François Brisset

Giant Amplification of Photoswitching by a Few Photons in Fluorescent Photochromic Organic Nanoparticles

Controlling or switching the optical signal from a large collection of molecules with the minimum of photons represents an extremely attractive concept. Promising fundamental and practical applications may be derived from such a photon-saving principle. With this aim in mind, we have prepared fluorescent photochromic organic nanoparticles (NPs), showing bright red emission, complete ON-OFF contrast with full reversibility, and excellent fatigue resistance. Most interestingly, upon successive UV and visible light irradiation, the NPs exhibit a complete fluorescence quenching and recovery at very low photochromic conversion levels (<5u2009%), leading to the fluorescence photoswitching of 420±20 molecules for only one converted photochromic molecule. This giant amplification of fluorescence photoswitching originates from efficient intermolecular energy-transfer processes within the NPs.

Cobalt‐Based Particles Formed upon Electrocatalytic Hydrogen Production by a Cobalt Pyridine Oxime Complex

An open-coordination-sphere cobalt(III) oximato-based complex was designed as a putative catalyst for the hydrogen evolution reaction (HER). Electrochemical alteration in the presence of acid occurs, leading to the formation of cobalt-based particles that act as an efficient catalyst for HER at pH 7. The exact chemical nature of these particles is yet to be determined. This study thus raises interesting issues regarding the fate of molecular-based complexes designed for the HER, and points to the challenging task of identifying the real catalytic species. Moreover, understanding and rationalizing the alteration pathways can be seen as a new route to reach catalytic particulates.

Size-controlled oriented crystallization in SiO 2 -based glasses by femtosecond laser irradiation

3D nano-crystallization has been investigated with a femtosecond laser beam in 33Li2O-33Nb2O5-34SiO2 glass by changing the pulse energy and polarization direction. Electron backscattered diffraction was used to characterize the size and the orientation of crystals in cross sections of laser tracks, while second harmonic generation was employed to investigate the nonlinear optical properties of laser lines macroscopically. We are the first, to the best of our knowledge, to reveal that crystals of nano-size were precipitated both at low pulse energy with laser polarization parallel to the laser motion direction and at high pulse energy in the perpendicular case, but they preferred to orientate perpendicularly to the written lines in the former case.

Growth and density control of nanometric nickel–iron cyanide-bridged objects on functionalized Si(100) surface

Isolated nanometric objects of the nickel-iron cyanide-bridged coordination network are obtained by a sequential growth on molecular seeds anchored on Si(100) surfaces. Control of the density and the size of the nano-objects is achieved by imposing a growth process without side nucleation.

Tunable angular-dependent second-harmonic generation in glass by controlling femtosecond laser polarization

Mastering second-harmonic generation (SHG) in glasses is essential for achieving second-order nonlinear devices. Three-dimensional SHG patterns with controllable angular dependence were achieved in Li2O-Nb2O5-SiO2 glass by femtosecond laser irradiation. Electron backscatter diffraction indicated that the angular dependence was associated with preferential nanocrystal orientation: crystal polar axes are distributed within a plane perpendicular to the incident laser polarization direction. An orientation mechanism based on the application of laser-induced torques on the nanocrystal electric dipole was proposed. This work may pave the way to design devices such as frequency agile lasers based on electro-optical waveguides.

Form birefringence induced in multicomponent glass by femtosecond laser direct writing.

We demonstrate a new kind of form birefringence in lithium niobium silicate glass induced by femtosecond laser direct writing. By combining electron backscatter diffraction and transmission electron microscopy, we reveal a self-assembled nanostructure consisting of periodic phase change: nonlinear optical nanocrystals embedded in a network of walls in a vitreous phase. These walls are aligned perpendicular to the laser polarization direction. This self-organized nanostructure may successfully explain the origin of the laser-induced birefringence in this multicomponent glass quite differently from pure silica. These findings highlight a spectacular modification of glass, and enable construction of a high contrast three-dimensional refractive index and birefringent structures at the micrometer scale in multicomponent glasses.

One-step photoinscription of asymmetrically oriented fresnoite-type crystals in glass by ultrafast laser.

Oriented fresnoite-type crystals (Sr(2)TiSi(2)O(8)) were photoinduced directly in bulk silica-based glass by femtosecond laser irradiation at high repetition rate (typ. 300 kHz). Unlike related results obtained from other researchers, asymmetrical polar-axis orientation of those nonlinear crystals in transverse direction of the cross section has been demonstrated by electron backscattered diffraction and micro-second-harmonic generation (SHG). The nonlinear optical property of laser lines has been further characterized by SHG measurement. We found that the preferential directions of the polar axis were in the laser motion direction with a small dispersion in part of the heated volume. The other part of the crystallized volume shows an axis perpendicular to the writing direction. The mechanism of asymmetric orientation of femtosecond-laser-induced crystallization also is discussed.

Grain boundary character distribution of CuNiSi and FeNi alloys processed by severe plastic deformation

In this work the Grain Boundary Character Distribution (GBCD) in general and the relative proportion of low-Σ CSL (Coincidence Site Lattice) grain boundaries are determined through EBSD in Cu-2.5Ni-0.6Si (wt.%) and Fe-36Ni (wt.%) alloys after processing by high-pressure torsion, equal- channel angular pressing and accumulative roll bonding.

Porous nanogratings and related form birefringence in silicate and germanate glasses

We prove the formation of birefringent nanoporous structures in the volume of various doped silica (TiO2, F, GeO2), GeO2 and borosilicate glasses by femtosecond laser. Using quantitative birefringence measurements and scanning electron microscopy we determined that this birefringence is mainly induced by porous nanogratings. So far, the form birefringence induced by these modifications was known mainly in fused silica. We determined the repetition rate of the irradiating laser pulses as crucial processing parameter. The retardation in the laser modified doped silica or GeO2 is comparable to fused silica, while the birefringence in Borosilicate is two times weaker.

Study of Microstructural and Mechanical Behavior of Mild Steel Wires Cold Drawn at TREFISOUD

The aim of the present work is to study the evolution of microstructure, texture and mechanical properties during drawing of mild steel wire of type F8Z used in the manufacture of welding electrodes TREFISOUD. It was found that the as received wire has a ferritic-pearlitic microstructure corresponding to an isotopic state (without texture). This microstructure is relatively heterogeneous in the wire section. On the other hand, after strong drawing structure appears relatively homogeneous, throughout the section of the drawn wire. Also the deformation process by drawing causes the hardening of wire as a function of deformation with a reinforcing component of the fiber texture <110> // ND (majority), typical for bcc materials. Characterization methods used in this work is: Optical microscopy (OM), Scanning Electron Microscopy (SEM), the Electron Back Scattered Diffraction EBSD, Vickers microhardness and the tensile tests.