Search

Discriminating between ballistic and diffuse components of light propagating through highly scattering media is not only important for imaging purposes but also for investigating the fundamental diffusion properties of the medium itself. Massively developed to this end over the past 20 years, nonlinear temporal gating remains limited to about 10e-10 transmission factors. Here, we report nonlinear time gated measurements of highly scattered femtosecond pulses with transmission factors as low as 10e-12. Our approach is based on third-order nonlinear cross-correlation of femtosecond pulses, a standard diagnostic used in high-power laser science, applied for the first time to the study of fundamental light scattering properties.

International audience; In laser-wakefield acceleration, an ultra-intense laser pulse is focused into an underdense plasma to accelerate electrons to relativistic velocities. In most cases, the pulses consist of multiple optical cycles and the interaction is well described in the framework of the ponderomotive force where only the envelope of the laser has to be considered. But when using single-cycle pulses, the ponderomotive approximation breaks down, and the actual waveform of the laser has to be taken into account. In this paper, we use nearsingle-cycle laser pulses to drive a laser-wakefield accelerator. We observe variations of the electron beam pointing on the order of 10 mrad in the polarization direction, as well as 30% variations of the beam charge, locked to the value of the controlled laser carrier-envelope phase, in both nitrogen and helium plasma. Those findings are explained through particle-in-cell simulations indicating that low-emittance, ultrashort electron bunches are periodically injected off-axis by the transversally oscillating bubble associated with the slipping carrier-envelope phase.

Near single-cycle pulses with 3.81fs of duration were achieved in the Voxel laboratory (GoLP/IPFN at Instituto Superior Técnico) by taking advantage of the Kerr effect, a nonlinear process. This was done with resource to a differentially pumped Hollow-Core Fiber (HCF) and a D-Scan system from Sphere Ultrafast Photonics.

We report on the first observation and control of carrier-envelope phase effects in Laser Wakefield Acceleration. These effects imply the breakdown of the cycle-averaged ponderomotive approximation and are explained through highly localized off-axis electron injection.

We report on cross-polarised wave generation in multi-pass cells at the milli-Joule level. Backed by simulations, results indicate 103 contrast enhancement, high spatio-temporal beam quality, and a record internal conversion efficiency of 50%.

Optics express 30(3), 3262 - 3288 (2022). doi:10.1364/OE.444564, The investigation of spatio-temporal couplings (STCs) of broadband light beams is becoming a key topic for the optimization as well as applications of ultrashort laser systems. This calls for accurate measurements of STCs. Yet, it is only recently that such complete spatio-temporal or spatio-spectral characterization has become possible, and it has so far mostly been implemented at the output of the laser systems, where experiments take place. In this survey, we present for the first time STC measurements at different stages of a collection of high-power ultrashort laser systems, all based on the chirped-pulse amplification (CPA) technique, but with very different output characteristics. This measurement campaign reveals spatio-temporal effects with various sources, and motivates the expanded use of STC characterization throughout CPA laser chains, as well as in a wider range of types of ultrafast laser systems. In this way knowledge will be gained not only about potential defects, but also about the fundamental dynamics and operating regimes of advanced ultrashort laser systems., Published by Optica, Washington, DC

International audience; Quantum-optical spectrometry is a recently developed shot-to-shot photon correlation-based method, namely using a quantum spectrometer (QS), that has been used to reveal the quantum optical nature of intense laser–matter interactions and connect the research domains of quantum optics (QO) and strong laser-field physics (SLFP). The method provides the probability of absorbing photons from a driving laser field towards the generation of a strong laser–field interaction product, such as high-order harmonics. In this case, the harmonic spectrum is reflected in the photon number distribution of the infrared (IR) driving field after its interaction with the high harmonic generation medium. The method was implemented in non-relativistic interactions using high harmonics produced by the interaction of strong laser pulses with atoms and semiconductors. Very recently, it was used for the generation of non-classical light states in intense laser–atom interaction, building the basis for studies of quantum electrodynamics in strong laser-field physics and the development of a new class of non-classical light sources for applications in quantum technology. Here, after a brief introduction of the QS method, we will discuss how the QS can be applied in relativistic laser–plasma interactions and become the driving factor for initiating investigations on relativistic quantum electrodynamics.

