PhD subjects

Laser technology now makes it possible to generate coherent light pulses with durations down to a few tens of femtosecondes only, with an energy per pulse of up to several Joules. These laser beams are likely to exhibit spatio-temporal coupling, i.e. a spatial dependence of their temporal properties across the beam, which can considerably degrade their performances. Our team has developed over the last few years different techniques to measure the full spatio-temporal structure of such lasers. These advanced measurement techniques have been demonstrated on different lasers, including some for the most powerful systems in operation to date. The next challenges in this field of optical metrology are, on the one hand, to develop single-shot measurement techniques (that is to say requiring only one laser shot, against several hundred currently), and to develop methods to control the spatio-temporal structure of ultrashort laser beams. The objective of this thesis will be to provide solutions to these two problems, using complex scattering media, which have been studied for several years by many research groups and whose properties are now better understood. Because they introduce deterministic correlations between spatial and spectral properties of light, these media are likely to be used in different configurations to measure as well as to control the spatio-temporal properties of ultrashort laser pulses.

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 Quantum Electrodynamics (QED) that are currently out of reach of conventional particle accelerators. In particular, above 10^29W/cm^2 also known as the Schwinger limit, vacuum breaks down and e-/e+ pairs can be produced out of vacuum. Such physical processes are only produced in the most extreme astrophysical events. 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 pradigm 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 non-perturbative 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 termes of computing time, the candidate will first have to develop and benchmark a new Adaptative Mesh Refinement (AMR) methode 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 paires 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.

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. Several applications may be foreseen: interpretation of recent opacity measurements performed on LULI2000 laser at Ecole Polytechnique, 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.

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/).

In the context of the Sanitation and Decommissioning of nuclear installations, it is important to locate very quickly alpha, beta and gamma source terms on the surfaces that can be treated, isolated and removed from the site in a regulated way. Gamma imaging is a technique that works very well now. On the other hand, the alpha or beta sources are localizable only in contact with materials, on the surface, because these emissions do not propagate over distances of more than a few cm. Fluorescence dosimetry and chemical scavenging during radiolysis processes have made tremendous progress recently. This allowed for example to highlight the effects of ionization density and Linear Energy Transfer (TEL effect) in radiolysis of water by heavy ions and alpha. Beta and alpha found in nuclear have very different TEL leading to very different yields of free radical production (H, OH, hydrated electron, HO2) and molecules (H2, H2O2), resulting from the ionization of the water. The objective of the proposed thesis is to exploit these differences by using non-toxic chemical sensors producing a fluorescent molecule detectable at long distance (objective, several meters), under laser illumination. Starting from the known chemical mechanisms, the doctoral student will have to give the experimental and applied conditions (on site) allowing the acquisition of images exploitable quickly.

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 atomic and molecular gases: electron ejection, electronic rearrangements in the ion, charge migration…



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 atomic and molecular systems. The objective is then to measure the instant of appearance of the charge particles, electrons and ions. Finally, the measurement of the photoelectron angular distribution, in combination with the temporal information, will allow the reconstruction of the full 3D movie of the electron ejection.

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)

For the past ten years, the generation of attosecond pulses has made it possible to study the fastest dynamics in matter, those associated with electrons, which take place naturally on this timescale. Attosecond spectroscopy thus allows the study of fundamental processes such as photoionization. In particular, studying the coherence of the ionization dynamics would provide access to very fine information on the atomic / molecular structure, such as electronic rearrangements in the ion following the ejection of an electron.



The objective of the thesis is to measure the complete quantum state of an electronic wave packet obtained after the photoemission. Experimentally, the student will study the coherence of the ionization dynamics of atomic and molecular gases using attosecond pulses generated from an intense Titanium:sapphire laser (ATTOLab Excellence Equipment). Theoretically, he / she will implement digital tools allowing the extraction of the quantum state of the photoelectron from experimental measurements.