Publications

2025

• Integration of acoustic constraints in trajectory generation
  
  • Hoareau Damien
  • Berrah Danil
  • Tillet Joris
  • Chapoutot Alexandre
  , 2025, pp.1-7. <div><p>Optimal control based trajectory generation offers the ability to formulate complex problems while optimizing the performance of the system control inputs. Nevertheless, adding acoustic constraints to the optimal control problem (OCP) can be highly challenging. The classical resolution approach employs a "first-discretize-then-optimize" strategy using direct methods. However, this approach leads to significant computational costs, which in turn limits its applicability. Recent studies suggest using the acoustic reciprocity theorem (ART) to formulate the problem as one of obstacles collision avoidance. Hence, this study proposes investigating the formulation and solution of an OCP based on this theorem. The ART is combined with the Boundary Element Method (BEM) for the acoustic part of the problem. The OCP is implemented using successive convexification (SCvx) approach which offers a convenient framework to take into account acoustic constraint in trajectory planning generation. Promising experimental results highlight the applicability of our formulation based on the ART and guidelines for further development are provided.</p></div> (10.1109/CoDIT66093.2025.11321899)
  DOI : 10.1109/CoDIT66093.2025.11321899
• SCvx-Frank-Wolfe Algorithm
  
  • Berrah Danil
  • Hoareau Damien
  • Chapoutot Alexandre
  , 2025, pp.737-742. Convex optimization algorithms are increasingly used in embedded trajectory planning for safety-critical systems, requiring verification to meet safety standards. Previous research has shown that formal verification methods can be applied to convex solvers for linear problems. This paper explores a specific implementation of the successive convexification approach for trajectory planning using linear programming. We focus on a new implementation of the successive convexification algorithm based on the Frank-Wolfe method (FW-SCVX), aiming at demonstrating that it maintains algorithm performance and is suitable for embedded systems. We also analyze the convergence of the FW-SCVX algorithm and provide numerical evaluations to support our proposal. This work contributes to the formal verification of advanced trajectory planning algorithms in safety-critical embedded systems. (10.23919/ECC65951.2025.11186948)
  DOI : 10.23919/ECC65951.2025.11186948
• Distributed Quantum Advantage for Local Problems
  
  • Balliu Alkida
  • Brandt Sebastian
  • Coiteux-Roy Xavier
  • d'Amore Francesco
  • Equi Massimo
  • Le Gall François
  • Lievonen Henrik
  • Modanese Augusto
  • Olivetti Dennis
  • Renou Marc-Olivier
  • Suomela Jukka
  • Tendick Lucas
  • Veeren Isadora
  , 2025, pp.451-462. We present the first local problem that shows a super-constant separation between the classical randomized LOCAL model of distributed computing and its quantum counterpart. By prior work, such a separation was known only for an artificial graph problem with an inherently global definition [Le Gall et al. 2019]. We present a problem that we call iterated GHZ, which is defined using only local constraints. Formally, it is a family of locally checkable labeling problems [Naor and Stockmeyer 1995]; in particular, solutions can be verified with a constant-round distributed algorithm. We show that in graphs of maximum degree $\Delta$, any classical (deterministic or randomized) LOCAL model algorithm will require $\Omega(\Delta)$ rounds to solve the iterated GHZ problem, while the problem can be solved in $1$ round in quantum-LOCAL. We use the round elimination technique to prove that the iterated GHZ problem requires $\Omega(\Delta)$ rounds for classical algorithms. This is the first work that shows that round elimination is indeed able to separate the two models, and this also demonstrates that round elimination cannot be used to prove lower bounds for quantum-LOCAL. To apply round elimination, we introduce a new technique that allows us to discover appropriate problem relaxations in a mechanical way; it turns out that this new technique extends beyond the scope of the iterated GHZ problem and can be used to e.g. reproduce prior results on maximal matchings [FOCS 2019, PODC 2020] in a systematic manner. (10.1145/3717823.3718233)
  DOI : 10.1145/3717823.3718233
• Fiber-based high-energy 1550 nm laser for generating high-order harmonics
  
