Quantum Electrodynamics predicts that the vacuum must behave as a nonlinear optical medium: the vacuum optical index should increase when it is stressed by intense electromagnetic fields. The DeLLight (Deflection of Light by Light) project aims to measure it by using intense and ultra-short laser pulses. The experiment uses a Sagnac interferometer to amplify the tiny deflection signal of a low-intensity probe pulse crossing the vacuum refractive-index gradient produced by an external high-intensity pump pulse. The measurement of the amplified signal by a CCD camera requires a high spatial resolution, which is limited by the ultimate quantum noise of the CCD. However, interferometric phase noise induced by the mechanical vibrations of the interferometer is also amplified and degrades spatial resolution. To overcome this, we propose a new method named High-Frequency Phase Noise Suppression (HFPNS), based on the addition of a delayed replica (5 ns) of the probe pulse. The delayed pulse, which is not affected by the pump but is subject to the same vibration noise, enables offline subtraction of correlated phase noise. In this work, we present an experimental proof-of-concept on a prototype interferometer operating with a limited amplification factor (𝒜≃25), about 10 times smaller than the required value of the final experiment. We have succeeded in reducing phase noise by a factor of 40, resulting in a residual noise level 2.3 times higher than the expected quantum noise. The residual noise is linked to delay-line instabilities and incident beam pointing fluctuations present during these tests. This result validates HFPNS as a robust method for future quantum-noise-limited interferometric measurements of vacuum optical nonlinearity, though additional stabilization and higher interferometric amplification are still needed.

The present article reports on the generation of stable 50 pC low-divergence electron beams above 150 MeV from laser-driven wakefield acceleration using a continuous-flow gas target prototype tested at the 60 TW Salle Jaune facility at LOA. The gas target design is meant to be easily transported and integrated as an element of the beamline with a differential pumping system offering some 10−4 mbar pressure in the rest of the line. A dedicated gas injection system allows for the control of the gas mixture concentration and gas pressure in two different regions of the target within the frame of controlled ionisation injection schemes. The measured electron beam parameters show the importance of gas density profiles and longitudinal gas mixture confinement.

In this work, we exploit HHG in a noble gas to merge the azimuthally twisted wavefront of a vortex beam and the spatially varying polarization of a vector beam, yielding EUV vector-vortex beams (VVB) that are tailored simultaneously in their SAM and OAM. Employing a high-resolution EUV Hartmann wavefront sensor (EUV HASO, Imagine Optic), we perform the complete spatial intensity and wavefront characterization of the vertical polarization component of the 25th harmonic beam centered at a wavelength of 32.6 nm. By driving the HHG using IR VVB, we show that HHG enables the production of EUV VVB exhibiting radial, azimuthal, or even intermediate polarization distribution. Furthermore, the wavefront characterization allows for the unambiguous confirmation of the topological charge and OAM helicity of the upconverted harmonic VVB. Notably, our work reveals that HHG provides a means for the synchronous and controlled manipulation of SAM and OAM. The production of ultrafast EUV VVB with high OAM and adjustable polarization distributions opens up promising prospects for their applications at nanometric spatial and sub-femtosecond temporal resolutions using a table-top harmonic source.

In Quantum Electrodynamics, vacuum becomes a nonlinear optical medium: its optical index should be modified in the presence of intense external electromagnetic fields. The DeLLight project (Deflection of Light by Light) aims to observe this effect using intense focused femtosecond laser pulses delivered by LASERIX. The principle is to measure with a Sagnac interferometer the deflection of a low-intensity focused pulse (probe) crossing the vacuum index gradient induced by a high-intensity pulse (pump). A Sagnac interferometer working with femtosecond laser pulses has been developed for the DeLLight project. Compared to previous prototypes, the interferometer now includes the focusing of the probe beam in the interaction area. In this article, we measure and characterize the critical experimental parameters limiting the sensitivity of the interferometer, namely the extinction factor, the spatial resolution, and the waist at focus of the probe pulse. We discuss future improvements.

