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Direct laser slab face-cooling by a fluid crossing the main and pump laser beams is an important method to reach high average laser powers. However, the flow regime is usually maintained at low Reynolds numbers, to prevent the onset of turbulence features in the flow, that would degrade wavefront quality. We show here how bringing the fluid temperature to the thermo-optical null point, close to the water/ice transition in the case of water, allows one to mitigate the optical consequences of hydrodynamic instabilities, by bleaching optically the temperature inhomogeneities within the flow. This optical process, dubbed 'index-leveling', opens the door to a highly efficient forced-flow, weakly turbulent face-cooling regime that should be instrumental to boost the kilowatt capabilities of next generation high-power lasers., Comment: 5 pages, 6 figures

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.

A general method to study the birefringence induced under laser pumping is presented. The method was tested on an Nd:Phosphate glass sample that is transversally and inhomogeneously pumped by a pulsed laser diode. Measurements to resolve the spatial and temporal stress-induced birefringence distribution are compared to a numerical model simulating the experimental setup. A quantitative agreement between measurement and simulation was found, showing that unmeasurable physical processes can be known in general cases. To compare the measurement and the simulation, a new formalism to extract the induced birefringence from initial and total birefringence measurement is presented.

Behaviour of a relativistic electron bunch, injected and trapped in a high intensity optical lattice resulting from the interference of two laser beams is studied. The optical lattice modifies the phase space distribution of the electron bunch due to the trapping and compression of the electrons by a ponderomotive force. High-frequency longitudinal beam eigenmodes of the trapped electron bunch are described in the framework of fluid and kinetic models. Such beam oscillations are expected to play a pivotal role in a stimulated Raman scattering of laser beams on the electrons.

Les lasers sont multiples : en fonction de leurs caracteristiques physiques intrinseques (quel milieu a gain ? quelle cavite ? quel pompage ?), ils produisent des rayonnements couvrant des plages extremement vastes, tant du point de vue spectral – de l’extreme ultraviolet au lointain infrarouge – que temporel – du regime continu aux attosecondes – ou encore energetique – du nano au megajoule. C’est de cette impressionnante diversite que les lasers tirent leur capacite a repondre a tant d’applications differentes.Nous vous proposons dans cet article de faire un rapide tour d’horizon des differents types de lasers existants et de leurs performances.

We present a numerical study of the injection and trapping process of a bunch of relativistic free electrons into a transverse high intensity optical lattice. We unravel different injection regimes depending on the characteristic length scale of the onset of the optical lattice , and explore how the characteristics of the electron beam and of the high intensity lattice affect the trapping rate. The average transverse kinetic energy , and the induced longitudinal energy broadening are studied. The analysis of the transverse phase space exhibits a complex behavior , leading to inhomogeneities that may affect the amplification dynamics of X-ray Free Electron Laser .

International audience; The burning process of high density (about 10**18cm-3), high temperature (tens to hundreds of keV) plasma trapped by a high mirror-like magnetic field in a Compact Magnetic Fusion (CMF) device is numerically investigated.. The initial high density and high temperature plasma in the CMF device is produced by ultrashort high intensity laser beam interaction with clusters or thin foils, and two fuels, D-T and p-11B are studied. The spatio-temporal evolution of D-T and p-11B plasmas, the production of alphas, the generated electric fields and the high external applied magnetic field are described by a 1-D multifluid code. The initial values for the plasma densities, temperatures and external applied magnetic field (about 100 T) correspond to high β plasmas. The main objectives of the numerical simulations are: to study the plasma trapping, the neutron and alpha production for both fuels, and compare the effect of the external applied magnetic field on the nuclear burning efficiency for the two fuels.. The comparisons and the advantages for each fuel will be presented. The proposed CMF device and the potential operation of the device within the ELI-NP pillar will be discussed. © (2015)

International audience; In order to develop an intense Extreme Ultra-Violet source based on High Harmonics Generation (HHG) for many applications, we have performed HHG signal optimization experiments. It has been shown that HHG is very sensitive to temporal fundamental pulse shape. For these experiments, we have used an acousto-optical programmable dispersive filter (Dazzler) in order to modify the spectral phase of the fundamental laser. We have optimized harmonic signals generated in argon gas cell with the help of an genetic algorithm up to a factor 6. We stress on the influence of macroscopic parameters on the optimization process. For the first time, we show and explain the algorithm sensitivity to phase-matching conditions.

