La physique des hautes pressions, et particulièrement la mesure des équations d'état, s'est développée pour répondre aux besoins d'autres disciplines où la connaissance quantitative du comportement de la matière fortement comprimée est fondamentale. Ainsi, en géophysique, la compréhension de la structure interne de la terre1 dépend de l'équation d'état du fer et de ses propriétés physiques 2 . Une situation similaire se rencontre en planétologie pour d'autres éléments (structure de Jupiter et équation d'état de l'hydrogène3 , structure d'Uranus et équation d'état de l'eau4 ...). Plus généralement, l'équation d'état des matériaux constituant un objet astrophysique intervient nécessairement dans l'étude théorique de sa structure interne pour fermer le système des équations de l'hydrostatique5 .

Th. 3e cycle--Optique et trait. du signal--Besançon, 1984. N°: 460.

Th.--Méd.--Reims, 1980. N°: 121.

Cette thèse vise à connaître et expliquer l’impact de la contamination moléculaire sur l’endommagement laser des optiques en silice d'un laser de puissance tel que le Laser Méga Joule (LMJ). L'une des causes de l'endommagement prématuré de ces optiques est l'adsorption de polluants moléculaires ou particulaires à leur surface. Dans le contexte particulier du LMJ, nos conditions d'études laser sont des fluences supérieures à 10 J/cm², une longueur d’onde de 351 nm et une durée d’impulsion de 3 ns pour une irradiation en monocoup. Des prélèvements moléculaires, l’analyse du dégazage des matériaux, et l’identification de la contamination condensée sur les surfaces des optiques présentes dans des environnements jugés critiques, ont permis de déterminer certains polluants critiques. Des expériences de contamination contrôlée impliquant ces polluants ont alors été menées afin de comprendre et modéliser leur effet sur l'endommagement laser des optiques. Ceci nous a conduits à proposer plusieurs mécanismes supposés. / Laser induced damage impact of molecular contamination on fused polished silica samples in a context of high power laser fusion facility, such as Laser MegaJoule (LMJ) has been studied. One of the possible causes of laser induced degradation of optical component is the adsorption of molecular or particular contamination on optical surfaces. In the peculiar case of LMJ, laser irradiation conditions are a fluence of 10 J/cm², a wavelength of 351 nm, a pulse duration of 3 ns for a single shot/days frequency. Critical compounds have been identified thanks to environmental measurements, analysis of material outgassing, and identification of surface contamination in the critical environments. Experiments of controlled contamination involving these compounds have been conducted in order to understand and model mechanisms of laser damage. Various hypotheses are proposed to explain the damage mechanism.

A la suite de la première démonstration de l'écriture de guide d'onde au sein de verres en 1996 par laser femtoseconde, l'écriture direct par Laser Femtoseconde (Femtoseconde Direct Laser Writing - FLDW) est apparu comme une technique souple pour la fabrication de structure photonique en trois dimensions au sein de matériaux pour l'optique. La thèse a porté sur l'inscription par laser femtoseconde de fonctionnalités optiques au sein de verres. Des éléments diffractifs par modification de l'indice de réfraction et des structures présentant des propriétés de luminescence ou d'optique non linéaire d'ordre deux ont pu être obtenus au sein de matériaux vitreux et étudiés. / Since the first demonstration of femtosecond laser written waveguides in 1996, femtosecond laser direct writing (FLDW) has been providing a versatile means to fabricate embedded 3-D microstructures in transparent materials. The key mechanisms are nonlinear absorption processes that occur when a laser beam is tightly focused into a material and the intensity of the focused beam reaches the range creating enough free electrons to induce structural modification. This dissertation was an attempt to make an improvement on the existing FLDW technique to achieve a reliable fabrication protocol for integrated optical devices involving micro diffractive optical elements and laser-structures exhibiting second order nonlinear optical properties. Relaxation processes of directly-written structures in chalcogenide glasses have been also investigated.

Made available in DSpace on 2016-12-12T20:15:53Z (GMT). No. of bitstreams: 1 parte1.pdf: 63647 bytes, checksum: f2fd5c831fcfe5e4d704fd18bec2cc27 (MD5) Previous issue date: 2010-03-11 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Laser cooling in high magnetic fields, presents a series of difficulties due to inhomogeneous broadening of the frequency due to Zeeman Effect. In the first part of this work we investigate the laser cooling of Li by a Monte Carlo simulation, the 2S(1/2)-> 2P(3/2) transition at 670.96 nm in a magnetic trap under the characteristics of trap operating at the LASER laboratory of the Institute of Physics, UFRJ. In the second part, we built a diode laser with extended Littrow cavity emitting in 972 nm using the configuration of extended cavity Littrow. The purpose of this laser is, after a double frequency doubling, to use it to study the hydrogen atom in the transition 1S -> 2S at 243 nm. / O resfriamento a laser em altos campos magnéticos, apresenta uma série de dificuldades devido ao alargamento inomogêneo da transição causado pelo efeito Zeeman. Na primeira parte desse trabalho investigamos o resfriamento a laser de Lítio através de uma simulação de Monte Carlo, na transição 2S1/2 -> 2P3/2 em 670.96 nm, em uma armadilha magnética com as características da armadilha em funcionamento no laboratório LASER do instituto de Física da UFRJ. Na segunda parte, foi construído um laser de diodo em cavidade estendida emitindo em 972 nm utilizando a configuração de Cavidade Estendida de Littrow. O propósito desse laser e após um duplo dobramento de freqüência, utilizá-lo para estudo do átomo de Hidrogênio na transição 1S -> 2S em 243 nm.

