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Proton beams with up to 100 pC bunch charge, 0.48 MeV cut-off energy and divergence as low as a $3^{\circ}$ were generated from solid targets at kHz repetition rate by a few-mJ femtosecond laser under controlled plasma conditions. The beam spatial profile was measured using a small aperture scanning time-of-flight detector. Detailed parametric studies were performed by varying the surface plasma scale length from 8 to 80 nm and the laser pulse duration from 4 fs to 1.5 ps. Numerical simulations are in good agreement with observations and, together with an in-depth theoretical analysis of the acceleration mechanism, indicate that high repetition rate femtosecond laser technology could be used to produce few-MeV protons beams for applications., Comment: 6 pages, 4 figures (main text). 7 pages, 6 figures (supplemental material)

International audience; Gas-filled multipass cells are an appealing alternative to capillaries to implement nonlinear temporal compression of high energy femtosecond lasers. Here, we provide an analytic expression for stationary beam coupling to multipass cells that takes into account nonlinear propagation. This allows a constant beam size on the mirrors and at the cell waist, thereby making the optical design more accurate, for example to avoid optical damage or significant ionization. The analysis is validated using spatio-temporal numerical simulations of the propagation in a near-concentric configuration. This is particularly important for multipass cells that are operated in a highly nonlinear regime, which is the current trend since it allows a lower number of roundtrips, relaxing the constraint on mirror coatings performance.

We present experiments using relativistic-intensity 1.5-cycle laser fields at 1 kHz repetition rate to drive surface high-harmonic generation (SHHG) from surface plasmas with controlled density gradient. As a function of the driving pulse carrier-envelope phase (CEP), we observe a transition from a modulated to a continuous SHHG spectrum, indicating the transition from double to isolated attosecond pulse emission. Single shot-acquisitions of XUV spectral continua support the emission of isolated attosecond pulses with an isolation degree of between 10 and 50 for the majority of the driving pulse CEPs. 2D Particle-in-cell simulations corroborate this interpretation and predict percent-level efficiencies for the generation of an isolated attosecond pulse even without spectral filtering.

International audience

International audience; Few-cycle pulse characterization methods face a serious challenge in providing sufficient signal-to-noise ratios together with superior spectral fidelity, as imposed by phase-matching conditions and linear dispersion effects. Here we investigate the effect of linear dispersion inside the nonlinear medium inherently present in such arrangements. We demonstrate that pulse characterization using a cross-polarized wave generation dispersion scan is surprisingly insensitive to the group-velocity dispersion itself. We characterize sub-4 fs pulses at 780 nm center the wavelength utilizing crystals of different thicknesses, yielding nearly identical pulse shapes. Numerical simulations shed light on this behavior indicating practical limits of usable medium lengths.

International audience; We report on recent progress on laser-plasma acceleration using a low energy and high-repetition rate laser system. Using only few milliJoule laser energy, in conjunction with extremely short pulses composed of a single optical cycle, we demonstrate that the laser-plasma accelerator ( LPA) can be operated close to the resonant blowout regime. This results in the production of high charge electron beams (> 10 pC) with peaked energy distributions in the few MeV range and relatively narrow divergence angles. We highlight the importance of the plasma density profile and gas jet design for the performance of the LPA. In this extreme regime of relativistic laser-plasma interaction with near-single-cycle laser pulses, we find that the effect of group velocity dispersion and carrier envelope phase can no longer be neglected. These advances bring LPAs closer to real scientific applications in ultrafast probing.

Controlled few-cycle light waveforms find numerous applications in attosecond science, most notably the production of isolated attosecond pulses in the XUV spectral region for studying ultrafast electronic processes in matter. Scaling up the pulse energy of few-cycle pulses could extend the scope of applications to even higher intensity processes, such as the generation of attosecond pulses with extreme brightness from relativistic plasma mirrors. Hollow-fiber compressors are widely used to produce few-cycle pulses with excellent spatiotemporal quality, whereby octave-spanning broadened spectra can be temporally compressed to near-single-cycle duration. In order to scale up the peak power of hollow-fiber compressors, the effective length and area mode of the fiber has to be increased proportionally, thereby requiring the use of longer waveguides with larger apertures. Thanks to an innovative design utilizing stretched flexible capillaries, we show that a stretched hollow-fiber compressor can generate pulses of TW peak power, the duration of which can be continuously tuned from the input seed laser pulse duration down to almost a single cycle (3.5fs at 750nm central wavelength) simply by increasing the gas pressure at the fiber end. The pulses are characterized online using an integrated d-scan device directly under vacuum. While the pulse duration and chirp are tuned, all other pulse characteristics, such as energy, pointing stability and focal distribution remain the same on target. This unique device makes it possible to explore the generation of high-energy attosecond XUV pulses from plasma mirrors using controllable relativistic-intensity light waveforms at 1kHz.

