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A Visual Dive into Conditional Flow Matching
No full texthttps://iclr-blogposts.github.io/2025/blog/conditional-flow-matching/Conditional flow matching (CFM) was introduced by three simultaneous papers at ICLR 2023, through different approaches (conditional matching, rectifying flows and stochastic interpolants).The main part of this post, Section 2, explains CFM by using both visual intuitions and insights on its probabilistic formulations. Section 1 introduces nomalizing flows; it can be skipped by reader familiar with the topic, or that wants to cover them later. Section 3 opens on the links between CFM and other approaches, and ends with a 'CFM playground'
Understanding and Exploiting Plasmonic-Photonic Hybridization in Laser-Induced Quasi-Random Plasmonic Metasurfaces
No full textInternational audienc
Reflections on the Spatial Exponential Growth of Electromagnetic Quasinormal Modes
No full textInternational audienceA major research objective across various fields is to represent the response of open systems using quasinormal mode (QNM) expansions, akin to the treatment of normal modes in closed systems. In electromagnetism, QNM expansions effectively describe modal physics inside resonators and in their near field. However, challenges arise in the intermediate and far field, where QNM fields grow exponentially, posing mathematical issues and often being considered unphysical. How can a near-field relevant concept lose its validity? Where does this transition occur? This perspective seeks to answer these questions by analyzing foundational concepts such as cavity perturbation theory and dissipative coupling using model problems. The analysis reveals no fundamental inconsistencies with exponential growth and sometimes yields surprising results, such as an increase in coupling coefficients between QNMs of two distant bodies as separation increases. These findings should be widely shared to prevent misunderstandings and enhance the understanding of contemporary electromagnetic QNM theories. The final section, intended for experts in electromagnetic QNMs, provides a thorough analysis of these theories
Modélisation physique de l’apparence colorée de milieux complexes pour la cosmétique
No full textFoundation is a colored, semi-transparent makeup product used to even out the complexion, conceal imperfections, or subtly alter skin tone. At the microscopic level, foundation is a complex and heterogeneous medium, generally composed, in its liquid form, of two phases (oil and water), solid particles (fillers, pigments, etc.) randomly distributed, and surfactants to ensure the stability of the formula. Establishing a relationship between the microscopic properties of such complex media and their visual appearance is a major challenge for digital formulation in cosmetics. This would allow, for example, to correct a priori variations in foundation ingredients or to predict the required composition for a specific targeted appearance. This thesis work contributes to this long-term objective by proposing a modeling approach for the prediction and analysis of the optical properties of complex heterogeneous media based on their composition and structure. An experimental study is first conducted on samples based on a simplified foundation formula to characterize their microscopic and optical properties. The foundation model used is that of a pigmented emulsion, namely a solution containing both solid, sub-micrometer particles and spherical, homogeneous, and transparent droplets of several tens of micrometers in size. A modeling tool based on a Monte Carlo method is developed to simulate light transport in such media, considering the scattering and absorption phenomena produced by the particles as well as the refraction and reflection phenomena produced by the droplets. This allows us to show how variations in microscopic parameters, such as the nature and density of particles, as well as the size and density of droplets, impact the coloration of a macroscopic material in reflection. In a second step, we propose an improvement of the method, based on an implicit description of the light interaction events with the microstructure, to simulate more efficiently the light transport in such media.This thesis work offers an increased understanding of the microscopic origin of the visual appearance of foundations, highlighting the importance of considering the heterogeneity of the materials due to the dispersed phase.Le fond de teint est un produit de maquillage coloré et semi-transparent qui est utilisé pour unifier la couleur du visage, masquer les imperfections ou altérer subtilement la carnation. A l’échelle microscopique, le fond de teint est un milieu complexe et hétérogène, généralement constitué, dans sa forme liquide, de deux phases (grasse et aqueuse), de particules solides (charges, pigments, etc.) distribués aléatoirement, et de tensioactifs pour assurer la stabilité de la formule. Etablir une relation entre les propriétés microscopiques de tels milieux complexes et leur apparence visuelle constitue un enjeu majeur pour la formulation digitale en cosmétique. Cela permettrait, par exemple, de corriger a priori les variations de nature des ingrédients des fonds de teint ou de prédire la composition requise pour une apparence spécifique ciblée .Ce travail de thèse s’inscrit dans cet objectif à long terme en proposant une approche de modélisation pour la prédiction et l’analyse des propriétés optiques de milieux complexes hétérogènes à partir de leurs composition et structuration. Une étude expérimentale est tout d’abord menée sur des échantillons reposant sur une formule simplifiée de fonds de teint afin de caractériser leurs propriétés microscopiques et optiques. Le modèle de fond de teint employé est celui d’une émulsion pigmentée, à savoir une solution contenant à la fois des particules solides sub-micrométriques et des gouttelettes sphériques, homogènes et transparentes de plusieurs dizaines de micromètres en taille. Un outil de modélisation basé sur une méthode Monte Carlo est développé pour simuler le transport de la lumière dans de tels milieux, en prenant en compte les phénomènes de diffusion et d’absorption produits par les particules ainsi que ceux de réfraction et de réflexion produits par les gouttelettes. Cela nous permet de montrer comment des variations de paramètres microscopiques, tels que la nature et densité de particules, ainsi que la taille et la densité des gouttelettes, impactent la coloration d’un matériau macroscopique en réflexion. Dans un second temps, nous proposons une amélioration de la méthode, basée sur une description implicite des évènements d’interaction de la lumière avec la microstructure, pour simuler de manière plus efficace le transport de la lumière dans de tels milieux.Ce travail de thèse offre une compréhension accrue de l’origine microscopique de l’apparence visuelle des fonds de teint, en soulignant notamment l’importance de prendre en compte l’hétérogénéité des matériaux due à la phase dispersée
