Cavity light bullets: three-dimensional localized structures in a nonlinear optical resonator

We consider the paraxial model for a nonlinear resonator with a saturable absorber beyond the mean-field limit. For accessible parametric domains we observe total radiation confinement and the formation of 3D localized bright structures. Different from freely propagating light bullets, here the self-organization proceeds from the resonator feedback, combined with diffraction and nonlinearity. Such "cavity" light bullets can be independently excited and erased by appropriate pulses, and once created, they endlessly travel the cavity round-trip.     

Dynamics of drag and force distributions for projectile impact in a granular medium

Our experiments and molecular dynamics simulations on a projectile penetrating a two-dimensional granular medium reveal that the mean deceleration of the projectile is constant and proportional to the impact velocity. Thus, the time taken for a projectile to decelerate to a stop is independent of its impact velocity. The simulations show that the probability distribution function of forces on grains is time independent during a projectile's deceleration in the medium. At all times the force distribution function decreases exponentially for large forces.     

Microorganic pollutants in the outskirts of Rome

A short field campaign was performed in the outskirts of Rome at four sites located pretty along the four rose wind directions to city centre. Both chlorinated (PCB and PCDD/F) and non-chlorinated (n-alkanes, PAH, nitrated-PAH, n-alkanoic acids) organic micropollutants were investigated for their contents in the atmosphere. Concentrations reached by these pollutants in the outskirts were compared to those found in downtown Rome, both inside and outside of its largest city garden. Although concentrations of organic pollutants found in the outskirts were quite low, however they seemed enough high to induce some health risk in humans. Rural sites were less affected than industrial and waste disposal/treatment areas.     

Electrochemical stability of steel,Ti, and Cu current collectors in water-in-salt electrolyte for green batteries and supercapacitors

Journal of Solid State Electrochemistry - The electrochemical behaviour of steel, copper, and titanium current collectors was studied in aqueous solutions of lithium...     

Different Topologies of Hg(II)-Bispidine 1D Coordination Polymers: Dynamic Behavior in Solvent Adsorption and Exchange Processes

One-dimensional (1D) coordination polymers (CPs) featuring three different topologies, comprising zig-zag, ribbon-like and poly-[n]-catenane structures, were obtained by reaction of Hg(II) ions with a novel bispidine ligand L3 , and structurally characterized by SC- and P-XRD methods. The CPs obtained in the form of microcrystalline powders were tested for their ability to undergo solvent adsorption and exchange by P-XRD and ¹H NMR spectroscopy. The extent of their dynamic behavior was then correlated to their structural features, highlighting the role of interchain interactions established among their constituting linear arrays. Zig-zag CPs proved to be resilient to external chemical stimuli, while they differently respond to thermal treatments, depending on the solvent originally included within the CP. In the case of polycatenated structures, we observed transformations where the original topology was maintained upon guest exchange, but also cases where it changed to zig-zag, even under solid/vapor conditions (i. e., no complete dissolution of the CP). Given the presence of linear interconnected 1D channels, 3 ⋅ ClBz-polycatenane^Pwd is also able to trap volatile guests such as n-hexane when exposed to its vapors.     

The Principle of Covariance and the Hamiltonian Formulation of General Relativity

The implications of the general covariance principle for the establishment of a Hamiltonian variational formulation of classical General Relativity are addressed. The analysis is performed in the framework of the Einstein-Hilbert variational theory. Preliminarily, customary Lagrangian variational principles are reviewed, pointing out the existence of a novel variational formulation in which the class of variations remains unconstrained. As a second step, the conditions of validity of the non-manifestly covariant ADM variational theory are questioned. The main result concerns the proof of its intrinsic non-Hamiltonian character and the failure of this approach in providing a symplectic structure of space-time. In contrast, it is demonstrated that a solution reconciling the physical requirements of covariance and manifest covariance of variational theory with the existence of a classical Hamiltonian structure for the gravitational field can be reached in the framework of synchronous variational principles. Both path-integral and volume-integral realizations of the Hamilton variational principle are explicitly determined and the corresponding physical interpretations are pointed out.     

Azobenzene: A Photoactive Building Block for Supramolecular Architectures

The development of nanoscale systems capable to perform specific functions under external control is a challenging task and a fascinating objective in Chemistry. Photochromic compounds undergo radical changes in their physico‐chemical properties upon light excitation, for this reason they are valuable building blocks for the construction of photo‐controllable molecular devices, machines and materials. The E–Z photoisomerization of azobenzene has been known for almost 80 years and – owing to its high efficiency and excellent reversibility – has been widely employed to introduce an element of photo‐control in a large variety of compounds, biomolecules, nanosystems and materials. Here we present some of our research results highlighting how this outstanding photochrome can be utilized to develop systems with light‐induced functionalities.     

Isothermal elongational behavior of liquid-crystalline polymers and LCPs based blends

The rheological behavior of two flexible thermoplastics, Nylon-6 (Ny) and bisphenol-A polysulfone (PSu), and two wholly aromatic liquid crystalline polymers, Vectra-A900 (VA) and Vectra-B950 (VB), as well as that of Ny/VB and PSu/VA blends with 10% LCP, has been investigated by the use of capillary viscometers equipped with cylindrical dies having different length-to-diameter ratios. The elongational viscosity of all materials was calculated, from the results of isothermal measurements carried out at 290°C, by means of the Cogswell's analysis, based on the estimation of the pressure drop due to the converging flow at the die inlet. The behavior in elongational flow was compared with the rheological behavior in shear flow conditions. It was found that the elongational viscosities of VA and VB are very large and account for a fairly marked pressure drop at the die entrance, due to the orientation of the LCP domains taking place in the converging flow zone. For these materials, the ratio of the elongational viscosity to the Newtonian shear viscosity is up to two orders of magnitude higher than the value expected on the basis of the Trouton rule. For the flexible resins, the Trouton ratio is 3 and ca. 3–10, are common values for high molar mass linear polymers. The addition of 10% LCP into the flexible resins strongly increases their elongational viscosity and makes the blends resemble neat LCPs in their extensional flow behavior. In shear flow, on the contrary, the addition of LCP was shown to induce a marked reduction of the melt viscosity, even when, as for the Ny/VB blend, the LCP is more viscous than the matrix.     

Radiant energy extraction from a gasdynamic laser. Static behavior

Summary The radiant energy extraction from a gasdynamic laser has been analyzed. The considered system of energy extraction is the MOPA one, in which the flowing gas medium is used as a Power Amplifier of a Master Oscillator, a primary laser beam introduced in the gas in which an inversion has been achieved. The analysis has been performed in general terms, in order to evaluate in a simple way the effect of the beam inclination, the effect of the losses due to the collisional relaxation and to the turbulent scattering and the effect of the ratio between the primary laser flux and the maximum available laser flux in static conditions.

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Sommario S'è studiata l'estrazione di energia radiante da un laser gasdinamico. Il sistema di estrazione analizzato è quello MOPA, cioè quello in cui il mezzo è sostanzialmente usato come amplificatore, (Power Amplifier), di un fascio primario, (Master Oscillator), immesso nel gas stesso in cui è stata realizzata l'inversione delle popolazioni. Lo studio è stato condotto con criteri di generalità, al fine di valutare il più semplicemente possibile l'effetto dell'inclinazione del fascio laser sull'asse, delle perdite per diseccitazione collisionale e per la diffusione nel mezzo, l'effetto del rapporto tra flusso primario entrante e flusso totale estraibile dal gas in condizioni stazionarie.