The aim of the DeLLight (Deflection of Light by Light) experiment is to observe for the first time the optical nonlinearity in vacuum, as predicted by Quantum Electrodynamics, by measuring the refraction of a low-intensity focused laser pulse (probe) when crossing the effective vacuum index gradient induced by a high-intensity focused laser pulse (pump). The deflection signal is amplified by using a Sagnac interferometer. Here, we report the first measurement performed with the DeLLight pilot interferometer, of the deflection of light by light in air, with a low-intensity pump. We show that the deflection signal measured by the interferometer is amplified, and is in agreement with the expected signal induced by the optical Kerr effect in air. Moreover, we verify that the signal varies as expected as a function of the pump intensity, the temporal delay between the pump and the probe, and their relative polarisation. These results represent a proof of concept of the DeLLight experimental method based on interferometric amplification.

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

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

Vigorous efforts to harness the topological properties of light have enabled a multitude of novel applications. Translating the applications of structured light to higher spatial and temporal resolutions mandates their controlled generation, manipulation, and thorough characterization in the short-wavelength regime. Here, we resort to high-order harmonic generation (HHG) in a noble gas to upconvert near-infrared (IR) vector, vortex, and vector-vortex driving beams that are tailored, respectively, in their spin angular momentum (SAM), orbital angular momentum (OAM), and simultaneously in their SAM and OAM. We show that HHG enables the controlled generation of extreme-ultraviolet (EUV) vector beams exhibiting various spatially dependent polarization distributions, or EUV vortex beams with a highly twisted phase. Moreover, we demonstrate the generation of EUV vector-vortex beams (VVB) bearing combined characteristics of vector and vortex beams. We rely on EUV wavefront sensing to unambiguously affirm the topological charge scaling of the HHG beams with the harmonic order. Interestingly, our work shows that HHG allows for a synchronous controlled manipulation of SAM and OAM. These EUV structured beams bring in the promising scenario of their applications at nanometric spatial and sub-femtosecond temporal resolutions using a table-top harmonic source.

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

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.

The thin-film post-compression technique has the ability to reduce the pulse duration in PW-class lasers, increasing the peak power. Here, the nonlinear response of an increasingly available optical thermoplastic demonstrates enhanced spectral broadening, with corresponding shorter pulse duration compared to fused silica glass. The thermoplastic can be used close to its damage threshold when refreshed using a roller mechanism, and the total amount of material can be varied by folding the film. As a proof-of-principle demonstration scalable to 10-PW, a roller mechanism capable of up to 6 passes through a sub-millimeter thermoplastic film is used in vacuum to produce two-fold post-compression of the pulse. The compact design makes it an ideal method to further boost ultrahigh laser pulse intensities with benefits to many areas, including driving high energy acceleration.

Recent developments of high harmonic generation (HHG) have enabled the production of structured extreme-ultraviolet (EUV) ultrafast laser beams with orbital angular momentum (OAM). Precise manipulation and characterization of their spatial structure are paramount for their application in state-of-the-art ultrafast studies. In this work, we report the generation and characterization of EUV vortex beams bearing a topological charge as high as 100. Thanks to OAM conservation, HHG in noble gases offers a unique opportunity to generate ultrafast harmonic beams with a high topological charge from low charge infrared vortex beams. A high-resolution Hartmann wavefront sensor allows us to perform a complete spatial characterization of the amplitude and phase of the 25th harmonic beam (32.6 nm), revealing very high-topological charges in the EUV spectral regime. Our experimental results, supported h numerical HHG simulations, demonstrate the linear upscaling of the OAM of the high-orth harmonics with that of low-charge driving vortex beams, showing the sensitiveness of the OAM content to the purity of the driving beam. The generation of structured EUV beams carrying large topological charges brings in the promising scenario of OAM transfer from light to matter at both macroscopic and microscopic scales.

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

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

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

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

We investigate the coherence of plasma-based soft X-ray laser (XRL) for different conditions that can alter the electron density in the gain region. We first measure the source temporal coherence in amplified spontaneous emission (ASE) mode. We develop a data analysis procedure to extract both its spectral width and pulse duration. These findings are in agreement with the spectral line shape simulations and seeded operation experimental results. Utilizing the deduced spectral width and pulse duration in a one-dimensional Bloch-Maxwell code, we reproduce the experimental temporal coherence properties of the seeded-XRL. Finally, we demonstrate efficient lasing in ASE and seeded mode at an electron density two times higher than the routine conditions. In this regime, using Bloch-Maxwell modeling, we predict the pulse duration of the seeded XRL to be ∼500fs.