Visualization and quantification of coral reef soundscapes using CoralSoundExplorer software
No full textInternational audienceDespite hosting some of the highest concentrations of biodiversity and providing invaluable goods and services in the oceans, coral reefs are under threat from global change and other local human impacts. Changes in living ecosystems often induce changes in their acoustic characteristics, but despite recent efforts in passive acoustic monitoring of coral reefs, rapid measurement and identification of changes in their soundscapes remains a challenge. Here we present the new open-source software CoralSoundExplorer , which is designed to study and monitor coral reef soundscapes. CoralSoundExplorer uses machine learning approaches and is designed to eliminate the need to extract conventional acoustic indices. To demonstrate CoralSoundExplorer ’s functionalities, we use and analyze a set of recordings from three coral reef sites, each with different purposes (undisturbed site, tourist site and boat site), located on the island of Bora-Bora in French Polynesia. We explain the CoralSoundExplorer analysis workflow, from raw sounds to ecological results, detailing and justifying each processing step. We detail the software settings, the graphical representations used for visual exploration of soundscapes and their temporal dynamics, along with the analysis methods and metrics proposed. We demonstrate that CoralSoundExplorer is a powerful tool for identifying disturbances affecting coral reef soundscapes, combining visualizations of the spatio-temporal distribution of sound recordings with new quantification methods to characterize soundscapes at different temporal scales
Magneto-optical properties of cobalt and Co3O4 thin films: Influence of microstructural properties
No full textInternational audienceTwo kinds of cobalt thin films, labeled smooth and nanostructured according to their surface properties, were deposited on a glass substrate by varying the deposition conditions during magnetron sputtering. Complementary characterization techniques were first used to investigate the internal microstructure of these two films, which give evidence that the smooth film structure is dense and close to bulk cobalt while the nanostructured film contains columnar-like structures of few nanometers' width surrounded by an oxide layer. The smooth film shows a linear magnetic behavior, while a hysteresis loop has been observed for the nanostructured film, due to the presence of pinning-type domain wall motion. The nanostructured film is more porous to oxygen diffusion than the smooth one, which explains that its oxidation starts at lower annealing temperature than that of the smooth one. The spectral magneto-optical properties of the oxidized films were studied, revealing for the first time, three diamagnetic transitions at around 700, 1330, and 1450 nm, involving Co 2+ and/or Co 3+ ions. The blue shift observed for the nanostructured film highlights the impact of the initial morphology of the films on the cation's crystal coordination.</div
Two-dimensional infrared spectroscopy using a fast-scanning interferometer and chirped pulse up-conversion at 100 kHz
No full textInternational audienceWe report on a 100-kHz two-dimensional infrared (2DIR) spectrometer in the pump-probe geometry, which we apply to the measurement of the 2DIR spectrum of carboxy-hemoglobin. The probe pulses are spectrally resolved by chirped-pulse upconversion (CPU) using a fast 2048-pixel linescan CMOS camera. The two-pulse pump sequence is generated using a conventional interferometer with a fast-scanning mechanical delay line allowing to achieve a scanning frequency of 2 Hz. The resulting modulation frequency of 3.1 kHz is large enough to shift the relevant signal away from the low-frequency noise of the laser source. The combined use of an interferometer on the pump side and of CPU on the probe side opens the way to an improved spectral resolution in both pump and probe dimensions, as compared to currently-available 100-kHz 2DIR spectrometers based on pulse shapers and Mercury-Cadmium Telluride (MCT) detector arrays
Iterative phase retrieval algorithm for space-variant PSF in optical systems with aberrations
No full textInternational audienceIterative phase retrieval algorithms are widely used in digital optics for their efficiency and simplicity. Conventionally, these algorithms do not consider aberrations as they assume an ideal, aberration-free optical system. Here, we propose modified iterative phase retrieval algorithms that take into account the space-invariant and space-variant point spread function of the optical system
Radiation Effects on Graded Index Germanosilicate Multimode Optical Fibers
No full textInternational audienc
Probing the magnetic origin of the pseudogap using a Fermi-Hubbard quantum simulator
No full textInternational audienceIn strongly correlated materials, interacting electrons are entangled and form collective quantum states, resulting in rich low-temperature phase diagrams. Notable examples include cuprate superconductors, in which superconductivity emerges at low doping out of an unusual ``pseudogap'' metallic state above the critical temperature. The Fermi-Hubbard model, describing a wide range of phenomena associated with strong electron correlations, still offers major computational challenges despite its simple formulation. In this context, ultracold atoms quantum simulators have provided invaluable insights into the microscopic nature of correlated quantum states. Here, we use a quantum gas microscope Fermi-Hubbard simulator to explore a wide range of doping levels and temperatures in a regime where a pseudogap is known to develop. By measuring multi-point correlation functions up to fifth order, we uncover a novel universal behaviour in magnetic and higher-order spin-charge correlations. This behaviour is characterized by a doping-dependent energy scale that governs the exponential growth of the magnetic correlation length upon cooling. Accurate comparisons with determinant Quantum Monte Carlo and Minimally Entangled Typical Thermal States simulations confirm that this energy scale agrees well with the pseudogap temperature . Our findings establish a qualitative and quantitative understanding of the magnetic origin and physical nature of the pseudogap and pave the way towards the exploration of pairing and collective phenomena among charge carriers expected to emerge at lower temperatures