International audience; We report on electron wakefield acceleration in the resonant bubble regime with few-millijoule near-single-cycle laser pulses at a kilohertz repetition rate. Using very tight focusing of the laser pulse in conjunction with microscale supersonic gas jets, we demonstrate a stable relativistic electron source with a high charge per pulse up to 24 pC/shot. The corresponding average current is 24 nA, making this kilohertz electron source useful for various applications.

Waveform-controlled few-cycle light pulses have become an important tool in contemporary ultrafast science not only because they provide high temporal resolution but also because they facilitate sub-cycle gating of highly nonlinear process [1]. Sub-cycle gating of relativistic laser-plasma interactions is of particular interest as it could be a potentially efficient source of high-energy attosecond XUV pulses [2] or MeV femtosecond electron bunches [3] for time-resolved applications. Achieving this requires near-single-cycle pulses with TW peak power and high spatiotemporal fidelity, so they can be focused down to relativistic intensity (>1018 W/cm2 for 800nm light).

Laser-plasma acceleration is an emerging technique for accelerating electrons to high energies over very short distances. The accelerated electron bunches have femtosecond duration, making them particularly relevant for applications such as ultrafast imaging or femtosecond X-ray generation. Current laser-plasma accelerators are typically driven by Joule-class laser systems that have two main drawbacks: their relatively large scale and their low repetition-rate, with a few shots per second at best. The accelerated electron beams have energies ranging from 100 MeV to multi-GeV, however a MeV electron source would be more suited to many societal and scientific applications. Here, we demonstrate a compact and reliable laser-plasma accelerator producing high-quality few-MeV electron beams at kilohertz repetition rate. This breakthrough was made possible by using near-single-cycle light pulses, which lowered the required laser energy for driving the accelerator by three orders of magnitude, thus enabling high repetition-rate operation and dramatic downsizing of the laser system. The measured electron bunches are collimated, with an energy distribution that peaks at 5 MeV and contains up to 1 pC of charge. Numerical simulations reproduce all experimental features and indicate that the electron bunches are only $\sim 1$ fs long. We anticipate that the advent of these kHz femtosecond relativistic electron sources will pave the way to wide-impact applications, such as ultrafast electron diffraction in materials with an unprecedented sub-10 fs resolution.

CEP-stable few-cycle light pulses find numerous applications in attosecond science, most notably the production of isolated attosecond pulses for studying ultrafast electronic processes in matter [1]. Scaling up the pulse energy of few-cycle pulses could extend the scope of applications to even higher intensity processes, such as attosecond dynamics of relativistic plasma mirrors [2]. Hollow fiber compressors are widely used to produce few-cycle pulses with excellent spatiotemporal quality [3], where octave-spanning broadened spectra can be temporally compressed to sub-2-cycle duration [4,5]. Several tricks help increase the output energy: using circularly polarized light [6], applying a pressure gradient along the fiber [7] or even temporal multiplexing [8]. The highest pulse energy of 5 mJ at 5 fs pulse duration was achieved by using a hollow fiber in pressure gradient mode [9] but in this case no CEP stabilization was achieved, which is crucial for most applications of few-cycle pulses. Nevertheless, it did show that in order to scale up the peak power, the effective length and area mode of the fiber had to be increased proportionally, thereby requiring the use of longer waveguides with larger apertures. Thanks to an innovative design utilizing stretched flexible capillaries [10], we recently demonstrated the generation CEP-stable sub-4fs pulses with 3mJ energy using a 2m length 450mm bore hollow fiber in pressure gradient mode [11]. Here, we show that a stretched hollow-fiber compressor operated in pressure gradient mode can generate relativistic intensity pulses with continuously tunable waveform down to almost a single cycle (3.5fs at 750nm central wavelength). The pulses are characterized online using an integrated d-scan device directly under vacuum [12]. While the pulse shape is tuned, all other pulse characteristics, such as energy, pointing stability and focal distribution remain the same on target, making it possible to explore the dynamics of plasma mirrors using controllable relativistic-intensity light waveforms at 1kHz. [1] Krausz and Ivanov, Rev. Mod. Phys. 81, 163 (2009). [2] Borot et al., Nature Phys. 8, 417-421 (2012). [3] Nisoli et al., Appl. Phys. Lett. 68, 2793–2795 (1996). [4] Park et al., Opt. Lett. 34, 2342–2344 (2009). [5] Schweinberger et al., Opt. Lett. 37, 3573–5 (2012). [6] Chen et al., Opt. Lett. 34, 1588–1590 (2009). [7] Suda et al., Appl. Phys. Lett. 86, 111116 (2005). [8] Jacqmin et al., Opt. Lett. 40, 709-712 (2015) [9] Bohman et al., Opt. Lett. 35, 1887–9 (2010). [10] Nagy et al., Appl. Opt. 47, 3264–3268 (2008). [11] Boehle et al., Las. Phys. Lett. 11, 095401 (2014). [12] Miranda et al., Opt. Express 20, 18732-43 (2012)

