HAL Portal IOGS (nstitut d'Optique Graduate School)
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Directional Superradiance in a Driven Ultracold Atomic Gas in Free-Space
No full textInternational audienceUltracold 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 [Ferioli Nat. Phys. 19, 1345 (2023)] has reported measurements that have 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 steady-state properties of the system as a function of the driving strength and atom number N . We observe signatures of collective effects in the weak-driving 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 nonequilibrium transition, also known as cooperative resonance fluorescence, we observe significant qualitative and quantitative differences, including a different scaling of the critical drive parameter (from N to N ). 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
Light-matter interaction at rough surfaces: A morphological perspective on laser-induced periodic surface structures
No full textInternational audienceWe use ab initio electromagnetic simulations to investigate light absorption by rough surfaces in the context of the formation of laser-induced periodic surface structures. Our approach involves modeling a rough surface using a statistical description of its continuous height distribution via a corresponding correlation function. We study the influence of incident light polarization and various statistical properties of surface roughness, such as root-mean-square height and correlation length, on the distribution of absorbed laser energy. By analyzing light absorption in different layers of the surface selvedge, we elucidate how different features of surface morphology influence the shape of the resulting periodic surface structures. We show that circularly polarized laser pulses are highly sensitive to initial or progressively developing asymmetries in surface roughness
Nonequilibrium electronic properties and stability consequences in metallic crystalline binary alloys under ultrafast laser excitation
No full textInternational audienceThis study undertakes an exhaustive exploration of properties under electron-phonon nonequilibrium for a series of ten industrially pertinent crystalline alloys, namely AlCu, AlNi, AlTi, AuCu, CuNi, CuTi, NbZr, NiTi, NiZr, and ZrCu. This comprehensive investigation encompasses electronic heat capacity, electron-phonon coupling factor, excited electron density, electronic pressure, dielectric function, refractive index, extinction coefficient, reflectivity, and optical penetration depth. Employing ab initio simulations, we meticulously analyze the nonequilibrium optoelectronic attributes inherent to these alloys change under electronic excitation across the temperature range going from 0 to 50,000 K. Additionally, to contextualize the findings, we employ classical Molecular Dynamics coupled with a Two Temperature Model simulation, notably to deliver a deep comprehension of the intricate mechanisms of the CuTi alloy ablation induced by femtosecond laser irradiation
QOSST : A Highly Modular Open Source Software for Continuous-Variable Quantum Key Distribution
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Unsupervised Learning and Effective Complexity: introducing JPG and Neural Sophistication
No full textInternational audienceMeasuring the complexity of arbitrary data has been of interest to many scientific domains, including machine learning and particularly unsupervised learning. In this paper, we cover relevant concepts including Kolmogorov complexity, entropy and minimum description length. We argue that these measures alone are failing to distinguish noise from meaningful complexity. We push for the concept sophistication which measures the complexity of the structured part of the data, ignoring unstructured noise. This concept is reified in two manners: using image compression algorithms and using autoencoder
Overview of Radiation Effects on Silica-Based Optical Fibers and Fiber Sensors
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Distillation des connaissances basée sur des enseignants monoclasses dans une infrastructure Edge utilisant des données non étiquetées
No full textInternational audienceEdge computing applications face significantchallenges due to the scarcity of labeled data and limitedcomputational resources. This paper introduces a noveldistillation method that leverages synthetic data and federatedlearning to address these challenges. The proposed methodinvolves pre-training multiple monoclass teacher models onsynthetic data, which are then used to distill knowledge into astudent network using real, unlabeled data on edge devices. Thisapproach optimizes model performance and resource utilizationwhile preserving data privacy. Experimental evaluations onMNIST, USPS, and grayscale SVHN datasets demonstrate thefeasibility and effectiveness of our method, highlighting itspotential and limitations for various edge computing scenarios
A bi-component model to assess the rheology of soft cellular aggregates probed using the micropipette aspiration technique
No full textInternational audienceThe micro-pipette aspiration technique is a classical experiment used to characterize the physical properties of inert fluids and biological soft materials such as cellular aggregates. The physical parameters of the fluid, as viscosity and interfacial tension, are obtained by studying how the fluid enters the pipette when the suction pressure is increased and how it relaxes when the suction pressure is put to zero. A mathematical model representative of the experiment is needed to extrapolate the physical parameters of the fluid-like matter; however, for biological materials as cells or cell aggregates mathematical models are always based on strong starting hypotheses that impact the significance of the identified parameters. In this article, starting from the bi-constituent nature of the cell aggregate, we derive a general mathematical model based of a Cahn–Hilliard–Navier–Stokes set of equations. The model is applied to describe quantitatively the aspiration-retraction dynamics of a cell-aggregate into and out of a pipette. We demonstrate the predictive capability of the model and highlight the impact of the assumptions made on the identified parameters by studying two cases: one with a non-wetting condition between the cells and the wall of the pipette (classical assumption in the literature) and the second one, which is more realistic, with a partial wetting condition (contact angle θs = 150°). Furthermore, our results provide a purely physical explanation to the asymmetry between the aspiration and retraction responses which is alternative to the proposed hypothesis of an mechano-responsive alteration of the surface tension of the cell aggregate
Surface plasmon resonance imaging signal amplification for the detection of micro-RNAs in the context of organ donation
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Stable 500 kW average power of infrared light in a finesse 35 000 enhancement cavity
No full textInternational audienceAdvances in laser technology over the past 25 years have been impressive, in particular, for the Ytterbium technology where, nowadays, kilowatt-class laser systems are available. This technology also led to the possibility to provide hundreds of kilowatts of laser power by the use of enhancement cavities. We report here on the demonstration of a stable 500 kW average laser power in a high-finesse enhancement cavity. It paves the way toward systems providing laser power in excess of 1 MW and opens the door to a breakthrough in a variety of future applications