  • Mikhneva Anastasiia
  • Boukhaoui Djamila
  • Idlahcen Saïd
  • Houard Jonathan
  • Blum Ivan
  • Godin Thomas
  • Guiramand Leo
  • Amrani Foued
  • Gérôme Frédéric
  • Benabid Fetah
  • Gauthier David
  • Merdji Hamed
  • Boutu Willem
  • Vella Angela
  • Hideur Ammar
  , 2025, pp.CG-4.4. The demand for precise, high-energy Extreme Ultraviolet (EUV) sources is growing across scientific and industrial fields. Studies have demonstrated that bulk crystals driven by mid-infrared laser pulses can generate high-order harmonics with enhanced intensities, extended cut-off energies, and improved damage thresholds [1]. While high-energy OPCPA and OPA systems have been successfully utilized to achieve these results, their complexity highlights the need for simpler, more compact solutions [1, 2, 3]. Ultrafast mid-IR fiber lasers have emerged as a promising alternative for high-harmonic generation (HHG). However, their pulse energies are limited to the nanojoule range [4]. Here, we report the first demonstration of high-order harmonics generated in bulk crystals using a high-energy fiber laser operating near 1550 nm. Our laser system integrates an erbium-doped fiber amplifier with a gas-filled hollow-core photonic crystal fiber (HC-PCF) for post-compression, delivering few-cycle pulses with durations under 50 fs, microjoule-level pulse energies, and a repetition rate of 660 kHz (fig. 1) [5].
• Petri nets and higher-dimensional automata
  
  • Amrane Amazigh
  • Bazille Hugo
  • Fahrenberg Uli
  • Hélouët Loïc
  • Schlehuber-Caissier Philipp
  , 2025, 15714, pp.18-40. Unlabelled - Coronal dimmings associated with coronal mass ejections (CMEs) from the Sun have gained much attention since the late 1990s when they were first observed in high-cadence imagery of the SOHO/EIT and Yohkoh/SXT instruments. They appear as localized sudden decreases of the coronal emission at extreme ultraviolet (EUV) and soft X-ray (SXR) wavelengths, that evolve impulsively during the lift-off and early expansion phase of a CME. Coronal dimmings have been interpreted as "footprints" of the erupting flux rope and also as indicators of the coronal mass loss by CMEs. However, these are only some aspects of coronal dimmings and how they relate to the overall CME/flare process. The goal of this review is to summarize our current understanding and observational findings on coronal dimmings, how they relate to CME simulations, and to discuss how they can be used to provide us with a deeper insight and diagnostics of the triggering of CMEs, the magnetic connectivities and coronal reconfigurations due to the CME as well as the replenishment of the corona after an eruption. In addition, we go beyond a pure review by introducing a new, physics-driven categorization of coronal dimmings based on the magnetic flux systems involved in the eruption process. Finally, we discuss the recent progress in studying coronal dimmings on solar-like and late-type stars, and how to use them as a diagnostics for stellar coronal mass ejections and their properties. Supplementary information - The online version contains supplementary material available at 10.1007/s41116-025-00041-4. (10.1007/978-3-031-94634-9_2)
  DOI : 10.1007/978-3-031-94634-9_2
• Low-complexity fully adaptive equalization algorithm for UWA communication systems
  
  • Khemir Melek
  • Arbi Tarak
  • Geller Benoit
  , 2025. <div><p>This paper presents an improved turbo equalization algorithm for underwater acoustic communications, designed to tackle severe time variations with reduced computational complexity. Our approach uses an enhanced optimization criterion that leverages soft information exchanged between the channel decoder and the equalizer to refine the equalizer's adaptation process. In contrast to previous works, our proposal has low computational complexity by selectively choosing the most relevant terms to be computed by the equalizer, and also by considering the sparsity of the Underwater Acoustic (UWA) channel. Our simulations, both on synthetic and real underwater channels, demonstrate that our method outperforms conventional equalization techniques. Compared to the fullcomplexity adaptive algorithm, our proposal shows better Bit Error Rate (BER) performance and lowers the computational complexity by about 90% for the 16-QAM constellation.</p></div>
• Low-complexity fully adaptive equalization algorithm for UWA communication systems
  
