Ultrashort light bullets described by the two-dimensional sine-Gordon equation

By using a reductive perturbation technique applied to a two-level model, this study puts forward a generic two-dimensional sine-Gordon evolution equation governing the propagation of femtosecond spatiotemporal optical solitons in Kerr media beyond the slowly varying envelope approximation. Direct numerical simulations show that, in contrast to the long-wave approximation, no collapse occurs, and that robust (2+1)-dimensional ultrashort light bullets may form from adequately chosen few-cycle input spatiotemporal wave forms. In contrast to the case of quadratic nonlinearity, the light bullets oscillate in both space and time and are therefore not steady-state lumps

Sexual Differences In Synaptogenesis In The Suprachiasmatic Nucleus Of The Rat Brain

1. A literature review of the effects of perinatal sex steroids on the development of the brain is presented. It can be shown that the hormones can modify the morphology of discrete areas of the brain. 2. It was concluded that there was a lack of information about how soon these morphological changes appeared. 3. It was therefore decided to study the effects of sex and perinatal hormonal levels on synaptogenesis in a discrete area of the brain. 4. The Suprachiasmatic Nucleus (SON) was chosen for anatomical convenience and because of its role in the photocontrol of ovulation. 5. In this thesis I describe synaptogenesis in the suprachiasmatic nucleus of rats of both sexes ranging from prenatal (20 days of gestation) to adult and in neonatally androgenised females. Preliminary results from neonatally castrated males are also included. 6. Synaptogenesis was studied using morphometric analysis of electron micrographs. It was found that the rate of development of synapses was similar in the SCN of both sexes, but that absolute values in males were higher than in females. Male type development cannot be mimicked by neonatal androgenization but initial results suggest that femal type development can be induced by neonatal castration. 7. My results suggest that both prenatal and postnatal androgens are essential to normal male development

A mechanical behavior law for the numerical simulation of the mushy zone in welding

The aim of this work is to propose a mechanical behavior law dedicated to the mushy zone located between the solid phase and the weld pool in welding. The objective is to take into account of the influence of the mushy zone in the simulation of welding in order to improve the computation of induced effects such as residual stresses

Finite-distance singularities in the tearing of thin sheets

We investigate the interaction between two cracks propagating in a thin sheet. Two different experimental geometries allow us to tear sheets by imposing an out-of-plane shear loading. We find that two tears converge along self-similar paths and annihilate each other. These finite-distance singularities display geometry-dependent similarity exponents, which we retrieve using scaling arguments based on a balance between the stretching and the bending of the sheet close to the tips of the cracks.Comment: 4 pages, 4 figure

Half-optical-cycle damped solitons in quadratic nonlinear media

In this paper, Using a classical model of the radiation-matter interaction, we show that the propagation of (1 + 1) dimensional few-optical-cycle pulses in quadratic nonlinear media, taking moderate absorption into account, can be described by the Korteweg–de Vries-Burgers’ (KdVB) equation without using the slowly varying envelope approximation. To fulfill this purpose we use the reductive perturbation method and consider the long-wave approximation, assuming that the characteristic frequency of the pulse is much lower than the resonance frequency of the atoms. We also study both analytical and numerical solution of the KdVB equation describing damped few-optical-cycle soliton propagation

Roles of resonance and dark irradiance for infrared photorefractive self-focusing and solitons in bi-polar InP:Fe

This paper shows experimental evidence of photorefractive steady state self-focusing in InP:Fe for a wide range of intensities, at both 1.06 and 1.55$\mu$m. To explain those results, it is shown that despite the bi-polar nature of InP:Fe where one photocarrier and one thermal carrier are to be considered, the long standing one photocarrier model for photorefractive solitons can be usefully applied. The relationship between the dark irradiance stemming out of this model and the known resonance intensity is then discussed

Spontaneous formation of optically induced surface relief gratings

A model based on Fick's law of diffusion as a phenomenological description of the molecular motion, and on the coupled mode theory, is developped to describe single-beam surface relief grating formation in azopolymers thin films. It allows to explain the mechanism of spontaneous patterning, and self-organization. It allows also to compute the surface relief profile and its evolution in time with good agreement with experiments

Candela photo-injector experimental results with a dispenser photocathode

International audienceThe CANDELA photo-injector is a two cell S-band photo-injector. The dispenser photocathode is illuminated by a 500 fs pulse from a frequency-tripled Ti:sapphire laser. In this paper we report charge measurements showing that the dispenser photocathode has a quantum efficiency as high as 10-3. This efficiency decreases with a lifetime of 12 hours, but can be recovered by heating the cathode for 5 minute

HIGH PULSED CURRENTS FROM PHOTO-FIELD EMITTERS

Différent microemitters - single or arrays - with various geometries and kinds of material have been irradiated with pulsed laser beams. These emitters working in photo-field emission regime delivered very high intensity electron bunches. Peak intensities as high as some tens of Amps with less than one ns duration have been obtained with U.V. light. New type of microemitters developed in collaboration with BNL have been tested since last year showing the possibility of obtaining charges above 20 nC with low energy laser puises, (εi = 100µJ). The main parameters affecting the choice of these emitters as quantum yield, photocurrent density, electron pulse length, repetition rate and vacuum system level are here discussed. Good performances obtained with these emitters as well as the absence of cesiation make these microemitters interesting candidates for the new generation of linac injectors as well as for multimegawatt RF sources. At LAL, Orsay efforts have been made since three years to develop such electron sources