Bas les masques ! GAMM—Une Taxonomie des Mécanismes d'Attributs Manquants dans les Graphes

International audienceExploring missing data in attributed graphs introduces unique challenges beyond those found in tabular datasets. In this work, we extend the taxonomy for missing data mechanisms to attributed graphs by proposing GAMM (Graph Attributes Missing Mechanisms), a framework that systematically links missingness probability to both node attributes and the underlying graph structure. Our taxonomy enriches the conventional definitions of masking mechanisms by introducing graph-specific dependencies. We empirically demonstrate that state-of-the-art imputation methods, while effective on traditional masks, significantly struggle when confronted with these more realistic graph-aware missingness scenarios

Decoding Ink in Historical Documents: Anomaly Localization and Source Separation

International audienceIn this paper, we attempt to make ink talk in historical documents by extracting visual information materialized by the trace left on the support, thus addressing two tasks: anomaly localization in hand-pressed ornaments in the framework of Rey's publishing and source separation in Queen Marie-Antoinette's correspondence where the letters are hidden with loops. Anomalies are defined as changes in shape with respect to the original skeleton. A logarithmic image processing is applied for both anomaly visualization and image generation by mixing letters with loops to train a source separation model, respectively. Such an image generation is tackled from spectral modeling in halftoning

Tailoring nanometric vanadium dioxide morphology to tune thermochromic optical properties

International audienc

Monte Carlo Analysis of the Parameters Impacting the Gain of Erbium-Doped Fiber Amplifiers

International audienc

Electromagnetic analysis of disordered metasurfaces: density of states and BSDF

International audienceWe review our current effort to model disordered metasurfaces, with a focus on statistical quasinormal mode (QNM) analysis and a numerical framework for computing the bidirectional scattering distribution function (BSDF)

Toward Accurate Monitoring of 14 MeV Neutron Flux: Investigating a 6 Lithium Glass Scintillator

International audienc

A high transmission tender X-ray monochromator employing a matched pair of multilayer grating and mirror

International audienceIt is now well established that multilayer coated gratings may offer high diffraction efficiencies over the tender X-ray range, from about 1 keV to 5 keV, covering the gap between single layer coated grating monochromators and crystal monochromators. Nevertheless, few beamlines in the world are using such gratings in their monochromator. The successful implementation of a multilayer grating monochromator requires producing a matched pair of a multilayer grating and a multilayer mirror, and this matching is not straightforward as it must account for different incidence angles and refraction effects on each element. Here we review the realization of the multilayer grating monochromator of the SIRIUS beamline which has been successfully in service for several years. We show how, by alternating computer simulation with our diffraction code and measurements, we could optimize the monochromator transmission on a very wide energy range. After the grating was coated, it was found that the angle of optimal efficiency versus photon energy was significantly different from what was predicted by a simple conformal model of binary layers. Layer interdiffusion and profile smoothing during the deposition process must be added to the multilayer model to reproduce the measured data. The critical adjustment of the mirror multilayer period is achieved by the lateral translation of the mirror, which was given a small transverse period gradient. The monochromator is thus providing high transmission efficiency in the 1 to 5 keV energy range, more than 30% over 2.5 keV and up to 46% at 4.6 keV

Equivariant Unsupervised Object Detection with Learnable Riesz Transform and Composite Spatial Transformers

International audienceBuilding models robust to transformations such as rotation, scale, and translation is a challenge in machine learning and computer vision. Existing approaches often provide only partial and discrete equivariance (group equivariance) or rely on supervision or very abundant data to learn equivariant representations. To achieve fine-grained equivariance from low data, we combine and improve over both approaches. We propose a novel, learnable, Riesz-transform-based architecture that achieves built-in group equivariance for translation, rotation, and scale. We combine it with a Spatial Transform Network (STN) tailored for the sequential estimation of composite transformations, reducing the combinatorial data requirements for learning fine-grained equivariance. Improved generalization guarantees and extensive experiments demonstrate that our approach brings improvements over state-of-the-art methods in unsupervised representation learning and object discovery, even more so in low-data regimes

Rhenium‐Based Dual‐Function Materials for Integrated CO₂ Capture and Methanation

International audienceIntegrated CO2 capture and utilization (ICCU) is a promising transition route for mitigating flue-gas emissions while producing useful energy-carrying chemicals. This study reveals the potential of rhenium as an alternative to nickel or ruthenium for catalyzing CO2 methanation. It is shown that the mixing of Re/γ-Al2O3 with CeO2 or a synthetic hydrotalcite-derived Mg–Al oxide (layered double oxide [LDO]) provides tunable low-Re-content dual-function materials (DFMs) with 100% CO2 conversion and 100% methane selectivity at 300°C under cyclic operation. The direct deposition of rhenium, in an atomically dispersed form, onto CeO2 or LDO increases the methane yield up to 128 µmol/g per 10 + 10 min cycle, suggesting a synergy between catalytic and adsorbing functions. In contrast, these two systems are poorly selective to methane in conventional gas-phase CO2–H2 reaction, showing the beneficial effect of sequential adsorption–hydrogenation operation on selectivity. In terms of stability, Re/CeO2 appears as the most efficient DFM, showing stable methane production over 50 cycles, moderate deactivation in the presence of water, and full recovery after return to dry conditions. An operando diffuse reflectance infrared spectroscopy (DRIFTS) investigation of this catalyst under both ICCU and conventional hydrogenation discloses the nature of molecular adsorbates (CO, formates) and their dependence on the reaction regime. In situ Raman spectroscopy shows that the oxidation state of the active ReOx species undergoes only minor modifications upon alternating CO2 and H2 steps, maintaining predominantly Re7+ moieties