We experimentally demonstrate the post-compression of radially polarized 25 fs pulses at 800 nm central wavelength in a multiple thin plate arrangement for the first time, to the best of our knowledge. Sub-7 fs pulses with 90 µJ energy were obtained after dispersion compensation, corresponding to a compression factor of more than 3.5. Preservation of radial polarization state was confirmed by polarized intensity distribution measurements. Linear projections of the radially polarized pulses were also fully characterized in the temporal domain.

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 report the first simultaneous measurements of high-harmonic generation, accelerated electron and proton beams generated on relativistic plasma mirrors with controlled scale length using laser pulses with duration tunable from 27 fs to sub-4 fs.

International audience; We report on the nonlinear temporal compression of mJ energy pulses from a Ti:Sa chirped pulse amplifier system ina multipass cell filled with argon. The pulses are compressed from 30 fs down to 5.3 fs, corresponding to two optical cycles. The post-compressed beam exhibits excellent spatial quality and homogeneity. These results provide guidelines for optimizing the compressed pulse quality and furtherscaling of multipass-cell-based post-compression down to the single-cycle regime.

We present a near-single-cycle near-infrared light source that can be focused to relativistic intensity at 1kHz. We observe first experimental indications of light waveform effects in laser-wakefield acceleration of relativistic energy electrons

International audience; We report on the stable and continuous operation of a kilohertz laser-plasma accelerator. Electron bunches with 2.6 pC charge and 2.5 MeV peak energy were generated via injection and trapping in a downward plasma density ramp. This density transition was produced in a specially designed asymmetrically shocked gas jet. The reproducibility of the electron source was also assessed over a period of a week and found to be satisfactory with similar values of the beam charge and energy. Particle in cell simulations confirm the role of the shock and the density transition in the electron injection mechanism. These results show that the reproducibility and stability of the laser-plasma accelerator are greatly enhanced on the long-term scale when using a robust scheme for density gradient injection.

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.

The development of ultra-intense and ultra-short light sources is currently a subject of intense research driven by the discovery of novel phenomena in the realm of relativistic optics, such as the production of ultrafast energetic particle and radiation beams for applications. It has been a long-standing challenge to unite two hitherto distinct classes of light sources: those achieving relativistic intensity and those with pulse durations approaching a single light cycle. While the former class traditionally involves large-scale amplification chains, the latter class places high demand on the spatiotemporal control of the electromagnetic laser field. Here, we present a light source producing waveform-controlled 1.5-cycle pulses with a 719 nm central wavelength that can be focused to relativistic intensity at a 1 kHz repetition rate based on nonlinear post-compression in a long hollow-core fiber. The unique capabilities of this source allow us to observe the first experimental indications of light waveform effects in laser wakefield acceleration of relativistic energy electrons., Relativistic optics: High intensity ultra-short fast-repeating light pulses A pioneering laser source combines extremely high intensity and fast-repeating pulses with ultra-short pulse duration, opening new opportunities in the field of research and technology called relativistic optics. This requires lasers that are sufficiently intense to accelerate particles such as electrons to close to light speed, when effects of relativity theory become increasingly significant. Marie Ouillé and colleagues at the CNRS Laboratoire d’Optique Appliquée in France, with co-workers in Germany, combined laser sources with light compression and manipulation methods to generate relativistic intensity pulses almost as short as a single cycle of the light wave. They say their system is currently the only light source capable of achieving pulses shorter than four femtoseconds combined with peak powers up to 1 terawatt. The researchers also demonstrated precise control over the fine structure of the light pulses.

We report on the progress of few-cycle lasers of ELI-ALPS user facility, operating at 100W average power level with peak powers and repetition rates ranging from 0.1TW at 100kHz up to PW at 10Hz.

We experimentally demonstrate the generation of radially polarized few-cycle pulses by post-compression of 25fs pulses in multiple thin plates. Around 100 µJ energy sub-7fs pulses with radial polarization were measured. This method can be scaled up to the millijoule energy level.

