Structure and relaxation processes of an anisotropic molecular fluid confined into 1D nanochannels

Structural order parameters of a smectic liquid crystal confined into the columnar form of porous silicon are studied using neutron scattering and optical spectroscopic techniques. It is shown that both the translational and orientational anisotropic properties of the confined phase strongly couple to the one-dimensional character of the porous silicon matrix. The influence of this confinement induced anisotropic local structure on the molecular reorientations occuring in the picosecond timescale is discussed

Raman-modes of index-identified free-standing single-walled carbon nanotubes

Using electron diffraction on free-standing single-walled carbon nanotubes we have determined the structural indices (n,m) of tubes in the diameter range from 1.4 to 3nm. On the same free-standing tubes we have recorded Raman spectra of the tangential modes and the radial breathing mode. For the smaller diameters (1.4-1.7nm) these measurements confirm previously established radial breathing mode frequency versus diameter relations, and would be consistent with the theoretically predicted proportionality to the inverse diameter. However, for extending the relation to larger diameters, either a yet unexplained environmental constant has to be assumed, or the linear relation has to be abandoned.Comment: 4 pages, 4 figures, +additional materials (select PostScript to obtain it

Design of fiber coupled Er3+: Chalcogenide microsphere amplifier via particle swarm optimization algorithm

International audienceA mid-IR amplifier consisting of a tapered chalcogenide fiber coupled to an Er3+-doped chalcogenide microsphere has been optimized via a particle swarm optimization (PSO) approach. More precisely, a dedicated three-dimensional numerical model, based on the coupled mode theory and solving the rate equations, has been integrated with the PSO procedure. The rate equations have included the main transitions among the erbium energy levels, the amplified spontaneous emission, and the most important secondary transitions pertaining to the ion-ion interactions. The PSO has allowed the optimal choice of the microsphere and fiber radius, taper angle, and fiber-microsphere gap in order to maximize the amplifier gain. The taper angle and the fiber-microsphere gap have been optimized to efficiently inject into the microsphere both the pump and the signal beams and to improve their spatial overlapping with the rare-earth-doped region. The employment of the PSO approach shows different attractive features, especially when many parameters have to be optimized. The numerical results demonstrate the effectiveness of the proposed approach for the design of amplifying systems. The PSO-based optimization approach has allowed the design of a microsphere-based amplifying system more efficient than a similar device designed by using a deterministic optimization method. In fact, the amplifier designed via the PSO exhibits a simulated gain G=33.7 dB, which is higher than the gain G=6.9 dB of the amplifier designed via the deterministic method

Sur l'utilisation de la résine UV210 se flashant en UV profond pour les circuits de photonique intégrée / optoélectroniques : processus de nanolithographie, propriétés physiques et structures réalisées

National audienceL'UV210 [1] est une résine chimiquement amplifiée (CA) basé sur poly(p-hydroxystyrène) (PHS) en combinaison avec poly(tbutyl acrylate) (PBA). Comme toutes les résines CA, un photo-générateur d'acide (PAG) est ajouté à la matrice de copolymère. Lorsqu'elle est exposée à la lumière UV-profonds (248 nm), le PAG produit une petite quantité d'acide qui agit comme catalyseur pendant la post-exposition de cuisson au four. La cascade des transformations chimiques activée par l'acide entraîne un changement de polarité dans le polymère de lipophile à hydrophile. Ainsi, les régions exposées deviennent solubles dans le révélateur basique comme le tétra-méthyle l'hydroxyde d'ammonium (TMAH). L'utilisation d'un catalyseur acide entraîne une augmentation spectaculaire de l'efficacité quantique et sensibilité de l'exposition. Les propriétés physiques, d'indices optiques, mécaniques excellentes et appropriées permettent le développement relativement simplement de nombreuses structures photoniques sub-longueur d'onde diverses, de quelques centaines de nanomètres, de types rubans [2] ou même photo-inscrites par insolations variables [3]. A ce titre, de nombreux composants capteurs dédiés au suivi de la matière molle (transition de phase) sur cette base de polymère ont pu ainsi être réalisés [4-7]