The XFROG technique has been applied to measuring temporal shifts of supercontinuum pulses propagating in scattering media consisting of spherical beads. Negative (subluminal) and positive (superluminal) changes in the group velocity have been measured. This behaviour can be explained in terms of spectral scattering losses and is directly related to the complex time formulation used in other phenomena where tunnelling times are used to describe the velocity changes. A numerical simulation of the XFROG measurement is consistent with the experimental results.

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.

The spectral and temporal characteristics of supercontinuum pulses propagating through monodisperse scattering media consisting of sub-lambda-sized particles have been measured with a broad-bandwidth cross-correlated frequency-resolved optical gating (XFROG) technique. The results show that the ballistic component of the supercontinuum preserves a phase relationship among its spectral components, which acquire a temporal shift in relation to propagation in a non-scattering medium.

Summarization: During the last few years there is an increasing interest on the development of alternative high power new negative ion source for Tokamak applications. The proposed new neutral beam device presents a number of advantages with respect to: the density current, the acceleration voltage, the relative compact dimension of the negative ion source, and the coupling of a high power laser beam for photo-neutralization of the negative ion beam. Here we numerically investigate, using a multi-fluid 1-D code, the acceleration and the extraction of high power ion beam from a Magnetically Insulated Diode (MID). The diode configuration will be coupled to a high power device capable of extracting a current up to few kA with an accelerating voltage up to MeV. An efficiency of up to 92% of the coupling of the laser beam, is required in order to obtain a high power, up to GW, neutral beam. The new high energy, high average power, high efficiency (up to 30%) ICAN fiber laser is proposed for both the plasma generation and the photo-neutralizer configuration [1]. Παρουσιάστηκε στο: The European Physical Journal Special Topics

We study experimentally how to control and improve phase matching of high-order harmonic generation. We use a birefringent lens and a birefringent compensator to obtain a fundamental laser pulse (150 fs, 800 nm, similar to 4 mJ) With two foci Separated by 6.2 mm along the propagation axis and with a controllable phase delay between the polarizations along the optic axes of the birefringent optical components. This enables us to enhance the high-order harmonic conversion efficiency for the high-order harmonics in neon to 3 x 10(-8), a factor of 4 higher compared to a single-focus setup in similar conditions. The enhancement is achieved by improving the phase matching and at the same time maintaining a high intensity in a large generating volume. [S1050-2947(99)04512-6]. (Less)

We present a new scheme of X-ray free electron laser, based on the interaction between a moderately relativistic electron bunch, and a transverse high intensity optical lattice. Gain scaling laws in the small signal regime are derived analytically, and confirmed from Particle-In-Cell simulations. The nature of amplification and saturation are discussed. The resulting all-optical Raman X-ray laser opens perspectives for ultra-compact X-ray coherent light sources.

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.

We demonstrate that a mm-scale free electron laser can operate in the X-ray range, in the interaction between a moderately relativistic electron bunch, and a transverse high intensity optical lattice. The corrugated light-induced ponderomotive potential acts simultaneously as a guide and as a low-frequency wiggler, triggering stimulated Raman scattering. The gain law in the small signal regime is derived in a fluid approach, and confirmed from Particle-In-Cell simulations. We describe the nature of bunching, and discuss the saturation properties. The resulting all-optical Raman X-ray laser opens perspectives for ultra-compact coherent light sources up to the hard X-ray range., accepted for publication in Phys. Rev. Lett. 03/11/2012

X-ray radiation is one of the most efficient tools to explore the properties of matter in multidisciplinary domains. Over the past fifty years three generations of synchrotrons have been developed delivering x-ray beams with always shorter wavelength and higher brightness to a rapidly growing users community. These large scale instruments have lead to important discoveries and outstanding applications. However, while femtosecond x-ray pulses are now essential to open new possibilities of research and applications (Ultrafast Phenomena proceedings 1992-2002; Zewail 2000; Bloembergen et al., 1999; Rousse et al., 2001), the shortest pulse duration at synchrotron is a few tens picosecond. To face these technological limits, several methods have been proposed and demonstrated, based, for example on electron bunch slicing or Thomson scattering off a part of the bunch (Schoenlein et al., 1996 ; Schoenlein et al., 2000). However, these mechanisms are limited by their very low efficiency. Major progresses have been made with the fourth generation of synchrotron: the free electrons lasers (FEL). Even larger than a synchrotron, X-ray FELs can produce femtosecond x-ray pulses billion times more intense than any conventional source (Emma et al., 2010).

