Thematic:
Quantum electrodynamics, or QED, is the physical theory that unifies electromagnetism and quantum physics to describe how light and matter interact. It is considered how one of the best demonstrated theories in its linear regime through the measurement of the fine structure constant with unprecedented accuracy. However, it is in the so-called ‘strong fields’ regime, when the light intensity becomes extreme, that QED reveals its most amazing predictions such as the generation of matter from vacuum in the form of electron/positron pairs. These extreme regimes have been studied theoretically for more than a century but have never been reached experimentally.
The recent development of a new generation of lasers which deliver femtosecond pulses with a power of several PetaWatt, makes it possible to develop experimental strategies for achieving the QED strong field regime. The most commonly considered strategy to reach it is to achieve the collision of a relativistic electron beam with an ultra-intense laser field [1].
The strategy that will be studied during this PhD is the so-called ‘Doppler boost’ [2]. The ultra-intense laser pulse focused at a few 1022W/cm2 on a solid is converted to extreme ultraviolet (XUV) by relativistic Doppler effect during reflection onto a plasma mirror. The XUV beam, of much shorter wavelength, can be focused on a focal spot much smaller than the laser which has the effect of multiplying by 3 orders of magnitude the light intensity. The interaction of any type of matter (gas, solid, relativistic electrons) with this XUV pulse is then in the highly non-linear QED regime of strong fields [3].

Objectives :
The objective of the thesis is to generate ‘Doppler boosted’ pulses to obtain the highest light intensity ever reached on Earth, of the order of 1025W/cm2 and to use them to carry out the very first QED experiments in the regime of strong fields. Two types of experiments will be carried out: – The collision of the boosted Doppler beam with relativistic electrons. This experiment will be carried out at the Applied Optics Laboratory. – The interaction of the boosted Doppler beam with a solid. These experiments will be carried out on the largest laser infrastructures in the world: APOLLON, BELLA, ELI. A first experiment on the BELLA laser installation located in Berkeley, California, is being prepared for December 2024.
The objective of the internship is the theoretical and experimental understanding of the generation of Doppler boosted pulses and strong field QED, as well as the preparation of the different experiments with the design of several experimental diagnostics such as a XUV spectrometer, a Gamma beam spectrometer, and an electron/positron spectrometer, and the preparation of the experimental chamber located at the Laboratory of Applied Optics.
An experience on the Petawatt APOLLON laser installation will also be part of the internship in May 2024 to gain experience on this type of large laser infrastructure.
Profile of the candidate:

The aim of the proposed thesis is to perform very high impact experiments in the field of physics with the first experimental observation of QED phenomenon in strong field.

To achieve such objectives, the candidate must be passionate about research and have a high capacity and autonomy of work. Pragmatism is also an essential quality for an experimental thesis. You must obviously be prepared to travel abroad for several weeks. Finally, sense of humor for long evenings in the experimental rooms is always welcomed by all colleagues.

Diploma: Master 2 research in physics
Important to know: Python, Matlab

Pursuing into PhD : Yes
Expected contract-funding ? Yes

Contact : Adrien Leblanc

Biobliography
[1] Lobet, M, et al. “Generation of high-energy electron-positron pairs in the collision of a laser-accelerated electron beam with a multipetawatt laser.” Physical Review Accelerators and Beams 20.4 (2017): 043401.
[2] Vincenti, H. “Achieving extreme light intensities using optically curved relativistic plasma mirrors.” Physical review letters 123.10 (2019): 105001.
[3] Fedeli, L, et al. “Probing strong-field QED with Doppler-boosted petawatt-class lasers.” Physical review letters 127.11 (2021): 114801.

Abstract :

Relativistic electron beams have many potential societal applications, particularly in medicine. Today two types of accelerators are used to generate them: either linear accelerators, very bulky and expensive, or laser acceleration in a plasma, more compact and less expensive but not yet directly applicable for the general public. This has been a very active area of research for two decades with several challenges to overcome such as (i) increasing the charge (number of electrons), and (ii) improving the mono-energetic aspect of the beams.

Using a very high power femtosecond laser (a few 100TW) focused at Ultra-High Intensity (UHI), two types of acceleration were demonstrated:

• 1) by reflection of the laser on a solid target: thanks to the high density, a strong charge of several tens of nanocoulombs (nC) is accelerated but quite weakly, a few MeV, because on a small dimension (some 10-100 μm);
• 2) by propagation of the laser in a gas, a plasma bubble is formed in the wake of the pulse. It traps and accelerates electrons over several centimeters to reach very high energies, around 100MeV, but with a more modest charge of around ~1nC (low density gas). Gas cells with shocks were developed to inject electrons into the bubble in a very localized manner in order to make the beam quasi-monoenergetic.

