HAL Portal IOGS (nstitut d'Optique Graduate School)
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Hybrid silica optical nanofibers for efficient Raman converters
No full textInternational audienceOver the last twenty years, silica optical nanofibers have been widely exploited for a large range of potential applications in several areas of research such as atomic manipulation, plasmonic or sensing. Indeed, these devices present original properties of light propagation characterized by a high confinement of the optical mode in the uniform part and the presence of a strong evanescent field in the vicinity of the surface. This last property offers a new degree of freedom for the realization of nonlinear effects by changing the surrounding medium. We will firstly illustrate this option by generating Stimulated Raman Scattering with very high conversion efficiencies using nanofibers immersed in different liquids. Secondly, we will investigate the functionalization of nanofibers by coating them with thin layers of nonlinear materials. These studies open the way to the realization of low cost and low-loss in-line devices based on hybrid silica optical nanofibers.Keywords—Optical Nanofiber, Stimulated Raman Scattering, coating, TiO2, PMM
LaPDA: Latent-Space Point Cloud Denoising With Adaptivity
No full textInternational audiencePoint cloud denoising is a fundamental yet challenging task in computer graphics. Existing solutions typically rely on supervised training on synthesized noise. However, real-world noise often exhibits greater complexity, causing learning-based methods trained on synthetic noise to struggle when encountering unseen noise–a phenomenon we refer to as noise misalignment. To address this challenge, we propose LaPDA (Latent-space Point cloud Denoising with Adaptivity), a neural network explicitly designed to mitigate noise misalignment and enhance denoising robustness. LaPDA consists of two key stages. First, we adaptively model noise in the latent space, aligning unseen noise distributions with the known training distributions or adjusting them toward distributions with lower noise scales. Training objectives at this stage are formulated based on controlled synthetic noise with varying intensity levels. Second, we introduce a gradual noise removal module that optimizes the spatial distribution of the adaptively adjusted noisy points. Extensive experiments conducted on both synthetic and scanned datasets demonstrate that LaPDA achieves enhanced accuracy and robustness compared to state-of-the-art methods. We will make the source code and test models publicly available
Detection module to increase the spatial bandwidth product of a microscope
No full textInternational audienceCapturing biological specimens at large scales with sub-micron resolution is crucial for biomedical research, but conventional cameras often cannot handle the pixel requirements. While most microscopes use motorized stages to move samples and capture images tile by tile, we propose a method that eliminates the need for sample movement. Our approach integrates a scanning mechanism within the microscope’s detection unit, enabling sequential capture of sub-areas of the field of view (FOV) without physically moving the sample. This “remote scanning” method works with all camera-based microscopes, and we demonstrate its effectiveness in both bright-field and epifluorescence microscopy. This technique is ideal for imaging motion-sensitive samples and large biological specimens, as evidenced in this study with millimeter-scale engineered tissues
Theory of superlensing with complex frequency illuminations
No full textRecent experiments have demonstrated that the resolution of superlensing slabs can be significantly enhanced with complex frequency illuminations. In this study, we introduce a novel theoretical framework for analyzing superlensing. The framework offers new and transparent insights. It helps clarify what resolution can be expected with complex frequency, or more generally pulse illuminations, but it also highlights inherent limitations and tempers high expectations raised by the recent electromagnetic experiments
Interaction Réglable de Champ Moyen et au-delà du Champ Moyen pour un Condensat de Bose-Einstein dans un mélange cohérent de spin
No full textThis thesis explores mean-field and beyond mean-field interactions in a coherently coupled two-component Bose-Einstein condensate (BEC) of ³⁹K atoms. We first establish the theoretical framework describing the atomic structure of potassium-39, interatomic interactions, and Feshbach resonances. We then introduce the mean-field and beyond mean-field theories relevant to ultracold Bose gases. In addition, we describe the physics of coherently coupled two-level systems, including radio-frequency coupling, adiabatic sweeps, and scattering lengths in the dressed-state basis. A major part of the experimental work focuses on developing a high-precision magnetic field stabilization system, essential for accurate and reliable measurements in spinor BECs. We investigate the mean-field interaction energy of a coherently coupled two-component BEC in the lower dressed-state basis, both theoretically and experimentally. The results reveal nonlinearities and saturation effects as functions of density. We then explore the regime of quantum droplets, aiming at observing self-bound states stabilized by quantum fluctuations beyond the mean-field approximation. Finally, we present an experiment on BEC tunneling through optical barriers created using a Digital Micromirror Device and a blue-detuned laser, demonstrating the transition from classical to quantum tunneling and revealing Fabry-Pérot-like interference in matter waves.Cette thèse explore les interactions de champ moyen et au-delà du champ moyen dans un condensat de Bose-Einstein (BEC) à deux composantes de ³⁹K couplé de manière cohérente. Nous établissons d'abord le cadre théorique décrivant la structure atomique du potassium-39, les interactions interatomiques et les résonances de Feshbach. Nous présentons ensuite les théories de champ moyen et les corrections au-delà du champ moyen pertinentes pour les gaz de Bose ultrafroids. Nous décrivons également la physique des systèmes à deux niveaux couplés de manière cohérente, y compris le couplage radiofréquence, les balayages adiabatiques et les longueurs de diffusion dans la base des états habillés. Une grande partie du travail expérimental porte sur le développement d'un système de stabilisation du champ magnétique à haute précision, essentiel pour des mesures fiables et précises dans un BEC à deux composantes. Nous étudions l'énergie d'interaction de champ moyen dans un BEC à deux composantes couplés de manière cohérente, préparé dans l'état habillé, à la fois théoriquement et expérimentalement. Les résultats révèlent des non-linéarités et des effets de saturation en fonction de la densité. Nous explorons ensuite le régime des gouttelettes quantiques, dans le but d'observer des états autoliés stabilisés par les fluctuations quantiques au-delà de l'approximation de champ moyen. Enfin, nous présentons une expérience de tunneling d'un BEC à travers des barrières optiques créées à l'aide d'un dispositif à micromiroirs numériques (DMD) et d'un laser désaccorder vers le bleu, démontrant la transition entre les régimes de l'effet tunnel classique et quantique, ainsi que l'apparition d'interférences de type Fabry-Pérot dans les ondes de matière