International audience; Laser–plasma acceleration at kilohertz repetition rates has recently been shown to work in two different regimes with pulse lengths of either 30 fs or 3.5 fs. We now report on a systematic study in which a large range of pulse durations and plasma densities were investigated through continuous tuning of the laser spectral bandwidth. Indeed, two laser–plasma accelerator (LPA) processes can be distinguished, where beams of the highest quality, with a charge of 5.4 pC and a spectrum peaked at 2–2.5 MeV, are obtained with short pulses propagating at moderate plasma densities. Through particle-in-cell (PIC) simulations, the two different acceleration processes are thoroughly explained. Finally, we proceed to show the results of a 5-h continuous and stable run of our LPA accelerator accumulating more than 18 × 10 6 consecutive shots, with a charge of 2.6 pC and a peaked 2.5 MeV spectrum. A parametric study of the influence of the laser driver energy through PIC simulations underlines that this unprecedented stability was obtained thanks to micro-scale density gradient injection. Together, these results represent an important step toward stable laser–plasma accelerated electron beams at kilohertz repetition rates.

Post-compression of radially polarized 30 fs pulses was performed in multiple thin fused silica plates for the first time. Sub-7 fs pulses with 90 µJ were obtained after re-compression. This approach is scalable in energy.

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.

When light sources approach the single cycle limit, both their dispersion compensation as well as pulse characterization become increasingly challenging. As all pulse characterization methods rely on some optical non-linearity, concomitant phase matching issues or dispersive broadening inside the nonlinear medium may severely corrupt the measurement result. One of the best suited methods for characterizing sub-2-cycle pulses is the dispersion scan (d-scan) technique that traditionally relies on second harmonic generation (SHG) as the nonlinear interaction [1]. However, using frequency conversion as a nonlinearity phase matching is the dominant limitation obscuring smaller propagation effects. By using cross-polarized wave (XPW) generation [2] in the d-scan arrangement [3], where no frequency conversion takes place, the phase matching issues are eliminated, uncovering the dispersion and self-phase modulation (SPM), inherently present in the propagation.

The Attosecond Light Pulse Source (ALPS) facility of the pan-European Extreme Light Infrastructure (ELI) project was designed as a laser-based research infrastructure in which light pulses of few optical cycles in the infrared or mid-infrared spectral range are used for basic and applied research. In particular, these pulses will be used as the driving source for generating even shorter extreme ultraviolet (XUV) pulses with durations as short as a few tens of attoseconds. All the six major laser systems available at ELI-ALPS were designed for stable and reliable operation, while featuring unique pulse parameters, such as unprecedented photon flux and extreme bandwidths. Each laser will run synchronized to the central facility clock, while femtosecond synchronization on target will be ensured by a dedicated timing system. Experimental beam time will be provided with uninterrupted operation of the primary driving lasers and associated secondary sources for at least eight hours per day. The primary focus of ELI-ALPS is the generation of the best quality attosecond XUV pulses in terms of pulse energy, repetition rate and photon energy. This goal is only achievable using the highest quality primary sources and expertly designed, innovative high-harmonic beamlines. The generation of high flux attosecond pulse trains and isolated attosecond pulses is targeted using Gas-based or Surface Plasma-based High Harmonic Generation. These secondary sources will feature dedicated target end stations (e.g. Reaction Microscope, Condensed matter end station, Velocity Map Imaging Spectrometer and Magnetic Bottle Electron Spectrometer) enabling users to perform state-of-the-art experiments. Experimental activities in the building complex started in 2018 with the installation of two 100 kHz repetition rate laser systems: the mid-infrared laser (MIR) and the first High Repetition Rate laser (HR1). They successfully served almost ten commissioning user experiments with external collaborators, for the investigation of phenomena such as electron migration in water, electron rescattering induced K-shell fluorescence, photoionization of droplets, photon statistics in harmonic generation in band gap materials etc., altogether for 51 operational weeks in 2018. In 2019 we expect to extend commissioning experiments to the SYLOS laser as well as to, at least, two attosecond and THz beamlines. The first attosecond beamline, driven by HR1 and dedicated to the investigation of ultrafast pheonemena in gas targets, is to be inaugurated mid 2019. In addition, the operation of the THz laboratory, as well as nanoplasmonic experiments are planned for 2019.