We report on an experimental investigation of the noise properties of an free-running, high-power, picosecond Nd:YVO 4 oscillator pumped by a 100 W laser diode. The amplitude noise has been measured with a photodiode and an electronic spectrum analyser, and then compared with other mode-locked oscillator's noise spectrum. We show that in terms of noise properties, our powerful oscillator is comparable with low-power oscillators such like low-noise Ti:Sapphire oscillators. We also show that the frequency doubling does not affect the amplitude noise of the oscillator. High power diode pumping is then not an issue to get low-noise high-power oscillators.

We report on a high-power passively mode-locked TEM(00)Nd:YVO(4) oscillator, 888 nm diode-pumped, with pulse durations adjustable between 46 ps and 12 ps. The duration tunability was obtained by varying the output coupler (OC) transmission while avoiding resorting to spatial hole burning (SHB) for pulse shortening. At a repetition rate of 91 MHz and for an output power ranging from 15 Wto45 W, we produced SHB-free 12-ps-to32-ps-long pulses. Within this range of power, these are the shortest pulse durations obtained directly from Nd:YVO(4) oscillators.

We report on a high-power, passively mode-locked, TEM 00 Nd:YVO 4 oscillator with adjustable pulse duration between 46 and 12ps. The laser is end-pumped by an 888nm laser diode and mode-locking is achieved with a semiconductor saturable absorber mirror (SESAM). The laser has a repetition rate of 91MHz and the M 2 beam quality factor is better than 1.2 at 15ps. At the optimum output coupler, it provides a maximum average output power of 45W with 32ps pulse duration. In literature, the presence of spatial hole burning (SHB) often helps to shorten the pulse length down to few picoseconds. However, SHB might be an issue for some specific application requiring e.g. low noise picosecond oscillators. In this contribution, we demonstrate that it is possible to shorten the pulse duration by lowering the intracavity losses without SHB. Pulse tunability from 46 to 12ps is achieved by changing the output coupler of the cavity while staying in the continuous-wave mode-locked regime. Pulse duration is almost linear with the output coupler transmission and increases from 12 to 32ps with average output power ranging from 15 to 45W. In this range of output power, we demonstrate the shortest pulses directly from a Nd:YVO 4 oscillator.

We report on a high-average-power 888nm diode-pumped passively mode-locked TEM 00 Nd∶YVO 4 oscillator with adjustable pulse durations. From 45 to 15W output power, we produced corresponding 32 to 12ps long pulses without spatial hole burning.

Compact X-rays sources raise a great enthusiasm in life sciences for applications such as protein crystallography, medical imaging or radiotherapy. Such X-rays can be produced by Thomson scattering of photons coming from a laser source colliding with electrons. However, Thomson scattering is very inefficient. Real-world life science on-site applications requiring high X-rays flux, a scheme based on a mode-locked picosecond laser injected in a Fabry-Perot cavity combined with an electron storage ring has been explored [1]. This technology can be pushed-up with a powerful high repetition-rate mode-locked laser injected in a high finesse cavity enabling to reach the intracavity average power up to few hundreds of kW required by the medical applications. The design of the laser system can then be based either on a low power oscillator combined with a relatively sophisticated high gain amplifier, or on a powerful oscillator seeding a low gain amplifier. However, a powerful single oscillator delivering a few tens of Watts average power with 15–20 ps pulses coupled with a 1000 gain Fabry-Perot cavity would be sufficient as a demonstrator for imaging application. Therefore, in this contribution, we propose a design for a 20W-sub-20 ps class mode-locked Nd:YAG oscillator.

International audience; We present a pump-probe experimental approach to study time-resolve coherent optical phonons using light pulses in the extreme ultraviolet wavelength range with femtosecond time resolution. Using this technique, a Bi (111)-oriented single crystal was excited by near-infrared (1.55 eV) pulses and probed by a high-order harmonic generation source, whose intrinsic flux instability was reduced by making use of a normalization procedure. This unconventional approach allowed us to perform measurements in a previously inaccessible range of phonon wavevectors within the Brillouin zone.

International audience; A novel multilayer mirror was designed and fabricated based on a recently developed three-material technology aimed both at reaching reflectivities of about 20% and at controlling dispersion over a bandwidth covering photon energies between 35 and 50 eV. The spectral phase upon reflection was retrieved by measuring interferences in a two-color ionization process using high-order harmonics produced from a titanium: sapphire laser. We demonstrate the feasibility of designing and characterizing phase-controlled broadband optics in the extreme-ultraviolet domain, which should facilitate the manipulation of attosecond pulses for applications.

