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LASERIX is a high-power laser facility intended to realize and use for applications transient collisional X-ray laser (XRL) beamlines at various wavelengths. In addition new types of XRL schemes giving rise to emission at short wavelengths will be developed using the high energy LASERIX driver. Thus, this laser facility will both offer Soft XRLs in the 30-7 nm range and auxiliary IR beam that could also be used to produce XUV sources. This experimental configuration highly enhances the scientific opportunities of the facility. Indeed it will be possible to realize both X-ray laser experiments and more generally pump/probe experiments, mixing IR and XUV sources. Then, this laser facility will be useful for the community, opening a large scale of XUV laser investigations.

In this paper we describe the experimental validation of the technique of correction of wavefront aberration in the middle of the laser amplifying chain. This technique allows the correction of the aberrations from the first part of the laser system, and the pre-compensation of the aberrations built in the second part. This approach will allow an effective aberration management in the laser chain, to protect the optical surfaces and optimize performances, and is the only possible approach for multi-Petawatt laser system from the technical and economical point of view. This approach is now possible after the introduction of new deformable mirrors with lower static aberrations and higher dynamic than the standard devices.

International audience; The surface laser-induced damage threshold fluence of sapphire is determined under various experimental conditions concerning the material irradiation (femtosecond, picosecond and nanosecond temporal regimes) and preparation (surface state). The results are of interest for optimising laser micromachining processes and for robust operation of high-peak power femtosecond Ti:sapphire laser chains.

We have recently demonstrated amplification in a Ti:Sapphire laser system up to 30 Joules before compression at the record repetition rate of 0.1 Hz. This CPA Ti: Sapphire laser system is the first femtosecond petawatt class laser system running at a high repetition rate of 0.1Hz. Such performances have been obtained thanks to the particular design of the last booster amplifier. In this paper, we will present the last results obtained on this system and the different strategies we have carried out for counteracting the two most limiting factors for high energy amplification that are the parasitic lasing in the large aperture Ti: Sapphire crystal and the spatial homogenisation of the pump laser beams. This laser will be used as a driver of the LASERIX facility.

In this paper, we present results of a ray tracing simulation of a stretcher-compressor arrangement for blue-green high contrast multiterawatt femtosecond laser Ti:sapphire laser chain, not only in terms of final pulse duration and contrast, but also with respect to the alignment sensitivity and the optical quality of components. The ray tracing simulation uses OSLO software and includes dispersion of the materials of the laser chain.

We have studied and designed an optical solutions to homogenize and shape the spatial profile of intense nanosecond pump beams to be used for a Petawatt-class Ti:sapphire amplifier. Simulations and experimental results will be presented.

The combination between both OPCPA and conventional amplification techniques is a hybrid scheme that takes advantage of the large, broadband single-pass gain provided by the non-linear parametric interaction as well as the large bandwidth achievable in low-gain Ti:S multipass amplifiers. We investigated different configurations with different spectral bandwidths of the pump beam. The second setup makes use of a 2.5 nm spectrum and a 400 /spl mu/J blue pump pulse energy which gave us the largest amplified spectrum: 120 nm, double the width of the spectrum produced by the oscillator.

Shearing interferometry applied to the spectral domain is used for femtoseconde pulse single-shot measurement. An innovating configuration delivering a bidimensional experimental direct representation of the spectral phase of the pulse is also presented.

We have tested two optical solutions to homogenize the spatial profile of nanosecond pump beams to be used for a Petawatt Ti:Sapphire. Simulations and experiments will be presented for microlens arrays and random phase plate.

We have demonstrated 33 J amplification in Ti: sapphire at the record repetition rate of 0.1 Hz. For this, we have successfully removed transverse lasing in the large crystal amplifier and homogenized the laser pump profiles.

International audience; We take advantage of nonlinear properties associated with χ^(3) tensor elements in BaF2 cubic crystal to improve the temporal contrast of femtosecond laser pulses. The technique presented is based on cross-polarized wave (XPW) generation. We have obtained a transmission efficiency of 10% and 10^−10 contrast with an input pulse in the millijoule range. This filter does not affect the spectral shape or the phase of the cleaned pulse. It also acts as an efficient spatial filter. In this method the contrast enhancement is limited only by the extinction ratio of the polarization discrimination device.

