PhD subjects

This subject is part of the fight against air pollution and global warming. Electron beam-based techniques for the treatment of gaseous effluents (EBFGT) are routinely implemented with energies from 300 keV to 1 MeV, in the absence of existing tools lighter than accelerators to produce them. The aim here is to develop a flexible mini electron source with a more relevant energy, then optimize it for a more energy-efficient conversion of CO2 or N2, based on the skills in miniaturized source and gas radiolysis developed at IRAMIS.

A vast range of topics in physics such as the star internal structure, the X-ray emission of accretion disks, the dynamics of inertial confinement fusion, or new radiation sources requires an accurate knowledge of the radiative properties of hot plasmas. Such plasmas exhibit spectra consisting of a large number of lines often merging in unresolved arrays. Statistical methods are used for the interpretation of such spectra.



Using second quantization and tensor algebra techniques, one may obtain quantities such as the average and variance of transition energies inside these unresolved arrays. Though a wide literature exists on this subject, certain types of transitions, e.g., magnetic dipole transitions inside a given configuration or processes involving several electron jumps, have not been considered up to now. In addition to this analytical study, a numerical work involving the Flexible Atomic Code will be proposed for this thesis. This study will concern plasmas either at thermodynamic equilibrium, or out of equilibrium, where level population is obtained by solving a system of kinetic equations.



This research program requires a deep knowledge in quantum physics and in atomic physics in plasmas. Several applications may be foreseen: interpretation of recent opacity measurements performed on LULI2000 laser at Ecole Polytechnique, extreme-UV source optimization for nanolithography, determination of radiative power losses in a tungsten plasma to characterize tokamak operation, or the open subject of the characterization of photoionized silicon plasma analyzed in Sandia Z-pinch in connection with astrophysical observations.

A major challenge faced at present by the accelerator community is to shrink the size of particle accelerators to enable the next generation of TeV electron-positron colliders. A very promising candidate in this regard are laser Wakefield Accelerators (LWFA) produced when a high-power laser is focused on a gas jet. These accelerators can deliver high accelerating gradients of 100 GV/m and have already demonstrated electron acceleration on up to 10 GeV energies on a cm-scale.



However, important limitations need to be addressed before enabling the use of LWFAs as medical devices or for building compact electron/positron high-energy colliders and X-ray Free Electron Lasers (X-FEL) light sources. In particular a major impediment of these accelerators is that they currently suffer from low charge at high-energy (10pC/bunch above 4GeV) far below the charge they could sustain (up to 50nC) or the ones obtained with conventional RF accelerators (> nC/bunch). In these conditions, building a LWFA-based collider mandating high number of collisions and hence much higher charge would require upgrading multi-TeraWatt or PetaWatt lasers repetition rates from present 1Hz to tens of kHz, to reach much higher average currents, which is beyond present laser technology. Solutions to increase the charge at high-energy with present injection techniques have been proposed (e.g. by using a high gas density pre-injection stage coupled to a second low-density acceleration stage with intermediate beam transport). Yet, scaling up charge to 1-10nC at high-energy (GeV) with these techniques is far from acquired and might degrade crucial beam features (e.g. emittance, energy spread) that would be a severe limitation for many applications necessitating high beam quality (e.g. X-FEL).



In this context, this PhD thesis aims at devising alternative and novel schemes using our kinetic numerical codes PICSAR and WARPX that should enable compact accelerators with up to 1-10nC/bunch up to multi-GeV energy levels while preserving a high beam quality. A very promising one would be to use plasma mirrors as electron injectors. Plasma mirrors are overdense plasmas formed when a high power laser is focused on a solid target. As such, they can provide a very large reservoir of electrons that could be coherently accelerated by the incident laser and injected into a LWFA. Preliminary simulations show that placing a plasma mirror just before a gas jet could allow for a highly localized spatio-temporal injection of sub-femtoseconds electron bunches in a LWFA. This highly localized injection is the pre-requisite to obtain very high quality LWFA electron bunches and seems to surpass by an order of magnitude (in terms of charge and beam quality) all schemes proposed so far in the literature.