Most of the laser sources of ELI-ALPS are operating in the 100 W average power regime, while the peak power and the repetition rate range from 0.1 TW at 100 kHz up to PW at 10 Hz. The pulse duration lasts typically a few optical cycle, feeding six attosecond beamlines, two particle and two THz beamlines, as well as many end stations associated with the beamlines [1]. The first 100 kHz repetition rate lasers have been installed by end 2017, and became available for experiments from the beginning of 2018. The Mid-InfraRed laser (MIR) provides 150 µJ, 40 fs pulses at 3.1µm [2]. The first High-Repetition rate laser (HR1) has been operating in long pulse mode (1.5 mJ, 40 fs pulses at 1030 nm). The short pulse operation mode (1mJ, 5 TW peak power pulses at sub-7 fs pulse duration. The system is available to run four beamlines from April 2019. In the meanwhile, parallel to its operation, the final upgrade stage will be developed, reaching an energy of >100 mJ by mid 2020. In order to serve the four beamlines efficiently, a so called SYLOS experimental alignment laser has been implemented. The laser “mimicking” the SYLOS laser at lower repetition rate, providing the similar beam size, peak intensity, and duration (3.8TW, 12 fs) at 10Hz repetition rate is available from end 2018. The High Field Petawatt (HF PW) laser of ALPS has been designed to operate at 10 Hz repetition rate with a pulse duration shorter than 17 fs. The system is under installation, 10 Hz operation at 0.4PW level will be reached by mid 2019, while the full 10Hz operation will be demonstrated by Fall 2020. We also carry on experiments towards the elimination of bottlenecks of high repetition rate sub-10fs operation of the 100Hz branch of the HF laser. The MIR and HR1 lasers have successfully served almost ten commissioning user experiments with external collaborators, investigating phenomena like electron migration in water, K-shell excited elections, etc., altogether for 51 operational weeks in 2018. Next year we expect to extend the commissioning experiments for the SYLOS laser as well as at least two attosecond and THz beamlines. The challenge of the laser R&D to design and implement such lasers was taken by the industry (EKSPLA, Light Conversion (both Lithuania), Fastlite, Amplitude Technologies (both France), AFS GmbH (Germany)), academia, and ELI-ALPS, resulting in a change of paradigm in laser engineering, leading to novel state of the art research grade lasers just in a few years. [1] S. Kuhn et al., Mol. Opt. Phys. 50 (2017) 132002 [2] R.Budrinas et al., Opt.Exp. 25 (2017) 5797 [3] N.Thiere et al., Opt.Exp. 26 (2018) 26907

EOS Annual Meeting (EOSAM 2021): Topical Meeting (TOM) 13- Ultrafast Optical Technologies and Applications, 13-17 September 201, Rome (Italy). Virtual meeting; International audience; Generating high-energy few-cycle pulses is key in the study of light-matter interaction in the regime of high field physics. Attosecond science possess the necessary time resolution to study the underlying fundamental processes but requires repetitions rates on the order the kilohertz and stabilization of the Carrier-Envelope Phase. We present here a post-compression stage delivering 3.8fs pulses with 2.5mJ coupled to a Ti: Sa based 1 kHz TW-class laser which can deliver 17.8fs pulses with 350mrad shot to shot CEP noise. This is the first step towards high-energy few-cycle post-compression of the FAB laser at ATTOLAB-Orme.

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.

We present the first carrier-envelope-phase stable chirped-pulse amplifier (CPA) featuring high temporal contrast for relativistic intensity laser-plasma interactions at 1 kHz repetition rate. The laser is based on a double-CPA architecture including cross-polarized wave (XPW) filtering technique and a high-energy grism-based compressor. The 8 mJ, 22 fs pulses feature 10⁻¹¹ temporal contrast at -20 ps and a carrier-envelope-phase drift of 240 mrad root mean square.

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.

We have used kHz single cycle laser pulses in order to resonantly drive a plasma wakefield, resulting in the acceleration of relativistic electron beams with 5 MeV energies and >20 pC/shot charges. Simulations indicate that the electron bunch duration can be as short as 1 femtosecond, making this source unique for probing structural dynamics on ultrafast time scales.

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.

We have used kHz single cycle laser pulses in order to resonantly drive a plasma wakefield, resulting in the acceleration of relativistic electron beam with 5 MeV energies and >20 pC/shot charges. Simulations indicate that the electron bunch duration can be as short as 1 femtosecond, making this source unique for probing structural dynamics on ultrafast time scales.

We report the generation of high-fidelity few-cycle pulses by nonlinear ellipse rotation in a hollow-fiber compressor. The use of flexible hollow-fiber technology and a pressure gradient enables simultaneous temporal filtering and spectral broadening with record-high efficiency.