International audience; We report for the first time on the anticorrelated emission of high-order harmonics and energetic electron beams from a solid-density plasma with a sharp vacuum interface-plasma mirror-driven by an intense ultrashort laser pulse. We highlight the key role played by the nanoscale structure of the plasma surface during the interaction by measuring the spatial and spectral properties of harmonics and electron beams emitted by a plasma mirror. We show that the nanoscale behavior of the plasma mirror can be controlled by tuning the scale length of the electron density gradient, which is measured in situ using spatial-domain interferometry.

International audience; It was recently proposed that ionization-induced self-compression could be used as an effective method to further compress femtosecond laser pulses propagating freely in a gas jet [He et al., Phys. Rev. Lett. 113, 263904 2014]. Here, we address the question of the homogeneity of the self-compression process and show experimentally that homogeneous self-compression down to 12fs can be obtained by finding the appropriate focusing geometry for the laser pulse. Simulations are used to reproduce the experimental results and give insight into the self-compression process and its limitations. Simulations suggest that the ionization process induces spatio-temporal couplings which lengthen the pulse duration at focus, possibly making this method ineffective for increasing the laser peak intensity. (C) 2016 Optical Society of America

International audience; We present a practical spatial-domain interferometer for characterizing the electronic density gradient of laser- induced plasma mirrors with sub-30-femtosecond temporal resolution. Time-resolved spatial imaging of an intensity- shaped pulse reflecting off an expanding plasma mirror in- duced by a time-delayed pre-pulse allows us to measure characteristic plasma gradients of 10–100 nm with an ex- pansion velocity of 10.8 nm/ps. Spatial-domain interferom- etry (SDI) can be generalized to the ultrafast imaging of nm to μm size laser-induced phenomena at surfaces.

International audience; We report on a compact energy-scalable device for generating high-fidelity femtosecond laser pulses based on spatial filtering through a hollow-core fiber followed by a nonlinear crystal for crosspolarized wave (XPW) generation. This versatile device is suited for temporal pulse cleaning over a wide range of input energies (from 0.1 to >10 mJ) and is successfully qualified on different ultrafast laser systems. Full characterization of the XPW output is presented. In particular, we demonstrate the generation of 1.6 mJ energy pulses starting from 11 mJ input pulse energy. The temporal contrast of the pulses is enhanced by more than 4 orders of magnitude. In addition, pulse shortening from 40 fs down to 15 fs Fourier-transform limit yields an overall peak-power transmission of up to 50%. This device not only serves as an integrated pulse contrast filter inside an ultrafast laser amplifier but also as a simple back-end solution for temporal post-compression of amplified pulses.

International audience; Experimental measurements of backward accelerated protons are presented. The beam is produced when an ultrashort (5 fs) laser pulse, delivered by a kHz laser system, with a high temporal contrast (10 8), interacts with a thick solid target. Under these conditions, proton cutoff energy dependence with laser parameters, such as pulse energy, polarization (from p to s), and pulse duration (from 5 to 500 fs), is studied. Theoretical model and two-dimensional particle-in-cell simulations, in good agreement with a large set of experimental results, indicate that proton acceleration is directly driven by Brunel electrons, in contrast to conventional target normal sheath acceleration that relies on electron thermal pressure.

International audience; The nonlinear interaction of an intense femtosecond laser pulse with matter can lead to the emission of a train of sub-laser-cycle--attosecond--bursts of short-wavelength radiation1, 2. Much effort has been devoted to producing isolated attosecond pulses, as these are better suited to real-time imaging of fundamental electronic processes3, 4, 5, 6. Successful methods developed so far rely on confining the nonlinear interaction to a single sub-cycle event7, 8, 9. Here, we demonstrate for the first time a simpler and more universal approach to this problem10, applied to nonlinear laser-plasma interactions. By rotating the instantaneous wavefront direction of an intense few-cycle laser field11, 12 as it interacts with a solid-density plasma, we separate the nonlinearly generated attosecond pulse train into multiple beams of isolated attosecond pulses propagating in different and controlled directions away from the plasma surface. This unique method produces a manifold of isolated attosecond pulses, ideally synchronized for initiating and probing ultrafast electron motion in matter.