The major bottleneck for the development of robust and cost-effective femtosecond amplification systems is the uncertainty concerning the damage threshold of Ti: Sapphire crystals. Up to now, Ti: Sapphire is the only material that supports the generation of temporally short pulses (few femtosecond) at high repetition rates, and overcoming this bottleneck will represent a major advance in laser performance for all the femtosecond community. Currently, when pumped at 532nm, the uncertainty on Ti:Sapphire damage threshold, is about a factor of ten. The empirically estimated threshold is 10J/cm 2 but for safety reasons the femtosecond laser community (especially the companies producing the lasers) uses the conservative value of 1J/cm 2 . Such a low pumping fluency means low extraction efficiency during the amplification process and a great waste of pumping energy, the most expensive part of a Ti:Sapphire amplifier. In order to remove this bottleneck, we launch a complete analysis of all the factors that influence the damage threshold in Ti:Sapphire Crystals. Our program is to first measure the bulk threshold to define the upper threshold limit, and the influence of Ti ion concentration in the crystal garnet. Then, we will analyze all the surface effects that influence the value of the threshold. These effects depend on the polishing, on the cleaning process, as well as the type of anti-reflective coating. Only a complete understanding of all the mechanisms involved in threshold limitation will allow us to produce Ti:Sa crystals with the best performances. The study of the characteristics of the Ti:Sapphire damage threshold will not be complete and reliable without a complete characterization of the pump beams (temporal and spatial modulations), and this analysis will be done with nanosecond and picosecond pulses at 532nm. Finally, to complete the exploration of the the behavior of the titanium doped sapphire crystal, we will characterize the damage threshold with femtosecond pulses, at 800nm to reach the deterministic dielectric threshold and validate fundamentals models and simulation results. To our knowledges this is the first time that such a complete characterization is done for Ti:Sapphire laser crystals. We will present the first conclusions about the experiments as well as the methods we will employ in our systematic analysis.

We have simulated Chirped Pulse Amplification process with a new software, allowing to extend the generally used 1D-models. We show here the importance of the beam spatial discretization in simulations in order to describe the gain saturation. Experimental measurements performed on our 100 TW laser validate this calculation, allowing us to carefully design the next generation of Ti:sapphire lasers at the Petawatt level.

International audience; We report the measurement of spatiotemporal distortions of an ultrashort pulse in singlet beam expanders. With a simple second-order autocorrelator the temporal broadening of the pulse from 23 to 40 fs, due to propagation time difference (PTD), is determined. The delay due to PTD between different parts of the beam is also measured. This effect was theoretically studied for the first time by Bor [J. Mod. Opt. 35, 1907 (1988)]. These experimental results are in good agreement with the calculations of a dedicated three-dimensional ray-tracing program developed to simulate the spatial and temporal transformation of femtosecond pulses in optical systems.

International audience; Nonlinear elliptical polarization rotation is used to improve the contrast of femtosecond pulses by several orders of magnitude. Using nonlinear induced birefringence in air, we produced cleaned pulses with an energy of a few hundreds of microjoules. This technique presents several major advantages, such as convenience and stability of the setup. We investigated the phase profile required for obtaining high-energy pulses. No phase distortion is observed, and the spatial quality of the beam is preserved.

Future progress of x-ray lasers call for specific facilities intended to x-ray laser studies and development of a largevariety of x-ray laser applications. This paper presents LASERIX, a future x-ray laser facility under construction foroptimization of transient collisional excitation x-ray lasers, study of new x-ray laser schemes, and development ofapplications.Keywords: X-ray laser, collisional pumping, laser and discharge plasmas, Ti:sapphire lasers 1. INTRODUCTION X-ray lasers are studied since more than 30 years and the first demonstration was made almost 20 years ago 1 . Thesehugely bright sources in the XUV range, brighter by orders of magnitude with respect to other sources, are however notwidely spread and used for applications. Such situation is due mainly to the size of drivers for early x-ray lasers, whichwere used mainly for inertial confinement fusion studies and left very few shots for x-ray lasers.However the required driving energy for x-ray lasers has strongly decreased with transient pumping on one hand andcapillary discharge pumping on the other hand. Several x-ray laser facilities are presently operating or underconstruction, and will give opportunities to step up application experiments. LASERIX will be one of these newfacilities. It is intended to produce transient collisional x-ray lasers and to realize application experiments using theunique properties of these sources. In addition LASERIX will offer a large variety of laser pulses to study new schemesof x-ray lasers.

