HAL Portal IOGS (nstitut d'Optique Graduate School)
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Chiral High-Harmonic Generation in Metasurfaces
No full textInternational audienceHigh-harmonic generation (HHG) provides the only source of attosecond pulsescurrently the shortest accessible time intervals, and it is employed as the only tabletop source of light in extreme UV and soft X-ray spectral regions. Chiral HHG can be employed as an efficient tool for studying the ultrafast response of chiral properties of matter, as well as for amplifying chiroptical effects. Traditionally, chiral high harmonics were associated with gases of enantiomer molecules or, more recently, solid surfaces with helicity in their crystalline structure. Here, we bring the concept of chiral high-harmonic generation to nanophotonics, specifically to metasurfaces consisting of arrays of nanoresonators. Our system is achiral at the material as well as at the level of individual nanoresonators. Chirality rises and falls in a controlled manner via an interplay of the nanoresonator symmetry and the symmetry of the metasurface lattice. Our calculations predict high contrast in harmonic brightness between the two orthogonal circular polarizations of the pump. Our findings, at the intersection of chiral nanophotonics and strong-field optics, pave the way for chiral attosecond physics and chiral extreme UV optics in nanostructured solids
Calibration Impact on the Characterization of Components at Temperature
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Observation of universal non-Gaussian statistics of the order parameter across a continuous phase transition
No full textInternational audienceSecond-order phase transitions are characterised by critical scaling and universality. The singular behaviour of thermodynamic quantities at the transition, in particular, is determined by critical exponents of the universality class of the transition. However, critical properties are also characterised by the probability distribution of order parameter across the transition, where non-Gaussian statistics are expected, but remain largely unexplored. Here, making use of single-atom-resolved detection in momentum space, we measure the full probability distribution of the amplitude of the order parameter across a continuous phase transition in an interacting lattice Bose gas. We find that fluctuations are captured by an effective potential -- reconstructed from the measured probability distribution by analogy with Landau theory -- displaying a non-trivial minimum in the superfluid (ordered) phase, which vanishes at the transition point. Additionally, we observe non-Gaussian statistics of the order parameter near the transition, distinguished by non-zero and oscillating high-order cumulants. We provide direct experimental evidence that these oscillations are universal, and show numerically that they exhibit critical scaling. Our experiments are conducted in inhomogeneous systems, challenging the conventional understanding of criticality, which is primarily based on homogeneous models. Our results underscore the crucial role of order parameter statistics in probing critical phenomena and universality
Caractérisation de composite magnétique à base de nanoparticules orientées par écoulement de cisaillement
No full textInternational audienceL'objectif de cette étude est la réalisation et la caractérisation hyperfréquence d'un matériau nanocomposite magnétique pour des applications dans le domaine des télécommunications. Ce matériau est composé d'une matrice de PMMA aux propriétés diélectriques faibles pertes et de nanoparticules d'hexaferrite de baryum orientées par écoulement de cisaillement. Ont été réalisées les caractérisation magnétiques statique par VSM et en hyperfréquence
Analyse de la gestion mémoire sur GPUs
No full textInternational audienceModern workloads increasingly rely on GPU acceleration. A wellknown performance bottleneck in this context is the warm-up effect, typically observed on the first memory copy call. Our analysis confirms this behavior for large data sizes (above 2 MB), where the initial transfer is notably slower. However, for smaller data sizes, the first call can be faster than subsequent ones, and in some cases, particularly involving memory allocations, the warm-up shifts to the second call. These findings offer new insights into the nuanced behavior of GPU memory instructions and help explain irregular warm-up patterns. By shedding light on these mechanisms, our work supports the development of more efficient and predictable GPU programming strategies.Les charges de travail modernes s'appuient de plus en plus sur l'accélération GPU. Dans ce contexte, l'effet de préchauffage, généralement observé lors du premier appel de copie de mémoire, constitue un goulot d'étranglement bien connu en termes de performances. Notre analyse confirme ce comportement pour les données volumineuses (supérieures à 2 Mo), où le transfert initial est nettement plus lent. Cependant, pour les données de plus petite taille, le premier appel peut être plus rapide que les suivants et, dans certains cas, notamment ceux impliquant des allocations de mémoire, le réchauffement se déplace vers le deuxième appel. Ces résultats offrent de nouvelles perspectives sur le comportement nuancé des instructions mémoire GPU et aident à expliquer les modèles de réchauffement irréguliers. En mettant en lumière ces mécanismes, nos travaux soutiennent le développement de stratégies de programmation GPU plus efficaces et plus prévisibles
Towards Organic Mimics of Metallic Nanoparticles: Tuning and Enhancing Fluorescence in Binary Dye-Based Nanoparticles
No full textInternational audienceA key characteristic of binary metallic nanoparticles is the extreme tuneability of their optical properties through small changes in their composition and structure. Organic mimics of such binary metallic nanoparticles with similar properties would be of major interest. Here, we present Binary Fluorescent Organic Nanoparticles (hereafter termed вFONs) made from two structurally similar but optically complementary polar and polarizable dyes (PPDs) (i.e., energy donor D and energy acceptor A) as such potential mimics. We designed dye-based coreshell and alloy ВFONS with varying composition (i.e. D versus A content) and investigated their fluorescence properties. In both core-shell and alloy ВFONS, the donor dyes efficiently harvest energy -through Förster Resonance Energy Transfer (FRET) -resulting in substantial brightness values (up to 1.3 10 8 M -1 cm -1 per nanoparticle). However, ВFONS' luminescence properties were found to strongly depend on their nanostructuration. Core-shell ВFONS exhibit a Nano Interfacial Emission Enhancement (NIEE) effect, consisting in a blue-shifted fluorescence and enhanced FRET-mediated fluorescence quantum yield, compared to alloy ВFONS of identical composition -but devoid of a core-shell topology. Moreover, we demonstrate for the first time that the extent of this NIEE effect directly depends on ВFONS composition, and increases with the D/A ratio. Furthermore, bioimaging in live cells reveals that ВFONS retain their binary nature and unique fluorescence properties in cellular environments
