Generalized Swift–Hohenberg and phase-field-crystal equations based on a second-gradient phase-field theory

Meccanica - The principle of virtual power is used derive a microforce balance for a second-gradient phase-field theory. In conjunction with constitutive relations consistent with a free-energy...     

Continua described by a microstructural field

Using a balance law for microforces and an appropriate statement of the second law of thermodynamics, a framework is provided for continuum theories that involve a microstructural variable. Examples of specific physical theories that fall within that framework—spanning internal state-variable theories for plasticity and polymeric solutions, order-parameter based theories for phase transitions, and various theories for liquid crystals-are given.     

Numerical developments for short-pulsed Near Infra-Red laser spectroscopy. Part II: inverse treatment

Indirect optical spectroscopy or tomography, that is, mapping of optical properties in scattering and absorption inside a medium given a set of measurements at the boundaries, is highly dependent on the radiative transfer model used to track radiative energy propagation in semi-transparent materials. In the first part of this study, a numerical tool adapted for treating radiative transfer in the frame of short-pulsed laser beam interaction with non-homogeneous matter has been presented. In this paper, it is intended to show how such numerical tools can undergo inversion through adjoint treatment or reverse differentiation.Adjoint models, as well as reverse differentiation, are used in order to allow an efficient computation of the gradient, in the unknown optical parameters space, of an objective or cost function estimating the residual between data obtained at the boundary and predictions by numerical simulations. This gradient is a crucial indication as to update, through line minimization, the set of internal optical properties of the medium.First, the theoretical background of the inverse treatments, both reverse differentiation and adjoint model, for the transient radiative transfer equation model introduced in Part I is developed. Second, different reconstruction configurations are presented. Time-dependent sampling and time filtering effects of the measurements are addressed. Image reconstructions from simulated data are achieved for material phantoms of simple geometry.     

Tensor Gauge Fields in Arbitrary Representations of GL( D, $${\mathbb{R})}$$ : II. Quadratic Actions

Quadratic, second-order, non-local actions for tensor gauge fields transforming in arbitrary irreducible representations of the general linear group in D-dimensional Minkowski space are explicitly written in a compact form by making use of Levi–Civita tensors. The field equations derived from these actions ensure the propagation of the correct massless physical degrees of freedom and are shown to be equivalent to non-Lagrangian local field equations proposed previously. Moreover, these actions allow a frame-like reformulation à la MacDowell–Mansouri, without any trace constraint in the tangent indices. Chargé de Recherches FNRS, Belgium     

5-epi-Canadensene and other novel metabolites of Taxus canadensis

The Canadian yew distinguishes itself from other yews by the nature of its taxane metabolites. We are now reporting a new canadensene taxane whose stereochemistry is rigorously established. The three-dimensional structures of canadensene, 5-epi-canadensene and three other related bicyclic taxanes isolated from other yews were calculated using distance constraints derived from NMR data. The stereochemistry of the substituents, the polar acetate groups and the double bonds determine the 3D models. In addition, three new taxanes were also characterized and some biosynthetic speculations are presented.     

Experimental demonstration of a pure third-order optical parametric downconversion process

We report what is, to the best of our knowledge, the first experiment on third-order optical difference frequency generation. The nonlinear medium is a phase-matched KTiOPO4 crystal pumped at 532 and 1665 nm in the picosecond regime. The proposed experimental configuration leads to an efficient and pure cubic process, with generation of 4.5 microJ/pulse at 1474 nm. This result opens the way to new quantum experiments on three-photon Greenberger-Horne-Zeilinger entanglement.     

Effective impedance spectra for predicting rough sea effects on atmospheric impulsive sounds

Two methods of calculating the effective impedance spectra of acoustically hard, randomly rough, two-dimensional surfaces valid for acoustic wavelengths large compared with the roughness scales have been explored. The first method uses the complex excess attenuation spectrum due to a point source above a rough boundary predicted by a boundary element method (BEM) and solves for effective impedance roots identified by a winding number integral method. The second method is based on an analytical theory in which the contributions from random distributions of surface scatterers are summed to obtain the total scattered field. Effective impedance spectra deduced from measurements of the complex excess attenuation above 2D randomly rough surfaces formed by semicylinders and wedges have been compared to predictions from the two approaches. Although the analytical theory gives relatively poor predictions, BEM-deduced effective impedance spectra agree tolerably well with measured data. Simple polynomials have been found to fit BEM-deduced spectra for surfaces formed by intersecting parabolas corresponding to average roughness heights between 0.25 and 7.5 m and for five incidence angles for each average height. Predicted effects of sea-surface roughness on sonic boom profiles and rise time are comparable to those due to turbulence and molecular relaxation effects.     

Theory of tunnel ionization in complex systems

A quasianalytical theory of tunnel ionization is developed that is applicable to general complex systems, such as large molecules. Our analysis reveals strong deviations from conventional tunnel ionization theories, dependent upon the system's geometry, angular momentum, and polarizability. A comparison of our theory with recent C(60) ionization experiments yields reasonable agreement.     

Thermo-optical effect and saturation of nonlinear absorption induced by gray tracking in a 532-nm-pumped KTP optical parametric oscillator

We present experiments that show that gray tracking modifies the parametric gain and the generated wavelengths of a KTP optical parametric oscillator pumped at 532 nm near degeneracy. These perturbations occur over a limited range of pump intensity. We propose a satisfactory model that takes into account photochromic damage, the thermo-optical effect, and the combined processes of creation and saturation of a two-photon absorber at 532 nm. The temperature dependence of Sellmeier equations of KTP is also established at 20-200 degrees C.