Automotive and aerospace industries consume a significant amount of Al alloys in structures and framing. There is, however, a significant challenge to join the alloy components by laser welding. A key problem is the presence of large amount of porosity in the welds. This research work aimed to understand factors affecting porosity formation in laser welding of AA6014 Al alloy and identification and verification of a suitable method for the porosity reduction and elimination. AC-170PX (AA6014) Al alloy was welded, for the first time, using a 5 kW disk laser in two different configurations: fillet edge and flange couch joints using a number of different filler wires. The experimental results showed that laser power (2-5 kW) and welding speed (20-50 mm/s) had a significant influence on porosity generation. Also, the introduction of a 0.2 mm gap between the sheets significantly reduced porosity for the fillet edge joint while it had a marginal effect for the flange couch joint. The effect of the chemical composition of the filler wire on the AA6014 weld quality was also evaluated for the first time by using different filler wires (AA3004, AA4043, AA4047 and AA5083) over a range of laser powers and welding speeds. The increase in Mg and Mn content in the filler wire's composition was found to reduce porosity in comparison with high silicon content filler wires. Nanosecond pulsed Nd:YAG laser cleaning was investigated as a surface preparation method for laser welding for AA6061, and its effect on porosity at various welding parameters was examined. The effect of laser cleaning on porosity reduction during laser welding using a filler wire has not been reported before. The surface characteristics before and after laser cleaning were analysed. The results showed that laser cleaning played an essential role in significantly reducing porosity in both the fillet edge and flange couch joints at different levels of power and laser welding speed. The developed surface preparation technology as a method for porosity reduction in laser welding has been successfully implemented in one of the largest UK/international car manufacturers. To study the laser cleaning process, a novel Smoothed particle hydrodynamics (SPH) meshless model has been implemented using a new 3-D multi-phase transient model. For the first time, a study was conducted to validate the temperature field distribution predicted in SPH method under nanosecond pulsed laser heating. The need for special surface treatment of the kernel truncation was also investigated. The proposed model accurately predicted the laser ablation depth and the crater shape and was validated using a significant number of experimental and numerical data reported in the literature. Moreover, a primitive laser welding model has been created to predict the material flow inside the welding pool. The research work has resulted in four publications in peer-reviewed journals. The research highlighted that future work should include the development of a more advanced SPH model for the prediction of porosity in laser welding and to fully describe the relationship between laser cleaning and porosity reduction in laser welding.

[pt] Esta dissertação faz parte de um projeto mais abrangente que visa caracterizar os parâmetros ideais para perfuração e corte a laser em rochas, metais e cimentos tipicamente utilizados na indústria do petróleo. Este trabalho teve por objetivo de caracterizar cortes a laser em placas de aço. Procurou-se correlacionar a morfologia do corte e a formação de rebarbas com parâmetros operacionais, tais como densidade de potência e posição do ponto focal. O estudo foi desenvolvido para o aço SAE 1020 com e sem anteparo rochoso (Travertino). Foram avaliadas também as mudanças na microestrutura de regiões adjacentes do aço duplex 2205 para diferentes faixas de densidade de potência. Finalmente, avaliou-se também o uso do laser no corte do aço inox submerso em água. Conclui-se que existe uma formação de rebarba, que aumenta conforme a densidade de potência diminui. Por outro lado, quanto maior a densidade de potência, maior é a extensão da mudança na microestrutura do aço. O corte de aço com anteparo rochoso abre novas perspectivas para o desenvolvimento de uma ferramenta baseada em laser de alta potência para completação de poços na indústria do petróleo. / [en] This work is part of a larger project that aims to characterize the optimal parameters for laser cutting and drilling into rocks, metals and cement typically used in the oil industry. This study had as objective to characterize laser cuts in steel plates. An attempt to establish a correlation between the morphology of the cut and the burrs with operating parameters such as power density and position of the focal point. The study was developed for the SAE 1020 steel with and without rocky bulkhead (Travertine). Also, the changes in the microstructure of adjacent regions of duplex steel in 2205 for different ranges of power density were assessed and finally it was evaluated the use of laser cutting of stainless steel submerged in water. It follows that there is a burr formation, which increases as the power density decreases. On the other hand, the higher the power density, the greater is the extent of change in the steel microstructure. Cutting steel with rocky bulkhead opens new perspectives for the tool development with high power laser for well completion in the oil industry tool.