We generate relativistic-intensity near-single-cycle laser waveforms that are used to drive the generation of intense isolated attosecond pulses from surface plasmas.

We characterized 4fs pulses of a high-energy Ti:Sa system under vacuum condition by d-scan arrangements using second harmonic generation and cross-polarized wave generation as nonlinearities. Both methods deliver similar temporal shapes and consistent pulse durations.

Laser-plasma accelerators are usually driven by 100-TW class laser systems with rather low repetition rates. However, recent years have seen the emergence of laser-plasma accelerators operating with kHz lasers and energies lower than 10 mJ. The high repetition-rate is particularly interesting for applications requiring high stability and high signal-to-noise ratio but lower energy electrons. For example, our group recently demonstrated that kHz laser-driven electron beams could be used to capture ultrafast structural dynamics in Silicon nano-membranes via electron diffraction with picosecond resolution. In these first experiments, electrons were injected in the density gradients located at the plasma exit, resulting in rather low energies in the 100 keV range. The electrons being nonrelativistic, the bunch duration quickly becomes picosecond long. Relativistic energies are required to mitigate space charge effects and maintain femtosecond bunches. In this paper, we will show very recent results where electrons are accelerated in laser-driven wakefields to relativistic energies, reaching up to 5 MeV at kHz repetition rate. The electron energy was increased by nearly two orders of magnitude by using single-cycle laser pulses of 3.5 fs, with only 2.5 mJ of energy. Using such short pulses of light allowed us to resonantly excite high amplitude and nonlinear plasma waves at high plasma density, ne=1.5-2×1020 cm-3, in a regime close to the blow-out regime. Electrons had a peaked distribution around 5 MeV, with a relative energy spread of ~30 %. Charges in the 100’s fC/shot and up to pC/shot where measured depending on plasma density. The electron beam was fairly collimated, ~20 mrad divergence at Full Width Half Maximum. The results show remarkable stability of the beam parameters in terms of beam pointing and electron distribution. 3D PIC simulations reproduce the results very well and indicate that electrons are injected by the ionization of Nitrogen atoms, N5+ to N6+, leading to the formation of an electron bunch of 1 fs duration. The interaction of single-cycle pulses with the plasma also leads to new physical effects. We have observed experimental evidence that plasma dispersion cannot be neglected in this regime. This is due to the extremely broad bandwidth of the laser, extending from 400 nm to 1000 nm, and to the high electron density. Therefore, the acceleration process is optimal when small positive chirps are introduced: the negative dispersion of the plasma then causes the re-compression of the laser pulse inside the plasma. Simulations indicate that this help localizing the injection process, leading to single femtosecond electron bunch. Such a kHz femtosecond electron source will pave to way to numerous innovative applications, such as sub-10 fs electron diffraction, radiolysis of water with unprecedented resolution or the generation of femtosecond X-ray at kHz.

ELI-ALPS in Hungary, one of the three pillars of the Extreme Light Infrastructure, aims at providing diverse light sources, including energetic attosecond pulses at the highest possible repetition rates. One of the main laser systems for driving plasma and gas-based HHG stages, is a state-of-the-art 1 kHz few-cycle laser called SYLOS. Targeted pulse parameters are an energy of 100 mJ and a duration shorter than two optical cycles (

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; Real-time observation of ultrafast dynamics in the microcosm is a fundamental approach for understanding the internal evolution of physical, chemical and biological systems. Tools for tracing such dynamics are flashes of light with duration comparable to or shorter than the characteristic evolution times of the system under investigation. While femtosecond (fs) pulses are successfully used to investigate vibrational dynamics in molecular systems, real time observation of electron motion in all states of matter requires temporal resolution in the attosecond (1 attosecond (asec) = 10(-18) s) time scale. During the last decades, continuous efforts in ultra-short pulse engineering led to the development of table-top sources which can produce asec pulses. These pulses have been synthesized by using broadband coherent radiation in the extreme ultraviolet (XUV) spectral region generated by the interaction of matter with intense fs pulses. Here, we will review asec pulses generated by the interaction of gas phase media and solid surfaces with intense fs IR laser fields. After a brief overview of the fundamental process underlying the XUV emission form these media, we will review the current technology, specifications and the ongoing developments of such asec sources.