds to a spectral linewidth of 0.7 mA which is narrow. We compare this experimental result to numerical simulations of the amplified line profile, performed along two different approaches. Both calculations predict a spectral linewidth that is about 3 times larger than the experimental value. We discuss several effects that might explain this discrepancy. We show that, due to the short duration of the X-ray laser pulse, the assumption of a stationary electromagnetic field used in both the experimental analysis and in the calculations need to be reconsidered.

Beam sections of transient collisional excitation (TCE) soft X-ray lasers exhibit highly contrasted intensity modulations forming complex patterns. We present model calculations that reproduce these experimental observations both qualitatively and quantitatively. The low spatial coherence, high longitudinal coherence and short pulse duration of the source are shown to control the visibility of the beam structures. Comparison with other types of laser-based soft X-ray lasers is discussed. We derive simple relationships that relate the spatial and temporal characteristics of the emitting source to the main features of the beam small-scale structures.

International audience; We study temporally localized electron wave packets, generated using a train of extreme ultraviolet (XUV) attosecond pulses to ionize the target atoms. Both the electron wave packets and the attosecond pulse train (APT) are characterized using the same technique, based on interference of two-photon transitions in the continuum. We study, in particular, the energy transfer from a moderately strong infrared (IR) field to the electron wave packets as a function of time delay between the XUV and the IR fields. The use of an APT to generate the electron wave packets enables the generation at times not accessible through tunneling ionization. We find that a significant amount of energy is transferred from the IR field to the electron wave packets, when they are generated at a zero-crossing of the IR laser field. This energy transfer results in a dramatically enhanced above-threshold ionization even at IR intensities that alone are not strong enough to induce any significant ionization.

We report the preliminary result of the application of the collisional Ne-like soft x-ray laser as radiation source to induce DNA damage. The goal of this experiment was a test bed study of the damage yields induced by soft x-ray radiation in dried plasmid DNA sample. The saturated Ne-like soft x-ray laser available at the PALS facility, delivering several millijoules in a single 100 ps pulse at 21.2 nm was used to irradiate two different plasmids: pSPl 89 and pBS. The study is centered on the dose effect leading to single and double strand break in DNA.

International audience; Synchrotrons have for decades provided invaluable sources of soft X-rays, the application of which has led to significant progress in many areas of science and technology. But future applications of soft X-rays—in structural biology, for example—anticipate the need for pulses with much shorter duration (femtoseconds) and much higher energy (millijoules) than those delivered by synchrotrons. Soft X-ray free-electron lasers1 should fulfil these requirements but will be limited in number; the pressure on beamtime is therefore likely to be considerable. Laser-driven soft X-ray sources offer a comparatively inexpensive and widely available alternative, but have encountered practical bottlenecks in the quest for high intensities. Here we establish and characterize a soft X-ray laser chain that shows how these bottlenecks can in principle be overcome. By combining the high optical quality available from high-harmonic laser sources (as a seed beam) with a highly energetic soft X-ray laser plasma amplifier, we produce a tabletop soft X-ray femtosecond laser operating at 10 Hz and exhibiting full saturation, high energy, high coherence and full polarization. This technique should be readily applicable on all existing laser-driven soft X-ray facilities.

International audience; A variety of complex phenomena occurs when an ultra-short laser pulse interacts with atoms in the 10^13 W/cm^2 range: non-sequential ionisation, electron recollision, and Freeman resonances. We show that high-harmonic spectra obtained experimentally in this parameter range also display anomalous features, which are difficult to understand in the framework of the three-step, semiclassical model. The results of a systematic study of these high harmonics generated in argon, xenon, and krypton are presented. From the experimental curves, complex high-order harmonic generation phenomena are discussed.

International audience; Following the theoretical predictions, the observation of two-photon processes by interaction of vacuum ultraviolet (VUV) radiation with inner-shell levels of atoms requires focused intensities in the 10^13-10^14 W/cm^2 range. Our aim is to reach this regime in order to study non-linear optics at these wavelengths. We first optimized the high harmonic conversion efficiency in argon by studying the best experimental conditions for phase-matching, concentrating on focus geometry related to laser energy, cell length and position relative to the focus. We then studied the resulting harmonic beam focusability by a toroidal mirror ( f=10 cm) and made an image of the harmonic focus. We conclude with an evaluation of the focused intensity that we are able to reach experimentally.