Atomistic-Continuum Modeling of Laser-Induced Phase Transitions in Silicon: Melting, Ablation, Solidification, and Amorphization
No full textInternational audienceModern semiconductor applications demand precise laser processing at the nanometer scale, requiring a detailed understanding of phase transition and structural modification mechanisms. Accurate control over laser-induced processes in semiconductors is essential for generating predesigned surface structures and modifying the surface properties. In this study, we present a numerical investigation of non-equilibrium laser-induced phase transitions in silicon (Si) using a hybrid atomistic-continuum model [1]. The model combines the strengths of Molecular Dynamics (MD) simulations for atomistic-scale descriptions of non-equilibrium phase transitions with a continuum approach to account for the effect of laser-generated free carriers. As compared to the ordinary continuum or MD approaches, this advanced framework, therefore, captures the kinetics of melting and ablation phenomena on one hand, and generation and diffusion of the electron-hole pairs, thermal diffusion, and the electron-phonon coupling processes during laser energy deposition on the other hand, Fig. 1. We applied the model to determine the melting depth as a function of fluence for a 100 fs laser pulse at 800 nm. The results show that the stand-alone continuum approach underestimates the melting threshold as compared to the hybrid atomistic-continuum model by 46% originating from the detailed description of the melting kinetics. Additionally, we explored the effect of crystal orientation on melting dynamics and compared the results with the corresponding experimental measurement. Finally, the MD model is used to identify the conditions leading to the amorphization of the Si surface and corresponding cooling rates are referred to the experimental conditions. These findings provide valuable insights into experimental observations of Si surface structuring induced by ultrashort laser pulses
SIFA on Nonce-based Authenticated Encryption: When Does It Fail? Application to Ascon
No full textInternational audienceIn nonce-based authenticated encryption schemes, fault attacks such as differential fault analysis are not applicable to due to the uniqueness of the nonce. In this context, Dobraunig et al. (SAC 2018) showed that Statistical Ineffective Fault Attacks (SIFA) remain applicable and powerful. The authors proposed a SIFA-based attack strategy targeting the initialization in nonce-based authenticated encryption schemes and demonstrated its practicality using a common fault method: instruction skip. In this work, we provide a more in-depth analysis of this attack strategy, with a focus on instruction skip as the fault method. First, we model common instruction skip scenarios in practice and formalize the probability that a fault is ineffective. Our analysis reveals that this probability depends on the instruction type and the device architecture. Notably, we show that it is practically inefficient to obtain a sufficient number of ineffective faults for SIFA when skipping an XOR instruction on 32-bit or 64-bit systems, where register data tends to be uniformly distributed. Second, we prove that, in certain authenticated encryption implementations, the intermediate value targeted by the attack unexpectedly remains unbiased under ineffective faults, making SIFA inapplicable. As a case study, we demonstrate this behavior in an 8-bit Ascon implementation
Generation of an Electromagnetic Jet Using a PTFE-Loaded WR90 Waveguide: Design and Characterization
No full textInternational audienceWe present a compact dielectric lens integrated at the aperture of a WR90 rectangular waveguide, achieved using polytetrafluoroethylene (PTFE). This innovative configuration enables, for the first time in the X- and Ku-bands, the direct generation of a subwavelength electromagnetic jet from a guided structure. The beam exhibits the hallmark features of an electromagnetic jet: strong near-field focusing, a subwavelength beam width surpassing the diffraction limit, and a quasi-planar wavefront sustained over a propagation distance of about 2λ. The lens design was systematically optimized, and its performance was assessed through full-wave finite element simulations and experimentally validated on a fabricated prototype. Excellent agreement between the simulation and measurement confirms the robustness of the approach. Beyond its simplicity and low cost, this solution achieves state-of-the-art focusing performance compared to free-space and guided-wave alternatives. It offers strong potential for applications in high-resolution imaging, precision sensing, and material characterization, particularly in opaque or highly lossy environments
Reconstruction 3D depuis des couples SAR ascendant/descendant
No full textNational audienceTo exploit the geometric disparities between SAR images acquired from multiple points of view, we introduce a surfacereconstruction by inverse rendering method inspired by NeRF. It optimises a DSM and a map of backscattering coefficients from asfew as two images, and relies on a custom differentiable SAR rendering model as well as a coarse-to-fine strategy ensuring fastconvergence. We verify the reconstruction abilities on realistic synthetic data generated by ONERA’s EMPRISE® simulator.Afin d'exploiter les disparités géométriques entre des images SAR acquises sous différents points de vue, nous présentons une méthode de reconstruction de surface par rendu inverse inspirée de NeRF. Elle optimise une carte d'élévation et une carte de coefficients de rétrodiffusion à partir d'un minimum de deux images, et s'appuie sur un modèle de rendu différentiable adapté à cette représentation en carte d'élévation ainsi qu'une stratégie multi-échelles assurant une convergence rapide. Nous validons les capacités de reconstruction sur des données synthétiques réalistes générées par le simulateur EMPRISE ® de l'ONERA