Amélioration du contraste temporel d'un laser de haute puissance pour optimiser la génération d'harmoniques d'ordre élevé sur miroir plasma relativiste M/F

Job Description

Thesis Subject

One of the major goals of laser–plasma interaction research today is to access the strong-field quantum electrodynamics (QED) regime using ultra-high-power laser systems. In this extreme regime, the quantum vacuum and matter exposed to ultra-intense electromagnetic fields exhibit novel nonlinear behaviors, opening the way to the experimental observation of fundamental processes that have so far remained largely unexplored.
Among the various approaches proposed to reach this regime, one of the most promising relies on the generation of high-order harmonics from relativistic plasma mirrors. When an ultra-intense laser pulse interacts with a solid target, the target is ionized and the plasma surface can oscillate at relativistic velocities, acting as a nonlinear mirror capable of up-converting the incident radiation to much higher frequencies. This process enables the generation of coherent extreme ultraviolet (XUV) radiation while simultaneously offering the possibility of spatially and temporally compressing the optical energy. The resulting intensities can exceed those of the incident laser by several orders of magnitude. At such intensities, matter placed at the focus of this secondary radiation could enter the strong-field QED regime, where new physical processes become accessible, including the emission of ultra-high-energy gamma rays and electron–positron pair creation through the Breit–Wheeler process.
However, we have recently demonstrated that high-order harmonic generation from relativistic plasma mirrors is severely limited, for laser intensities exceeding 1e21W/cm2 on target, by the sub-picosecond temporal contrast of the laser pulse. In practice, an overly intense temporal pedestal modifies the target well before the arrival of the main pulse, significantly degrading the optimal interaction conditions. Improving the sub-picosecond temporal contrast is therefore essential to remove this limitation on relativistic harmonic generation and, ultimately, to enable access to the strong-field QED regime.
In this context, this PhD project proposes to investigate an entirely new approach based on placing a gas medium upstream of a solid target. The idea is to tailor the temporal evolution of the laser focusing process on the target. The focal distance of the laser beam is deliberately increased so that the low-intensity temporal pedestal remains weak at the solid target, while the much more intense main pulse undergoes nonlinear self-focusing in the gas, allowing it to maintain a high intensity at the interaction point. This approach selectively enhances the effective on-target contrast between the temporal pedestal and the main pulse while simultaneously relaxing several experimental constraints associated with the use of very short focal lengths to achieve ultra-high laser intensities.
The first objective of the PhD project will be to determine the optimal parameters of this approach—such as the focal position, focal intensity, and plasma density—using particle-in-cell (PIC) simulations. The second objective will be to demonstrate its experimental feasibility on a 100 TW laser system. Finally, the project will assess its scalability to petawatt-class laser facilities, with the long-term goal of establishing a new route toward the experimental exploration of the strong-field QED regime.

Your Work Environment

The Laboratoire d'Optique Appliquée (LOA) is a joint research laboratory (UMR 7639) affiliated with the CNRS, École Polytechnique, and ENSTA Paris, and is located on the Palaiseau campus.
The laboratory is internationally recognized for its research in ultrafast lasers, photonics, laser–plasma interactions, and laser-driven particle accelerators.
It brings together approximately 80 researchers, faculty members, engineers, PhD students, and postdoctoral fellows.
The LOA conducts research ranging from fundamental physics to applications in medicine, high-energy physics, imaging, and scientific instrumentation.
The laboratory is organized into several research groups whose main topics include femtosecond laser development, laser–matter interactions, relativistic plasmas, compact X-ray sources, plasma-based particle accelerators, and laser filamentation.
Host Research Group: UPX (Ultrafast Particles and X-rays)
The UPX (Ultrafast Particles and X-rays) group is one of the main research teams within the LOA. Its research focuses on the generation of charged particles—primarily electrons—and ultrashort X-ray radiation through the interaction of ultra-intense laser pulses with plasmas.
Its main research activities include:
-the development of compact plasma-based particle accelerators;
- the investigation of the fundamental mechanisms governing relativistic laser–plasma interactions;
-the generation of ultrafast X-ray sources for imaging and the diagnosis of ultrafast phenomena;
-the development of new applications in physics, materials science, and imaging;
-the improvement of electron beam performance in terms of energy, stability, and beam quality.
The UPX group combines experimental research, numerical simulations, and advanced instrumentation development to investigate and optimize laser-driven particle and radiation sources.

Constraints and risks

Laser safety training will be provided on site at the beginning of the PhD.

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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