Thèse Caractérisation Temporelle de la Génération d'Harmoniques d'Ordre Élevé dans les Cristaux Semi-Conducteurs 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 : Willem BOUTU ORCID 0000000311375724
Début de la thèse : 2026-10-01
Date limite de candidature : 2026-04-30T23:59:59

Il est possible de caractériser des impulsions de rayonnement extrême-ultraviolet femtosecondes, issues d'harmoniques générées dans des gaz, en exploitant une variation d'absorption induite par un laser infrarouge. Cette approche, fondée sur l'absorption multiphotonique au voisinage des seuils d'ionisation des gaz, se distingue par sa simplicité de mise en oeuvre. Nous proposons de l'étendre à l'étude des sources d'harmoniques d'ordre élevé générées dans des cristaux semi-conducteurs. En raison du flux de photons relativement faible associé à l'émission harmonique dans les cristaux, les méthodes traditionnelles de caractérisation s'avèrent inadaptées ou difficiles à appliquer. L'objectif du projet de thèse est double : d'une part, démontrer la faisabilité de cette nouvelle méthode de caractérisation temporelle des impulsions issues de sources à faible flux photonique, et d'autre part, développer une approche robuste et accessible pour analyser les dynamiques temporelles, femtosecondes et sub-femtosecondes, de l'émission harmonique, qui résulte des dynamiques électroniques sous champ intense dans les cristaux.

The development of picosecond to femtosecond UV and extreme-UV (XUV) laser sources over the past two decades has driven significant interest in methods for their full characterization. While techniques such as FROG and SPIDER are highly effective in the infrared domain, they are not easily adaptable to the XUV regime. In response, alternative methods, including RABBIT, XUV-XFROG, and FROG CRAB, have been developed to characterize ultrashort XUV pulses. However, these approaches rely on electron time-of-flight spectrometers, particularly constraining experimentally and not suitable for every light source. For instance, they are poorly suited characterize high-harmonic generation (HHG) from semiconductor crystals.
HHG in semiconductor crystals offers distinct advantages over gas-phase targets, such as robustness, compactness, and easier integration with other optical components. Yet, the temporal characterization of HHG emission remains challenging, primarily due to the relatively low photon flux. HHG arises from the coherent interaction between the electromagnetic field and electrons in their environment. In the spectral domain, this interaction produces successive high-order harmonics of the incident radiation, while in the temporal domain, it results in the emission of attosecond pulse trains. As such, HHG emission encodes valuable information about electron dynamics and can be used to probe complex electronic behaviours in non-trivial materials. Temporal characterization is therefore essential for studying these dynamics. To address this challenge, we propose to adapt a novel method known as TIBETAN-FOX [1] for the temporal characterization of HHG emission from semiconductor crystals.

The objective of this PhD project is to implement the TIBETAN-FOX method based on the transient absorption of femtosecond XUV pulses near gas ionization thresholds, induced by multiphoton absorption with the assistance of a dressing infrared laser. The method enables full characterization of the temporal properties of XUV pulses using only an optical spectrometer. The candidate will initially collaborate with the team at the Laboratoire d'Optique Appliquée (LOA) [2], which developed this characterization technique, to gain both theoretical and experimental expertise using gas-phase HHG. Following this training, the candidate will design and implement a new experimental configuration tailored to the specific features of HHG from semiconductor crystals. This work will be carried out at the NanoLight platform at LIDYL [3], which is fully equipped for crystal HHG generation in vacuum. The platform uses a laser delivering pulses of 40 fs down to 12 fs (2 cycles) at a central wavelength of 1800 nm, focused to achieve intensities on the order of a few TW/cm² in the generating crystal. The initial crystal for this study is MgO, with targeted harmonics corresponding to energy transitions between the valence and conduction bands at the Brillouin zone edge: H23 (16 eV), H25 (17.4 eV), and H27 (18.8 eV) [4]. Depending on the generating laser pulse duration, these harmonics can merge, resulting in a broad spectrum. The goal is to extend this to a quasi-continuum spanning 2 to 3 harmonics around H25, with an expected Fourier-limited pulse duration of approximately ~1 fs.
TIBETAN-FOX relies on cross-correlation between the harmonic pulse and the driving laser in a gas medium. A key advantage of this method is its simplicity, as it does not require a complex electron spectrometer. To adapt it for HHG in crystals, the candidate will design a compact apparatus that integrates the HHG generation target (the MgO crystal) with a gas medium for cross-correlation. The initial proposed design could be a gas cell where the MgO sample itself serves as the entrance window, minimizing the need for additional refocusing optics and enabling seamless integration with the existing setup and spectrometer. For the gas medium, neon will be used, focusing on its ionization energy levels: 2s²2p at 21 eV and 2s²2p3p at 18 eV. Nonlinear mixing of one photon from the driving laser with the HHG (H23, H25, or H27) will induce transient absorption from these levels, which can be detected in the harmonic spectrum.
In the second phase of the PhD project, the candidate will exploit the resulting temporal characterization to investigate the fundamental electron dynamics under strong-field conditions, which are the origin of HHG emission [5]. This will be achieved for example, by studying the temporal response of the HHG emission as a function of inputs from temporal shaping of the IR driving laser.

Etudiant niveau master 2 ou école d'ingénieur, avec un profil orienté vers la physique des lasers, la physique des milieux condensés ou l'interaction laser/matière. Profil de candidat qui aime faire de nombreuses expériences.