HAL Portal IOGS (nstitut d'Optique Graduate School)
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Amélioration de la qualité spatiale des lasers multi-joules à haute cadence : développement de miroirs à conjugaison de phase par diffusion Brillouin stimulée pour la correction de front d'onde
No full textHigh-energy lasers are important tools in a large variety of scientific and industrial domains such as plasma physics, machining or energy sources for secondary lasers. Currently, the main drawback of such systems is their low repetition rate, limiting the number of shots provided. This limit rises from thermal energy accumulation shot after shot inside the amplifiers of the lasers systems during operation, generating laser beam wavefront distortion up to complete laser dysfunction. In this thesis, a collaboration between Amplitude laser, an industrial laser manufacturer, Laboratoire Charles Fabry, and Laboratoire pour l'Utilisation des Lasers Intenses (LULI), aim to develop a component capable of correcting those wavefront distortions. This component, called phase conjugated mirrors, uses a nonlinear effect called stimulated Brillouin scattering to invert the wavefront and allow self-correction of the laser wavefront aberration in a double pass amplifying scheme. In this work, the existing theoretical framework of the nonlinear effect is used for the development of numerical models simulating the reflection. Those models are applied to the design of experimental phase conjugate mirrors whose properties are investigated. The thesis places an emphasis on the fidelity of the mirror, that is to say, its capacity to conserve the spatial and temporal pulse properties while properly inverting the wavefront. In particular, the optical configuration used, and the input laser parameters are shown to have a large influence on the reflection quality. The capacity to be applied to arbitrary temporal shapes is demonstrated for the first time and paves the way to the usage of phase conjugate mirrors for broader applications requiring unusual temporal shapes. The phase conjugate mirror stability and reliability are considered for the usage in industrial commercial laser systems and no particular erratic behaviour is identified. The limits of the components are investigated up to unprecedented input energy and wavefront aberrations levels making this component compatible for laser sources up to the kilojoule energy level.Les lasers de forte énergie sont des outils d'importance croissante dans de nombreux domaines scientifiques et industriels tel que la physique des plasma, l'usinage ou encore en tant que source d'énergie pour d'autres types de lasers. A l'heure actuelle, une des problématiques liées à ces lasers est leur cadence de tir, limitant le nombre total de tir pouvant être réalisés par les utilisateurs. Cette limite provient de l'accumulation de chaleur tirs après tirs au sein des amplificateurs et dégradant le front d'onde des impulsions lasers jusqu'à le rendre inutilisable. Cette thèse, issue de la collaboration entre un industriel du laser: Amplitude laser, le Laboratoire Charles Fabry et le Laboratoire pour l'Utilisation des Lasers Intenses (LULI), cherche à développer un composant optique permettant de corriger ces aberrations de front d'onde. Ce composant, appelé miroir à conjugaison de phase, permet la réflexion d'une impulsion laser en utilisant un effet non linéaire: la diffusion Brillouin stimulée. Lors de cette réflexion, le front d'onde et les aberrations optiques sont inversée et il est alors possible d'autocompenser tous les défauts introduit lors de l'amplification en réalisant un aller-retour dans les amplificateurs, avec inversion du front d'onde au milieu de cet aller-retour. Ce travail reprend la description physique de la diffusion Brillouin stimulée pour développer des modèles numériques permettant la simulation de la réflexion. Ces modèles numériques sont utilisés pour le design de miroirs à conjugaison de phase expérimentaux dont les propriétés sont caractérisées. Cette thèse cherche en particulier à optimiser la fidélité de la réflexion, c'est à dire la capacité à conserver les propriétés spatiales et temporelles de l'impulsion incidente tout en inversant le front d'onde. Les études numériques montrent que la configuration optique et les paramètres du laser ont une forte influence sur la qualité de la réflexion. Nous démontrons pour la première fois la capacité à conserver une forme temporelle arbitraire lors de la conjugaison de phase et rend possible l'utilisation du composant pour une nouvelle gamme d'application exigeant des formes temporelles diverses. La stabilité et fiabilité du miroir à conjugaison de phase est étudiée dans le but de l'introduire à terme dans des systèmes commerciaux, aucun comportement erratique n'est identifié lors de son utilisation sous les conditions d'utilisation nominales. Les limites d'utilisations du composant sont recherchées et son bon fonctionnement est démontré jusqu'à des niveaux d'énergie et d'aberration incidente record, rendant pour la première fois son utilisation possible sur des chaînes lasers de classe kilojoule
