Ultrabroadband conical emission generated from the ultraviolet up to the far-infrared during the optical filamentation in air

Ultraviolet and infrared conical emissions were observed during the filamentation in air of powerful femtosecond laser pulses produced by a portable terawatt laser system. The broadband spectrum was measured from 200 nm up to 14 microm and covered the complete optical transmission window of the atmosphere. The angularly resolved spectrum showed some X-wave structure across the frequency range analyzed. However, we demonstrated that the strong conical emission observed in the mid- and far-infrared is mainly owing to the four-wave mixing between the pump pulse and its blueshifted conical emission.     

Pinning and depinning process of an incommensurate CDW as revealed by coherent X-ray diffraction

Experiments on the coherent X-ray diffraction, and their modeling, have been performed on the Charge Density Wave (CDW) system NbSe₃. The 2k_F$2k_F$ satellite reflection associated with the CDW has been measured with respect to external dc currents. Below the threshold current, reflection displays several fringes in the transverse direction which disappear when the threshold current is exceeded. In the sliding state, the transverse satellite profile has a form of two nonsymmetric peaks, one of them being centered at the same position as below the threshold and another one being shifted. The shift of the longitudinal peak position below the threshold current and the nonsymmetric peak in the transverse direction above the threshold one is interpreted as the influence of strong linear defect like a crystal step present on the sample surface, combined with induced arrays of dislocations. Coherent X-rays provide a new access to processes in a CDW driven by an external force in a random pinning potential.     

Linear and nonlinear optical properties of carbon nanotube-coated single-mode optical fiber gratings

Single-wall carbon nanotube deposition on the cladding of optical fibers has been carried out to fabricate an all-fiber nonlinear device. Two different nanotube deposition techniques were studied. The first consisted of repeatedly immersing the optical fiber into a nanotube supension, increasing the thickness of the coating in each step. The second deposition involved wrapping a thin film of nanotubes around the optical fiber. For both cases, interaction of transmitted light through the fiber core with the external coating was assisted by the cladding mode resonances of a tilted fiber Bragg grating. Ultrafast nonlinear effects of the nanotube-coated fiber were measured by means of a pump-probe pulses experiment.     

The Contribution of Raman Scattering to the Fluorescence of the Polyene Antibiotic Amphotericin B

Fluorescence has recently been applied to the analysis of the molecular organization state of the polyene antibiotic amphotericin B (AmB) in solution or in lipid membranes. The polyene chain of AmB monomer gives rise to two fluorescence emissions; S₁(2¹A_g) → S₀(1¹A_g) between 500 and 700 nm, S₂(1¹B_u) → S₀(1¹A_g) between 400 and 500 nm. However, Raman scattering might interfere with the S₂ → S₀ emission fluorescence due to the weak fluorescence quantum yield and close proximity to the exciting lines. In fact, we show here that a change in the excitation wavelength results in a shift of three emission bands, an effect which excludes their assignment to fluorescence. These bands originate from the water Raman at 3382 cm^-1and AmB resonance Raman at 1556 and 1153 cm^-1. As a consequence, some former conclusions on the molecular organization state of AmB should be reconsidered.     

Stereospecific Synthesis and Acylation of cis 1-Halo 2-Trimethylsilyl Ethylenes

Several routes to 1-halo 2-trimethylsilyl ethylenes have been previously proposed ^1-6: from ethyn7yltrimethylilane (addition of HBr^1,2), ethyltrichlorosilane (addition H by C1, followed by dehydrohalogenation and further methylation of th Si-C1 bonds)³ or by other ways ^4,5, especially by reaction of (2-trimethylsilyl) vinyl lithium with dibromoethane⁶.     

Analytic modeling of the angular sensitivity function and modulation transfer function of ultrathin multichannel imaging systems

We propose what we believe to be a novel, refined model of the angular sensitivity function of artificial apposition compound eyes. Compared with the formerly used Gaussian approximation that was derived for natural compound eyes, our model is better suited to describe the resolution capacity of artificial compound eyes accounting for the cylindrical sensitivity function of technical receptors. It is shown that this analytic model is valid over a broad range of parameters of the optical system, which was not fulfilled by one of the previous models. Finally, an analytic approach is used to derive the modulation transfer function of these multichannel imaging systems.     

Reactive polymers incorporating silyl enol ether groups. I. synthesis by radical polymerization of 2-(trimethylsiloxy)-butadiene and 2-(tert)-butyldimethylsiloxy butadiene

2-(Trimethylsiloxy)butadiene (TMSBD) and 2-(tert-butyldimethylsiloxy)butadiene (TBMSBD) were copolymerized with styrene (St) and methyl methacrylate (MMA) under free-radical conditions. The obtained polymers were found to contain reactive silyl enol ether groups in a ratio identical to the TMSBD or TBMSBD molar fraction in the copolymer. All investigated samples displayed only 1,4- and 3,4-microstructures. The influence of several experimental factors on the yields, rates of polymerization, microstructures, and copolymer compositions were examined. Monomer reactivity ratios r₁ and r₂ at 60°C were determined from copolymer composition curves at low conversions. The homopolymerization of TBMSBD was also investigated and results were compared with those previously obtained for TMSBD. A slight increase in rates was observed and was rationalized on the basis of the higher viscosity resulting from the structural change in the monomer. Thermal stabilities of the synthesized polymers were investigated by TGA and their glass transition temperatures were determined by DSC. All measurements are compatible with a possible use of TMSBD and TBMSBD copolymers as reactive polymers. © 1996 John Wiley & Sons, Inc.     

GC-MS headspace analysis of the volatile components of soya oil without heating the sample

The analysis of volatile compounds, and particularly of certain hydrocarbons, e.g. pentane, and some aldehydes, e.g. hexanal, has been used to estimate the degree of oxidation of vegetable oil. We have tested a dynamic headspace technique without heating the sample during the purge time of the analysis. Some tests have been performed after heating the sample at 180°C for a specified time before the analysis, to compare the results of analysis with and without heating. The best results were obtained at room temperature.     

Estimation of the surface energetic heterogeneity of fillers by inverse gas chromatography

The surface properties of the fillers are determining for obtaining high performance filled polymers. Yet, their determination is most difficult. This paper proposes to use inverse gas chromatography, either at infinite dilution or finite concentration conditions, to estimate surface energy and nanomorphological characteristics, but also surface heterogeneity of different clays, talcs of various origins and ground muscovites.