Leveraging on the numerical tools developed by the Physics at High Intensity Group in the last five years, the goal of this PhD thesis will be to design numerically a high-quality electron injector for LWFA using plasma mirrors. It will include several important milestones:



(i) A first phase where preliminary simulations will be refined and a detailed proof-of-concept of the injection will be established (A patent will be written),



(ii) A second phase where a model of electron injection from the plasma mirror into the LWFA is developed and optimal regimes are found (in terms of laser-plasma parameters). The optimization step will involve the development of surrogate models using deep neural networks,



(iii) A final phase involving the coupling with experiments where the full experimental set-up will be numerically simulated. This will involve the coupling of 2D/3D hydrodynamic simulations (to efficiently model the gas density profile at the gas-plasma mirror interface) with kinetic simulations (to model injection in the LWFA).



Succeeding in this task would alleviate by several orders of magnitude constraints in terms of required laser repetition rate for building a compact collider. In addition, achieving an ultra-compact accelerator with high-charge and high beam-quality could be used to produce table-top ultra-short electron, X-FEL or Bremsstrahlung/Compton X-ray light sources that are paramount to many applications such as cancer treatment, femtosecond chemistry, radiobiology, radiotherapy or industrial radiography.

Nowadays, the major challenge of high-field physics or Ultra-High Intensity (UHI) physics is to design a very intense light source capable of exploring new regimes of Strong-Field Quantum Electrodynamics (SF-QED) that are currently out of reach of conventional particle accelerators. The most famous example occurs above 10^29W/cm^2 (the so-called Schwinger limit) around which vacuum breaks down and e-/e+ pairs can be produced out of vacuum. Actually, light-matter interactions at extreme intensities (>10^25W/cm^2) are entirely dominated by SF-QED processes. Such physical regimes are only accessible in the extreme astrophysical events and could reveal new physics beyond the standard model (such as the presence of axions or millicharged fermions). Being able to reproduce and control them in the lab represents a huge fundamental interest.



Yet, the most intense light source on earth (presently, high-power PetaWatt -PW- lasers) only deliver intensities around 10^22W/cm^2. Reaching the Schwinger limit therefore requires a paradigm shift that we recently proposed in the Physics at High Intensity (PHI) group at CEA. Our solution consists in using a remarkable optical component, generated by a high-power laser itself when interacting with a solid target and known as an ’optically-curved relativistic plasma mirror’. Upon reflection on such a curved mirror, the reflected laser light is strongly intensified due to a temporal compression by Doppler effect and a spatial compression by focusing to tinier spots than the ones possible with the incident light. The PHI group recently proposed to use the plasma mirror optical deformation by the incident laser radiation pressure to tightly focus the reflected light. Preliminary 3D simulations show that intensities of 10^25W/cm^2 can be reached with this scheme at plasma mirror focus. At such intensities, yet unexplored SF-QED processes would occur during the interaction of the reflected field with matter. This constitutes the first milestone towards the Schwinger limit.



Now, the major challenge to reach the Schwinger limit is to design novel realistic schemes to optically-curve the plasma mirror surface more strongly than with radiation pressure. In this context, the candidate will develop and validate numerically these novel schemes with Particle-In-Cell (PIC) codes. As the simulations envisaged are extremely costly in terms of computing time, the candidate will first have to develop and benchmark a new Adaptative Mesh Refinement (AMR) method developed by the group of Dr. J-L Vay at Lawrence Berkeley National Lab (LBNL), in which the first phase of the PhD will start. During the second phase (at CEA);, the candidate will use the code to validate the new schemes and answer the following questions: what are the optimal laser-plasma conditions to reach the Schwinger limit? At which intensities does the reflected field start to produce e-/e+ pairs? Are these pairs detectable? How to find clear signatures of the achieved intensities in experiments? The candidate will also support the interpretation of the very first QED experiments performed with plasma mirrors during his PhD.