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].

Summary form only given. Recent theoretical results have indicated that the spectral continuum is sensitive to third order dispersion in microstructured fibers. In particular, the position of the blue edge of the continuum is determined by the amount of residual third order dispersion (TOD) in the fiber. For a given propagation length, the TOD is fixed. However, the phase of the input pulse also plays an important role in nonlinear pulse propagation. The pulse envelope shape, and therefore the effective potential in the NLSE, is determined by the input phase. Moreover, fiber TOD can be compensated by the pulse /spl Phi//sup (3)/(/spl omega/). Here, we investigate the spectral character of the continuum as a function of /spl Phi//sup (2)/(/spl omega/) and /spl Phi//sup (3)/(/spl omega/), the second and third order phase of the input pulse. In addition, we use adaptive pulse shaping to tailor the continuum emerging from the fiber.

Summary form only given. Different possible solutions for improving the contrast consist in seeding the preamplifier with more energetic pulses, amplifying these energetic pulses via a low ASE preamplifier in the femtosecond regime and also temporally filtering the ASE versus the femtosecond pulse in a nonlinear interferometer before the power amplifiers. Preliminary results have been obtained by temporal filtering a millijoule level femtosecond pulse with a Sagnac interferometer in the nonlinear regime.

Summary from only given. The propagation through many optical components, and thermal effects during the amplification processes produce some spatial aberrations onto the beam. The thermal aberrations in the power amplifier (third amplification stage) are very well corrected by cooling the Ti:sapphire crystal down to 120 K where its index remains almost constant. The remaining aberrations will be corrected with a deformable mirror in order to optimise the size of the focal spot and increase the peak intensity. The deformable mirror which is already implemented in our laser chain will also be used to correct the aberrations induced by the focusing spherical mirror. The deformable mirror is silver coated.

International audience; We studied the possibility of focusing an ultra-intense laser beam with a spherical mirror instead of a parabolic mirror. To correct the induced wavefront aberrations, we used a deformable mirror in a closed-loop configuration with a Shack–Hartmann Wavefront Sensor (SHWS). We demonstrate in this paper that we were able to correct the aberrations induced by spherical mirrors down to an f-number equal to 3.3.

International audience; Plasmas are an attractive medium for the next generation of particle accelerators because they can support electric fields greater than several hundred gigavolts per meter. These accelerating fields are generated by relativistic plasma waves--space-charge oscillations--that can be excited when a high-intensity laser propagates through a plasma. Large currents of background electrons can then be trapped and subsequently accelerated by these relativistic waves. In the forced laser wake field regime, where the laser pulse length is of the order of the plasma wavelength, we show that a gain in maximum electron energy of up to 200 megaelectronvolts can be achieved, along with an improvement in the quality of the ultrashort electron beam.

Summary form only given. We present diode-pumped Yb:GdCOB laser producing 3.2 W CW at 1043 nm with a slope efficiency of 68% and an optical-to-optical conversion efficiency of 36%.

International audience; We present the design and characterization of a femtosecond high-intensity laser system emitting a near-diffraction-limited beam. This system was dimensioned in order to reach intensities in excess of 10^20 W/cm^2 at a high repetition rate for ultrahigh-field physics experiments. We describe the improvements that were added to a conventional chirp pulse amplification configuration in order to decrease the deleterious effects of gain narrowing, gain shifting, thermal focusing in the amplifier stages, and spatial degradation due to multipass amplification processes.