The THz regime is widely appealing across many disciplines including solid-state physics, life sciences, and increasingly in particle acceleration. Multicycle THz pulses are typically formed via optical rectification in periodically poled crystals. However the manufacturing procedures of these crystals limit their apertures to below ~1 cm, which from damage limitations of the crystal, limits the total pump power which can be employed, and ultimately, the total THz power which can be produced. Here we report on the simple in-house fabrication of a periodically poled crystal using ~300 μm thick wafers. Each wafer is consecutively rotated by 180 ∘ to support quasi-phase matching. We validate the concept with a Joule-class laser system operating at 10 Hz and measure up to 1.3 mJ of energy at 160 GHz, corresponding to an average peak power of approximately 35 MW and a conversion efficiency of 0.14%. In addition, a redshifting of the pump spectrum of ~50 nm is measured. Our results indicate that high-power THz radiation can be produced with existing and future high-power lasers in a scalable way, setting a course toward multi-gigawatt multicycle THz pulses.

Objective of ESCULAP project is the experimental study of Laser-Plasma Acceleration (LPA) of relativistic electron bunch from photo-injector in 9 cm length plasma cell [1]. In parallel, numerical tools have been developed in order to optimize the setup configuration and the analysis of the expected results. The most important issue when dealing with numerical simulation over such large interaction distances is to obtain a good accuracy at a limited computing cost in order to be able to perform parametric studies. Reduction of the computational cost can be obtained either by using state-of-the-art numerical technics and/or by introducing adapted approximation in the physical model. Concerning LPA, the relevant Maxwell-Vlasov equations can be numerically solved by Particle-In-Cell (PIC) methods without any additional approximation, but can be very computationally expensive. On the other hand, the quasi-static approximation [2], which yields a drastic reduction of the computational cost, appears to be well adapted to the LPA regime. In this paper we present a detailed comparison of the performance, in terms of CPU, of LPA calculations and of the accuracies of their results obtained either with a highly optimized PIC code (FBPIC [3]) or with the well known quasi-static code WAKE [3]. We first show that, when considering a sufficiently low charge bunch for which the beam loading effect can be neglected, the quasi-static approximation is fully validated in the LPA regime. The case of a higher bunch charge, with significant beam loading effects, has also been investigated using an enhanced version of WAKE, named WAKE-EP. Additionally, a cost evaluation, in terms of used energy per calculation, has been done using the multi-CPU and multi-GPU versions of FBPIC.

High-power multicycle THz radiation is highly sought after with applications in medicine, imaging, spectroscopy, characterization and manipulation of condensed matter, and could support the development of next-generation compact laser-based accelerators with applications in electron microscopy, ultrafast X-ray sources and sub-femtosecond longitudinal diagnostics. Multicycle THz-radiation can be generated by shooting an appropriate laser through a periodically poled nonlinear crystal, e.g. lithium niobate (PPLN). Unfortunately, the manufacturing processes of PPLNs require substantially strong electric fields $\mathcal{O}(10~kV/mm)$ across the crystal width to locally reverse the polarization domains; this limits the crystal apertures to below 1 cm. Damage threshold limitations of lithium niobate thereby limits the laser power which can be shone onto the crystal, which inherently limits the production of high-power THz pulses. Here we show that in the THz regime, a PPLN crystal can be mechanically constructed in-air by stacking lithium niobate wafers together with 180$^{\circ}$ rotations to each other. The relatively long (mm) wavelengths of the generated THz radiation compared to the small gaps ($\sim$10 $\mu$m) between wafers supports a near-ideal THz transmission between wafers. We demonstrate the concept using a Joule-class laser system with $\sim$50 mm diameter wafers and measure up to 1.3 mJ of THz radiation corresponding to a peak power of $\sim$35 MW, a 50 times increase in THz power compared to previous demonstrations. Our results indicate that high-power THz radiation can be produced with existing and future high-power lasers in a scalable way, setting a course toward multi-gigawatt THz pulses. Moreover the simplicity of the scheme provides a simple way to synthesize waveforms for a variety of applications.