The fundamental TOCYDYS research program aims to probe the dynamics of solids with temporal resolution at the optical cycle scale and to cross the femtosecond resolution limit. We will initially concentrate on insulators such as silica and quartz (SiO2) or sapphire (Al2o3).



The experiments will be carried out on the facilities recently opened at LOA and LIDYL of Equipex Attolab (http://attolab.fr/), where we will have access to phase-stabilized lasers and associated ultra short VUV pulses.



The experiments will consist of exciting the samples with pulses of a few optical cycles (intensity in the range 1012 to 1015 W/cm2) and probing the dynamics by measuring change of reflectivity, in the IR and visible domains, then with attosecond pulse trains in the VUV.



We will have direct access to the physical mechanisms of the material laser interaction and to the initial stages of the electronic relaxation of the solid: multiphoton, tunnel or Zener ionization, modulation of the band gap, inelastic diffusion of the carriers, impact ionization, Auger effect, etc…



During the first part of the program, at the Laboratory of Applied Optics- LOA, the measurements will be made in the visible and near IR domains, with the objective to achieve the resolution of the optical cycle. Then, in the second part, we will construct a set-up for the reflectivity measurement in the VUV domain, capable of recording variations in the amplitude of the probe pulse, but also of the phase using spatial interferometry in the VUV domain.



The TOCYDYS research program received funding from the National Research Agency (ANR) for the period 2020-2023. So the Masters internship is funded. The experimental part will be conducted at LOA in collaboration with Davide Boschetto (https://loa.ensta-paristech.fr/research/appli-research-group/).

Light in the extreme ultraviolet (XUV) is a universal probe of mater, may it be in diluted or condensed phase: photons associated with this spectral range carry energy of 10 to 100 eV, sufficient to directly ionize atoms, molecules or solids. Large scale instruments such as synchrotrons or the lately developed free electron lasers (FEL) work in this spectral range and are used to both study fundamental light matter interaction and develop diagnosis tools. However these instruments do not offer the temporal resolution require to study light matter interactions at their ultimate timescales, which is in the attosecond range (1as = 10-18s). An alternative is offered by the recent development of XUV sources based on high order harmonic generation (HHG). They are based on the extremely nonlinear interaction of a femtosecond intense laser beam with a gas target. Our laboratory has pioneered the development, control and design of these sources providing XUV attosecond pulses.



During this PhD project, we will develop specific setups to allow these attosecond pulse to carry angular momenta, may it be spin or orbital angular momenta. This will open new applications roads through the observations of currently ignored spectroscopic signatures. On the one hand, the fundamental aspects of the coupling of spin and orbital angular momentum of light in the highly nonlinear regime will be investigated, and on the other hand, we will tack attosecond novel spectroscopies, may it be in diluted or condensed phase. In particular, we will chase helical dichroism, which manifest as different absorptions of beams carrying opposite orbital angular moments. These effects are largely ignored to date.

The student will acquire practical knowledge about lasers, in particular femtosecond lasers, and hands on spectrometric techniques of charged particles. He/she will also study strong field physical processes which form the basis for high harmonic generation. He/she will become an expert in attosecond physics. The acquisition of analysis skills, computer controlled experiments skills will be encouraged although not required.



Full and detailed subject at http://iramis.cea.fr/LIDYL/Pisp/thierry.ruchon/.

Summary :

The student will first generate XUV attosecond pulses using an intense Titanium:Sapphire laser (ATTOLab Excellence Equipment), and then use them to investigate the ionization dynamics of molecular gases: electron ejection, electronic rearrangements in the ion, charge migration, quantum decoherence…



Detailed summary :

Recently, the generation of sub-femtosecond pulses, so-called attosecond pulses (1 as=10-18 s), has made impressive progress. These ultrashort pulses open new perspectives for the exploration of matter at unprecedented timescale. Their generation result from the strong nonlinear interaction of short intense infrared (IR) laser pulses (~20 femtoseconds) with atomic or molecular gases. High order harmonics of the fundamental frequency are produced, covering a large spectral bandwidth in the extreme ultraviolet (XUV) range. In the temporal domain, this coherent radiation forms a train of 100 attosecond pulses [1]. In order to generate isolated pulses, it is necessary to confine the generation in an ultrashort temporal window, which implies the development of various optical confinement techniques.