International audience; High harmonic amplification is shown in an optical-field ionized X-ray laser. The 4d–4p X-ray line of Kr^8+ at 32.8 nm, which has a close match to the 25th harmonic of the infrared laser, was amplified up to 200 times. The energy-extraction regime was also achieved, which depended on the level of seeding. This second-generation X-ray laser is fully polarized, has low divergence and shows a high degree of coherence. The duration is also expected to be short, fulfilling the requirements for an ultra-intense tabletop X-ray laser. Conditions for higher energy output are also suggested.

In the race towards attosecond (as) pulses for which high order harmonics generated in rare gases are the best candidates, both the Harmonic spectral range and spectral phase have to be controlled. We present in this proceeding four mirrors numerically optimized and designed to compensate for the intrinsic Harmonic chirp recently discovered and which is responsible for a temporal broadening of the pulses. They are capable of compressing the duration down to 100 as. We present the fabrication of those aperiodic multilayers and show the measurement of reflectivity, which prooves that those multilayers are in agreement with the specifications and so let us think that they will be able to compress attosecond high harmonics trains.

International audience; Au LOA, nous avons réalisé la première chaîne d'amplification laser fonctionnant dans la gamme spectrale XUV. La chaîne est constituée d'une source, d'un relais d'image et d'un amplificateur. La source de génération d'harmoniques d'ordre élevé avait de bonnes qualités optiques, tandis que l'amplificateur produit intrinsèquement des impulsions de forte énergie. Afin de garder les qualités optiques de la source durant l'amplification, le laser X était pompé par ionisation par effet de champs dans un gaz neutre. La métrologie de ce laser X injecté a permis de mesurer une énergie de sortie de 0.7 µJ et un haut degré de cohérence et de polarisation. La durée de l'impulsion a été estimée à 500 fs. On peut espérer atteindre un éclairement de 1.5x10^16 Wcm^-2 en focalisant le faisceau.

International audience; In the race toward attosecond pulses, for which high-order harmonics generated in rare gases are the best candidates, both the harmonic spectral range and the spectral phase have to be controlled. We demonstrate that multilayer extreme-ultraviolet chirped mirrors can be numerically optimized and designed to compensate for the intrinsic harmonic chirp that was recently discovered and that is responsible for temporal broadening of pulses. A simulation shows that an optimized mirror is capable of compressing the duration from 260 to 90 as. This new technique is an interesting solution because of its ability to cover a wider spectral range than other technical devices that have already been proposed to overcome the chirp of high harmonics.

International audience; We give on overview of recent advances in collisionally pumped optical field-ionization soft X-ray lasers developed at LOA. Saturated amplification has been achieved on the 5d-5p transition in Xe^8+ at 41.8 nm, and on the 4d-4p transition in Kr^8+ at 32.8 nm. We demonstrate a significant increase of the energy output from the Xe^8+ laser driven within two types of wave-guide. Finally, we present results of a pioneering work aimed to set up and characterize the first true soft X-ray laser chain.

International audience; We study the phase of the atomic polarization in the process of high-order harmonic generation. Its dependence on the laser intensity and the harmonic order induce a frequency variation in time (chirp) respectively of the harmonic pulses and attosecond pulses. We review the recent experimental results on the temporal characterization of the harmonic emission and show that measurements performed using very different techniques (like XFROG and RABITT), probing the phase in different parameter spaces, can be connected through the mixed phase derivatives, demonstrating the common underlying physics.

International audience; We use a train of sub-200 attosecond extreme ultraviolet (XUV) pulses with energies just above the ionization threshold in argon to create a train of temporally localized electron wave packets. We study the energy transfer from a strong infrared (IR) laser field to the ionized electrons as a function of the delay between the XUV and IR fields. When the wave packets are born at the zero crossings of the IR field, a significant amount of energy (∼20 eV) is transferred from the field to the electrons. This results in dramatically enhanced above-threshold ionization in conditions where the IR field alone does not induce any significant ionization. Because both the energy and duration of the wave packets can be varied independently of the IR laser, they are valuable tools for studying and controlling strong-field processes.

