Spatiotemporal Characterization of Supercontinuum Extending from the Visible to the Mid-Infrared in Multimode Graded-Index Optical Fiber

We experimentally demonstrate that pumping a graded-index multimode fiber with sub-ns pulses from a microchip Nd:YAG laser leads to spectrally flat supercontinuum generation with a uniform bell-shaped spatial beam profile extending from the visible to the mid-infrared at 2500\,nm. We study the development of the supercontinuum along the multimode fiber by the cut-back method, which permits us to analyze the competition between the Kerr-induced geometric parametric instability and stimulated Raman scattering. We also performed a spectrally resolved temporal analysis of the supercontinuum emission.Comment: 5 pages 7 figure

Molecular dynamics simulations of elementary chemical processes in liquid water using combined density functional and molecular mechanics potentials. I. Proton transfer in strongly H-bonded complexes

The first molecular dynamics (MD) simulation of a chemical process in solution with an ab initio description of the reactant species and a classical representation of the solvent is presented. We study the dynamics of proton (deuterium) transfer in strongly hydrogen-bonded systems characterized by an energy surface presenting a double well separated by a low activation barrier. We have chosen the hydroxyl-water complex in liquid water to analyze the coupling between the reactive system and the environment. The proton is transferred from one well to the other with a frequency close to 1 ps−1 which is comparable to the low-frequency band associated to hindered translations, diffusional translation and reorientation of water molecules in water. The proton transfer takes place in 20–30 fs whereas the solvent response is delayed by about 50 fs. Therefore, the reaction occurs in an essentially frozen-solvent configuration. In principle, this would produce a barrier increase with respect to the equilibrium reaction path. However, solvent fluctuations play a substantial role by catalyzing the proton transfer. The solvent relaxation time after proton transfer has been evaluated. Since it falls in the same time scale than the reactive events (0.6 ps) it substantially influences the proton dynamics. The present study is intended to model charge transfer processes in polar media having a low activation barrier for which many reactive events may be predicted in a MD simulation. The case of reactions with large activation barriers would require the use of special techniques to simulate rare events. But still in that case, hybrid QM/MM simulations represent a suitable tool to analyze reaction dynamics and non-equilibrium solvent effects in solution [email protected]

The Benthic Boundary Layer: geochemical and oceanographic data from the GEOSTAR-2 observatory

Geochemical and oceanographic data, acquired throughout 6 months by the GEOSTAR-2 benthic observatory in southern Tyrrhenian Sea, evidenced ocean-lithosphere interactions in the 1900-m deep Benthic Boundary Layer (BBL), distinguishing two water masses with different origin and, possibly, benthic residence time. Gas concentration, helium isotopic ratios, radioactivity, temperature, salinity and vertical component of the current converged towards the indication of a BBL characterised by a colder and fresher western water (WW), which is episodically displaced by the cascading of the warmer and saltier Eastern Overflow Water (EOW). The benthic WW has higher concentration of geochemical tracers diffusing from the seafloor sediments. The data set shows the potential of long-term, continuous and multiparametric monitoring in providing unique information which cannot be acquired by traditional, short-term or single-sensor investigations

Molecular dynamics simulations of elementary chemical processes in liquid water using combined density functional and molecular mechanics potentials. II. Charge separation processes

A new approach to carry out molecular dynamics simulations of chemical reactions in solution using combined density functional theory/molecular mechanics potentials is presented. We focus our attention on the analysis of reactive trajectories, dynamic solvent effects and transmission coefficient rather than on the evaluation of free energy which is another important topic that will be examined elsewhere. In a previous paper we have described the generalities of this hybrid molecular dynamics method and it has been employed to investigate low energy barrier proton transfer process in water. The study of processes with activation energies larger than a few kT requires the use of specific techniques adapted to “rare events” simulations. We describe here a method that consists in the simulation of short trajectories starting from an equilibrated transition state in solution, the structure of which has been approximately established. This calculation is particularly efficient when carried out with parallel computers since the study of a reactive process is decomposed in a set of short time trajectories that are completely independent. The procedure is close to that used by other authors in the context of classical molecular dynamics but present the advantage of describing the chemical system with rigorous quantum mechanical calculations. It is illustrated through the study of the first reaction step in electrophilic bromination of ethylene in water. This elementary process is representative of many charge separation reactions for which static and dynamic solvent effects play a fundamental [email protected]

Pressure dependence of Raman modes in double wall carbon nanotubes filled with α-Fe.