International audience; Nous avons utilisé une technique récente de mesure d'impulsions ultra-courtes afin d'étudier la dynamique de l'ionisation et la propagation dans un milieu gazeux. Nous présentons les mesures de variation de l'intensité, de la phase temporelle et du spectre d'une impulsion laser intense (10^16W/cm^2), ultracourte (35 fs) après son interaction avec un milieu gazeux. Les variations de l'indice de réfraction du plasma sont rapides et modifient la structure spatiale et temporelle de l'impulsion lors de sa propagation dans le gaz. Ces variations d'intensité et de phase temporelles peuvent être utilisées pour connaitre la dyriamique de l'interaction laser-plasma. En particulier, cette expérience permet de mettre en évidencc les effets sur la génération d'un milieu de gain pour le laser X de type OFI (Optical Field Ionization).

International audience; In this paper we report on results obtained with a compact double-stage molybdenum x-ray laser (XRL), operated with a total pump energy of 600 mJ. The two gain regions were pumped using the double-pulse grazing incidence pumping technique, which includes travelling wave excitation for both the seed- and the amplifier-target. In addition, the influence of an additional pre-pulse has been studied. Seeded XRL operation has been demonstrated in both schemes, resulting in XRL pulses with a divergence of 2 × 2 mrad. The peak brilliance of the amplified XRL of 4 × 10^23 photons/s/mm2/mrad2 in 5 × 10^−5 relative bandwidth was more than two orders of magnitude larger compared to the original seed pulses. The presented experimental concept provides an alternative approach to the currently more common use of high-order harmonic pulses as a seed source, well suited for applications like laser spectroscopy of highly-charged ions at a storage ring.

International audience; We report a systematic study of high order harmonic generation with an infrared laser apertured by an iris, as a function of the aperture size. Measurements were made of harmonic generation efficiency for different gas species, laser energies and focal geometries. Harmonic efficiencies as a function of aperture show a characteristic peaked shape, which is independent of gas species and harmonic order. A one dimensional, time dependent simulation of harmonic generation in a gas cell, taking into account experimentally measured transverse coherence of the laser, closely reproduces the observed behaviours. We show that the aperture diameter which maximizes harmonic yield is the result of a compromise between considerations of focal geometry and ionization (which favour small apertures); and harmonic dipole amplitude and phase (which favour large apertures).

We have investigated a new scheme for laser plasma transient collisional soft x-ray lasers based on the use of an additional laser to produce the preplasma. Soft x-ray emission measurements made for different solid targets are presented and discussed. A significant enhancement of the SXRL emission as compared to double-pulse single-beam grazing incidence (DGRIP) using the same pump laser is reported for 13.9- and 32.6-nm SXRL wavelengths.

An alternative, novel multiple pulse generation scheme was implemented directly after the optical compressor output of an x-ray pump laser. The new method uses a polarization sensitive thin film beam splitter and a half-wavelength wave plate for tuning the energy ratio in the multiple short pulses. Based on this method, an extensive study was made of the running parameters for a grazing incidence pumped silver x-ray laser (XRL) pumped with a long pulse of 145 mJ in 6 ns at 532 nm and up to 1.45 J in few picoseconds at 810 nm. Fivefold enhancement in the emission of the silver XRL was demonstrated using the new pump method.

We show that a significant enhancement of the photon flux produced by high harmonic generation can be obtained through guided configuration at high laser intensity largely above the saturation intensity. We identify two regimes. At low pressure, we observe an intense second plateau in the high harmonic spectrum in argon. At relatively high pressure, complex interplay between strongly time-dependent ionization processes and propagation effects leads to important spectral broadening without loss of spectral brightness. We show that the relevant parameter for this physical process is the product of laser peak power by gas pressure. We compare source performances with high harmonic generation using a gas jet in loose focusing geometry and conclude that the source developed is a good candidate for injection devices such as seeded soft x-ray lasers or free electron lasers in the soft x-ray range.