International audience; Metrology of XUV beams (X-ray lasers, high-harmonic generation and VUV free-electron lasers) is of crucial importance for the development of applications. We have thus developed several new optical systems enabling us to measure the optical properties of XUV beams. By use of a Michelson interferometer working as a Fourier-transform spectrometer, the line shapes of different X-ray lasers have been measured with a very high accuracy (Δλ/λ~10^-6). Achievement of the first XUV wavefront sensor has enabled us to measure the beam quality of laser-pumped as well as discharge-pumped X-ray lasers. A capillary discharge X-ray laser has demonstrated a very good wavefront allowing us to achieve an intensity as high as 3×10^14 W cm^-2 by focusing with a f=5 cm mirror. The sensor accuracy has been measured using a calibrated spherical wave generated by diffraction. The accuracy has been estimated to be as good as λ/120 at 13 nm. Commercial developments are underway. At Laboratoire drsquoOptique Appliquée, we are setting up a new beamline based on high-harmonic generation in order to start the femtosecond, coherent XUV optic .

We give an overview of recent experimental results on optical-field-ionization collisional soft X-ray lasers developed at LOA. By focusing a 30-fs, circularly polarized Ti-sapphire laser pulse at an intensity of up to 8×1017 Wcm-2 into a low-density gas cell containing Xe or Kr, we produced a few mm long plasma column for soft X-ray amplifier. Saturated amplification has been achieved on the 4d95d(1 S 0)–4d95p(1 P 1) transition at 41.8 nm in Xe8+, and on the 3d94d(1 S 0)–3d94p(1 P 1) transition in Kr8+ at 32.8 nm. Under optimum pumping conditions the Xe8+ laser provides about (5±2)×109 photons per pulse whilst the Kr8+ laser delivers up to (2.5±1)×109 photons per shot. The repetition rate of these soft X-ray lasers is 10 Hz. The beam wavefront of the Xe8+ laser has been measured by a Shack–Hartmann soft X-ray wavefront sensor, and the pulse duration by a cross-correlation technique.

International audience; An adaptive learning loop enhances the efficiency and tuning of high-order harmonic generation. In comparison with simple chirp tuning, we observe a broader tuning range and a twofold to threefold enhancement in integrated photon flux in the cutoff region. The driving pulse temporal phase varies significantly for different tunings and is more complicated than a simple chirp. We compare our experimental results with a one-dimensional, time-dependent model that incorporates the intrinsic atomic response, the experimental pulse temporal phase, ionization effects, and transverse coherence of the spatial mode of the laser. The model agrees with our experimental results and indicates that a specific quantum path coupled with ionization effects determines the optimized harmonic spectrum.

The optimization of high-order harmonic generation (HHG) processes continues to be an active area of investigation, not only for improving specific aspects of HHG (such as phase-matching, extending the cutoff frequency, generating tunable harmonics, and generating ultrashort harmonics), but also for developing a better understanding of fundamental HHG processes. While many intuitive schemes exist for improving HHG (such as chirping the laser pulse to tune the harmonic spectrum [1–3]), adaptive pulse shaping has also proven effective. The use of genetic algorithms coupled with pulse shaping has become a popular method in coherent control experiments [4]. Using sub-20 fs pulses, specific orders near the cutoff can be enhanced through shaping of the input pulse spectral phase [5]. Recent theoretical work using genetic algorithms have investigated the optimal focus, gas density, and interaction length to optimize the output flux, the pulse duration, and temporal coherence [6].

We give an overview of recent results on optical-field-ionization collisional soft X-ray lasers developed at LOA. By focusing a 30-fs, circularly polarized Ti-sapphire laser pulse at an intensity of up to 8 x 10 17 Wcm -2 into a low-density gas cell containing xenon or krypton, we produced a few mm long plasma column for soft X-ray amplifier. Saturated amplification has been achieved on the 4d 9 5d( 1 S 0 ) - 4d 9 5p( 1 P 1 ) transition at 41.8 nm in Pd-like Xe, and strong lasing at 32.8 nm on the 3d 9 4d( 1 S 0 ) - 3d 9 4p( 1 P 1 ) transition in Ni-like Kr has been demonstrated. Under optimum pumping conditions the Xe IX laser provides about 5 x 109 photons per pulse while the Kr IX laser delivers up to (2-3) x 10 9 photons per shot. The repetition rate of these X-ray lasers is 10 Hz. The experimental results are discussed and compared to computational simulations in which issues related to plasma formation, electron energy distribution, and atomic processes are addressed.

Summary form only given. Femtosecond time-resolved x-ray diffraction has been demonstrated using the technique of laser-produced plasma x-ray sources. The main drawbacks of laser-produced plasmas are the weak x-ray flux available for the experiment and the monochromaticity of the x-rays. Ultrafast time resolution is at present limited to very simple systems with high x-ray diffraction power. On the basis of our experiments, we discuss the possibility to extend this technique to more complex processes such as the characterization of the fundamental atomic vibrations in solids (optical phonons) and the elementary steps of biochemical reactions.

