Bruno Capoen

Sol-gel derived ionic copper-doped microstructured optical fiber: a potential selective ultraviolet radiation dosimeter

We report the fabrication and characterization of a photonic crystal fiber (PCF) having a sol-gel core doped with ionic copper. Optical measurements demonstrate that the ionic copper is preserved in the silica glass all along the preparation steps up to fiber drawing. The photoluminescence results clearly show that such an ionic copper-doped fiber constitutes a potential candidate for UV-C (200-280 nm) radiation dosimetry. Indeed, the Cu⁺-related visible photoluminescence of the fiber shows a linear response to 244 nm light excitation measured for an irradiation power up to 2.7 mW at least on the Cu-doped PCF core. Moreover, this response was found to be fully reversible within the measurement accuracy of this study ( ± 1%), underlying the remarkable stability of copper in the Cu⁺ oxidation state within the pure silica core prepared by a sol-gel route. This reversibility offers possibilities for the achievement of reusable real-time optical fiber UV-C dosimeters.

From molecular precursors in solution to microstructured optical fiber: a Sol-gel polymeric route

Solid-core photonic crystal fibers with the core derived from non-doped or Erbium-doped sol-gel silica rods are fabricated. The results demonstrate that the direct polymeric sol-gel route constitutes a promising method to prepare large high quality glass pieces that can be integrated into microstructured optical fibers suitable for passive and active optical fiber applications.

Linear and nonlinear optical properties of gold nanoparticle-doped photonic crystal fiber.

The concept of photonic crystal fibers (PCFs) has opened the route to a new family of optical fibers available for a wide range of applications in nonlinear optics. The nonlinear performances are based both on the easy chromatic dispersion management possible with PCFs and on the increased fiber nonlinearity, thanks to the small effective mode areas accessible with these fibers. Hence, even if necessary, the intrinsic nonlinearity of the material used to realize the fiber generally plays a secondary role. This is true for most of the realizations which are based on pure silica glass, a matrix with modest nonlinear coefficients. However, various glass matrices are known to offer larger nonlinear coefficients and could be useful to further improve the performances of systems based on highly nonlinear fibers. Among these, conventional glasses containing dopants presenting nonlinear properties, like gold nanoparticles (Au NPs), appear as promising candidates [1]. However, realization of glasses doped by Au NPs is mainly limited to films and bulk materials and there are only two reports of insertion of such systems in the core of a conventional optical fiber, mainly due to the difficulty induced by the high temperatures commonly used to synthesize optical fibers (typically 1500–2000°C) as compared to the gold melting point [2,3].

Multicolor photochromism of silver-containing mesoporous films of amorphous or anatase TiO2

Mesoporous TiO2 films loaded with silver nanoparticles grown photocatalytically, which are initially brown, change their color under visible laser irradiations. In this article, we compare the multicolor photochromisms of amorphous and anatase phases of TiO2. The mesoporous films are impregnated with silver salt and then exposed to a low-intensity UV laser light to grow silver nanoparticles. The Ag–TiO2 films are then exposed to visible laser beams, and the influences of several exposure parameters on the photochromic behavior are examined. Most of the previous studies have reported a poor stability of the photoinduced colors under day light or even in the dark, and few of them demonstrated the ability to get various colors on the same sample. These inconveniences limit the application field of such materials. On the other hand, except in our previous studies, only crystalline TiO2 is generally used, in its anatase or rutile phase. In this article we show that mesoporous films of amorphous and anatase phases of TiO2 respond in an efficient manner to light excitation and that multiple colors can be obtained on both kinds of films. For the first time on such Ag–TiO2 films we show that the various photoinduced colors are stable over considerable months. Visible intensity is shown to have a significant influence on the film behavior, which was not identified in previous studies. The laser-induced spectral changes are also shown to depend on the incident laser polarization. The photochromic behaviors are characterized in terms of color changes and spectral variations. The reproducibility of the photochromic process along reduction/oxidation cycles is demonstrated, and the stability of different laser-induced colors is reported on 6-month-old samples.

H2-induced copper and silver nanoparticle precipitation inside sol-gel silica optical fiber preforms

Ionic copper- or silver-doped dense silica rods have been prepared by sintering sol-gel porous silica xerogels doped with ionic precursors. The precipitation of Cu or Ag nanoparticles was achieved by heat treatment under hydrogen followed by annealing under air atmosphere. The surface plasmon resonance bands of copper and silver nanoparticles have been clearly observed in the absorption spectra. The spectral positions of these bands were found to depend slightly on the particle size, which could be tuned by varying the annealing conditions. Hence, transmission electron microscopy showed the formation of spherical copper nanoparticles with diameters in the range of 3.3 to 5.6 nm. On the other hand, in the case of silver, both spherical nanoparticles with diameters in the range of 3 to 6 nm and nano-rods were obtained.

Environment segregation of Er3+ emission in bulk sol–gel-derived SiO2–SnO2 glass ceramics

Er-doped (100-x) SiO2–x SnO2 glass–ceramic monoliths were prepared using a sol–gel method. Raman spectroscopic measurements showed the structural evolution of the silica matrix caused by the formation and the growth of SnO2 nanocrystals. Analysis of the photoluminescence properties shows that the quantity of Er3+ ions embedded in the vicinity of SnO2 nanocrystals could be controlled by the SnO2 concentration. We give spectroscopic evidence of energy transfer to erbium ions provided by SnO2 nanocrystals in the silica matrix. The 4I13/2 level decay curves present a double-exponential profile with two lifetimes associated to rare-earth ions in two different environments.