At the Applied Optics Laboratory, we have demonstrated a new type of injection by combining a plasma mirror and a gas jet in order to combine the forces of the two systems, see figure, panel a.

• Step 1, injection: the laser reflects on a solid target in order to remove a large charge from the high density and this in a very localized manner, just on the surface of the target.
• Step 2, acceleration: then, the laser propagates a few millimeters in the gas to accelerate the electron bunch to high energies and monoenergetically. Experimental proof of such has been achieved very recently.

Objectives:

The electron beams generated by this new type of accelerator increase the performance of these laser-plasma accelerators tenfold and make it possible to consider medium-term applications. On the one hand, medical applications using lower energy but high cadence lasers, and on the other hand, the drastic increase in the electronic charge available on the largest laser installations in the world (PetaWatt laser) could allow to demonstrate for the first time experimentally the effects of Quantum ElectroDynamics (which predicts, for example, the creation of matter in a vacuum). These two axes will be studied during the thesis.

Internship/thesis objectives :

90% experimental, 10% theory/simulations. (i) The main objective is to adapt the experimental schemes developed at 100TW towards both 10TW lasers and PW lasers. Numerous experiments on the largest laser infrastructures in the world (APOLLON (Fr), ELI (Romania), BELLA (California)) will take place. (ii) Design the hybrid target with hydrodynamic simulations in order to minimize shocks during the expansion of the gas against the solid target. (iii) Carry out numerical simulations of the interaction in order to understand the different injection parameters and to determine which configuration makes it possible to optimize the acceleration conditions. (iv) Experiments on biological samples are planned on 10TW lasers.

Environment :

The experimental implementation and development parts of the hybrid target are carried out within the Ultra-fast Sources of Particles and X-rays (UPX) team at the LOA. Experiments on different lasers (Bordeaux, Marseille, California, Romania) will be carried out. A dynamic team that is at the world forefront in laser electron acceleration. The theory/numerical part is done in collaboration with CEA Saclay.

Possibility of thesis Yes
Probable contract-financing / Expected contract-funding? IPP Doctoral School

Contact :

Adrien Leblanc / Cedric Thaury

State-of-the-art multi-PW lasers, such as APOLLON (France), and km-scale accelerators such as FACET-II at Stanford (USA) [1], allow to create on Earth the extreme conditions for fundamental interactions between particles and fields. Such interactions follow the laws of strong-field quantum electrodynamics (QED) that has emerged as a promising discovery science area with exciting opportunities. Strong-field QED effects appear when the electric field experienced by the electron in its rest frame approaches the Schwinger field E_S=m²c³/(eℏ), and their most spectacular manifestations include the production of electron-positron pairs through the inverse-Compton scattering and the nonlinear Breit-Wheeler processes. At multi-PW laser facilities such as APOLLON, strong-field QED experiments can be performed by colliding high-energy electrons produced by a novel type of particle accelerator, namely a laser-driven plasma accelerator, with a high-intensity counterpropagating laser pulse or another source of strong fields. Several avenues will be explored in the ANR g4QED project (Gamma photon sources as a path for strong-field QED experiments), specifically including two promising concepts: (i) the use of a plasma mirror to reflect the laser pulse and enable the collision between the high-energy electrons and the laser pulse [2], and (ii) the use of multiple foils to focus the electron beam to very small size, and use the focused electron beam itself as the source of strong fields [3,4].

The internship and the PhD will aim to model, theoretically and numerically, these two concepts and to provide the best strategies to implement them experimentally on APOLLON and FACET-II. The work will involve a rich variety of physics: the interaction between laser pulses and plasmas, between particle beams and plasma, as well as the strong-field QED physics that we aim to unveil experimentally. The development of advanced numerical modeling for this project will also be key to provide accurate physical interpretation for the experiments. During the internship, the student will work more specifically on the first concept with the modeling of the plasma channel formation, the laser focusing and compression in the plasma, the laser-driven plasma acceleration, the generation of the plasma mirror for the laser reflection, and finally the strong-field QED collision. The goal is to understand its potential for strong-field QED experiments, as well as possible limitations or paths for further improvements.

[1] V. Yakimenko et al., Phys. Rev. Accel. Beams 22, 101301 (2019).

[2] K. Ta Phuoc et al., Nature Photonics 6, 308 (2012).

[3] A. Sampath et al., Phys. Rev. Lett. 126, 064801 (2021).

[4] A. Matheron et al., Comm. Phys. 6, 141 (2023).

PhD conditions: The M2 internship can be continued by a PhD with co-supervision by CEA and LOA, which will be funded by CEA through the ANR g4QED (collaborative project between CEA, LOA and LULI). In addition, the PhD student can apply to be Teaching Assistant at Ecole Polytechnique and other graduate schools.