International audience; We present a review of some recent results on high-order-harmonic generation, aiming at optimizing the photon flux to allow for future applications in extreme-ultra-violet non-linear optics. We first present new schemes to control phase matching of high harmonics in gases, by using the effect of the spatially varying atomic phase displayed by the high harmonics. An enhancement by a factor of 50 is observed in neon in conditions for which the gradient of the atomic dispersion balances the electronic dispersion. A new scheme to manipulate the laser field was demonstrated, and shown to improve phase matching. We then turn to high-harmonic generation by solid targets, and show that high harmonics generated by an intense 30-fs laser pulse remain collimated even at the threshold of the relativistic regime.

It was shown in 1999 that D2 cluster explosion under ultrashort intense laser irradiation can lead to ion energies sufficient to drive nuclear fusion reactions [Ditmire et al. Nature 398, 491]. We show how the use of molecular clusters allows to further enhance the ionic acceleration, allowing one to reach optimal fusion cross sections. A new low density high energy regime is described, in which fusion occurs via a spallation-like process. The process increases with laser intensity up to the relativistic threshold, at which the neutron production is overwhelmed by gamma production resulting from electron acceleration.

From the experience of several French teams involved in X‐ray laser research, a new concept of X‐ray laser facility is presented. This idea, based on a high power Ti:sapphire CPA driver would give a large versatility to the future facility “LASERIX”.

Adaptive femtosecond pulse shaping has become a powerful tool for optimizing the characteristics of ultrafast pulses [1,2] or ultrafast laser-matter interactions [3]. Recently, it has been shown in high order harmonic generation that specific harmonic orders near the cutoff can be significantly enhanced by optimizing the spectral phase of the driving pulse via adaptive optimization [4]. These experiments were conducted in a waveguide geometry using a hollow core fiber. Nonetheless, it is interesting to consider if adaptive learning can be effective for high order harmonic generation in a free propagation geometry. Here, we present results of experiments that demonstrate that harmonic emission can be tuned more effectively using adaptive optimization than via simple pulse chirping.

We present experimental results concerning the observation and acceleration of negative and positive ions produced from high‐intensity laser beam interactions with deuterated and hydrogenated clusters. The 30‐fs, 10‐Hz rep. rate, 30 TW Ti‐Sa laser of LOA was focused on clusters produced from a pulsed gas nozzle. The novelty of our experimental work was the gas composition (CD4 and CH4), which was used for the cluster formation. Both kinds of ions, negative and positive, were observed with approximately equal energies. The maximum energy of both kinds of ions as well their energy distribution was measured using a Thomson parabola mass spectrometer. Negative and positive ions of H, D, and C were observed with a maximum energy up to 70 keV for a maximum laser beam intensity of 1018 W/cm2. The mass spectrometer allowed us to correlate both the positive and negative ion energy spectrums with the laser beam intensity in order to study the process of cluster explosions, ion acceleration, and negative ion formation in the fs regime. We summarize the more important and new results as follows: (1) only single positive and negative ions were measured on the mass spectrometer; (2) the number of negative ions is equal to the number of positive ions; (3) similar energy spectrums for positive and negative ions were measured; (4) the positive and the negative ions are accelerated in the same radial direction, outwards from the interaction volume; and (5) similar energy spectrums for positive and negative ions at different laser energies (the laser intensity increases by a factor of 5) were measured. There are two possible “scenarios” concerning the process of negative ion formation. The first process is related with the formation of different ion species during the Coulomb explosion of the clusters. The second process for negative ion formation is the double electron attachment caused by collisions between the positive ions with the backing gas. A number of arguments confirm that the second process is more probable. The fact that from the mass spectrometer measurements the value for the more probable energy of the positive ions is the same as the energy at the maximum of the double electron attachment cross section leads to the second process of negative ion formation. For a complete description of our observation, however, we need an efficient physical process for negative ion formation, which will be the subject of our future work. The process of cluster explosion accelerates the D ions sufficiently to produce neutrons from DD nuclear fusion reactions. The efficiency of both the negative ions and the neutron alternative source will be discussed.

International audience; Following the interaction of superintense, short pulse lasers and plasmas, ions can be accelerated to velocities sufficient to drive nuclear fusion reactions, in particular, by the process of Coulomb explosion of clusters [T. Ditmire et al., Nature (London) 398, 491 (1999)]. We show here how short bursts of neutrons can be produced using a jet of low-density deuterated methane clusters. Ion velocity distributions were simultaneously measured by a Thomson parabola mass spectrometer, demonstrating deuteron energies up to 120 keV. We show that, in such conditions, nuclear fusion will occur not only in the hot plasma core, but also in the cold outer region by collision processes.

