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Experimental demonstration of negative refraction of water waves using metamaterials with hyperbolic dispersion
No full textInternational audienceIn this study, we experimentally demonstrate the possibility of negative refraction of water waves, using a locally resonant metamaterial. This metamaterial exhibits a dispersion that encompasses elliptical and hyperbolic regimes, characterized by theoretical analysis and supported by experimental validation. In the frequency range associated with hyperbolic dispersion, we confirm and characterize the appearance of negative refraction. Our experimental achievement provides convincing evidence of the potential of metamaterials for water wave contro
Perfect active absorption of water waves in a channel by a dipole source
No full textInternational audienceThis study investigates the potential use of an active device to efficiently absorb water waves propagating in a channel. The active device comprises a dipole source consisting of two sources in quasi-opposition of phase. We explore the feasibility of this approach to achieve perfect absorption of guided waves through interference phenomena. To accomplish this, we establish the law governing the waves emitted by the dipole source to optimize the absorption of specific incident waves. The validity of this law is demonstrated through numerical simulations and laboratory experiments, encompassing both the harmonic and transient regimes of the experimental set-up
Toward Dynamic Path Planning of Unmanned Aerial Vehicles (UAVs) in Windy Conditions 1
No full textInternational audienceUnmanned Aerial Vehicle (UAV) path planning optimizes trajectory for efficient target reach with minimal energy consumption. This study enhances the Target Interception method (TI), to address wind-related challenges. We introduce two adaptations: TI Dynamic Path and TI External Influences. Implementing these methods in a dynamic wind environment yields results demonstrating enhanced reliability and efficiency. This research marks a significant stride in developing more effective UAV path planning algorithms
Transient dielectric function dynamics driven by coherent phonons in Bismuth crystal
No full textInternational audienceIn this study, we investigate the ultrafast transient dynamics of the dielectric function in bismuth crystal, excited by femtosecond laser pulses and modulated by coherent phonons. The primary aim is to understand the influence of the coherent A1g phonon mode on the dielectric function and to characterize the nature of the quasi-steady state that persists for tens of picoseconds after the coherent oscillations vanish. Our findings reveal that the dielectric function undergoes damped oscillations corresponding to the A1g phonon mode, with the real and imaginary parts of the dielectric function oscillating out of phase but sharing the same frequency and lifetime of the oscillatory component. Once the oscilla?ons vanish, the system reaches a quasi-steady state around 20 ps after excitation. In this state, the dielectric function deviates significantly from the values expected for the liquid phase, indicating that no phase transition occurs, even though the calculated lattice temperature exceeds the melting point of bismuth. To probe the nature of this quasi-steady state, we compare the transient dielectric function to equilibrium ellipsometry measurements taken at various temperatures, ranging from room temperature to temperatures approaching the melting point. This comparison allows us to estimate the real and imaginary parts of the dielectric function as a function of temperature, particularly in a warmed state, where the crystal temperature is elevated but still below the melting threshold. The comparison reveals a clear discrepancy between the dielectric function values in the quasi-steady state and those measured in a thermally equilibrated warmed state. This suggests that the quasi-steady state cannot be solely attributed to crystal heating. Instead, we propose that the persistence of the quasi-steady state is due to the fact that electron-hole recombination has not fully occurred within the measured time range
OAM driven nucleation of sub-50 nm compact antiferromagnetic skyrmions
No full textInternational audienceAbstract Owing to their high mobility and immunity to topological deflection, skyrmions in antiferromagnetic (AFM) systems are gaining attention as a potential solution for next‐generation magnetic data storage. Synthetic antiferromagnets (SAFs) offer a promising avenue to tune the properties of the individual magnetic layers, facilitating the conditions necessary for skyrmions to be used in practical devices. Despite recent advancements achieving fast skyrmion mobility, the nucleation of small and rigid circular skyrmions without an external field remains challenging in SAFs. Theoretical predictions suggest that optical vortex (OAM) beams can stabilize skyrmionic spin textures by transferring their spin and orbital angular momentum to the magnetic material. Here, this intriguing proposal is delved into and the creation of sub‐50 nm compact skyrmions in SAFs using OAM beams is successfully demonstrated, eliminating the need for external magnetic fields. Additionally, the results underscore the importance of beam energy and the number of pulses, as both factors play critical roles in the stabilization of these AFM skyrmionic textures. This breakthrough is significant as it paves the way for stabilizing true zero‐field skyrmions in AFM systems, where magnetization is minimally affected by external magnetic fields. This work will open a potential avenue for stabilizing small, compact skyrmions in antiferroic systems, facilitating their implementation in logic and memory devices