Contact : Igor Andriyash / Sebastien Corde

Twenty-five years after the discovery of the laser filamentation phenomena in air by Pr. Gerard Mourou, who received the Nobel Price in 2018, the field of femtosecond filamentation is still very active [1]. In particular, the most striking and promising effect observed in the plasma channel produced in the filament is the UV lasing effect from the plasma.

Indeed, a cavity-free laser in the sky could lead to revolutionary improvements in optical remote sensing for atmospheric science. N2 molecules, when pumped by an intense femtosecond laser, exhibit an important optical gain in the near UV regime. We reported intense forward emission around 391 or 428 nm with energy up to several µJ during filamentation of femtosecond laser pulses in atmosphere [2]. Many laboratories are studying this physical effect in USA, Canada, Austria, Russia, Japan, China and France [3-5]. Despite numerous works, the physical origin of this lasing is still not understood and subject to controversies. Also, the intensity of the backward lasing emission in air at atmospheric pressure would need to be improved for real scale remote application.

In this project we want to study on one side the fundamental origin of the lasing emission and, on the other side, improve the efficiency of the lasing effect for remote sensing applications. A large part of the work will be experimental measurements performed on the laser facilities of LOA.

[1] A. Braun, G. Korn, X. Liu, D. Du, G. Mourou, “Self-channeling of high-peak-power femtosecond laser pulses in air”, Optics Letters 20, 73,‎ (1995)

[2] G. Point, Y. Liu, Y. Brelet, S. Mitryukovskiy, P. J. Ding, A. Houard, and A. Mysyrowicz, “Lasing of  ambient air with microjoule pulse energy pumped by a multi-terawatt  femtosecond laser,” Opt. Lett. 39,1725 (2014)

[3] Y. Liu, et al., “Unexpected sensitivity of nitrogen ions superradiant emission on pump laser wavelength and duration,” Phys. Rev. Lett. 119, 203205 (2017)

[4] H. Xu, et al., Sub-10-fs population inversion in N2+ in air lasing through multiple state coupling, Nat. Commun. 6, 8347 (2015)

[5] A. Mysyrowicz, R. Danylo, A. Houard, V. Tikhonchuk, X. Zhang, Z. Fan, Q. Liang, S. Zhuang, L. Yuan, Y. Liu, “Lasing without population inversion in N2+,” APL Photonics 4, 110807 (2019)

Contact : Aurelien Houard

Thesis possibility after internship: YES

Funding: NO

Condensed Matter Physics: YES

Soft Matter and Biological Physics: NO

Quantum Physics: YES

Theoretical Physics: YES

A laser beam usually exhibits a plane wavefront (flat phase) et can be circularly polarized (E field spinning around the propagation axis). This polarization state defines the spin angular momentum of the photons. On the contrary, if the polarization is linear but the wavefront spins around the propagation axis (spiral phase), the photons possesses an orbital angular momentum. It can be quantified using a number ℓ called topological charge characterizing the shape of the wavefront (ℓ=0 flat, ℓ=1 simple helix, ℓ=2 double helix like ADN, etc…).

Such beams can be generated with good conversion efficiencies by a ‘vortex’ phase plate applying a azimuthally-varying phase difference, giving the wavefront its helical shape. Visible or infrared vortex beams have many applications, such as rotating molecules or nanostructures, stimulating forbidden dipolar transitions or allowing STED microscopy measurements with resolutions better than the diffraction limit.

A vortex beam can also be used to generate XUV high-order harmonics in gases. It was demonstrated that the vortex is conserved during this highly nonlinear process. However the topological charge of the photons of a given harmonic is equal to that of the driver beam multiplied by the harmonic order. Therefore we typically work with beams at 32nm with a charge ℓ=25. Although it is too high to rotate molecules, new phanomena such as helical dichroism (different absorption of photons with opposed orbital momentum) have been predicted, and the conversion to shorter wavelengths is promising towards STED microscopy with even higher resolutions.

We propose to amplify a topologically charged harmonic pulse at 32nm to drastically increase its fluence. XUV laser amplification occurs, as in the visible or infrared domain, in a population-inverted medium. To reach the high energy difference corresponding to XUV photons, the medium is a highly charged ion. The ions are created by an intense laser pulse focused into a high density gas jet. Electrons in the plasma are heated by the laser field and transfer energy to the ions by collisions, thus allowing a population inversion to take place. Since the high-density plasma is strongly defocusing, we set up a plasma waveguide beforehand to allow proper propagation of the main laser beam. This harmonic amplification technique is well-mastered at LOA and has its own dedicated beamline.

