Thèse Structuration Tri-Dimensionnelle de Champs Optiques Ultrabrefs et Nouvelles Spectroscopies Attosecondes Associées H/F Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Physique École doctorale : Ondes et Matière Laboratoire de recherche : CEA/LIDYL - Laboratoire Interactions, Dynamique et Lasers Direction de la thèse : Thierry RUCHON ORCID 0000000179338933 Début de la thèse : 2027-10-01 Date limite de candidature : 2027-04-15T23:59:59 Les impulsions attosecondes (1 as = 10¹ s), produites par génération d'harmoniques d'ordre élevé (GHOE), permettent de suivre le mouvement des électrons et de sonder chaque élément chimique dans l'extrême ultraviolet (XUV, 10-150 eV), des mesures autrefois réservées aux grands instruments comme les synchrotrons. En parallèle, on sait désormais sculpter la lumière en polarisation, en front d'onde et dans ses trois dimensions, mais l'interaction de ces champs exotiques avec la matière reste presque inexplorée. Le groupe Attophysique du LIDYL, pionnier des sources attosecondes et de leur extension aux moments angulaires de spin et orbital, propose de faire de ces champs de nouvelles sondes du magnétisme et de la chiralité. La thèse vise notamment à synthétiser des champs dont la chiralité varie dans l'espace et dans le temps, en exploitant la composante longitudinale du champ électrique. Elle explorera trois régimes : linéaire (vortex de lumière XUV pour lire des vortex magnétiques), fortement non linéaire (champs visibles-IR structurés dans des milieux chiraux) et faiblement non linéaire (pompe IR structurée, sonde XUV, pour suivre le renversement d'aimantation). Principalement expérimentale, sur la plateforme ATTOLab, la thèse vous formera aux lasers femtosecondes, à l'optique XUV, à la spectrométrie de particules chargées et à la physique des champs forts, au sein d'une équipe internationale d'environ 20 personnes.
Sujet complet : http://iramis.cea.fr/LIDYL/Pisp/thierry.ruchon/
The team and environment
The Attophysics group at LIDYL pioneered the generation and characterization of attosecond pulses [MdF+03, KZR+13, BBBG+22] and their use to study ultrafast dynamics [HFH+09, HCB+10, CMH+11, GBJG+16]. More recently, it developed attosecond sources driven by beams carrying spin (SAM) [FHD+15] or orbital (OAM) angular momentum [GCA+16, GRA+17, CBA+19]:
- SAM sources, associated with circular polarization, are suited to studies of chiral molecules and homogeneous magnetic materials;
- OAM sources, with their helical wavefronts, are suited to chiral mesoscopic structures such as magnetic textures.
By combining both, the team opened a new field of nonlinear optics based on generalized angular momentum [LVG+23].
The team has six permanent researchers and trains on average two to three PhD students a year, alongside one to three postdocs: about 20 people in total. It runs the ATTOLab platform, whose state-of-the-art femtosecond lasers feed four beamlines, open since 2018-2019 to a growing community of physicists and physical chemists. National and international collaborators visit year-round through competitive calls (PEPR LUMA, Lasers4EU). ATTOLab pursues two complementary activities: using increasingly reliable and precise attosecond sources in atomic, molecular and condensed-matter physics, and studying the nonlinear mechanisms that will produce tomorrow's sources. This PhD belongs to the second.
Your project: three regimes, three experiments
One common thread, light with tailored angular momentum and three-dimensional structure, explored at three levels of nonlinearity.
1. Linear regime: reading the winding of a magnetic vortex with a vortex of XUV light
A beam carrying orbital angular momentum (OAM) has a helical wavefront. Through theory and experiments at the FERMI free-electron laser (Italy), we have shown that such a beam can identify the sense of winding (the 'toroidal moment') of a vortex-like magnetic structure, an effect called helicoidal dichroism [FBV+21, FPP+22, FPS+25]. We have also shown that reflection from a magnetic texture can control the OAM of light [LFC+25]. But access to FERMI is very limited. You will therefore develop the tools to perform these experiments on a laboratory attosecond source. This will involve:
- Qualifying a tight-focusing setup for high harmonics, with performance expected beyond the current state of the art;
- Testing new combinations of structured laser fields for HHG;
- Producing an attosecond pulse carrying a single, easily reversible OAM, which current sources cannot provide.
This source will enable in-depth studies of helicoidal dichroism and its femtosecond, even attosecond, dynamics.
2. Strongly nonlinear regime: when light becomes chiral within an optical cycle
When a beam is tightly focused, its field acquires a longitudinal component. The usual notions of polarization and orbital angular momentum, defined as averages, then lose their meaning. Instead, one must consider a three-dimensional, spatially inhomogeneous field that evolves in time, and that can carry a local chirality varying within the optical cycle. Theory predicts that in extreme nonlinear optics, and in HHG in particular, the chiral symmetry of a sample should then be imprinted on the emitted harmonics. Synthesizing such fields requires superposing several beams (typically and 2) carrying tailored spin and orbital angular momenta. You will build on our experience with these fields [VLG+23, LVG+23] to identify specific chiral signatures. Here the degrees of freedom of light grow from a single one (the wavelength) to a multitude of parameters. The application prospects are considerable, and you will map them out as completely as possible.
3. Weakly nonlinear regime: 'super-resolution' for magnetization switching -- STED microscopy, recognized by the 2014 Nobel Prize in Chemistry, uses a 'doughnut' beam (the intensity zero at the centre of an OAM beam) to switch off fluorescence everywhere except a tiny spot, which is then probed with a resolution far below the beam size. We propose to extend this idea with a Poincaré beam, which carries all polarization states and drives a weakly nonlinear (second- or third-order), polarization-sensitive effect. The first target is the inverse Faraday effect, through which circularly or elliptically polarized light acts on the magnetization of a material, up to reversing it. Promising for spintronics, this effect still lacks a consensus explanation. By triggering it with an ultrashort Poincaré beam and probing it with an XUV pulse, you will be able to follow how magnetization reversal propagates on length and time scales that are currently out of reach. More exploratory, this part will be tackled in a second phase, and it will open the way to other polarization-dependent nonlinear effects.