Séminaires

Effet Casimir Dynamique avec des atomes: de l’émission de paires de photons à une phase Sagnac quantique

L’interaction entre la matière et le champ électromagnétique quantique est particulièrement intrigante lorsque les effets de mouvement entrent en jeu. L’effet Casimir dynamique (ECD) est généralement considéré dans une approche macroscopique basée sur les conditions aux limites [1] pour le champ quantique. À un niveau microscopique plus fondamental, l’ECD peut être décrit à l’échelle atomique à l’aide de modèles hamiltoniens d’optique quantique pour le couplage atome-champ. Une observation expérimentale d’un analogue de l’ECD a été rapportée avec des supraconducteurs. Néanmoins, une mesure directe du rayonnement ECD produit par des corps en mouvement reste hors de portée car ce rayonnement est extrêmement faible dans des conditions expérimentales réalistes. Une voie alternative consiste à suivre des signatures indirectes d’aspects analogues à l’ECD dans les interféromètres atomiques, produites par le mouvement relatif entre l’atome et une surface conductrice. Dans cette présentation, nous présentons une théorie microscopique de l’effet Casimir dynamique [2,3], et utilisons ce formalisme pour prédire des effets analogues à l’ECD dans les interféromètres atomiques [4,5]. Notre approche révèle des connexions entre des aspects non triviaux de la physique moderne tels que la retardation électrodynamique, les phases géométriques, la non-localité et les effets non-inertiels.

La diffraction d’atomes rapides au dessus d’une surface cristalline

Découverte, il y a presque vingt ans la diffraction d’atomes légers entre 200 eV et 5 keV au dessus d’une surface trouve des applications pour le suivie de la croissance directement en bâti d’épitaxie ou elle permet un diagnostique fin du mode de croissance et de la topologie/reconstruction de la dernière couche. Je présenterai les évolutions récentes sur les processus inélastiques et les mesures des forces attractives vers la surface.

—

Diffraction of fast atoms above a crystalline surface.

Discovered almost twenty years ago, the diffraction of light atoms between 200 eV and 5 keV above a surface is finding applications in the monitoring of growth directly in the epitaxial frame, where it enables fine diagnosis of the growth mode and the topology/reconstruction of the last layer. I will present recent developments in inelastic processes and measurements of attractive forces towards the surface.

Laser absorption with non-resonance cavities and Faraday rotation spectroscopy for gas sensing applications, towards laboratory detection of OH and HO2 radicals.

Laser absorption spectroscopy is a fundamental technique widely used in trace gas sensing, with versatile applications in atmospheric chemistry, environmental monitoring, biomedical diagnostics, and molecular astrophysics. By quantifying the attenuation of light passing through a gas sample, this non-invasive method enables in-situ and absolute quantification of sample concentration. The utilization of optical cavities can significantly increase the light-sample interaction length, thereby enhancing detection limits according to the Beer-Lambert law. Among laser-based cavity techniques, off-axis Cavity-Enhanced Absorption Spectroscopy (off-axis CEAS) stands out as a robust approach for trace gas detection. A key feature of this approach involves eliminating the cavity mode structure through the un-resonance coupling of a narrow bandwidth laser source into a cavity, facilitating continuous scanning of laser frequency for rapid spectral measurements. This strategy offers simple optical alignment and circumvents the need for high-bandwidth devices or laser frequency-locking, ensuring robustness against misalignment and mechanical vibration.

In this work, we present the developments and applications of non-resonance cavities for the direct quantification of OH and HO2 radicals, key species in atmospheric chemistry. Furthermore, the enhancement of optical effective path length not only amplifies the absorption of the target molecules but also extends to common atmospheric species such as water vapor and CO2. The occurrence of spectral interference, resulting from the overlapping absorptions of various species, poses challenges in accurately determining the concentration of the species of interest. To address these challenges, we introduce Faraday Rotation Spectroscopy, leveraging the magneto-optic effect observed only for paramagnetic species (e.g., NO, NO2, OH, HO2) to facilitate interference-free measurements. The combination of the optical cavity and Faraday rotation technique enhances both sensitivity and selectivity for gas concentration determination.

Earth – Moon – Mars crustal attenuation properties comparison

Following its deployement at the surface of Mars, the SEIS seismometer of the NASA-InSight mission recorded tens of high-frequency Martian seismic events (> 1Hz) which we analyzed to characterize the attenuation properties of the Martian lithosphere from an Earth-Moon-Mars comparison perspective. The Martian waveforms are generally depolarized and show P and S arrivals with a gradual beginning, a broad maximum and a very long coda decay. These characteristics are highly reminiscent of the seismic wavefield in the terrestrial oceanic lithosphere at high frequency (Po and So above 2Hz), where the stratification of the absorption was used to estimate the Asthenosphere-lithosphere boundary (Takeuchi et al., 2017). To constrain the attenuation properties on Mars, we modeled the energy envelopes of the Martian events using a multiple-scattering approach in which we considered a stratification of the velocity and the attenuation properties in the medium. We observed that a simple model composed of a highly scattered crust and a weakly inhomogeneous mantle is sufficient to explain Martian events. We found that the Martian crustal diffusivity (10-12 km²/s) is similar to the estimation obtained in the lithosphere of the Atlantic Ocean (15–60 km²/s, Hannemann et al. 2022), but higher than the Lunar crust value (2 km²/s). The absorption attenuation results indicate that the Martian crust is globally dry ~ 10^-4 compared to the terrestrial crust ~ 10^-3. Our results suggest that the basaltic nature and the heterogeneities of the crust are the main source of the scattering in the Martian lithosphere unlike the extend of the fractures with depth observed on the Moon.

Experimental strategies to track conformational changes in the gas phase

Complex molecular ions, like synthetic polymers or biomolecules, also display complex conformational landscapes, in relation with their hierarchical structuration. Characterizing such complex landscapes also poses experimental challenges. Gas phase approaches, based on mass spectrometry allow some degree of simplification, by isolating species of well-defined composition and environment. However, gas-phase structure-sensitive techniques, often based on optical spectroscopy, are not easy to apply to large, flexible polymers. In this context, ion mobility spectrometry (IMS) is an interesting compromise, as a low-resolution structural probe applicable to large (bio-)molecular species. Indeed, the “collision cross sections” (CCS) determined by IMS-MS have been widely used to propose structures for protein complexes. Our group has proposed several strategies to exploit IMS-MS beyond CCS measurements. In particular we demonstrated that tandem IMS measurements could be used to explore the (photo-)isomerization of complex molecular systems. More recently, we developed approaches to follow the kinetics of conformational changes in such systems, and derive the associated thermodynamical quantities. This seminar will describe the principle of tandem ion mobility measurements, and give an overview of the approaches developed in our lab.

Rydberg excitons : a nonlinear optics perspective

For the last decade, highly excited (Rydberg) states of electron-hole bound states (excitons) in the semiconductor Cu2O have been showing more and more similarities with their atomic relatives, enabling solid-state Rydberg physics. After a rapid overview on Cu2O Rydberg excitons, I will focus on two recent experimental results where Rydberg-induced optical nonlinearities are central. In the first case, the giant interactions between Rydberg states are used to produce a large self-Kerr effect. This is directly observed with spatially-resolved interferometric imaging. In a second example, I will discuss very recent result where a second harmonic generation process is time-resolved with sub-picosecond resolution to directly unveil the Rydberg states lifetime along with several other intriguing dynamics.