International audience; A 80 μJ, 6 fs, CEP-stable high-contrast injector is demonstrated. The device relies on standard pulse post-compression in hollow-core fiber followed by a temporal filter based on cross-polarized wave generation. Pulses with a Gaussian spectrum over 350 nm, centered at 750 nm, are generated. Temporal measurements show that the contrast of the few-cycle pulses is enhanced on a femtosecond and picosecond time scale. The carrier-envelope phase stability is preserved (0.3 rad RMS). These performances make the system an ideal seed laser for high-power, high-contrast OPCPA systems.

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

International audience; We show that a standard hollow-core fiber (HCF) compressor device can be used to efficiently compress multi-mJ energy laser pulses down to few-cycle duration, when seeded with linearly chirped, circularly polarized pulses. With this approach, we routinely generate carrier-envelope phase (CEP)-locked, 1.6 mJ, 4.8 fs pulses using only 3 mJ, 25 fs pulses as the seed. © 2011 Pleiades Publishing, Ltd.

International audience; We report the first demonstration of high-harmonic generation from plasma mirrors at a 1 kHz repetition rate. Harmonics up to nineteenth order are generated at peak intensities close to 10$^{18}$ W=cm$^2$ by focusing 1 mJ, 25 fs laser pulses down to 1:7 $\mu$m FWHM spot size without any prior wavefront correction onto a moving target. We minimize target surface motion with respect to the laser focus using online interferometry to ensure reproducible interaction conditions for every shot and record data at 1 kHz with unprecedented statistics. This allows us to unambiguously identify coherent wake emission as the main generation mechanism

International audience; We report the routine generation of sub-5-fs laser pulses with 1-mJ energy and stable carrier-envelope phase at 1-kHz repetition rate, obtained by compressing the multi-mJ output from a phase-locked Ti:sapphire amplifier in a rare-gas-filled hollow fiber. The dual-stage amplifier features a hybrid transmission grating/chirped mirror compressor providing 2.2-mJ, 26-fs pulses at 1 kHz with standard phase deviation of 190 mrad rms. We demonstrate hour-long phase stability without feedback control of grating position or rigorous control of the laser environment, simply by using small pulse stretching factors in the amplifier, which minimize the beam pathway in the compressor. The amplifier also integrates a versatile AOPDF (acousto-optic programmable dispersive filter) for closed-loop spectral phase optimization. The various factors influencing the overall phase stability of the system are discussed in detail. Using the optimized output, 1-mJ, 4.5-fs pulses are generated by seeding the neon gas filled hollow fiber with a circularly polarized input beam. A standard phase deviation of 230 mrad after the HCF is obtained by direct f-to-2f detection and slow-loop feedback to the oscillator locking electronics without any additional spectral broadening.

International audience; We demonstrate arbitrary carrier-envelope (CE) phase control of femtosecond laser pulses by an acousto-optic programmable dispersive filter (AOPDF), with an accuracy better than π/100 at a repetition rate of 1 kHz. We also demonstrate, for the first time to the best of our knowledge, 15 Hz closed-loop CE phase stabilization using an AOPDF inside a 1 kHz chirped pulse amplifier to correct for slow CE phase drifts.

International audience; We report the generation of 4.3 fs, 1 mJ pulses at 1 kHz using a hollow-core fiber compressor seeded with circularly polarized laser pulses. We observe up to 30% more energy throughput compared to the case of linearly polarized laser input, together with significantly improved output spectral stability. Seeding with circularly polarized pulses proves to be an effective approach for high-energy operation of the hollow-fiber compression technique.

International audience; Carrier-envelope phase (CEP) stabilization of a femtosecond chirped-pulse amplification system featuring a compact transmission grating compressor is demonstrated. The system includes two amplification stages and routinely generates phase-stable (~250 mrad rms) 2 mJ, 25 fs pulses at 1 kHz. Minimizing the optical pathway in the compressor enables phase stabilization without feedback control of the grating separation or beam pointing. We also demonstrate for the first time to the best of our knowledge, out-of-loop control of the CEP using an acousto-optic programmable dispersive filter inside the laser chain.

International audience; The characterization of a temporal filter based on cross-polarized wave generation working in the few-optical-cycle regime is presented. We show that this device dramatically improves the spectral quality of the ultrashort seed pulses, opening the way to the production of sub-10 fs pulses with high incoherent and coherent contrast. The dispersion compensation conditions for an optimized behavior of the process are experimentally and theoretically discussed.

International audience; Theoretical studies and computer simulations predict efficient generation of attosecond electromagnetic pulses from overdense plasma targets, driven by relativistically strong laser pulses. These predictions need to be validated in time resolved experiments in order to provide a route for applications. The first available femtosecond sources for these experiments are likely to be 10 fs pulses of a few millijoules, which could provide focal intensities at about the relativistic threshold. With particle-in-cell simulations, we demonstrate that the radiation resulting from interaction of such pulses with solid targets is expected to be attosecond trains with very high conversion efficiency as relativistic effects start to act.

