Characterization of a complex dispersion of multilamellar vesicles

Spherulites ^® are multilamellar vesicles made up of surfactant bilayers. These vesicles would potentially be very useful for the encapsulation and protection of molecules; however, traditional formulations of these vesicles are poor at retaining small hydrophilic molecules (below 1000 g/mol). In this study, we present new systems of Spherulites called complex dispersions. These are prepared by dispersing Spherulites in an oil medium, and then emulsifying this oily dispersion of Spherulites within an aqueous solvent. These new systems provide an additional oil barrier between encapsulated molecules and an external aqueous phase. We have used polarized light optical microscopy, X-ray diffraction and freeze–fracture electron microscopy to study a complex dispersion of Spherulites at all stages of its preparation. We first studied the sheared lamellar phase, followed by the dispersion of the multilamellar vesicles in the oily medium and finally the emulsification of the oily dispersion within the aqueous solvent. We compared our results on lamellar phases with previous results obtained with Spherulites directly dispersible in an aqueous medium. Since the formulation of our lamellar phase included a large percentage of oil as a component, we studied the localization of the oil in the lamellar structure. We also studied the influence of osmotic pressure on complex dispersions, because complex dispersions possess a double structure similar to that of water-in-oil-in-water emulsions and multiple emulsions are known to be sensitive to osmotic pressure. In conclusion, complex dispersions proved to be new potential carriers exhibiting some unique physical properties.     

Extreme events in optics: Challenges of the MANUREVA project

In this contribution we describe and discuss a series of challenges and questions relating to understanding extreme wave phenomena in optics. Many aspects of these questions are being studied in the framework of the MANUREVA project: a multidisciplinary consortium aiming to carry out mathematical, numerical and experimental studies in this field. The central motivation of this work is the 2007 results from optical physics [D. Solli et al., Nature 450, 1054 (2007)] that showed how a fibre-optical system can generate large amplitude extreme wave events with similar statistical properties to the infamous hydrodynamic rogue waves on the surface of the ocean. We review our recent work in this area, and discuss how this observation may open the possibility for an optical system to be used to directly study both the dynamics and statistics of extreme-value processes, a potential advance comparable to the introduction of optical systems to study chaos in the 1970s.     

Route to broadband blue-light generation in microstructured fibers

We explore theoretically the possibility of generating broadband blue light by copropagating a short soliton pump pulse and a broader signal pulse in a microstructured fiber with a zero-dispersion wavelength located between the center wavelength of the pump and the signal pulses. We show that the unique properties of microstructured fibers should allow for broadening of the signal pulse's spectrum by as much as a factor of 50 through the conjugate action of cross-phase modulation and a soliton self-frequency shift. The physical mechanism that leads to this large spectral broadening is analyzed by use of an extended nonlinear Schr?dinger equation.     

Interaction d'un jet moléculaire avec un gaz tampon : relaxation collisionnelle d'un système à deux niveaux d'énergie

Using a molecular beam, we study the transitions induced by collisions with molecules of a scattering gas. The experimental method uses a H₂CO beam apparatus which allows the observation of rotational transitions . The target gas is NH₃. The application of a static electric field E considerably modifies the collision induced transition probabilities; the transitions become forbidden when Eis intense. An experimental method is deduced to select transitions on a molecular beam. We initially prepare the system to be in the level (i) before entering the scattering chamber. After scattering, the population ratio n _f/n _i is measured as a function of the target gas pressure P _T. We determine the state to state cross-section of H₂CO. When the pressure P _T becomes intense, the effect is non linear. It is shown that this effect results of many collisions. We compare theory with experimental results. This experimental method gives a model for relaxation process studies in two energy level systems. Re?u le 30 juin 1999 et re?u sous forme finale le 14 décembre 1999     

Robust design of Si/Si3N4 high contrast grating mirror for mid-infrared VCSEL application

A Si/Si₃N₄ high contrast grating mirror has been designed for a VCSEL integration in mid-infrared (λ = 2.65?μm). The use of an optimization algorithm which maximizes a VCSEL mirror quality factor allowed the adjustment of the grating parameters while keeping large and shallow grating pattern. The robustness with respect to fabrication error has been enhanced thanks to a precise study of the grating dimension tolerances. The final mirror exhibits large high reflectivity bandwidth with a polarization selectivity and several percent of tolerance on the grating dimensions.     

Recurrence phase shift in Fermi-Pasta-Ulam nonlinear dynamics

We show that the dynamics of Fermi-Pasta-Ulam recurrence is associated with a nonlinear phase shift between initial and final states that are otherwise identical, after a full growth-return cycle. The properties of this phase shift are studied for the particular case of the self-focussing nonlinear Schrödinger equation, and we describe the magnitude of the phase shift in terms of the system parameters. This phase shift, accumulated during the nonlinear recurrence cycle, is a previously-unremarked feature of the Fermi-Pasta-Ulam problem, and we anticipate its wide significance as an essential feature of related dynamics in other systems.     

Maximum and Minimum Principles for Radionuclide Transport Calculations in Geological Radioactive Waste Repository: Comparison Between a Mixed Hybrid Finite Element Method and Finite Volume Element discretizations

In the context of high-level radioactive waste repository simulations, specific waste repository system properties require the use of a highly heterogeneous and anisotropic diffusion tensors. Classical finite volume or mixed hybrid finite element schemes produce non-physical negative concentrations and therefore are unsuitable for simulations which couple transport and chemical reactivity models. In this article, the authors use a new finite volume scheme satisfying a minimum and maximum principle to solve the transport equations and demonstrate that this scheme avoids the negative concentration values generated by other schemes.     

Supercontinuum and gas cell in a single microstructured fiber

We exploit both the high nonlinearity and the holey structure of microstructured fibers to combine a broad-band light source and a gas cell in a single microstructured fiber. A broadband supercontinuum is formed by launching nanosecond pulses from a compact, Q-switched Nd:YAG laser into a microstructured fiber filled with acetylene. This continuum is self-referenced to the acetylene lines in the 1500 nm region. The performance of different index-guiding narrow-core microstructured fibers as nonlinear and host media is evaluated. The concept offers many possibilities and can be applied to various gases absorbing at different wavelengths.