  • Khemir Melek
  • Arbi Tarak
  • Geller Benoit
  , 2025. <div><p>This paper presents an improved turbo equalization algorithm for underwater acoustic communications, designed to tackle severe time variations with reduced computational complexity. Our approach uses an enhanced optimization criterion that leverages soft information exchanged between the channel decoder and the equalizer to refine the equalizer's adaptation process. In contrast to previous works, our proposal has low computational complexity by selectively choosing the most relevant terms to be computed by the equalizer, and also by considering the sparsity of the Underwater Acoustic (UWA) channel. Our simulations, both on synthetic and real underwater channels, demonstrate that our method outperforms conventional equalization techniques. Compared to the fullcomplexity adaptive algorithm, our proposal shows better Bit Error Rate (BER) performance and lowers the computational complexity by about 90% for the 16-QAM constellation.</p></div>
• Categorical semantics of compositional reinforcement learning
  
  • Bakirtzis Georgios
  • Savvas Michail
  • Topcu Ufuk
  Journal of Machine Learning Research, Microtome Publishing, 2025, 26 (130), pp.1-37. Compositional knowledge representations in reinforcement learning (RL) facilitate modular, interpretable, and safe task specifications. However, generating compositional models requires the characterization of minimal assumptions for the robustness of the compositionality feature, especially in the case of functional decompositions. Using a categorical point of view, we develop a knowledge representation framework for a compositional theory of RL. Our approach relies on the theoretical study of the category MDP, whose objects are Markov decision processes (MDPs) acting as models of tasks. The categorical semantics models the compositionality of tasks through the application of pushout operations akin to combining puzzle pieces. As a practical application of these pushout operations, we introduce zig-zag diagrams that rely on the compositional guarantees engendered by the category MDP. We further prove that properties of the category MDP unify concepts, such as enforcing safety requirements and exploiting symmetries, generalizing previous abstraction theories for RL.
• Navigating the sociotechnical labyrinth: Dynamic certification for responsible embodied AI
  
  • Bakirtzis Georgios
  • Aler Tubella Andrea
  • Theodorou Andreas
  • Danks David
  • Topcu Ufuk
  , 2025. Sociotechnical requirements shape the governance of artificially intelligent (AI) systems. In an era where embodied AI technologies are rapidly reshaping various facets of contemporary society, their inherent dynamic adaptability presents a unique blend of opportunities and challenges. Traditional regulatory mechanisms, often designed for static---or slower-paced---technologies, find themselves at a crossroads when faced with the fluid and evolving nature of AI systems. Moreover, typical problems in AI, for example, the frequent opacity and unpredictability of the behaviour of the systems, add additional sociotechnical challenges. To address these interconnected issues, we introduce the concept of dynamic certification, an adaptive regulatory framework specifically crafted to keep pace with the continuous evolution of AI systems. The complexity of these challenges requires common progress in multiple domains: technical, socio-governmental, and regulatory. Our proposed transdisciplinary approach is designed to ensure the safe, ethical, and practical deployment of AI systems, aligning them bidirectionally with the real-world contexts in which they operate. By doing so, we aim to bridge the gap between rapid technological advancement and effective regulatory oversight, ensuring that AI systems not only achieve their intended goals but also adhere to ethical standards and societal values. (10.1016/B978-0-44-340553-2.00019-8)
  DOI : 10.1016/B978-0-44-340553-2.00019-8
• Improved monocular depth prediction using distance transform over pre-semantic contours with self-supervised neural networks
  
  • Hariat Marwane
  • Manzanera Antoine
  • Filliat David
  , 2025, pp.21868-21879. Monocular depth estimation (MDE) with self-supervised training approaches struggles in low-texture areas, where photometric losses may lead to ambiguous depth predictions. To address this, we propose a novel technique that enhances spatial information by applying a distance transform over pre-semantic contours, augmenting discriminative power in low texture regions. Our approach jointly estimates pre-semantic contours, depth and ego-motion. The pre-semantic contours are leveraged to produce new input images, with variance augmented by the distance transform in uniform areas. This approach results in more effective loss functions, enhancing the training process for depth and ego-motion. We demonstrate theoretically that the distance transform is the optimal variance-augmenting technique in this context. Through extensive experiments on KITTI, Cityscapes, Waymo, NYUv2 and ScanNet our model demonstrates robust performance, surpassing competing self-supervised methods in MDE. (10.1109/CVPR52734.2025.02037)
  DOI : 10.1109/CVPR52734.2025.02037
• Toughening effects of out-of-crack-path architected zones
  