We have investigated the high harmonic generation driven by two quasi-collinear IR pulses separated by picosecond delay. Experiments were performed at the LASERIX facility using 50 fs pulses at 800 nm with energy up to 25 mJ. We have studied the generation of two high harmonic pulses from the same medium. In particular, we show how the ratio in energy of each pulse can be used to balance the perturbation induced by the first pulse in the generating gas.

Different pumping schemes for soft X-ray lasers have been investigated at the PHELIX laser facility, including a double-target seeding approach at 18.9 nm. A technical feasibility study of using a Mo XRL beam of several μJ as an excitation source for heavy-ion spectroscopy in a storage ring has been carried out. XRL photon numbers and the beam transport under ultra-high vacuum conditions over almost 30 m are the major challenges.

The LASERIX facility has restarted operation in the middle of the year 2010. Since then, important work has been achieved on the XUV sources performance, reliability and stability, which lead to the construction of two beamlines. The first one is based on transient collisional soft X-ray laser. In the second one, the high order harmonic generation process is used to convert infrared laser into coherent XUV radiations. Characteristics and some recent works on both of them will be presented. Besides, a complete setup for pump-probe experiments has been developed and successfully tested. Some examples involving this device and dealing with plasma opacity, radiobiology and nanomagnetism will be presented. We will conclude this paper by a discussion on how application experiments can be turned into valuable diagnostics for XUV sources.

The influence of temporal pump laser profile on the Soft X-ray laser (SXRL) efficiency is investigated in double grazing incidence pumping (DGRIP) transient collisional scheme. We have demonstrated a simple method that allows to produce and control one or several small prepulses and the ASE level of the pump laser beam by modifying the regenerative amplifier parameters. By changing the temporal structure of the pump beam we observed both an increase of the SXRL energy and an improvement of the target lifetime. Measurements made for different configurations are presented and discussed.

We present in this paper a laser-driven coherent EUV beamline resulting from the combination of a versatile high-order laser harmonic generator with a robust plasma-based EUV laser amplifier. Both devices can be used separetely or in synergy. Seeding of the plasma amplifier by a high-order harmonic beam leads to a strong improvement of the EUV laser beam divergence and uniformity. Moreover the system can be turned easily into a IR pump-XUV probe setup for plasma opacity probing. The possibility to generate two separate harmonic sources from the same gas cell offers the opportunity to explore EUV pump-EUV probe experiments.

The potential for coherent extreme ultra-violet (EUV) light in probing laser-produced plasmas is investigated. New results are presented to demonstrate that EUV radiation can be employed to measure heat penetration into solid targets from electrons using the signature of a change of opacity due to heating. We examine, in particular, the effects of hot electron heating of targets. In addition, phase variations after transmission through a laser-irradiated target change the subsequent propagation of the radiation, suggesting a simple diagnostic measuring the far-field footprint of coherent EUV radiation can be a useful measurement of the uniformity of target heating.

The accurate characterization of thermal electron transport and the determination of heating by suprathermal electrons in laser driven solid targets are both issues of great importance to the current experiments being performed at the National Ignition Facility, which aims to achieve thermonuclear fusion ignition using lasers. Ionization, induced by electronic heat conduction, can cause the opacity of a material to drop significantly once bound-free photoionization is no longer energetically possible. We show that this drop in opacity enables measurements of the transmission of extreme ultraviolet (EUV) laser pulses at 13.9 nm to act as a signature of the heating of thin (50 nm) iron layers with a 50-nm thick parylene-N (CH) overlay irradiated by 35-fs pulses at irradiance $3\ifmmode\times\else\texttimes\fi{}{10}^{16}$ Wcm${}^{\ensuremath{-}2}$. Comparing EUV transmission measurements at different times after irradiation to fluid code simulations shows that the target is instantaneously heated by hot electrons (with approximately 10$%$ of the laser energy), followed by thermal conduction with a flux limiter of $\ensuremath{\approx}$0.05.