Optical ionization effects in kHz laser wakefield acceleration with few-cycle pulses
No full textInternational audienceWe present significant advances in laser wakefield acceleration (LWFA) operating at a 1 kHz repetition rate, employing a sub-TW, few-femtosecond laser and a continuously flowing hydrogen gas target. We conducted a comprehensive study assessing how the nature of the gas within the target influences accelerator performance. This work confirms and elucidates the superior performance of hydrogen in LWFA driven by few-cycle, low-energy laser pulses. Our system generates quasimonoenergetic electron bunches with energies up to 10 MeV, bunch charges of 2 pC, and angular divergences of 15 mrad. Notably, our scheme relying on differential pumping enables continuous operation at kHz repetition rates, contrasting with previous systems that operated in burst mode to achieve similar beam properties. Particle-in-cell simulations explain hydrogen's superior performances: the ionization effects in nitrogen and helium distort the laser pulse, negatively impacting the accelerator performance. These effects are strongly mitigated in hydrogen plasma, thereby enhancing beam quality. This analysis represents a significant step forward in optimizing and understanding kHz LWFA. It underscores the critical role of hydrogen and the imperative need to develop hydrogen-compatible target systems capable of managing high repetition rates, as exemplified by our differential pumping system. These advances lay the groundwork for further developments in high-repetition-rate laser plasma accelerator technology. Published by the American Physical Society 202
The Sphericity Paradox and the Role of Hoop Stresses in Free Subduction on a Sphere
No full textInternational audienceOceanic plates are doubly curved spherical shells, which influences how they respond to loading during subduction. Here we study a viscous fluid model for gravity‐driven subduction of a shell comprising a spherical plate and an attached slab. The shell is 100–1,000 times more viscous than the upper mantle. We use the boundary‐element method to solve for the flow. Solutions of an axisymmetric model show that the effect of sphericity on the flexure of shells is greater for smaller shells that are more nearly flat (the “sphericity paradox”). Both axisymmetric and three‐dimensional models predict that the deviatoric membrane stress in the slab should be dominated by the longitudinal normal stress (hoop stress), which is typically about twice as large as the downdip stress and of opposite sign. Our models also predict that concave‐landward slabs can exhibit both compressive and tensile hoop stress depending on the depth, whereas the hoop stress in convex slabs is always compressive. We test these two predictions against slab shape and earthquake focal mechanism data from the Mariana subduction zone, assuming that the deviatoric stress in our flow models corresponds to that implied by centroid moment tensors. The magnitude of the hoop stress exceeds that of the downdip stress for about half the earthquakes surveyed, partially verifying our first prediction. Our second prediction is supported by the near‐absence of earthquakes under tensile hoop stress in the portion of the slab having convex geometry
Full-field measurements of the microstructure’s effect on the mechanical behaviour of a wire and arc additively manufactured duplex stainless steel
No full textInternational audienceWire and Arc Additive Manufacturing (WAAM) is particularly well suited to the fabrication of large-scale structural components. Its use for the deposition of Duplex Stainless Steel (DSS) structures is especially sought upon by the naval industry. Although additively manufactured materials usually come with heterogeneous microstructure, residual stresses, internal and surface defects that must not jeopardize the structural integrity of components. In this context, the present study aims to analyse the as-deposited microstructure of a WAAM DSS and its influence on the mechanical behaviour via mechanical field measurement using both Digital Image Correlation (DIC) and Thermoelastic Stress Analysis (TSA).<=
Regio-and Chemoselective Double Allylic Substitution of Alkenyl vic-Diols
No full textInternational audienceDouble allylic substitution is an attractive approach to build molecular complexity from simple starting materials by creating two new bonds in one-pot. However, this type of reaction has been doomed by chemoselectivity and regioselectivity issues. In this manuscript, we describe a new approach to introduce a-la-carte two new C-C, C-N, C-O or C-S bonds in a chemo-and regioselective fashion. The reaction relies on sequential dual catalysis with Lewis acid and palladium. The scope is remarkably broad, and the reaction can be diastereoselective using secondary alcohols as the first nucleophile.</div