Seminars

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.

Observation of relaxation stages in out-of-equilibrium closed quantum systems

The temporal evolution of a non-equilibrium quantum system is the most crucial aspect of its dynamics. We introduce ro-translational excitations in an 87Rb Bose-Einstein condensate, which, under certain conditions, induces turbulence in the system . Once this non-equilibrium state is achieved, the system is allowed to evolve until it reaches equilibrium. Throughout this dynamical process, the characteristic particle cascade is observed, featured by the appearance of a power-law in the momentum distribution along with temporal windows of meta-stability showing the emergence of universal scaling. Subsequently, the system reaches a pre-thermalization stage with minimal variation, followed by a reverse cascade that forms a new condensate with fraction and temperature different from the initial one. This allows the assessment of various energy inputs during relaxation. The system is investigated under different initial excitations and represents a closed system suitable for exploring new physics related to the relaxation dynamics of non-equilibrium quantum systems. 

Fiber based high power lasers for quantum computing applications