In this article, we report on experimental electron spectra generated by SMLWF in the ultra-relativistic regime a>1,where a0 is the normalized vector potential of the laser. For the first time, we have observed that in our range of parameters themaximum electron energy increases when the electron density decreases. A maximum of 70 MeV was observed for n/n1 .5%.1. INTRODUCTION Laser plasma interaction in the relativistic regime is crucial for laser plasma acceleration 13 and for Inertial Confinement Fusion(ICF) with the fast ignitor scheme . The success of these applications depends on the efficient transfer of energy from the laserto electrons. Electrons are trapped in high amplitude relativistic plasma waves (RPW) driven by the Self-Modulated Laser WakeField instability (SMLWF) In this relativistic regime plasma waves can reach amplitude of a few tens of percent 810,accelerating electrons to energies of several tens MeV1 1,12 Various mechanisms, such as relativistic effects and ponderomotiveself-channel4ing13 must be taken into account in estimating the maximum electron energy. On the other hand, the measurement ofmulti-MeV electrons was imputed to direct laser acceleration in a plasma channel14, and the effective temperature deduced fromelectron spectra was observed to grow with electron density 15•

International audience; We report the first saturated amplification of an optical-field-ionization soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 330-mJ, 35-fs, 10-Hz Ti:sapphire laser system in a few-mm cell filled with xenon. 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 about 5×10^9 photons per pulse. The influence of the pumping energy and the laser polarization on the lasing output are also presented.

The use of a new crystal: Yb:GdCOB for the development of a regenerative amplifier is reported. This chirped pulse amplification system generates 21-mJ pulses with a bandwidth of 8 nm leading to 350-fs, 12-mJ pulses after compression.

Summary form only given. The chirped pulse amplification method reduced dramatically non-linear effects, enabling thereby higher amplification levels. But when increasing average power needed to pump the Ti:sapphire amplifier crystals, strong thermal effects appear which affect beam propagation by inducing unwanted wavefront distortions. Moreover, amplification process narrows the spectrum of a chirped pulse because of gain narrowing and gain saturation effects. Our laser chain is composed of a Kerr lens mode locked Ti:sapphire laser oscillator delivering a 80 Mhz/300 mW a pulse train with a spectrum of 65 nm FWHM centered around 810 nm. Pulses are stretched through an Offner triplet, and amplified at -10 Hz through three multipass amplifiers to reach, first the millijoule regime through a high-gain amplifier.

Summary form only given. One of the important issue in high intensity CPA chains, is the determination of the output pulse duration. The seed pulse is modified through the CPA amplification by different processes (dispersion through amplifying materials, spatial and temporal transformation through lenses or other optical components...) and it is often difficult to predict the pulse duration after compression. This duration can be measured with techniques such as spider or autocorrelators, but to design such complex systems, it is of crucial importance to be able to foresee what the pulse duration should be. Moreover spatio-temporal effects are often neglected for ultrashort pulses and however strongly modify the temporal characteristics of the laser pulse on interaction targets. We present a set of simulations of stretchers-compressors systems including dispersive beam expanders and show for pulses shorter than 30 femtoseconds how crucial are these spatio-temporal coupling effects.

International audience; Detailed measurements of electron spectra and charges from the interaction of 10 Hz, 600 mJ laser pulses in the relativistic regime with a gas jet have been done over a wide range of intensities (10^18–2×10^19 W/cm^2) and electron densities (1.5×10^18–1.5×10^20 cm^−3), from the “classical laser wakefield regime” to the “self-modulated laser wakefield” regime. In the best case the maximum electron energy reaches 70 MeV. It increases at lower electron densities and higher laser intensities. A total charge of 8 nC was measured. The presented simulation results indicate that the electrons are accelerated mainly by relativistic plasma waves, and, to some extent, by direct laser acceleration.