  • Triclot Julie
  • Corre Thomas
  • Lazarus Véronique
  • Gravouil Anthony
  , 2024. The increasing use of architected materials has broadened the possibilities of mechanical behaviour. In this article, we aim to explore these new possibilities in terms of in-service behaviour, especially in terms of crack propagation by performing an in-depth study in the framework of Linear Elastic Fracture Mechanics (LEFM). The specific configuration studied here is the case where the architected zones are symmetrically positioned adjacently to the crack path and no propagation occurs within the zone. This problem is addressed both numerically and experimentally. Numerically, an path-following algorithm is used to simulate the crack propagation. Different toughening aspects of the addition of architected zones are identified. First, a temporary increase in crack propagation resistance is shown. It comes from a temporary increase of stored elastic energy in the architected zones, thus acting as mechanical springs. Second, a snap-back instability appears, linked to the release of the previously stored energy. It leads to a higher energy dissipated by the crack propagation process. Experimentally, we evidence the possibility to reproduce the theoretical results using 3D printing. A good quantitative comparison is obtained between numerical and experimental approaches. This study shows that it is possible to improve crack propagation resistance while lightening the component by addition of architected zones outside the crack path. This opens up the way to tune finely, through the use of optimization tools, the crack propagation response.
• Correct-by-construction requirement decomposition
  
  • Sun Minghui
  • Bakirtzis Georgios
  • Jafarzadeh Hassan
  • Fleming Cody
  Software and Systems Modeling, Springer Verlag, 2025, pp.1-16. In systems engineering, accurately decomposing requirements is crucial for creating well-defined and manageable system components, particularly in safety-critical domains. Despite the critical need, rigorous, top-down methodologies for effectively breaking down complex requirements into precise, actionable sub-requirements are scarce, especially compared to the wealth of bottom-up verification techniques. Addressing this gap, we introduce a formal decomposition for contract-based design that guarantees the correctness of decomposed requirements if specific conditions are met. Our (semi-)automated methodology augments contract-based design with reachability analysis and constraint programming to systematically identify, verify, and validate sub-requirements representable by continuous bounded sets---continuous relations between real-valued inputs and outputs. We demonstrate the efficacy and practicality of a correct-by-construction approach through a comprehensive case study on a cruise control system, highlighting how our methodology improves the interpretability, tractability, and verifiability of system requirements. (10.1007/s10270-025-01291-4)
  DOI : 10.1007/s10270-025-01291-4
• Quantitative estimates of the magnetic flux variations in the inner magnetosphere during an intense storm.
  
  • Alqeeq Soboh
  • Fontaine Dominique
  • Le Contel Olivier
  • Akhavan-Tafti Mojtaba
  • Cazzola Emanuele
  • Atilaw Tsige
  , 2025. In the present study, we analyzed the Earth's magnetospheric dynamics in response to the intense geomagnetic storm of 19th December 2015, marked by a substantial decrease in the SYM-H index to -188 nT. We focushere on the variations of the magnetic flux content (MFC) within closed magnetic shells in the inner magnetosphe up to a distance roughly corresponding to the magnetopause. During this event, we had the chance to have observations on the dayside and on the nightside and at different distances in the magnetosphere (OMNI, Van Allen Probes, GOES, THEMIS, MMS, Cluster). Using these various observations together with the Tsyganenko T96 model, we estimated the MFC in the inner magnetosphere. It is found that in comparison to pre-storm conditions, MCF decreased during SSC by 17% and in the main phase by 27% but it gradually rebounded (swelled) during 3 following days of the recovery phase reducing the decrease to 22%, 14% and 8% respectively. The importance of storm-time magnetospheric dynamics in the field of space weather forecasting is emphasized by these findings and calls for further studies. (10.5194/egusphere-egu25-8724)
  DOI : 10.5194/egusphere-egu25-8724
• Caractérisation expérimentale du bruit d’un drone en vol stationnaire en environnements réel et contrôlé
  