Reversible superdeformability of hiPSC epithelial cortinoids
No full textAbstract Epithelial cortinoids, fluid filled shells formed from induced pluripotent stem cells (iPSCs), must accommodate large deformations during growth and morphogenesis. Using inflation–deflation assays and high-resolution imaging, we find that these fluid-filled shells are weakly-pressurized and achieve extreme deformability through reversible soft modes of deformation accommodated by the cytoskeleton. We show that cytoskeletal elements such as actin localized along lateral cell edges undergo tilt and bend instabilities that buffer mechanical load by decoupling apico–basal stretching from lateral extension. These reversible instabilities act as elastic safety valves, permitting large shape changes without loss of epithelial hydraulic and topological integrity. A minimal theoretical and computational model demonstrates how tilt and bend reduce effective resistance to radial thinning and explains the observed pressure–strain softening. Thus, iPSC shells exploit reversible cytoskeletal instabilities as mechanical buffers, enabling robust tolerance of large deformations in developing epithelia
Probing the local rapidity distribution of a one-dimensional Bose gas
No full textInternational audienceOne dimensional Bose gases with contact repulsive interactions belong to the class of integrable systems, which renders their dynamics and the description of the stationary states very different from that of chaotic systems. The stationary states are parameterized by a function, dubbed the rapidity distribution. The latter can be seen as the velocity distribution of the quasiparticles of infinite lifetime in the system. The rapidity distribution is also homothetic to the density profile of the gas after expansion in 1D to very large distances. The latter property can be used to measure the rapidity distribution. In system with long scale spatial variations, one can describe the system introducing a spatially-dependent local rapidity distribution. This key notion is at the heart of the Generalized Hydrodynamic theory, which gives a prediction for the time evolution of the spatially-dependent rapidity-distribution. In our cold atom experiment, we implemented a method to measure the local rapidity distribution, based on a method to select a slice of the cloud [1]. This permits us to reconstruct the spatially-dependent rapidity distribution of a gas. We used this techniques to probe different out-of-equilibrium dynamics, including the bi-partide quench protocol [2]
Effective pulse number and energy distribution uniformity in laser scanning with Gaussian and cylindrical beams
No full textInternational audienceThis study presents a method for calculating the effective pulse number and energy dose uniformity in pulsed laser scanning for surface irradiation. The analysis focuses on Gaussian and cylindrical beams, comparing their effects on surface uniformity and energy distribution. Using a polynomial decomposition approaches from Taylor series expansion and surface regression, derived closed-form solutions for the effective pulse number are proposed, leading to a uniformity index to quantify energy distribution across surfaces. The results demonstrate that a hexagonal irradiation grid provides as comparable surface uniformity of the deposited energy with the same effective pulse number compared to a square grid for both beam types. The proposed method enhances accuracy in predicting the effective pulse number in reasonable computing time, simplifying dose computation and optimizing laser surface treatment for industrial applications and research activities
Evaluating the impact of LED spectral variations on color discrimination in low vision
No full textInternational audiencePurposeLED lighting systems exhibit peak wavelength deviations over time due to manufacturing differences and environmental factors such as temperature fluctuations. Additionally, functional white light composed of multiple colored LEDs may present wavelength variations for the same perceived color across different productions. Understanding how these variations influence color perception of low vision people is essential for developing smart white LED sources tailored to their visual needs. This study investigates whether small shifts in the spectral power distribution (SPD) significantly impact color perception of low vision people.MethodsA psychophysical experiment was conducted in which participants without color arranged 15 Munsell color samples under 13 different narrow-band lighting conditions. The lighting systems used -LEDcube-I14 (Thouslite), Dittosizer Light Player (Telelumen) and LED Flood Lights (L&E) -covered peak wavelengths from 468nm to 663nm providing narrow-band illumination in the blue, green and red ranges.The experiment involved two groups of 16 participants wearing low vision simulation goggles (central scotoma, 20/200 visual acuity) and a control group of 16 participants with normal or corrected-to-normal vision. Color discrimination scores were calculated using a modified Farnsworth-Munsell 100-Hue test. Statistical analysis was performed using the Shapiro-Wilk test and a nonparametric permutation test with Bonferroni correction (p < 0.05).ResultsRed-light channels (622nm-663nm) yielded the highest error scores but showed more homogenous distributions. The control group performed better for the blue and green channels (468nm-477nm and 515nm-529nm) whereas participants with simulated low vision experienced more difficulties with these wavelengths. However, statistical analysis using permutation tests revealed no significant differences across lighting conditions and color patches.ConclusionsStatistical analysis of experimental data suggests that small SPD shifts do not significantly affect color perception of individuals with simulated central scotoma. These results are highly encouraging as they suggest that the small differences and instabilities over time of the LED lighting systems do not introduce biases into psychophysical experiments aimed at studying how light affects color discrimination in a low vision context. Future studies should extend this analysis to other visual impairments, such as blurred or tunnel vision, to assess broader implications for lighting design and accessibility