During the internship, the student will set up the vortex beam conversion and subsequent high harmonic generation, the harmonic amplification and caracterization using a XUV wavefront sensor. An experimental campaign will take place at LOA, and a strong collaboration with the LASERIX team from Université Paris Saclay is also expected. LASERIX also operates an installation dedicated to high harmonic generation and amplification

The candidate should have a solid training in general physics and optics. Additional knowledge of plasma physics, nonlinear physics or X/XUV opticswill be appreciated

Contact : Fabien Tissandier

Laser-plasma acceleration is a technique that accelerates particles to relativistic energies over millimeter lengths. Physically, the mechanism is as follows: a femtosecond laser pulse is focused in a plasma and excites a very intense plasma wave there. This plasma wave carries an electric field of some 100GV / m which can be used to accelerate the electrons in the plasma. This method makes it possible on the one hand to drastically reduce the size of the particle accelerators: for example, electrons of 1 GeV (the energy of the SOLEIL synchrotron beam) are obtained over a distance of only 1 cm. On the other hand, it is possible to obtain particle beams whose properties are proving to be extremely interesting for a good number of applications of the medical type, or in X-ray imaging.

One of the key points of current research is to be able to realize a laser-plasma accelerator operating at high speed, in particular for applications. To do this, the LOA and Thalès have just launched a joint laboratory whose goal is to develop laser and plasma technologies in order to build a laser-plasma accelerator delivering relativistic electron beams at 100 Hz. This accelerator will then be used for generating an X-ray source by Compton scattering.

The proposed internship fits into this context. The student will participate in this project and will work on two aspects:

– Participation in the development of 100 Hz laser technology in the Thalès laser laboratories in Elancourt. This includes: participation in the development and characterization of new laser amplifier technology operating at 100 Hz

– Modeling work on laser-plasma acceleration with the parameters of the laser built by Thales. The study will be carried out using Particle In Cell simulations and will model the acceleration of electrons as well as the X-rays obtained by Compton scattering.

The candidate will have a solid general training in physics. Knowledge in the following fields will be appreciated: plasma physics, nonlinear physics, optics and laser. The work will mix the experimental as well as numerical modeling.

We strongly hope that the intern will continue his internship with a CIFRE thesis which will be supported by Thalès.

Nota Bene :

The LOA team is a pioneer in laser-plasma acceleration and in particular in the use of high-speed laser in this field of research, see https://loa.ensta-paris.fr/fr/recherche/groupe- app-researcher /

The Thales team is a world leader in the design and manufacture of ultra-intense short-pulse lasers.

Internship manager: Jérôme Faure

Quand une impulsion laser intense de durée femtoseconde se propage dans l’air, elle donne lieu à la filamentation, un processus spectaculaire où le faisceau se contracte spatialement pour former un mince canal de lumière dans lequel l’intensité est maintenue à ~1015 W /cm2. La filamentation s’accompagne de la formation d’une longue colonne de plasma de courte durée de vie générée dans le sillage de l’impulsion laser. Cette colonne présente notamment la capacité d’initier et de guider des arcs électriques de plusieurs mètres avec une grande reproductibilité (voir photo ci-dessous [1]).

Ces dernières années plusieurs applications basées sur les filaments de plasma ont été proposées telles que le paratonnerre laser [2] et l’antenne virtuelle radiofréquence [3].

Pour rendre possibles ces applications, il est nécessaire de caractériser et d’optimiser les paramètres du plasma produit par le laser femtoseconde ainsi que l’arc électrique guidé. Pour ce faire, différentes techniques de spectroscopie résolue en temps, d’interférométrie et d’imagerie [4] seront mises en œuvre dans le cadre de ce stage. Elles seront testées sur des expériences de guidage de décharges électriques en laboratoire dans le cadre du développement d’une antenne plasma et d’une application de paratonnerre laser.

Le candidat devra avoir des connaissances de base en optique ou en physique des plasmas, un bon niveau d’anglais et présenter de solides références scolaires.

Ce stage sera rémunéré et pourra donner lieu à une prolongation en thèse.

[1] B. Forestier, et al., “Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament”, AIP Advances 2, 012151-13 (2012)

[2] J. Kasparian et al. Science 301, 61 (2003)

[3] Y. Brelet, et al., Radiofrequency plasma antenna generated by femtosecond laser filaments in air”, Applied Physics Letters 101, 264106 (2012)

[4] Improving supersonic flights with femtosecond laser filamentation, P.-Q. Elias, et al., Science Advances 4, eaau5239 (2018)

Ce stage pourra-t-il se prolonger en thèse ? Possibility of a PhD ? : OUI

Rémunération du stage/ financial support for the internship : OUI

Financement de thèse envisagé / financial support for the PhD : Ecole doctorale IP Paris

Type de stage et/ou de thèse (expérience/théorie/simulations) : Expérience

Contact : Houard Aurelien