  • Cotté Benjamin
  • Simião Pitta Vinícius
  • Hoareau Damien
  • Beausse Yoann
  • Toralba Thibault
  • Doaré Olivier
  • Chapoutot Alexandre
  , 2025. La mesure du bruit des drones dans un environnement réaliste est importante afin d’évaluer leur impact sur les riverains, mais aussi pour comprendre les sources de bruit dominantes en fonction des opérations de vol et ainsi améliorer les outils de modélisation et de simulation du bruit. Cette caractérisation expérimentale est une tâche difficile à cause de la variabilité des conditions météorologiques en milieu extérieur, du bruit de fond, et du bruit du drone. En effet, les différentes hélices du drone voient leur vitesse de rotation varier indépendamment pour assurer la stabilité en vol. C’est pourquoi des mesures en environnement contrôlé comme des chambres anéchoïques sont également réalisées afin de diminuer la variabilité du bruit, même si la taille limitée de ces installations limitent les opérations de vol réalisables. L’objectif de cette étude est de comparer les mesures de bruit en chambre anéchoique et en environnement extérieur pour un drone quadricoptère de petite taille en vol stationnaire, afin de déterminer les similitudes et différences entre les deux types de mesures. Les mesures en chambre anéchoïque ont été réalisées sur une structure fixe, ce qui permet de caractériser les hélices soit de façon isolée, soit par paire soit pour l’ensemble des quatre hélices. Pour les mesures en milieu extérieur, six microphones placés sur des plate-formes rigides ont été utilisées, et des mesures en vol stationnaire à trois hauteurs ont été réalisées. Même si les signaux acoustiques mesurés en extérieur présentent une plus grande complexité, les mêmes composantes spectrales sont retrouvées dans les deux types d’environnement. En particulier, une forte émergence du bruit des moteurs à environ 18 fois la fréquence de passage des pales est observée.
• Birth of a bubble: drop impact onto a thin liquid film for an immiscible three-fluid system
  
  • Maës Pierre-Antoine
  • Amirfazli Alidad
  • Josserand Christophe
  Journal of Fluid Mechanics, Cambridge University Press (CUP), 2025, 1009, pp.A8. When a drop impacts a solid substrate or a thin liquid film, a thin gas disc is entrapped due to surface tension, the gas disc retracting into one or several bubbles. While the evolution of the gas disc for impact on solid substrate or film of the same fluid as the drop has been largely studied, little is known on how it varies when the liquid of the film is different from that of the drop. We study numerically the latter unexplored area, focusing on the contact between the drop and the film, leading to the formation of an air bubble. The volume of fluid method was adapted to three fluids in the framework of the Basilisk solver. The numerical simulations show that the deformation of the liquid film due to air cushioning plays a crucial role in bubble entrapment. A new model for the contact time and the entrapment geometry was deduced from the case of the impact on a solid substrate. This was done by considering the deformation of the thin immiscible liquid layer during impact depending mainly on its thickness and viscosity. The lubrication of the gas layer was found to be the major effect governing bubble entrapment. However, the film viscosity was also identified as having a critical role in bubble formation and evolution; the magnitude of its influence was also quantified. (10.1017/jfm.2025.195)
  DOI : 10.1017/jfm.2025.195
• Free surface topography of capillary flows using spatiotemporal phase shifting profilometry
  
  • de Miramon Hélie
  • Sarlin Wladimir
  • Huerre Axel
  • Cobelli Pablo
  • Séon Thomas
  • Josserand Christophe
  Experiments in Fluids, Springer Verlag (Germany), 2025, 66 (5), pp.89. We present a novel experimental technique for characterizing the free surface of capillary flows using the spatiotemporal phase shifting profilometry (ST-PSP) method. This study specifically addresses various regimes of capillary flows over inclined surfaces, including drops, rivulets, meanders, and braided films. The technique is explained step by step with a detailed discussion of the calibration process, which is carried out on a solid wedge to determine the optical distances required for the phase-to-height relationship. In addition, the minimal dye concentration for accurately reconstructing the free surface of a dyed water flow is investigated. The ST-PSP method is then applied to profile different liquid flows, achieving large signal-to-noise ratios in all experiments. Notably, the analysis of a sessile droplet shows excellent agreement between the ST-PSP results and side-view visualizations, as demonstrated by the precise recovery of its apparent contact angle. Moreover, free surface reconstructions of rivulet flows align well with previous theoretical predictions. These findings suggest that the ST-PSP method is highly effective for obtaining precise height maps of capillary flows, offering a valuable tool for future validation of theoretical models. (10.1007/s00348-025-04006-z)
  DOI : 10.1007/s00348-025-04006-z
• Observation of a Multimode Displaced Squeezed State in High-Harmonic Generation
  