Poisson's Ratio and the Densification of Glass under High Pressure

International audienceBecause of a relatively low atomic packing density, (Cg) glasses experience significant densification under high hydrostatic pressure. Poisson's ratio ( ) is correlated to Cg and typically varies from 0.15 for glasses with low Cg such as amorphous silica to 0.38 for close-packed atomic networks such as in bulk metallic glasses. Pressure experiments were conducted up to 25 GPa at 293 K on silica, soda-lime-silica, chalcogenide, and bulk metallic glasses. We show from these high-pressure data that there is a direct correlation between and the maximum post-decompression density change

Giant optical activity of sugar in thin soap films

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Highlighting excitonic optical properties of bundled carbon nanotubes to tailor novel saturable absorbers

International audienceWe present the ability of bundled carbon nanotubes to replace classical III-V semiconductor quantum-wells in nanomaterials-based devices, such as efficient saturable absorbers. Preliminary study of bundled carbon nanotubes films is presented using scanning electronic microscopy and Raman spectroscopy. Then, performing time-resolved pump-probe experiments, we show that excitonic nonlinear optical properties of bundled carbon nanotubes are simultaneously marked by ultrafast subpicosecond absorption recovery time, relative great contrast ratio and total recovery of absorption, in direct comparison with quantum-wells nanostructures. This first direct comparison attributes strong potential to bundled carbon nanotubes for all-optical switching devices, as lower-cost nanomaterials solution

Raman analysis of silicon and germanium nanowires

International audienceSilicon and germanium nanowires are synthesized by the VLS (VaporLiquid Solid) growth process in a LPCVD reactor (Low Pressure Chemical VaporDeposition), using gold catalysts and silane (SiH4) and germane (GeH4)respectively as gas precursors. The gold catalyst droplets can be formed byeither by dewetting of gold continuous layers or deposition of a colloidal solutionon the substrate. The second solution was used in the present work to get acontrol on the nanowire diameter during growth, which is not possible whenstarting with a continuous gold layer. These nanowires are characterized byRaman spectroscopy to study their structure and electronic properties. In fact,the Raman spectrum is sensitive to many factors that can cause its modification(effect of inhomogeneous distribution of the diameters, inhomogeneous heatingeffect and laser-induced phonon confinement effect). These effects can lead tothe shifting and broadening of Raman spectra compared to a Si or Gemonocrystalline reference. On the Raman spectra from our individual nanowires,we observe in the case of tapered nanowires, oscillations on the phonon lineintensities which we attribute to resonances related the local nanowire diameter.Such effects have been predicted from theoretical simulations. [1] 1. Cao, L., B.Nabet, and J.E. Spanier, Enhanced Raman scattering from individualsemiconductor nanocones and nanowires. Physical review letters, 2006. 96(15):p. 157402

From Fabrication to Characterization of 3D Organic Micro-Resonators: a Complementary Alliance of Microfluidics and Optics

International audienceThis paper introduces a preview of targeted current research on organic optofluidic materials and devices devoted to 3D photonics microresonators (MRs). First, such an approach takes advantage from a significant know-how on optical simulations of 3D spherical MRs by complementary and coupled ways based on electromagnetism and quantum mechanics principle. Such simulations have allowed to preset the quantization of whispering gallery modes (WGMs) and to define a new formulation of optical caustics in global 3D MRs cavities. Leaning from these simulations, an interdisciplinary approach has been achieved by combining microfluidics techniques and thin layer processes that allowed with flow rates control the realization of 3D droplets MRs of several tens of micrometers in radii. Finally, free-space optical characterizations have been performed on such 3D polymeric MRs by judicious protocols based, respectively, on a modified Raman spectroscopy laser excitation and an adequate direct beam waist optical coupling. Spectral analysis on such 3D MRs of various sizes confirms the excitation of the expected WGMs revealing free spectral range (FSR) and caustics values close to the analytical one