Silica aerogels are known to be materials with exceptional characteristics, such as ultra-low density, high surface area, high porosity, high adsorption, and low-thermal conductivity. In addition, these unique properties are mainly related to their specific processing. Depending on the aerogel synthesis procedure, the aerogels texture can be tailored with meso and/or macroporosity. Fractal geometry has been observed and used to describe silica aerogels at nanoscales in certain conditions. In this review paper, we describe the fractal structure of silica aerogels that can develop depending on the synthesis conditions. X-ray and neutron scattering measurements allow to show that silica aerogels can exhibit a fractal structure over one or even more than two orders of magnitude in length. The fractal dimension does not depend directly on the material density but can vary with the synthesis conditions. It ranges typically between 1.6 and 2.4. The effect of the introduction of silica particles or of further thermal treatment or compression of the silica aerogels on their microstructure and their fractal characteristics is also resumed.

Low frequency noise in P-InAsSbP/n-InAs infrared light emitting diode- photodiode pairs is investigated for the first time at 300 K. It is shown that photocurrent fluctuations under LED illumination are smaller than photocurrent fluctuations under a black body illumination. When the photodiode is illuminated by LED, the spectral noise density follows the 1/f dependence. In the case of a black body illumination we observe a significant contribution of generation-recombination noise.

La microfluidique est l’ensemble de technologies permettant la manipulation et la caractérisation de fluides à des échelles micrométriques ou nanométriques. Elle peut être considérée à la fois comme une science visant à l’étude du comportement des fluides à l’intérieur de ces micro-canaux, et une technologie permettant la fabrication et l’utilisation de dispositifs microfluidiques. Ces dispositifs sont constitués d’une surface plane sur laquelle des capillaires et des chambres de réactions sont formés par photolithographie, moulage ou emboutissage. Des liquides en volumes micrométriques ou nanométriques sont ensuite insérés dans ces capillaires où ils sont déplacés, fractionnés, séparés ou mélangés pour obtenir des réactions chimiques. L’intérêt de travailler avec des volumes micrométriques, outre l’économie de moyens utilisés, réside dans l’accélération et l’automatisation du processus.Les puces et réacteurs microfluidiques peuvent être simples ou instrumentés (électrodes, capteurs, optiques, pompes…), la tendance étant à la complexification des outils, intégrant de plus en plus d’instruments sans pour autant être accompagnée d’une augmentation du prix. Lorsqu'une intégration optique est effectuée, on parle d'opto-microfluidique.Le travail proposé sera de construire un prototype de dispositif avec un double objectif :-Amélioration de la résolution des circuits de microfluidique par utilisation des nouvelles technologies de photoinscription Laser 2D et 3D haute résolution-Pousser l'intégration en implantant sur la même puce, à la fois le circuit microfluidique et un composant optique intégré.

Despite the ultimate performance of the existing cascade lasers, simple interband emitters in the mid-infrared (IR) can still be of interest as a cheaper and widely tunable alternative for some applications. In this work, we show mid-infrared stimulated emission (SE) at 5–6 μm wavelength from an optically pumped mercury–cadmium–telluride quantum well (QW) heterostructures at temperatures up to 200 K. At lower temperatures, the SE threshold appears to be mostly determined by conventional eeh Auger recombination, while the contribution of alternative QW-specific ehh Auger processes is limited. At higher temperatures, we establish heating of the electron gas by pumping radiation as a primary factor responsible for the thermal quenching of the SE. Consequently, both pumping scheme and QW designs should be carefully revised to minimize carrier heating in order to realize near-to-mid-IR optical converters operating close to ambient temperature. We suggest using low-barrier QWs to minimize excessive heat introduced in the QW upon carrier capture and also to eliminate eeh Auger processes involving excited QW subbands. Thus, mid-infrared HgCdTe lasers are expected to reach operating temperatures readily attainable under thermoelectric cooling

We have calculated two-dimensional plasmon energy spectra in HgTe/CdHgTe quantum wells with normal, gapless, and inverted energy spectra with different electron concentrations, taking into account spatial dispersion of electron polarizability and plasmon interaction with the optical phonons. The spectra of the absorption coefficients of two-dimensional plasmons are found. It is shown that an increase of electron concentration in a quantum well leads to a decrease in the plasmon absorption coefficient. We have calculated the probabilities to recombine via the plasmon emission for nonequilibrium holes. The threshold concentrations of the nonequilibrium holes, above which the plasmon amplification is possible, have been calculated for various electron concentrations. It is shown that the presence of equilibrium electrons can significantly reduce the threshold hole concentration required for amplification of plasmon in the terahertz wavelength region. The dependencies of threshold hole concentration on electron concentration for different quantum wells are discussed. Gain spectra of the two-dimension plasmon are calculated.