Patrick Weisbecker

Topological Darkness in Self‐Assembled Plasmonic Metamaterials

Self-assembled plasmonic metamaterials are fabricated from silver nanoparticles covered with a silica shell. These metamaterials demonstrate topological darkness or selective suppression of reflection connected to global properties of the Fresnel coefficients. The optical properties of the studied structures are in good agreement with effective medium theory. The results suggest a practical way of achieving high phase sensitivity in plasmonic metamaterials.

An image-guided atomistic reconstruction of pyrolytic carbons

A method for the generation of atomistic models of dense nanotextured carbons is presented. This method is based on the statistical analysis of high resolution transmission electron microscopy images and their three-dimensional (3D) extension through image synthesis under constraint. The resulting 3D images then serve as an external potential bringing the atoms to settle preferentially on the black areas during a conventional simulated annealing simulation. Application of this method to the case of two laminar pyrocarbons, differing in their degree of disorder, highlights the promising nature of this approach.

Bottom-up fabrication and optical characterization of dense films of meta-atoms made of core-shell plasmonic nanoparticles.

In an attempt to fabricate low index metamaterials by a bottom-up approach, meta-atoms constituted of silica-coated silver nanoparticles are assembled by a Langmuir-Schaefer technique into thin films of large area and well-controlled thickness. The silica shells ensure a constant distance between the silver cores, hence providing a constant coupling of the localized surface plasmon resonance (LSPR) of the nanoparticles in the assembled composite material. The optical response is studied by normal angle spectral reflectance measurements and by variable angle spectroscopic ellipsometry. The normal incidence data are described well in the framework of a single effective Lorentz oscillator model. The resonance of the assembled material is blue-shifted and shows no significant broadening with respect to the absorption band of the individual nanoparticles. The observation of these two effects is enabled by the core-shell structure of the meta-atoms that prevents aggregation of the metallic cores. The ellipsometry study confirms the general behavior and reveals the natural birefringence of the few-layer materials. The amplitude of the observed resonance is weaker than expected from the Maxwell-Garnett mixing rule. This well-characterized system may constitute a good model for numerical simulations.

Formation and properties of Al composites reinforced by quasicrystalline AlCuFeB particles

Al-based composites reinforced by icosahedral (i-) Al59Cu25.5Fe12.5B3 quasicrystalline particles were prepared by solid-state sintering. It was found that Al diffusion from the matrix to the quasicrystalline particles induces phase transformation into the ω-Al7Cu2Fe tetragonal phase. In order to preserve the i phase, we used an oxidation pre-treatment of the particles and studied its influence on the kinetics of the phase transformation (Al + i → ω) as a function of temperature by high energy X-ray diffraction. The oxide layer acts as a barrier, reducing efficiently the diffusion of Al up to a sintering temperature of 823 K, allowing the control of the phases in the composites. The mechanical properties and the friction behaviour of the composites were investigated and show the negative influence of the oxide on the interface strength.

Influence of oxidation of i -AlCuFeB particles on the formation of Al-based composites prepared by solid state sintering

We present new results concerning the formation of Al-based composites reinforced by icosahedral i-Al59Cu25.5Fe12.5B3 quasicrystalline particles. The issue is to limit the diffusion of Al from the matrix to the quasicrystalline particles during sintering in order to overcome the transformation of the i-phase into the ω-Al7Cu2Fe tetragonal phase. Here we have investigated the influence of an oxidation pre-treatment of the quasicrystalline particles onto the kinetics of this phase transformation by high energy X-ray diffraction. The quasicrystalline particles were annealed either in vacuum or in air, leading to a different thickness of the oxide layer. We show that the oxide barrier efficiently reduces the kinetics of Al diffusion, enabling the formation of composite materials with no phase transformation during the sintering process. The dependence of the tribological properties of these composites is investigated as a function of the volume fraction of i-AlCuFeB particles.

Chimie douce route to novel acoustic waveguides based on biphenylene-bridged silsesquioxanes

Novel silsesquioxane thin films were conveniently synthesized by the sol–gel route under mild conditions. Thus, a sol was prepared from 4,4′-bis(triethoxysilyl)-1,1′-biphenyl (4,4′-BTEBp) under a hydroalcoholic medium without a surfactant, deposited by spin-coating on silica supports, and dried at 280 °C. The resulting biphenylene-bridged films exhibited very smooth surfaces, excellent mechanical behavior and no cracks. Moreover, SEM and TEM studies showed flawless, homogeneous interfaces with silica substrates. The acoustic waveguide properties of these films on piezoelectric quartz microsensors were investigated at high frequency through a network analyzer. For an optimal thickness close to 1 μm, a marked frequency shift of the main broad lobe and a decrease of the insertion loss were observed. These results show a strong effect of the waveguide for trapping the acoustic energy, promising an unprecedented ability to provide a new generation of sensors. The capability of this device in the detection of mass is also demonstrated through the immobilization of Bovine Serum Albumin (BSA). This novel approach appears very promising in view of the development of surface acoustic wave sensors and biosensors.

An atomistic reconstruction of the nanostructure of pyrolitic carbons guided by HRTEM data

A new method for the generation of atomistic models of dense nanotextured carbons is presented. This method is based on the statistical analysis of HRTEM images and their threedimensional extension through image synthesis under constraint. The resulting 3D images then serve as an external potential bringing the atoms to settle preferentially on the white areas during a conventional simulated annealing simulation. Application of this method to the case of two laminar pyrocarbons, differing in their degree of disorder, highlights the promising nature of this approach.

Elaboration of C/C Composites Based on the Infiltration of a Hydrocarbon Precursor in Supercritical State into the Preform

This paper reports on the development of a new process for the fabrication of a C/C composite for aeronautic and aerospace industries, in order to reduce the infiltration time of the carbon perform. The infiltration of a carbon fibrous preform by means of a hydrocarbon in the supercritical fluid state is done at high temperature in order to obtain a carbon matrix. The microstructure of the pyrocarbon coating is characterized by SEM and TEM. The experimental parameters (temperature, hydrocarbon pressure, residence time) are tuned in order to elaborate the carbon matrix. The best conditions lead to in-depth rapid densification from the top of the preform. After several experiments, the densification has been improved.

Influence of substrate temperature on the achievement of Ti-Ni-Zr quasi-crystalline films grown by pulsed laser deposition

Pulsed laser deposition has been used to prepare films in the Ti- Ni-Zr system. Morphology and structure of the obtained films have been studied as a function of substrate temperature being in the range 25 - 350 degree(s)C. Morphological and structural modifications have been followed by grazing incidence and (theta) -2(theta) X-ray diffraction, transmission, electron diffraction and imaging. Chemical composition has been analysed by electron probe micro- analysis. The in-depth variation of composition has been studied by secondary neutral mass spectroscopy. This study leads to the determination of the best growth temperature giving rise to the known quasicrystalline phase recently discovered in the Ti-Ni-Zr ternary system.