  • Theidel David
  • Cotte Viviane
  • Heinzel Philip
  • Griguer Houssna
  • Weis Mateusz
  • Sondenheimer René
  • Merdji Hamed
  , 2024. High harmonic generation is a resource of extremely broad frequency combs of ultrashort light pulses. The non-classical nature of this new quantum source has been recently evidenced in semiconductors by showing that high harmonic generation generates multimode squeezed states of light. Applications in quantum information science require the knowledge of the mode structure of the created states, defining how the quantum properties distribute over the spectral modes. To achieve that, an effective Schmidt decomposition of the reduced photonic state is performed on a tripartite harmonic set by simultaneously measuring the second- and third-order intensity correlation function. The Schmidt number is estimated which indicates an almost single-mode structure for each harmonic, a useful resource in quantum technology. By modelling our data with a displaced squeezed state, we retrieve the dependencies of the measured correlation as a function of the high harmonic driving laser intensity. The effective high-harmonic mode distribution is retrieved, and the strength of the contributing squeezing modes is estimated. Additionally, we demonstrate a significant violation of a Cauchy-Schwarz-type inequality for three biseparable partitions by multiple standard deviations. Our results confirm non-classicality of the high-harmonic generation process in semiconductors. The source operates at room temperature with compact lasers, and it could become a useful resource for future applications in quantum technologies. (10.48550/arXiv.2411.02311)
  DOI : 10.48550/arXiv.2411.02311
• Les spécialisations industrielles des PECO membres de l’Union européenne sur la période 1992-2018 : Une lecture structurale
  
  • Anedda Raffaele
  • Lebert Didier
  Economie appliquée, Classiques Garnier, 2025 (6), pp.119-147. L’histoire de la transformation des systèmes productifs des pays anciennement socialistes ayant rejoint l’Union européenne durant les années 2000 est aujourd’hui établie. Après une période de transition marquée par de fortes perturbations macro- et méso-économiques débordant pour certains d’entre eux leurs dates d’entrée dans l’Union, ces pays ont stabilisé ces systèmes quelquefois caractérisés désormais par des spécialisations industrielles high tech prononcées. L’objectif de l’article consiste à proposer une architecture empirique fondée sur la théorie de la dominance économique pour représenter cette transformation à l’échelle de chacun des onze pays concernés. Une typologie des trajectoires constatées est ensuite établie. (10.48611/isbn.978-2-406-18063-0.p.0119)
  DOI : 10.48611/isbn.978-2-406-18063-0.p.0119
• Les inégalités d’intégration au sein de l’espace économique méditerranéen au tournant du xxie siècle
  
  • Lebert Didier
  Economie appliquée, Classiques Garnier, 2025 (6), pp.11-35. Nous proposons un nouvel indicateur mesurant les inégalités d’intégration des pays au sein d’un espace d’échanges commerciaux. Cet indicateur associe une approche globale à une autre révélant la contribution spécifique de chaque pays à sa valeur. Nous l’appliquons à l’espace économique méditerranéen (EEM) sur la période 1980-2004, période précédant une intense dynamique institutionnelle visant à intégrer davantage le Nord et le Sud de la Méditerranée. Avant ces transformations institutionnelles, l’EEM reste une zone globalement peu intégrée, et cette faible intégration est entretenue par les principaux pays d’Europe qui en sont membres. (10.48611/isbn.978-2-406-18063-0.p.0011)
  DOI : 10.48611/isbn.978-2-406-18063-0.p.0011
• POLSAR2POLSAR: A SEMI-SUPERVISED DESPECKLING ALGORITHM FOR POLARIMETRIC SAR IMAGES
  