With such attosecond pulses, it becomes possible to investigate the fastest dynamics in matter, i.e., electronic dynamics that occur naturally on this timescale. Attosecond spectroscopy thus allows studying fundamental processes such as photo-ionization, in order to answer questions such as: how long does it take to remove one electron from an atom or a molecule? The measurement of such tiny ionization delays is currently a “hot topic” in the scientific community. In particular, the study of the ionization dynamics close to resonances gives access to detailed information on the atomic/molecular structure, such as the electronic rearrangements in the remaining ion upon electron ejection [2].



The objective of the thesis is first to generate attosecond pulses with duration and central frequency adequate for the excitation of various molecular systems. The objective is then to measure the instant of appearance and the angular distribution of the charged particles, electrons and ions. These spatial and temporal informations will allow the reconstruction of the full 3D movie of the electron ejection, as well as of the hole migration in the ion leading to fragmentation. Finally, quantum decoherence, e.g., induced by ion-photoelectron entanglement, will be studied using a new technique recently demonstrated in our laboratory [3].



The experimental work will include the development and operation of a setup installed on the FAB1 laser of the ATTOLab Excellence Equipment allowing: i) the generation of attosecond XUV radiation, ii) its characterization using quantum interferometry, iii) its use in photo-ionization spectroscopy. The theoretical aspects will also be developed. The student will be trained in ultrafast optics, atomic and molecular physics, quantum chemistry, and will acquire a good mastery of charged particle spectrometry. A background in ultrafast optics, nonlinear optics, atomic and molecular physics is required.

Part of this work will be performed in collaboration with partner French and European laboratories through joint experiments in the different associated laboratories (Milano, Lund).



References :

[1] Y. Mairesse, et al., Science 302, 1540 (2003)

[2] V. Gruson, et al., Science 354, 734 (2016)

[3] C. Bourassin-Bouchet, et al., Phys. Rev. X 10, 031048 (2020)

The development of ultrashort lasers, controlled at a sub cycle scale, has given rise to a new discipline of physics, attosecond physics, dedicated to the study of electron dynamics during the matter interaction with the laser. Long limited to the study of gas phase phenomena, high order laser harmonic generation in semiconducting crystals opens the way to the study of those ultrafast dynamics in condensed matter. The objective of this PhD thesis is to transpose the techniques of spectral and temporal characterization developed in LIDYL for the gas phase to this new phenomenon, in order to image the electronic band structure in exotic materials such as 2D materials (graphene) or strongly correlated materials (NiO for instance), and to measure the attosecond electron currents generated during the interaction. This experimental work will take place on the new NanoLight platform in a brand new laboratory. However, it will benefit from a strong theoretical and numerical support from our collaborators at MPSD in Hamburg.

Today, gigahertz electronics are under control and the terahertz regime is barely accessible. Quantum technologies must now anticipate recent advances in Moore's law evolutions but in the quantum field. Indeed, thanks to the innovative technologies offered by femtosecond lasers, electronic components will progress towards the petahertz range, which involves controlling electronic dynamics at the attosecond scale. The candidate will study in dielectrics and semiconductors the ultra-fast and high mobility properties of electrons when exposed to intense femtosecond laser fields. We will study how the strong current of electrons can be controlled at petahertz frequencies in the conduction band, by the laser field. In addition to these temporal aspects, it has been shown theoretically that these lasers can transfer spin or angular momentum, thus making it possible to shape the quantum state of the system. The thesis will focus on applications in quantum information by topology on 2D semiconductors.