International audience; We use semiconductor (Si) and metallic (Al, Zr) transmission filters to shape, in amplitude and phase, high-order harmonics generated from the interaction of an intense titanium sapphire laser field with a pulsed neon gas target. Depending on the properties of the filter, the emitted attosecond pulses can be optimized in bandwidth and/or pulse length. We demonstrate the generation of attosecond pulses centered at energies from 50 to 80 eV, with bandwidths as large as 45 eV and with pulse durations compressed to 130 as.

International audience; The temporal confinement of high harmonic generation (HHG) via modulation of the polarization of the fundamental pulse is studied in both temporal and spectral domains. In the temporal domain, a collinear cross-correlation setup using a 40 fs IR pump for the HHG and a 9 fs IR pulse to probe the generated emission is used to measure the XUV pulse duration. The observed temporal confinement is found to be consistent with theoretical predictions. An increased confinement is observed when a 9 fs pulse is used to generate the harmonics. An important spectral broadening, including a continuum background, is also measured. Theoretical calculations show that with 10 fs driving pulses, either one or two main attosecond pulses are created depending on the value of the carrier envelope phase.

International audience; The authors present a simple add on for multicycle, multimJ Ti:sapphire-based amplifier systems to efficiently generate 9.5 fs pulses with 1.8 mJ. Filamentary propagation yields spectrally broadened pulses carrying an unexpected large group-delay dispersion of -500 fs2 which allows linear propagation of the few-cycle pulses towards target areas without need of any sophisticated dispersion control. Pulse compression could easily be achieved by transmission through a combination of different glass substrates. The pulse wave front, the low energy fluctuations, and the good temporal contrast make this source well suited for relativistic laser-solid experiments with intense few-cycle pulses at kilohertz repetition rate.

International audience; We report the generation, compression, and delivery on target of ultrashort extreme-ultraviolet light pulses using external amplitude and phase control. Broadband harmonic radiation is first generated by focusing an infrared laser with a carefully chosen intensity into a gas cell containing argon atoms. The emitted light then goes through a hard aperture and a thin aluminum filter that selects a 30-eV bandwidth around a 30-eV photon energy and synchronizes all of the components, thereby enabling the formation of a train of almost Fourier-transform-limited single-cycle 170 attosecond pulses. Our experiment demonstrates a practical method for synthesizing and controlling attosecond waveforms.

International audience; Nous présentons les résultats de deux campagnes expérimentales, étudiant le confinement temporel de la génération d'harmoniques d'ordres élevés par modulation de l'ellipticité du champ fondamental. Une première expérience réalisée au LLC (Lund Laser Center) valide cette technique de « porte d'ellipticité » dans le régime femtoseconde et nous permet par une mesure temporelle directe de comparer la durée de l'émission XUV avec la durée calculée de la porte d'ellipticité. Une seconde expérience réalisée au CELIA met en évidence unéelargissement spectral des harmoniques jusqu'à l'obtention d'un quasi-continuum. Ce quasicontinuum est théoriquement consistant avec l'émission d'une impulsion ou de deux impulsions attosecondes selon la phase absolue du champ fondamental.

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.