Radiative Corbino effect in nonreciprocal many-body systems
No full textInternational audienceWhen a magnetic field is applied in the perpendicular direction to a metallic disk under the action of a radial bias voltage, a tangential electric current superimposes to the radial current due to the presence of the Lorentz force which acts on electrons. Here we introduce a thermal analog of this Corbino effect in many-body systems made of nonreciprocal bodies which interact by exchanging photons in near-field regime. In systems out of thermal equilibrium with a radial temperature gradient, we demonstrate that the Poynting field in the Corbino geometry is bent in presence of an external magnetic field, giving rise to a tangential heat flux. This thermomagnetic effect could find applications in the field of thermal management and energy conversion at nanoscale
Choice of numerical implementation of spatial contrast calculation impacts microcirculation quantitation in laser speckle contrast imaging
No full textInternational audienceLaser speckle contrast imaging (LSCI) allows noninvasive imaging of microcirculation. Its scope of clinical applications is growing, yet the literature lacks a comparison of the accuracy of methods used to compute the spatial contrast Ks from which the blood flow index is derived.Aim: We aim to evaluate the impact on flow quantitation of different computational approaches used to derive Ks .Approach: We compare numerical calculation of Ks in Python and ImageJ applied to noise-free simulated data and to experimental data acquired in vivo in anesthetized mice. The estimation of the decorrelation time τc, inversely proportional to the blood flow index, is carried out following two approaches: LSCI asymptotic estimation and fitting the multiple exposure speckle imaging (MESI) model to Ks(T).Results: For simulation data, we found variations of up to 58% for the blood flow index in the LSCI approach. Nonlinear fitting of the MESI model was less affected with discrepancies of only a few percent. Considering experimental data, the LSCI approximation led to K s with relative differences (up to 35%) depending on the calculation methods. The noise and limited exposure time strongly limited the accuracy of the LSCI asymptotic estimation. Adjustment of the MESI model to the data led to consistent values of τc in the 0.05 to 1 ms range with significant variations depending on the method used to calculate s.Conclusions: Numerical methods used to calculate Ks should be precisely acknowledged and validated against direct calculation to ensure accuracy. Uniform filter approach leads to accurate Ks values and is 100 times more computationally efficient than the Dir ect calculation. Other investigated methods lead to various levels of errors in flow index estimation using LSCI. Errors are minimized using larger kernels. MESI derivation of τc is not immune but less affected by such methodological biases
Hybrid integration of crystalline doped oxides on silicon photonics platform for optical communications (Conference Presentation)
No full textInternational audienceSilicon photonics has been extensively developed as a platform to address future challenges across several applications, including datacom, sensing, and optical communications, among others. However, the inherent properties of silicon alone are insufficient to overcome all limitations in terms of speed, power consumption, and scalability. Consequently, new strategies have been pursued, focusing on the hybrid integration of novel materials within the silicon photonics platform. In this context, we will introduce recent advances in the hybrid integration of doped crystalline oxides on silicon and silicon nitride photonics platforms. Particularly, the integration of doped Zirconia (ZrO2) based oxides is promising due to its compatibility with silicon technology. ZrO2 exhibits excellent linear and nonlinear optical properties. Furthermore, the doping of ZrO2 can be carried out to develop active photonic devices with strong second- and third-order nonlinearities and light emission. We will present an overview of the recent advances in the hybrid integration of zirconia-based crystalline oxides
Relax and penalize: a new bilevel approach to mixed-binary hyperparameter optimization