This paper discusses the generation of high-order optical harmonics from solid targets using laser pulses of 35 and 120 femtoseconds. Harmonics up to the 35th order were observed. High conversion efficiency has been achieved, e.g., ${10}^{\ensuremath{-}6}$ to the $10\mathrm{th}$ harmonic. It is demonstrated that the harmonic emission is highly directional and that the harmonic efficiency decreases rapidly with increasing plasma scale length.

Summary form only given. The amplification, at a repetition rate rather high (10 Hz or more), of femtosecond pulses in a Ti-doped sapphire laser at an energy level of 3.5 joules is produced in order to obtain focused intensities in the range of 10/sup 20/ W/cm/sup 2/, which rely on the use of 80 watts of average pump power.

We have experimentally studied thermal effects in highly pumped Ti:sapphire crystals in the 140–340 K temperature range. We demonstrate that a critical temperature exists, which depends on pumping and probing conditions, below which detrimental thermal effects completely disappear. In particular, we show that in a typical high-intensity Ti:sapphire amplifier, thermal lensing and wave-front distortions can be suppressed keeping the crystal temperature lower than ∼230 K.

An extension to dilute systems of the measurement of the non-linear refractive index by spectral analysis after non-linear propagation is presented. We show that one can deduce the magnitude and temporal characteristics of the induced nonlinearity in air and other gases. Our results obtained with femtosecond pulses are compared to those measured using much longer pulses.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We demonstrate a femtosecond laser chain that generates 20 TW at 10 Hz in less than 50 fs with focused intensities higher than 3 X 1019 W/cm2. This system is optimized for a broad transmission bandwidth and includes an aberration-free stretcher compressor.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe the generation of optical harmonics of high order during the interaction of intense femtosecond laser pulses with the surface of insulating and metallic solid targets. With a p- polarized fundamental beam from a titanium sapphire, chirped pulse amplification laser system ((lambda) equals 800 nm) we have observed even and odd harmonics up to the eighteenth order (lambda approximately equals 45 nm) for fundamental laser intensities of about 1017 to 1018 W/cm2.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

International audience; We present the results of the spectral analysis of intense ultrashort laser pulses propagating through nonlinear transparent media. We discuss the spectral features characteristic of the self-phase-modulation process, which is the most prominent process during nonguided propagation in the weakly nonlinear regime. We describe an application of automatic pulse phase retrieval based on the Kerr-like nonlinearity.

Recent progress in the understanding of the physics of femtosecond oscillators has made it possible to produce 10 fs pulses. In order to use these pulses in high field physics experiments they have to be amplified while keeping an extremely good contrast ratio between the peak of the pulse and its wings. We will show that several effects can cause a severe loss in the pulse quality and will present new solutions.

The application of the chirped pulse amplification technique to Ti:sapphire has made possible the generation of energetic sub-100 fs pulses. Sub- 20 fs Ti:Sapphire oscillators are now available, and amplification of pulses shorter than 40 fs has been demonstrated [1,2]. Although these pulses have very high peak power they can be useful to high field physics and specially to solid-state target experiments, only if they are free of any prepulse or wing. We describe here a Ti:sapphire amplifier which has been designed to produce pedestal free, 20TW,40fs pulses at 10 Hz. In order to minimize prepulses, we designed the amplifier with a broad transmission bandwidth, minimum material path length, high gain, and introduced an aberration free system for stretching and recompressing the pulses [3].

Relativistic self-focusing, self-channeling and filamentation are observed in the propagation of a 50 fs, 5 TW laser pulse in an underdense Argon plasma.

During the interaction of an intense, sharply rising femtosecond laser pulse with the surface of a solid target a thin layer of plasma is formed in which the density falls from approximately solid density to vacuum in a very short distance. It has been recognized more than a decade ago that the dynamics of electrons driven across such a sharp plasma-vacuum boundary is highly nonlinear, giving rise to the generation of optical harmonics of high order1-2. With the extremely high laser intensity available today the laser-induced excursion of the electrons may reach a considerable fraction of the wavelength, and significant contributions to the electronic anharmonicity from relativistic effects come into play. The relativistic interaction regime offers higher efficiency and an extension of harmonic generation to much shorter wavelengths. Thus the generation of harmonics using a solid target is a promising tool for the generation of coherent beams of ultrashort soft x-ray pulses.