Very short light pulses allow us to resolve ultrafast processes in molecules, atoms and condensed matter. This started with the advent of Femtochemistry, for which Ahmed Zewail received the Novel Prize in Chemistry in 1999. Ever since, researcher have been trying to push the temporal resolution further and we have now reached attosecond pulse durations. Their generation, however, remains very challenging and various different generation mechanisms are the topic of heated research around the world.Our group focuses on attosecond pulse generation and ultrashort electron bunch acceleration on solid targets. In particular, this thesis deals with the upgrade of a high intensity, high contrast, kHz, femtosecond laser chain to reach the relativistic interaction regime on solid targets. Few cycle driving laser pulses should allow the generation of intense isolated attosecond pulses. A requirement to perform true attosecond pump-probe exeriments.To achive this, a HCF postcompression scheme has been conceived and implemented to shorten the duration of a traditional laser amplifier. With this a peak intensity of 1TW was achieved with near-single-cycle pulse duration. For controlled experiments, a vacuum beamline was developed and implemented to accurately control the laser and plasma conditions on target.During the second part of this thesis, this laser chain was put in action to drive relativistic harmonic generation on solid targets. It was the first time ever that this has been achieved at 1 kHz. By CEP gating the few-cycle-pulses, single attosecond pulses were generated. This conclusion has been supported by numerical simulations. Additionally a new regime to accelerate electron bunches on soft gradients has been detected., Les impulsions laser ultrabrèves nous permettent de suivre en temps réel les phénomènes ultrarapides au sein de la matière à l’échelle microscopique. C’est précisément pour l’invention de la chimie à l’échelle femtoseconde, ou femtochimie, qu’Ahmed Zewail se vit décerner le prix Nobel de chimie en 1999. Depuis les utilisateurs du laser cherchent à augmenter la résolution temporelle, c’est-à-dire réduire la durée des impulsions laser. Aujourd’hui, nous savons générer des flashs lumineux à l’échelle attoseconde dans le domaine spectral de l’extrême ultraviolet (XUV) mais l’efficacité de génération reste faible et le développement de sources laser attosecondes intenses constitue un sujet de recherche très actif sur le plan international.Notre groupe au LOA se concentre sur la génération d’impulsions attoseconde sur miroir plasma en régime relativiste. Pour cela, il cherche à développer une source d’impulsions femtosecondes à forte cadence et fort contraste et suffisamment énergétiques pour atteindre des intensités relativistes (>> 10^18W/cm2) lorsqu’elles sont fortement focalisées sur un plasma surdense. Un plasma surdense réfléchit la lumière incidente et par conséquent agit comme un miroir qui se déplaçant à vitesse relativiste et qui comprime l’impulsion incidente, produisant ainsi un flash attoseconde par cycle optique. En utilisant des impulsions proches d’un cycle optique, il est donc envisageable de générer une seule impulsion attoseconde intense pendant l’interaction.Dans la première partie de mon travail de thèse, j’ai réalisé un compresseur nonlinéaire pour réduire la durée des impulsions issues d’une chaîne à double dérive de fréquence (10mJ, 25fs, 1kHz) à phase enveloppe-porteuse (CEP) stabilisée. En propageant les impulsions du laser à haute intensité dans une fibre creuse remplie de gaz rare, j’ai réussi à générer des impulsions de 1.3 cycle optique avec une puissance crête autour de 1TW avec une CEP stabilisée. Dans un deuxième temps, j’ai mis en forme spatialement et temporellement les impulsions issues du compresseur à fibre pour générer à la fois des impulsions attosecondes intenses et des faisceaux d’électrons énergétiques sur un miroir plasma à gradient de densité contrôlé. Ces expériences nous permis, pour la première fois, de mettre en évidence la production d’impulsions attosecondes isolées dans l’XUV, l’émission corrélée de faisceaux d’électrons énergétiques en régime relativiste ainsi qu’un nouveau régime d’accélération d’électrons à très long gradient plasma.

The framework of this thesis is the design and development of a TW-class, high-repetition rate, CEP-stabilized, few-cycle laser system devoted to attosecond physics. Few-cycle pulses includes only a few oscillations of the carrier wave (duration about 5 fs for 800nm central wavelength) and are not directly available at the output of typical femtosecond sources. One of the most popular techniques used for producing such pulses with high spatial quality is nonlinear spectral broadening in a gas-filled hollow-core fiber followed by temporal compression with chirped mirrors. However, as the input pulse energy approaches the milliJoule level, both the transmission and stability of hollow fiber compressors rapidly drop with the onset of self-focusing and ionization. A way of overcoming this limitation is to divide the input pulse into several lower energy replicas that can be subsequently recombined after independent spectral broadening in the fiber. In this thesis, the passive coherent combining of millijoule energy laser pulses down to few-cycle duration in a gas-filled hollow fiber is demonstrated for the first time. High combining efficiency is achieved by using carefully oriented calcite plates for temporal pulse division and recombination. Carrier-envelope phase (CEP)- stable, 6-fs, 800-nm pulses with more than 0.6 mJ energy were routinely generated in the case of twofold division and recombination. A detailed theoretical and experimental analysis of this temporal multiplexing technique is proposed to explain the conditions required for producing few-cycle pulses with high fidelity. In particular, an interferometric method for measuring the relative spectral phase between two replicas is demonstrated. This gives a measure of the phase mismatch in the combining plate, as well as that induced by eventual cross-phase modulation or ionization during propagation in the fiber. The effects degrading the combining process, as polarization change or nonlinear interactions between pulse replicas are analyzed in details. A method is proposed to overcome these limitations, even in the critical case of fourfold pulse division and combination., Cette thèse s’inscrit dans le cadre du développement d’une source laser TW, de cadence élevée, stabilisée en phase, et délivrant des impulsions de quelques cycles optiques pour explorer la physique attoseconde. De telles impulsions contiennent seulement quelques oscillations de l’onde porteuse (durée de 5 fs à une longueur d’onde centrale de 800 nm) et ne sont pas directement disponibles à la sortie d’une source laser femtoseconde classique. Une technique de post-compression efficace pour obtenir de telles impulsions consiste à élargir le spectre des impulsions laser par automodulation de phase dans une fibre creuse remplie de gaz, puis à compenser la phase spectrale introduite avec des miroirs chirpés. Cette technique convient à des impulsions dont l’énergie est inférieure au millijoule. Au-delà, la transmission et la stabilité du compresseur chutent fortement à cause d'effets non linéaires tels que l'autofocalisation et l'ionisation. Pour comprimer des impulsions énergétiques et dont la phase de l’enveloppe est stabilisée par rapport à la porteuse (stabilisation de la CEP), il est possible de diviser l'impulsion initiale en plusieurs répliques d'énergie moindre et de réduire ainsi l'intensité crête en entrée de fibre. Le spectre de chaque réplique est alors élargi indépendamment. Dans le cadre de cette thèse, la combinaison cohérente passive d'impulsions de quelques cycles optiques issues d'une fibre creuse remplie de gaz est démontrée pour la première fois. L'utilisation de lames biréfringentes (calcite) dont l’orientation est soigneusement déterminée permet de générer et combiner des répliques avec une efficacité élevée. Ainsi, dans le cas d’une division en deux répliques, des impulsions stabilisées en phase (CEP), de durée 6 fs et d'énergie 0.6 mJ ont été générées de manière fiable et reproductible. L’étude détaillée de cette technique, aussi bien théorique qu’expérimentale, a permis de mettre en évidence les conditions requises pour générer des impulsions de quelques cycles optiques et présentant un bon contraste temporel. Plus précisément, la phase spectrale relative entre les répliques peut être mesurée à l'aide d'une méthode interférométrique permettant de quantifier les déphasages résiduels dus à la lame qui recombine les répliques, ainsi que ceux induits lors de la propagation dans la fibre par d'éventuels effets de modulation de phase croisée ou d'ionisation. Les effets qui affectent le processus de combinaison des répliques, tels que les modifications des états de polarisation des répliques ou bien les interactions non linéaires entre les répliques, sont analysés en détail. Une méthode est proposée pour minimiser ces effets, même dans le cas plus critique de la division et combinaison d'impulsions à quatre répliques.

