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Directional superradiance in a driven ultracold atomic gas in free-space
No full text25 pages, 19 figuresUltra-cold atomic systems are among the most promising platforms that have the potential to shed light on the complex behavior of many-body quantum systems. One prominent example is the case of a dense ensemble illuminated by a strong coherent drive while interacting via dipole-dipole interactions. Despite being subjected to intense investigations, this system retains many open questions. A recent experiment carried out in a pencil-shaped geometry reported measurements that seemed consistent with the emergence of strong collective effects in the form of a ``superradiant'' phase transition in free space, when looking at the light emission properties in the forward direction. Motivated by the experimental observations, we carry out a systematic theoretical analysis of the system's steady-state properties as a function of the driving strength and atom number, . We observe signatures of collective effects in the weak drive regime, which disappear with increasing drive strength as the system evolves into a single-particle-like mixed state comprised of randomly aligned dipoles. Although the steady-state features some similarities to the reported superradiant to normal non-equilibrium transition, also known as cooperative resonance fluorescence, we observe significant qualitative and quantitative differences, including a different scaling of the critical drive parameter (from to ). We validate the applicability of a mean-field treatment to capture the steady-state dynamics under currently accessible conditions. Furthermore, we develop a simple theoretical model that explains the scaling properties by accounting for interaction-induced inhomogeneous effects and spontaneous emission, which are intrinsic features of interacting disordered arrays in free space
Première application d’un dispositif couplant LC-OCT et microspectroscopie Raman confocale pour la caractérisation de pièces opératoires de carcinomes basocellulaires à l’échelle micrométrique
No full textInternational audienc
Ultra-fast Laser texturing : A New Approach for Deterministic Graphene Folds
No full textInternational audienceGraphene exhibits physical and chemical properties that depend on its curvature [1,2]. The creation of deterministic or stochastic folded graphene fields is an active research area [3]. The goal of the presented work is to create an original method for the realization of such folded graphene surfaces with tunable multiscale texturing through the use of ultra fast lasers. The idea is to attach planar graphene to a pre-patterned elastomer so that, very similarly to japanese origami, folds related to the patterns will appear when compressing the substrate. Ultrashort pulsed-laser irradiation is a well-known technique for nanotexturing surfaces, due in particular to its ability to spontaneously create periodic nanostructures with a spatial period close to the laser wavelength or lower. It is also known for creating "smooth" craters as the heat affected zone can be minimal. Which allows using tight focusing and spatial shaping such as Bessel beams to obtain well defined holes in the elastomer at the sub-micrometric scale without relying on periodic surface generation. The advantages of using ultrashort pulses for nanopatterning over other methods are numerous: there is no chemistry involved, the sample can be processed in air, at very high speed, a lot of different patterns at the nanoscale can be attained, which makes it one of the most versatile methods for what we intend.We present here our first results on this innovative approach. The laser induced pattern was carved out in the form of matrices at first, reaching the nanometric scale for the periodicity of our folds. We demonstrate the creation of wrinkles on the graphene surface with a deterministic, reversible and even repeatable (several fold/unfold processes) behavior. In order to characterize this behavior we use Atomic Force Microscopy, to provide the graphene height profile as function of the elastomer stress percentage, and Raman (D-band) Spectroscopy probes the defaults (or lack thereof) created during the process
Femtosecond Laser-Induced Oxidation Mechanism on Tungsten Surfaces