The preparation of highly anisotropic one-dimensional (1D) structures confined into carbon nanotubes (CNTs) in general is a key objective in CNTs research. In this work, the capillary effect was used to fill double wall carbon nanotubes with iron. The samples are characterized by Mössbauer and Raman spectroscopy, transmission electron microscopy, scanning area electron diffraction, and magnetization. In order to investigate their structural stability and compare it with that of single wall carbon nanotubes (SWNTs), elucidating the differences induced by the inner-outer tube interaction, unpolarized Raman spectra of tangential modes of double wall carbon nanotubes (DWNTs) filled with 1D nanocrystallin α-Fe excited with 514 nm were studied at room temperature and elevated pressure. Up to 16 GPa we find a pressure coefficient for the internal tube of 4.3 cm−1 GPa−1 and for the external tube of 5.5 cm−1 GPa−1. In addition, the tangential band of the external and internal tubes broadens and decreases in amplitude. All findings lead to the conclusion that the outer tube acts as a protection shield for the inner tubes (at least up 16 GPa). Structural phase transitions were not observed in this range of pressure

Atomic diffraction from nanostructured optical potentials

We develop a versatile theoretical approach to the study of cold-atom diffractive scattering from light-field gratings by combining calculations of the optical near-field, generated by evanescent waves close to the surface of periodic nanostructured arrays, together with advanced atom wavepacket propagation on this optical potential.Comment: 8 figures, 10 pages, submitted to Phys. Rev.

GEOSTAR, an observatory for deep sea geophysical and oceanographic researches: characteristics, first scientific mission and future activity

GEOSTAR (GEophysical and Oceanographic STation for Abyssal Research) is a project funded by in the 4th Framework Programme of the European Commission, with the aim of developing an innovative deep sea benthic observatory capable of carrying out long-term (up to 1 year) scientific observations at abyssal depths. The configuration of the observatory, conceived to be a node of monitoring networks, is made up of two main subsystems: the Bottom Station, which in addition to the acquisition and power systems and all the sensors also hosts the communications systems; and the Mobile Docker, a dedicated tool for surface-assisted deployment and recovery. At present the Bottom Station is equipped with a triaxial broad-band seismometer, two magnetometers (fluxgate and scalar), CTD, transmissometer, ADCP, but it can easily host other sensors for different experiments. The first phase of the project, started in November 1995, was concluded with the demonstration mission in Adriatic Sea at shallow water depth (42 m) in August - September 1998. Some preliminary results of this first scientific experiment are presented and discussed. The second phase, started in 1999, will end with a long-term deep sea scientific mission, scheduled during 2000 for 6-8 months at 3400 m.w.d. in the southern Tyrrhenian bathyal plain.Published491-4973A. Ambiente MarinoN/A or not JCRrestricte

Mission results from the first GEOSTAR observatory (Adriatic Sea, 1998)

We assess the first mission of the GEOSTAR (GEophysical and Oceanographic STation for Abyssal Research) deep-sea multidisciplinary observatory for its technical capacity, performance and quality of recorded data. The functioning of the system was verified by analyzing oceanographic, seismological and geomagnetic measurements. Despite the mission’s short duration (21 days), its data demonstrated the observatory’s technological reliability and scientific value. After analyzing the oceanographic data, we found two different regimes of seawater circulation and a sharp and deepening pycnocline, linked to a down-welling phenomenon. The reliability of the magnetic and seismological measurements was evaluated by comparison with those made using on-land sensors. Such comparison of magnetic signals recorded by permanent land geomagnetic stations and GEOSTAR during a “quiet” day and one with a magnetic storm confirmed the correct functioning of the sensor and allowed us to estimate the seafloor observatory’s orientation. The magnitudes of regional seismic events recorded by our GEOSTAR seismometer agreed with those computed from land stations. GEOSTAR has thus proven itself reliable for integrating other deep-sea observation systems, such as modular observatories, arrays, and instrumented submarine cablesPublished361-373ope

Acoustic and optical variations during rapid downward motion episodes in the deep north-western Mediterranean Sea

An Acoustic Doppler Current Profiler (ADCP) was moored at the deep-sea site of the ANTARES neutrino telescope near Toulon, France, thus providing a unique opportunity to compare high-resolution acoustic and optical observations between 70 and 170 m above the sea bed at 2475 m. The ADCP measured downward vertical currents of magnitudes up to 0.03 m s-1 in late winter and early spring 2006. In the same period, observations were made of enhanced levels of acoustic reflection, interpreted as suspended particles including zooplankton, by a factor of about 10 and of horizontal currents reaching 0.35 m s-1. These observations coincided with high light levels detected by the telescope, interpreted as increased bioluminescence. During winter 2006 deep dense-water formation occurred in the Ligurian subbasin, thus providing a possible explanation for these observations. However, the 10-20 days quasi-periodic episodes of high levels of acoustic reflection, light and large vertical currents continuing into the summer are not direct evidence of this process. It is hypothesized that the main process allowing for suspended material to be moved vertically later in the year is local advection, linked with topographic boundary current instabilities along the rim of the 'Northern Current'.Comment: 30 pages, 7 figure