Jérôme Lhermite’s research while affiliated with Centre Lasers Intenses et Applications and other places

ThomX is a compact x-ray source based on Compton scattering, installed at IJCLab (Laboratoire de physique des 2 infinis-Irène Joliot-Curie) in Orsay. The machine uses a small electron storage ring and an intense laser pulse stored in a high-finesse optical cavity. This article describes the various subsystems of the machine and their initial results of the commissioning, which began in mid-2021. This first commissioning phase led to the production of 10 10 x-rays/s with an on-axis energy of 45 keV. The main steps to be taken to reach the nominal flux are outlined at the end. Published by the American Physical Society 2025

Very high-average optical enhancement cavities (OECs) are being used both in fundamental and applied research. The most demanding applications require stable megawatt level average power of infrared picosecond pulses with repetition rates of several tens of MHz. Toward reaching this goal, we report on the achievement of 710 kW of stable average power in a two-mirror hemispherical optical enhancement cavity. This result further improves the state of the art. So far, in compact high-power systems, cavity geometry optimization has been driven by the need to limit the deformation of radii of curvatures due to thermal effects. Here we explicitly demonstrate that thermal lensing must be accounted for, too, and that it can be used to assess the absorption of coatings. Experimental observations are matched with a simple model of thermal effects in the mirror’s coatings. These results set a further stage for designing an optimized optical system for several applications where very high-average power enhancement cavities are expected to be operated.

With the increase in laser power and finesse of optical cavities over the last decade, laboratory-size Compton sources are very promising. These sources produce X-rays through interactions between relativistic electrons and laser photons and, in term of brightness, fall between large synchrotron facilities and classical laboratory X-ray sources. The ThomX source is the French project in this field. This article first presents a state of the art of high-intensity Compton sources, then the ThomX source is briefly described, and the first results are detailed, in particular the production of the first X-rays, the acquisition of the first spectrum and the first image of the beam. Finally, the next objectives are discussed.

We present the simulation, fabrication, and characterization of large area microstructured fiber tapers which enables broadband phasematching conditions of the four wave-mixing process. These silica-based tapers are intended to serve as a nonlinear gain medium for intense and high average power Fiber Optical Parametric Chirped Pulse Amplifier emitting at 2 μm\upmu \hbox {m} and strongly pumped at Yb wavelength. Different geometries (tapered/untapered, aspect ratio, etc.) are fabricated, analyzed and their broadening properties—key for supporting ultrashort pulses amplification—are compared and discussed. The characterization of nonlinear gain bandwidth of the tapers relies on a tunable source of stochastic pulses based on tunable amplified spontaneous emission in Yb-doped amplifiers. The strong overshoots of this source allows degenerate four-wave mixing process to occur thus generating broadband incoherent visible signal and mid-infrared idler waves at much lower average power than usually needed with coherent pumping. The idler centered around 1.85 μ$\mu$m is broadened due to zero-dispersion wavelength shift along the taper.