No full textInternational audienceLaser-induced surface chemistry modifications, specially laser-assisted oxidation on metals is gaining a lot of attention in recent years. There have been several studies on its necessity for nanostructure formation by femtosecond laser irradiation, which seems to be insignificant for HSFLs (High Spatial frequency LIPSS) on tungsten (W) [1]. The questions associated with the incorporation of oxygen during laser irradiation is still under debate, with the deposition of a large amount of energy over a small region involving the sudden variation of the temperature gradients that is linked with ultrafast laser processing. Femtosecond laser-assisted processes result in fast cooling rates, in order of 1011 K/s - 1013 K/s [2,3] of the irradiated surfaces. Hence, the oxidation associated becomes comparable to the collision rate of atoms on the surface making it complex to combine the related surface chemical modifications with classical oxidation kinetics.In this work, we are analyzing the oxidation mechanism during HSFL formation on Tungsten samples by femtosecond irradiation, by utilizing surface chemical characterization techniques like XPS, STEM-EDX and contact angle measurements. HSFLs obtained in ambient and vacuum conditions upon femtosecond irradiations help us to address the chemical state after laser irradiation. By associating it with oxidation mechanisms and phase diagram of the resulting tungsten oxide and by combining it with numerical simulations, an oxidation mechanism associated with HSFL formation on W is established.References[1] P. Dominic, F. Bourquard, S. Reynaud, A. Weck, J.-P. Colombier, and F. Garrelie, “On the Insignificant Role of the Oxidation Process on Ultrafast High-Spatial-Frequency LIPSS Formation on Tungsten,” Nanomaterials, vol. 11, no. 5, p. 1069, Apr. 2021, doi: 10.3390/nano11051069.[2] W. H. Duff and L. V. Zhigilei, “Computational study of cooling rates and recrystallization kinetics in short pulse laser quenching of metal targets,” J. Phys. Conf. Ser., vol. 59, pp. 413–417, Apr. 2007, doi: 10.1088/1742-6596/59/1/088.[3] M. He, E. T. Karim, M. V. Shugaev, and L. V. Zhigilei, “Atomistic simulation of the generation of vacancies in rapid crystallization of metals,” Acta Mater., vol. 203, p. 116465, Jan. 2021, doi: 10.1016/j.actamat.2020.11.007
Radiation Detection with Radiosensitive Pure-Silica Core Ultra-Low Loss Optical Fiber
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Deep stacking ensemble learning applied to profiling side-channel attacks
No full textCARDIS 2023 - 22nd International Conference, CARDIS 2023, Amsterdam, The Netherlands, November 14–16, 2023, Revised Selected PapersInternational audienceDeep Learning is nowadays widely used by security evaluators to conduct side-channel attacks, especially in profiling attacks that allow a supervised learning phase. However, designing an efficient neural network model in a side-channel attack context can be a difficult task that may require a laborious hyperparameterization process. Hyperparameter selection is known to be a challenging problem in Deep Learning, while being a crucial factor for neural networks performances. Recent works investigate the so-called Deep Ensemble Learning in the side-channel context. It consists in using multiple neural networks in a single predictive task and aggregating the several predictions in an opportune way. The intuition behind is to use the power of numbers to improve the attack performance. In this work, we propose to use Stacking as an aggregation method, in which a meta-model is trained to learn the best way to combine the output class probabilities of the ensemble networks. Our proposal is supported by several experimental results, that allow to conclude that the use of Stacking can relieve the security evaluator from performing a fine hyperparameterization
Bloch mode analysis of subwavelength polarizing planar optics
No full textInternational audienceThe purpose of this paper is to investigate the impact of advanced immersion lithography process for the development of polarization optics at pixel level on CMOS image sensors. In the first part of this paper, we use Bloch formalism to define regimes that depend on the number of propagative Bloch modes within the structure. The presented analysis gives estimations of required features size to operate in NIR and visible range. The second part of this paper present optical characterization of silicon lamellar grating made on 300 mm wafer using advanced immersion lithography. Characterization results are discussed with respect to optical simulations and reconstructed grating profile is compared to patterning features estimated during first part
Étude des propriétés optiques des matériaux nanocomposites métal-diélectrique induites par laser