  • Mendes Cristiano Ulondu
  • Dalsasso Emanuele
  • Zhang Yi
  • Denis Loïc
  • Tupin Florence
  ISPRS Journal of Photogrammetry and Remote Sensing, Elsevier, 2025, 220 (0924-2716), pp.783-798. <div><p>Polarimetric Synthetic Aperture Radar (PolSAR) imagery is a valuable tool for Earth observation. This imaging technique finds wide application in various fields, including agriculture, forestry, geology, and disaster monitoring. However, due to the inherent presence of speckle noise, filtering is often necessary to improve the interpretability and reliability of PolSAR data. The effectiveness of a speckle filter is measured by its ability to attenuate fluctuations without introducing artifacts or degrading spatial and polarimetric information. Recent advancements in this domain leverage the power of deep learning. These approaches adopt a supervised learning strategy, which requires a large amount of speckle-free images that are costly to produce. In contrast, this paper presents PolSAR2PolSAR, a semi-supervised learning strategy that only requires, from the sensor under consideration, pairs of noisy images of the same location and acquired in the same configuration (same incidence angle and mode as during the revisit of the satellite on its orbit). Our approach applies to a wide range of sensors. Experiments on Radarsat-2 and RCM data demonstrate the capacity of the proposed method to effectively reduce speckle noise and retrieve fine details. The code of the trained models is made freely available at https://gitlab.telecom-paris.fr/ring/polsar2polsar. The repository additionally contains a model fine-tuned on SLC PolSAR images from NASA's UAVSAR sensor.</p></div>
• Small-scale interface dynamic modelling based on the geometric method of moments for a two-scale two-phase flow model with a disperse small scale
  
  • Loison Arthur
  • Pichard Teddy
  • Kokh Samuel
  • Massot Marc
  Journal of Fluid Mechanics, Cambridge University Press (CUP), 2025, 1003 (A27), pp.1--42. In this contribution, we develop a versatile formalism to derive unified two-phase models describing both the separated and disperse regimes as introduced by Loison et al. (2024). It relies on the stationary action principle and interface geometric variables. This contribution provides a novel method to derive small-scale models for the dynamics of the interface geometry. They are introduced here on a simplified case where all the scales and phases have the same velocity and that does not take into account large-scale capillary forces. The derivation tools yield a proper mathematical framework through hyperbolicity and signed entropy evolution. The formalism encompasses a hierarchy of small-scale reduced-order models based on a statistical description at a mesoscopic kinetic level and is naturally able to include the description of a disperse phase with polydispersity in size. This hierarchy includes both a cloud of spherical droplets and non-spherical droplets experiencing a dynamical behaviour through incompressible oscillations. The associated small-scale variables are moments of a number density function resulting from the geometric method of moments (GeoMOM). This method selects moments as small-scale geometric variables compatible with the structure and dynamics of the interface; they are defined independently of the flow topology and, therefore, this model allows the coupling of the two-scale flow with an inter-scale transfer. It is shown in particular that the resulting dynamics provides partial closures for the interface area density equation obtained from the averaging approach. (10.1017/jfm.2024.1200)
  DOI : 10.1017/jfm.2024.1200
• Derivation of a 4-moment model for electron transport in Hall thrusters from a gyrokinetic model
  
  • Tazakkati Zoubaïr
  • Laguna Alejandro Alvarez
  • Massot Marc
  • Pichard Teddy
  , 2025. <div><p>We model the motion of a population of electrons in a strong electromagnetic field undergoing elastic electron/electron collisions. This regime is derived from a dimensional analysis of the electron confinement in Hall-effect thrusters. The electrons exhibit a very high cyclotron frequency and a E × B-drift, modelled by stiff PDEs at the mesoscopic scale. We obtain a gyrokinetic model in which the fastest oscillations of the system are filtered out by averaging the rotation of the electrons around the magnetic field lines. The model is derived in the strong electromagnetic field limit. Based on this gyrokinetic model, we then develop a 10-moment model. The averaging operation performed at the kinetic scale leads to symmetry properties that allow to reduce the 10-moment model to a 4-moment model.</p></div>
• High-harmonic generation in solids from a high-energy fiber laser system
  