Direct-writing of PbS nanoparticles inside transparent porous silica monoliths using pulsed femtosecond laser irradiation

Pulsed femtosecond laser irradiation at low repetition rate, without any annealing, has been used to localize the growth of PbS nanoparticles, for the first time, inside a transparent porous silica matrix prepared by a sol-gel route. Before the irradiation, the porous silica host has been soaked within a solution containing PbS precursors. The effect of the incident laser power on the particle size was studied. X-ray diffraction was used to identify the PbS crystallites inside the irradiated areas and to estimate the average particle size. The localized laser irradiation led to PbS crystallite size ranging between 4 and 8 nm, depending on the incident femtosecond laser power. The optical properties of the obtained PbS-silica nanocomposites have been investigated using absorption and photoluminescence spectroscopies. Finally, the stability of PbS nanoparticles embedded inside the host matrices has been followed as a function of time, and it has been shown that this stability depends on the nanoparticle mean size.

Room temperature direct space-selective growth of gold nanoparticles inside a silica matrix based on a femtosecond laser irradiation

Melting point phenomena of micron-sized indium particles embedded in an aluminum matrix were studied by means of acoustic emission. The acoustic energy measured during melting increased with indium content. Acoustic emission during the melting transformation suggests a dislocation generation mechanism to accommodate the 2.5% volume strain required for melting of the embedded particles. A geometrically necessary increase in dislocation density of 4.1 x 10^13 m^-2 was calculated for the 17 wt% indium composition.

Ice mixtures formed by simultaneous condensation of formaldehyde and water: an in situ study by micro-Raman scattering

Thin films of formaldehyde-water mixtures are co-deposited at 88 K and 10(-1) Torr from gas collected above formaldehyde aqueous solutions of different concentrations (5, 10, 15, 20, 30 mol%). They are analyzed in situ by micro-Raman scattering in the 2700-3800 cm(-1) spectral range. The spectral characteristic of H2CO distributed molecularly in amorphous solid water is obtained under vacuum conditions. As temperature is increased formaldehyde is released during the crystallization of ice between 118 and 138 K. On the other hand, under controlled nitrogen atmosphere, the deposits crystallize in hydrate phases (or solid H2CO(s)) during annealing. A new phase (metastable FOR-A) of H2CO(s) (or a low hydrate after rejection by crystallizing ice) can be spectroscopically identified at 138 K before transformation into a hydrate (with molecular H2CO distributed within the cages of the clathrate FOR-B) takes place at 148 K. This latter phase decomposes between ca. 180 and 200 K. The significant spectral differences between these hydrates and those formed in frozen formaldehyde aqueous solutions reflect the existence of H2CO-clusters of distinctive structural nature relative to those resulting from important oligomerization process in the liquid. Moreover, the structure, the gas distribution and relative gas population in the formaldehyde clathrate cages are influenced by the relative amount of trapped nitrogen at the surface, which moreover depends on the ice film morphology. The dependence on the crystallization temperature of the deposits is explained by the relative amounts of occluded H2CO/N2 and the external pressure conditions. The distinct behavior observed between vacuum and N2-atmosphere conditions certainly reflects a complex mechanism of surface mediated nucleation in which the transport of the reactants to the hydrate reaction zone is facilitated by the presence of a polar dopant.

Laser-induced growth of nanocrystals embedded in porous materials

Space localization of the linear and nonlinear optical properties in a transparent medium at the submicron scale is still a challenge to yield the future generation of photonic devices. Laser irradiation techniques have always been thought to structure the matter at the nanometer scale, but combining them with doping methods made it possible to generate local growth of several types of nanocrystals in different kinds of silicate matrices. This paper summarizes the most recent works developed in our group, where the investigated nanoparticles are either made of metal (gold) or chalcogenide semiconductors (CdS, PbS), grown in precursor-impregnated porous xerogels under different laser irradiations. This review is associated to new results on silver nanocrystals in the same kind of matrices. It is shown that, depending on the employed laser, the particles can be formed near the sample surface or deep inside the silica matrix. Photothermal and/or photochemical mechanisms may be invoked to explain the nanoparticle growth, depending on the laser, precursor, and matrix. One striking result is that metal salt reduction, necessary to the production of the corresponding nanoparticles, can efficiently occur due to the thermal wrenching of electrons from the matrix itself or due to multiphoton absorption of the laser light by a reducer additive in femtosecond regime. Very localized semiconductor quantum dots could also be generated using ultrashort pulses, but while PbS nanoparticles grow faster than CdS particles due to one-photon absorption, this better efficiency is counterbalanced by a sensitivity to oxidation. In most cases where the reaction efficiency is high, particles larger than the pores have been obtained, showing that a fast diffusion of the species through the interconnected porosity can modify the matrix itself. Based on our experience in these techniques, we compare several examples of laser-induced nanocrystal growth in porous silica xerogels, which allows extracting the best experimental conditions to obtain an efficient particle production and to avoid stability or oxidation problems.