The experimental work presented in this manuscript focuses on the non-linear response of plasma mirrors when driven by a sub-relativistic (~10^18 W/cm^2) ultra-short (~30fs) laser pulse. In particular, we studied the generation of attosecond pulses (1as=10^(-18) s) and electron beams from plasma mirror generated in controlled pump-probe experiment. One first important result exposed in this manuscript is the experimental observation of the anticorrelated emission behavior between high-order harmonics and electron beams with respect to plasma scale length. The second important result is the presentation of the « spatial domain interferometry » (SDI) diagnostic, developed during this PhD to measure the plasma expansion in vacuum. Finally, we will discuss the implementation of phase retrieval algorithms for both spatial and temporal phase reconstructions.From a more general point of view, we replace this PhD in its historical context. We hope to convince the reader that through laser-plasma mirror interaction schemes, we could tomorrow conceive cost-efficient X-UV and energetic electron sources with unprecedented temporal resolutions., Dans cette thèse expérimentale, nous nous intéressons à la réponse non-linéaire d’un miroir plasma sous l’influence d’un laser d’intensité sous-relativiste (~10^18 W/cm^2), et de très courte durée (~30fs). Nous avons en particulier étudié la génération d’impulsions attosecondes (1as=10^(-18) s) et de faisceaux d’électrons en effectuant des expériences dites de « pompe-sonde » contrôlées. Un premier résultat important est l’observation d’une anti-corrélation entre l’émission X-UV attoseconde et l’accélération d’électron lorsque l’on change la longueur caractéristique du plasma, résultats confirmés par des simulations numériques. Un second résultat important concerne le diagnostique de l’expansion du plasma sous vide par « interférométrie en domaine spatial » (SDI), technique élaborée dans le cadre de cette thèse. Enfin nous discutons à deux reprises l’utilisation d’algorithmes de reconstruction de phase dans le domaine spatiale ou temporel.De manière plus générale, nous avons cherché à replacer ce travail de thèse dans un contexte scientifique plus général. En particulier, nous tentons de convaincre le lecteur qu’à travers l’intéraction laser-miroir plasma, il devient concevable de fournir un jour aux utilisateurs des sources peu onéreuses d’impulsions X-UV et de faisceaux d’électrons de résolutions temporelles inégalées.