Summary form only given. Laser-matter interaction at high intensity provides a variety of new schemes for the production of ultrashort pulses of short wavelength radiation. Rapid progress has been made in the generation of optical harmonics of high order in noble gases, and a few hundred harmonic orders have been observed. More recently, harmonic generation of laser light upon reflection from the plasma-vacuum boundary has attracted considerable attention both in experiment and in theory. High conversion efficiencies and no high-frequency limit are predicted for harmonic generation at the intensities approaching relativistic level. A sharp plasma-vacuum interface is essential for this type of harmonic generation. With the use of relatively long pumping pulses, when the hydrodynamic expansion of the target was counteracted by the pondermotive forces, up to 75 harmonic has been observed. However the harmonics were generated not in the specular direction, but distributed in a wide angle. This paper describes new results on the observation of high order harmonics using femtosecond pulses, when the target hydrodynamic expansion during the laser pulse can be neglected and spatially coherent harmonics are expected.

International audience; We report recent investigations of optical field ionization soft-x-ray lasers. We generated the amplifying medium by focusing a circularly polarized 760-mJ, 30-fs 10-Hz Ti:sapphire laser system into a gas cell a few millimeters long filled with xenon or krypton gas. A gain of 67 cm^−1 on the 4d^95p–4d^95d transition at 41.8 nm in Pd-like xenon and a gain–length product of 15 have been inferred at saturation. This source delivers ∼5×10^9 photons per pulse. More recently we demonstrated lasing at 32.8 nm in Ni-like krypton. The influence of the pumping energy and the laser polarization on the lasing output as well as on the far-field pattern of the x-ray laser beam are reported and discussed.

International audience; Nous présentons de récents travaux sur la production de sources lasers XUV collisionels pompés par des impulsions laser femtoseconde. Le milieu amplificateur est un plasma créé d'ions multi-chargés et l'effet laser est observé sur les transitions 5p-5d à 41,8 nm du xénon IX et 4d-4p du krypton IX à 32,8 nm. L'influence des conditions d'irradiation et de densité du milieu lasant sur l'intensité de la source est présentée et discutée.

International audience; We report the first experimental demonstration of a Ni-like optical-field ionization collisional soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 760 mJ, 30 fs, 10-Hz Ti:sapphire laser beam in a few mm cell filled with krypton. We have measured a gain coefficient of 78 cm^-1 on the 3d^94d^1S0-3d^94p^1P1 transition at 32.8 nm, which is here amplified for the first time. This radiation source represents the shortest wavelength optical-field ionization collisional soft x-ray laser ever produced. The influence of the gas pressure and the pumping energy on the lasing output are also presented.

Obtaining a reliable XUV laser operating in an optical field ionization scheme has long been sought by numerous groups, both in recombination and collisional schemes. We report a successful observation of lasing in Pd-like xenon at 41.8 nm. High photon flux is obtained with respect to what can be expected in that scheme, so that the results are consistent with the obtention of amplification saturation. Moreover, more recent results demonstrate a large amplification of the 4d-4p transition of Ni-like Kr at 31.2 nm.

Summary form only given. We used the technique of time-resolved X-ray diffraction with an X-ray source produced from the interaction of a 10 Hz, 120 fs, 16 mJ Ti:sapphire laser beam focused on a silicium target. The emitted 7.12 /spl Aring/ K/sub /spl alpha//, X-ray radiation is expected to last 100 fs which can give a clear mapping of the atomic motions during the phase transition. The X-ray radiation is collected by a toroidal quartz crystal and focussed onto the InSb. We used a second laser pulse from this laser system to excite the sample at fluences ranging from 200 mJ/cm/sup 2/ to 5 mJ/cm/sup 2/, which is well below the damage threshold of the semiconductor. The depth and the timescale of nonthermal process have been fully characterized.

Summary form only given. The increasing peak power of laser chains based on the chirped pulse amplification leads to the use of a compressor or a part of the compressor under vacuum in order to avoid the nonlinear effect when the pulse has been fully recompressed. We propose an alternative solution that is to replace the compressor under vacuum by a compressor in the air combined with a set of chirped mirrors that is under vacuum. The "air-compressor" will be aligned such that the residual spectral phase after the chirped mirror's set will be minimized in order to obtain the optimal pulse duration.