No full textStructural color arises from the interaction between light and nanostructured materials. The ability to control the structural colors of nanomaterials has generated significant interest in metasurface research due to their potential for a wide range of industrial applications, such as vision, display technologies, data encoding, and anti-counterfeiting. However, scaling up production presents a major challenge for customizing the metasurfaces. Laser-based techniques offer a potential approach to overcome such challenge owning to the simplicity, cost-effectiveness and scalability. Nonetheless, the laser-induced materials typically suffer from intrinsic imperfections, including variable particle sizes, imperfect organization, and different surface morphologies. The optical responses of the material are thus characterized by their statistical properties, making the researches in color properties challenging.This thesis investigates the optical properties of mesoporous TiO2 thin films impregnated with small silver particles deposited on glass substrates processed by lasers. The main objectives of this thesis are to understand how structural inhomogeneities affect the optical properties of the materials and the mechanisms that lead to different diffracted and dichroic properties arising from their statistical properties.The thesis consists of five main parts. The first part presents the theoretical background, including the plasmonic resonance properties of silver particles under varying opto-geometrical parameters. The simulation method based on T-matrix and scattering matrix formalisms to solve the multiple scattering problem is also discussed. The second part presents the experimental techniques used in this thesis, including the preparation of sol-gel-based samples, laser printing experiments, and color characterization. The following three parts contain the main results of the thesis. Part 3 focuses on the study of dichroism arising from the coupling between plasmonic and photonic resonances excited within laser-structured films. It investigates how statistical properties affect simulation results and the influence of imperfections on the optical properties. Part 4 concentrates on the colored properties, diffraction, and dichroism of materials controlled by nanosecond laser pulses. The mechanisms by which certain samples exhibit diffraction, some show dichroism, while others exhibit both optical phenomena simultaneously, are also addressed. Finally, Part 5 examines the diffraction properties of structures with three superimposed gratings induced by a continuous laser.La couleur structurale résulte de l'interaction entre la lumière et les matériaux nanostructurés. La capacité à contrôler les couleurs structurales des nanomatériaux a suscité un intérêt significatif dans la recherche sur les métasurfaces en raison de leur potentiel pour une large gamme d'applications industrielles, telles que la vision, les technologies d'affichage, l'encodage de données et la lutte contre la contrefaçon. Cependant, l'augmentation de la production représente un défi majeur pour la personnalisation des métasurfaces. Les techniques basées sur le laser offrent une approche potentielle pour surmonter un tel défi en raison de leur simplicité, de leur rentabilité et de leur évolutivité. Cependant, les matériaux induits par laser souffrent généralement d'imperfections intrinsèques, notamment des tailles de particules variables, une organisation imparfaite et des morphologies de surface différentes. Les réponses optiques du matériau sont donc caractérisées par leurs propriétés statistiques, ce qui rend la recherche sur les propriétés colorées difficile.Cette thèse étudie les propriétés optiques de films minces de TiO2 mésoporeux imprégnés de petites particules d'argent déposées sur des substrats en verre et traitées par des lasers. Les principaux objectifs de cette thèse sont de comprendre comment les inhomogénéités structurales affectent les propriétés optiques des matériaux et les mécanismes qui conduisent à différentes propriétés de diffraction et de dichroïsme résultant de leurs propriétés statistiques.La thèse se compose de cinq parties principales. La première partie présente le contexte théorique, y compris les propriétés de résonance plasmonique des particules d'argent en fonction des paramètres opto-géométriques variables. La méthode de simulation basée sur les formalismes de la matrice T et de la matrice de diffusion pour résoudre le problème de la diffusion multiple est également discutée. La deuxième partie présente les techniques expérimentales utilisées dans cette thèse, notamment la préparation d'échantillons à base de sol-gel, les expériences d'impression laser et la caractérisation des couleurs. Les trois parties suivantes contiennent les principaux résultats de la thèse. La partie 3 se concentre sur l'étude du dichroïsme résultant du couplage entre les résonances plasmoniques et photoniques excitées dans les films structurés par laser. Elle examine comment les propriétés statistiques affectent les résultats des simulations et l'influence des imperfections sur les propriétés optiques. La partie 4 se concentre sur les propriétés colorées, la diffraction et le dichroïsme des matériaux contrôlés par des impulsions laser de nanoseconde. Les mécanismes par lesquels certains échantillons présentent une diffraction, d'autres montrent un dichroïsme, tandis que d'autres présentent simultanément les deux phénomènes optiques sont également abordés. Enfin, la partie 5 examine les propriétés de diffraction de structures comportant trois réseaux superposés induits par un laser continu