Laser-plasma interaction at high intensities (I " 1015 W/cm2) with ultra-short pulses ( t " 100 fs) is a booming research domain as it offers the opportunity to study increasingly short phenomena, comprised under the trivial name " attosecond science ". Laser-plasma interaction also bears the promise to achieve new light sources for the generation of energetic particle beams and X-rays. Our approach focuses on the generation of attosecond pulses from plasma mirrors using few-cycle laser pulses ( 5 fs at 800 nm) at high repetition rate (1 kHz) and with fine control of laser parameters. The generation and spatio-temporal control of high-order harmonics emission in the non-relativistic regime has recently been demosntrated by our group. The following objective is to reach the relativistic regime thanks to higher intensities on target. This thesis pursued the goals of improving the existing laser chain performances in termes of energy and temporal contrast, while preserving, at least, all the other key parameters. Meeting these requirements was achieved while developing a laser architecture based on chirped-pulse amplification (CPA) delivering 5 mJ, 5 fs pulses at 1 kHz, with a temporal contrast C = 1011 and an absolute phase (CEP) stabilized down to 200 mrad rms. Contrast improvement by cross-polarized wave (XPW) generation is at the centre of this dissertation. An extensive study of the mechanism was carried out. It allowed to confront experimental results and theoretical developments in " extremes " regimes such as the generation at very high efficiency and few-cycle pulse filtering. Besides, a new energy-scalable scheme was proposed to filter efficiently (20%) a wide range of pulse energies (from 100 μJ to 10 mJ). Pulse compression by a factor larger than two was also demonstrated. The new laser chain includes such filter in a double-CPA configuration delivering pulses of 10 mJ, 20 fs, C = 1011 and CEP = 200 mrad rms. A detailed study was carried out to develop a new stretcher/compressor scheme achieving highly stretched pulses (50 ps) while remaining compact to preserve CEP stability. The chosen configuration consists of a bulk stretcher, an acousto-optic programmable dispersive filter and a grisms compressor. The manuscript finally presents the perspectives of post-compression in a rare-gas filled hollow-core fibre in order to achieve pulses of 5 mJ, 5 fs, C = 1011, CEP = 200 mrad rms at 1 kHz.; L'interaction laser-plasma à très haute intensité (I " 1015 W/cm2) avec des impulsions ultra-courtes ( t " 100 fs) est un domaine en plein essor car il offre l'opportunité d'étudier des phénomènes physiques toujours plus brefs, regroupés sous l'appellation " science attoseconde ". L'interaction laser-plasma promet aussi l'avènement de nouvelles sources pour la génération de faisceaux de particules et de rayonnement X très énergétiques. Notre démarche se concentre sur la génération d'impulsions attosecondes sur miroir plasma à partir d'impulsions de quelques cycles optiques ( 5 fs à 800 nm), à haut taux de répétition (1 kHz) et avec un contrôle fin des paramètres laser. La génération et le contrôle temporel et spatial de l'émission d'harmoniques d'ordres élevés dans le régime non-relativiste ont récemment été démontrés par le groupe. L'objectif suivant est d'atteindre le régime relativiste, qui nécessite une intensité sur cible plus élevée. Ce travail de thèse s'inscrit donc dans la logique d'améliorer les performances de la chaine existante en termes d'énergie et de contraste temporel, tout en préservant les autres paramètres clés. Pour répondre à ces défis, l'objectif a été de mettre au point une architecture laser basée sur l'amplification à dérive de fréquence (CPA) délivrant des impulsions de 5 mJ, 5 fs à 1 kHz, avec un contraste temporel C = 1011 et une phase absolue (CEP) stabilisée à 200 mrad rms. La problématique de l'amélioration du contraste par la technique de génération de polarisation croisée (XPW) occupe une place centrale dans ce mémoire. Une étude extensive du mécanisme XPW a été menée. Elle a permis de confronter résultats expérimentaux et développements théoriques dans les régimes dits " extrêmes " tels que la génération à très haut rendement et le filtrage d'impulsions de quelques cycles optiques. En outre, un nouveau schéma de filtrage adaptable sur une large gamme d'énergie (de 100 μJ à 10 mJ) et efficace (20%) a été réalisé. La compression des impulsions d'un facteur supérieur à deux avec ce schéma a également été démontrée. La nouvelle chaine laser inclut un tel filtre dans une configuration en double CPA dont les performances finales visées sont les suivantes : 10 mJ, 20 fs, C = 1011 et CEP = 200 mrad rms. Le schéma d'étirement/compression a fait l'objet d'une étude détaillée pour permettre un étirement élevé (50 ps) tout en restant compact pour préserver la stabilité CEP. La configuration adoptée consiste en un étireur verre, un filtre acousto-optique dispersif programmable et un compresseur "grismes". Le mémoire présente enfin les perspectives de post-compression dans une fibre creuse remplie d'un gaz rare pour obtenir des impulsions de 5 mJ, 5 fs, C = 1011, CEP = 200 mrad rms à 1 kHz.

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