No full textInternational audienceIn recent years, bilevel approaches have become very popular to efficiently estimate high-dimensional hyperparameters of machine learning models. However, to date, binary parameters are handled by continuous relaxation and rounding strategies, which could lead to inconsistent solutions. In this context, we tackle the challenging optimization of mixed-binary hyperparameters by resorting to an equivalent continuous bilevel reformulation based on an appropriate penalty term. We propose an algorithmic framework that, under suitable assumptions, is guaranteed to provide mixed-binary solutions. Moreover, the generality of the method allows to safely use existing continuous bilevel solvers within the proposed framework. We evaluate the performance of our approach for two specific machine learning problems, i.e., the estimation of the group-sparsity structure in regression problems and the data distillation problem. The reported results show that our method is competitive with state-of-the-art approaches based on relaxation and roundin
Multimodal Explainable Automated Diagnosis of Autistic Spectrum Disorder
No full textInternational audienceAutism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterized by symptoms affecting social interaction, communication, and behavior, with diagnosis complicated by significant individual variability and the absence of definitive biomarkers. Current artificial intelligence methods have improved diagnostic accuracy, but their reliance on subjective assessments or single-modal data, coupled with their ``black-box" nature, limits consistency and clinical applicability. Addressing current limitations, this paper introduces a multimodal ASD detection framework using deep neural networks (DNN) with explainable AI (xAI) to enhance model transparency. Our model achieves a mean 5-fold cross-validation accuracy of 98.64% ( 0.86%), surpassing existing methods and demonstrating potential for clinical dependability of ASD diagnoses. The source code is available at: https://github.com/mebenyahia/Multimodal-Explainable-Automated-Diagnosis-of-Autistic-Spectrum-Disorde
In situ infrared spectroscopic ellipsometry as a tool to probe the formation of sol–gel based mesoporous films
No full textInternational audienceProbing the formation of sol-gel mesoporous films and characterizing them under environmental/in-operando conditions represents an important challenge to optimize their performances. Obtaining a complete picture of the system usually requires a combination of multiple techniques. In this work, we introduce in situ infrared (IR) ellipsometry equipped with an environmental chamber as a tool to follow simultaneously the evolution of structural, optical and chemical properties during the formation of sol-gel derived mesoporous films. As a case study, we investigate the formation of mesoporous TiO2 by comparing a conventional thermal treatment and a low-temperature annealing by UV irradiation. In both cases, the structural optical and chemical evolution could be monitored during the IR ellipsometric experiment. Interestingly, UV-annealing allows the fabrication of mesoporous TiO2 films at low temperatures enabling the formation of plasmonic mesoporous composites. At last, we critically discuss the advantages and drawbacks of IR ellipsometry for in situ investigations compared to conventional UV-visible ellipsometry by providing additional insights for future developments
Classification of S-Boxes According to Hardware Implementation Cost
No full textInternational audienc
Unveiling Solvent Effects on Femtosecond Laser-Irradiated Au/Fe₃O₄ Colloidal Nanoparticles: The Acetone Effect
No full textThe interplay between laser parameters and liquid environments dictates the outcome of femtosecond laser-induced nanoparticle modification. We present a study of gold and iron oxide nanoparticles in water and a water-acetone mixture, irradiated with femtosecond lasers at 808 nm and 404 nm. While aggregation was observed in pure water at both wavelengths, the results revealed a strong stability and a rather unexpected wavelength-dependency in the acetone-water mixture. In this case, 808 nm irradiation produced some decrease in nanoparticle sizes, while 404 nm led to some nanoparticle growth. As a result, the acetone effect is found to be twofold: (i) on one hand, it helps to prevent aggregation; (ii) on the other hand, it acts as a reactive medium allowing to tune the nanoparticle size and composition simply by changing laser wavelength. So, this work emphasizes that solvent physical properties as well as laser-induced chemical processes in the solvent are not merely secondary effects but can dominate the final morphological outcome, providing a predictive framework for nanoparticle synthesis