  • Boukhaoui D.
  • Mikhneva A.
  • Idlahcen S.
  • Houard Jonathan
  • Godin T.
  • Guiramand L.
  • Blum Ivan
  • Amrani F.
  • Gérôme F.
  • Benabid F.
  • Gauthier D.
  • Boutu W.
  • Merdji H.
  • Vella Angela
  • Hideur A.
  APL Photonics, AIP Publishing LLC, 2025, 10 (2), pp.026106. We demonstrate high-harmonic generation (HHG) in solids using a high-energy fiber laser system operating at 1550 nm. The driving few-cycle source consists of an erbium-doped fiber chirped pulse amplifier combined with a postcompression stage featuring a gas-filled hollow-core photonic crystal fiber (HC-PCF). The nonlinear self-compression process in the HC-PCF enables the generation of ultrashort pulses with sub-50 fs durations and µJ-levels energies at a 660 kHz repetition rate. Perturbative and non-perturbative harmonics were subsequently generated when focusing the few-cycle pulses into zinc oxide (ZnO) and magnesium oxide (MgO) bulk samples. In the latter, in particular, we observed the generation of EUV harmonics up to H29 (below 55 nm), highlighting the remarkable potential of such a platform for the development of compact HHG sources. (10.1063/5.0244415)
  DOI : 10.1063/5.0244415
• High-Harmonic Generation in an Optical Fiber Functionalized with Zinc Oxide Thin Films
  
  • Tiliouine Idris
  • Leventoux Yann
  • Orlianges Jean-Christophe
  • Crunteanu Aurelian
  • Froidevaux Marie
  • Merdji Hamed
  • Février Sébastien
  Photonics, MDPI, 2025, 12 (1), pp.82. High-order harmonic generation (HHG) in semiconductor thin films from ultrashort mid-infrared laser drivers holds the potential for the realization of integrated sources of extreme ultraviolet light. Here, we demonstrate solid-state HHG in zinc oxide thin films synthesized by the radiofrequency reactive magnetron sputtering process directly on the cleaved facets of optical fibers. Harmonics 3 to 13 of the radiation from a fiber-based laser system delivering 500 kW, 96 fs pulses at 3130 nm are produced in the thin film and guided along the fiber. A proper choice of the laser wavelength and fiber material allows for filtering out the mid-IR pump laser and achieving the HHG mode selection. The possibility to nanostructure the fiber exit by, e.g., focused ion beam milling paves the way to an increased control over the HHG spatial mode. (10.3390/photonics12010082)
  DOI : 10.3390/photonics12010082
• Step-by-step verification of particle-in-cell Monte Carlo collision codes
  
  • Parodi Pietro
  • Petronio Federico
  Physics of Plasmas, American Institute of Physics, 2025, 32 (1). The particle-in-cell (PIC) method with Monte Carlo collisions (MCC) is widely used in the simulation of non-equilibrium plasmas for electric propulsion and laboratory applications. Due to the simplicity of the basic PIC algorithm and the specific modeling needs of the different research groups, many codes have been independently developed. Verification of these codes, i.e., ensuring that the computational code correctly implements the intended mathematical models and algorithms, is of fundamental importance. Different benchmark cases, such as one from Turner et al. [Phys. Plasmas 20, 013507 (2013)], Charoy et al. [Plasma Sources Sci. Technol. 28, 105010 (2019)], and Villafana et al. [Plasma Sources Sci. Technol. 30, 075002 (2021)], have been published in recent years. These have consisted of a complex physical setup, in which many computation modules interact to yield the final result. Although this approach has the advantage of testing the code in a realistic case, it may hide some implementation errors. Moreover, in the case of disagreement, the previous works do not provide an easy way to identify the faulty code modules. In this work, we propose a step-by-step approach for the verification of PIC-MCC codes in a 2D-3V electrostatic setup. The criteria for the test cases are (i) they should highlight possible implementation errors by testing the modules separately, whenever possible (ii) they should be free from physical instabilities to avoid chaotic behavior, and (iii) the numerical result should be accompanied by analytical calculations, for confirmation purposes. The seven test cases identified all show excellent agreement between the authors' codes. (10.1063/5.0241527)
  DOI : 10.1063/5.0241527