Maggy Colas

Determination by x-ray reflectivity and small angle x-ray scattering of the porous properties of mesoporous silica thin films

Two-dimensional hexagonal silica thin films templated by a triblock copolymer were investigated by grazing incident small angle x-ray scattering (GISAXS) and x-ray reflectivity (XR) before and after removing the surfactant from the silica matrix. XR curves—analyzed above and below the critical angle of the substrate—are evaluated by the matrix technique to obtain the average electron density of the films, the wall thickness, the electron density of the walls, the radius of the pores, and subsequently the porosity of such mesoporous films. In combination with GISAXS, the surface area of the mesopores is ascertained, thereby providing a complete analysis of the porosity in thin films by x-ray scattering methods.

Electric field-assisted metal insulator transition in vanadium dioxide (VO2) thin films: optical switching behavior and anomalous far-infrared emissivity variation

We present the vanadium dioxide (VO2) thin films deposition using e-beam evaporation of a vanadium target under oxygen atmosphere on different substrates (sapphire, Si, SiO2/Si…) and we focus on their electrical and optical properties variations as the material undergoes a metal-insulator transition under thermal and electrical stimuli. The phase transition induces extremely abrupt changes in the electronic and optical properties of the material: the electrical resistivity increases up to 5 orders of magnitude while the optical properties (transmission, reflection, refractive index) are drastically modified. We present the integration of these films in simple planar optical devices and we demonstrate electrical-activated optical modulators for visible-infrared signals with high discrimination between the two states. We will highlight a peculiar behavior of the VO2 material in the infrared and far infrared regions (2- 20 μm), namely its anomalous emissivity change under thermal- end electrical activation (negative differential emittance phenomenon) with potential applications in active coatings for thermal regulation, optical limiting or camouflage coatings.

Nd 3+ -doped transparent tellurite ceramics bulk lasers

We report on the laser emission of the polycrystalline ceramic obtained from the full and congruent crystallization of the parent glass 1Nd3+:75TeO2-12.5Bi2O3-12.5Nb2O5 composition. In particular, the current work underlines the importance of carefully controlling the heat treatment in order to solely crystallize the Bi0.8Nb0.8Te2.4O8 cubic phase and consequently avoid the formation of the BiNbTe2O8 orthorhombic phase that would be detrimental for optical purpose. The structure, microstructure and photoluminescence properties of the resulting transparent tellurite ceramics are characterized. The continuous-wave and gain-switching laser performances reveal that the emission remains perfectly single transversal mode in the range of pump powers explored. The maximum output power achieved was ~28.5 mW, for a pump power threshold of ~67 mW, and with associated efficiency and slope efficiency of ~22.5% and ~50%, respectively. These data definitely stand among the best results obtained so far for bulk laser tellurite materials and thus demonstrate the potential of such polycrystalline transparent ceramics as optically active materials. Finally, the laser emission characteristics in pulsed regime, at low and high repetition rates, are also provided: more than 6.5 W of peak power at a repetition rate of 728 kHz can be obtained.

TIME-RESOLVED IN SITU GISAXS EXPERIMENT OF EVAPORATION-INDUCED SELF-ASSEMBLY OF CTAB TEMPLATED SILICA THIN FILMS UNDER CONTROLLED HUMIDITY

The influence of controlled humidity on the phase transformation of CTAB templated silica thin films is studied by time-resolved in situ GISAXS measurements. It is shown that during a short period of time the silica matrix is flexible enough to follow the morphologic changes of the cylindrical micelles into spherical ones. During this time water can be uptaken and outtaken from the film. The evolution of the lattice parameters upon cycling dry and humid air is presented and discussed.

Third order nonlinear optical properties of a paratellurite single crystal

The (a,b) plane angular dependence of the third-order nonlinear optical susceptibility, χ(3), of a c-cut paratellurite (α-TeO2) single crystal was quantitatively evaluated here by the Z-scan technique, using a Ti:sapphire femtosecond laser operated at 800 nm. In particular, the mean value Re( ⟨χ(3)⟩a,b)(α-TeO2) of the optical tensor has been extracted from such experiments via a direct comparison with the data collected for a fused silica reference glass plate. A Re(⟨χ(3)⟩(a,b)(α−TeO2)):Re(χ(3))(SiO2 glass) ratio roughly equal to 49.1 is found, and our result compares thus very favourably with the unique experimental value (a ratio of ∼50) reported by Kim et al. [J. Am. Ceram. Soc. 76, 2486 (1993)] for a pure TeO2 glass. In addition, it is shown that the angular dependence of the phase modulation within the (a,b) plane can be fully understood in the light of the strong dextro-rotatory power known for TeO2 materials. Taking into account the optical activity, some analytical model serving to estimate the di...