Développement d'amplificateurs lasers pompés par LEDs via concentrateurs luminescents pour l'imagerie acousto-optique

In optically scattering media, the penetration depth and spatial resolution of light imaging are greatly limited. Acousto-optic imaging (AOI) is a promising hybrid approach based on frequency tagging of photons by an ultrasonic wave. However, AOI imposes stringent laser requirements: single-frequency emission, wide tunability within the therapeutic window (700-1000 nm), high peak power (~100 W) in a so-called ‘quasi-continuous' mode with long pulses of around 100 µs and a repetition rate of up to 100 Hz.The objective of this thesis is to develop a new laser source, adapted to these constraints, based on an oscillator-amplifier architecture with laser crystals doped with transition metal ions. Three amplifier materials were studied: Ti:sapphire, Cr:LiSAF and alexandrite, selected for their wide gain band in the near infrared.A central focus of the work is the development of innovative LED pumping technology using a luminescent concentrator, which had to be scaled up to meet the specifications of the laser source. To achieve this, three levers were used: increasing the unit power of the LEDs, multiplying their number, and optimising a Ce:YAG luminescent concentrator, including an innovative ‘edge' geometry. An advanced optical model was developed and validated experimentally, highlighting the critical role of losses associated with the average propagation distance, particularly due to absorption by the excited state. The pump head resulting from this work delivers up to 2.6 kW of peak power between 530 nm and 630 nm, setting a record level of performance in this spectral range.The integration of this technology into geometric multi-pass amplifiers made it possible to evaluate the performance of the three crystals. Ti:sapphire proved to be limited by insufficient gain in the quasi-continuous regime. Cr:LiSAF demonstrated a single-frequency signal at 830 nm reaching 9 W over 100 µs, but remains constrained by gain saturation and unfavourable thermal behaviour.Alexandrite has proven to be the most promising material: thanks to the long lifetime of the excited state and increasing gain with temperature, a multi-pass amplifier delivering a net gain of 34 after 16 passes and a peak power of 14 W at 761 nm has been demonstrated, setting a record for IAO. Interference effects limiting operation to 100 µs were identified and partially eliminated.This work marks a significant step forward towards more powerful optical imaging devices with greater penetration into biological tissue. Furthermore, it demonstrates the relevance of a new laser technology based on LED-pumped alexandrite, whose potential applications extend far beyond acousto-optic imaging: LiDAR, femtosecond chains, and UV lasers for microelectronics are just a few examples.Dans les milieux optiquement diffusants, la profondeur de pénétration et la résolution spatiale de l'imagerie par la lumière est grandement limitée. L'imagerie acousto-optique (IAO) constitue une approche hybride prometteuse, reposant sur le marquage en fréquence des photons par une onde ultrasonore. Toutefois, l'IAO impose des exigences laser contraignantes : émission monofréquence, large accordabilité dans la fenêtre thérapeutique (700-1000 nm), puissance crête élevée (~100 W) dans un régime dit "quasi-continu" avec des impulsions longues de l'ordre de 100 µs et une cadence pouvant atteindre 100 Hz. L'objectif de cette thèse est de développer une source laser nouvelle, adaptée à ces contraintes, fondée sur une architecture oscillateur-amplificateur avec des cristaux laser dopés aux ions de transition métallique. Trois matériaux amplificateurs ont été étudiés : le Ti:saphir, le Cr:LiSAF et l'alexandrite, sélectionnés pour leur large bande de gain dans le proche infrarouge. Un axe central du travail consiste au développement d’une technologie de pompage innovante par LEDs via concentrateur luminescent dont il fallait assurer la montée en puissance pour répondre au cahier des charges de la source laser. Pour cela, trois leviers ont été utilisés : l'augmentation de la puissance unitaire des LEDs, la multiplication de leur nombre et l'optimisation d'un concentrateur luminescent en Ce:YAG, incluant une géométrie innovante dite « en arête ». Un modèle optique avancé a été développé et validé expérimentalement, mettant en évidence le rôle critique des pertes associées à la distance moyenne de propagation, notamment dues à l'absorption par l'état excité. La tête de pompage issue de ces travaux délivre jusqu'à 2,6 kW de puissance crête entre 530 nm et 630 nm, établissant un niveau de performance record dans cette gamme spectrale. L'intégration de cette technologie dans des amplificateurs multipassages géométriques a permis d'évaluer les performances des trois cristaux. Le Ti:saphir s'est révélé limité par un gain insuffisant dans le régime quasi-continu. Le Cr:LiSAF a permis de démontrer un signal monofréquence à 830 nm atteignant 9 W sur 100 µs, mais reste contraint par la saturation du gain et une tenue thermique défavorable. L'alexandrite s'est avérée être le matériau le plus prometteur : grâce au long temps de vie du niveau excité et à un gain croissant avec la température, un amplificateur multipassage délivrant un gain net de 34 après 16 passages et une puissance crête de 14 W à 761 nm a été démontré, établissant un record pour l'IAO. Des effets d'interférences limitant le fonctionnement à 100 µs ont été identifiés et partiellement levés. Ces travaux marquent une progression significative vers des dispositifs d'imagerie optique plus performants et plus pénétrants dans les tissus biologiques. De plus, ils démontrent la pertinence d'une nouvelle technologie laser basée sur l'alexandrite pompé par LED, dont le potentiel d'application s'étend bien au-delà de l'imagerie acousto-optique : LiDAR, chaînes femtoseconde, ou lasers UV pour la micro-électronique en sont quelques exemples