Terahertz (THz) radiation has become increasingly important in many scientific and commercial fields in recent years. It possesses many remarkable features resulting in an increased use of THz radiation for various applications, like biomedical imaging, security screening, and industrial quality control. The capability of these applications depends directly on the availability of powerful THz sources and high-responsivity, fast THz detectors. Current commercial products used to detect THz radiation, like Golay cells and pyroelectric detectors, have only slow detection rates and poor sensitivities. Other commercial THz detectors, like bolometers, are more sensitive but require liquid helium cooling. In this thesis, two types of room-temperature high-responsivity graphene-based THz detectors are presented, relying on the unique properties of graphene and the function of plasmonic antenna arrays which boost the interaction between THz waves and graphene. Graphene has been demonstrated as a promising material for THz detection. However, the challenge is its insufficient light absorption that largely limits the responsivity. The first design is based on an array of planar antennas arranged in series and shorted by graphene squares. Highly efficient photodetection can be achieved by using the metallic antenna to simultaneously improve both light absorption, as resonant elements, and photocarrier collection, as electrodes. The device has been characterized with quantum cascade lasers, yielding a maximum responsivity of ~2 mA/W at 2 THz. The second detector is based on an array of interdigitated bow-tie antennas connected in parallel and shunted by graphene squares. The arms of the bow-tie antennas were made of two metals with different work functions to create a built-in electric field and improve the responsivity. The device has been characterized and yields a maximum responsivity of ∼34 μA/W at 2 THz. Efficient THz imaging is presented by integrating the detector in a QCL-based THz imaging system.

This thesis is concerned with the non-linear effects observed in the absorption spectra obtained with swept continuous wave quantum cascade laser (cw-QCL) radiation. The slower chirp rate afforded by cw-QCLs, compared with pulsed systems, allows different aspects of the population transfer and polarisation induced in a molecular transition to be elucidated: namely, adiabatic rapid passage and coherent transient effects. The thesis commences with a brief outline of the architecture of QCLs, and an introduction to the non-linear processes inherent in their utilisation. In the following chapter, an outline of the methods required to characterise and drive a QCL chip are described along with the theory and characterisation of a Herriott cell, the latter allowing optically thick but minimally damped samples to be investigated. In chapter 3, a cw-QCL operating at ~10 μm is employed at chirp rates near to the adiabatic limit, in the range 0.1-1.1 MHz ns-1. A ro-vibrational overtone transition of OCS is studied at two path lengths within an astigmatic Herriott cell, 238 and 47 m. The population transfer and non-linear effects induced by the cw-QCL are described and modelled using the Maxwell-Bloch equations, which incorporate different experimental parameters. As such, the maximum population transfer found at a low pressure of 4 mTorr was determined to be 12 %. Additionally, the maximum amplitude of the coherent transient is forced to shift markedly to later times as the sample becomes increasingly optically thick, whilst remaining minimally damped. Intrapulse spectroscopy with a cw-QCL operating at 4.6 μm is investigated in chapter 4. The laser's response to short current pulses, in the range of 20-400 ns, applied in conjunction with a slow ramp, 2.75 kHz ns-1, is characterised. The initial chirp rate during the pulse is found to be ~6 MHz ns-1 by observation of the temporal width of transitions within the pulse. After the pulse a down chirp, at a rate of ~1 MHz ns-1, caused the laser frequency to relax back to the frequency position prior to the pulse. Subsequently, the effect of the pulse on a strongly absorbing N2O transition is monitored. In particular, the pulse position is altered with ii Quantum Cascade Laser Spectroscopy: Non-Linear Optics and Population Transfer respect to the transition line-centre and the coherent transient response of the molecular sample is probed. The free induction decay and RP effects noted are found to have a beat frequency which alters in line with the chirp rate of the laser, and the decay time of the transient signal was found to decrease as the range of velocity groups swept through is increased. The non-linear response of optically thick NH3 is investigated in chapter 5 with a 30 mW laser operating at ~10 μm. The effect of changing the gas pressure, laser intensity, and chirp rate on optically thick transitions is investigated for this molecule, which has a markedly larger dipole moment than OCS, and a comparison between the three molecular systems studied in this thesis is presented. Due to the importance of velocity dephasing in this work, the linewidth of the QCL was measured by Lamb-dip spectroscopy and found to be ~3 MHz. Two noise sources are then employed to alter the linewidth of the QCL: a single frequency modulation and a random white noise source. The noise, applied through the bias-T of the laser, leads to a change in the linewidth and lineshape, and as such, user-selectable linewidths in the range 3-20 MHz can be created. Increasing the laser linewidth has been found to increase the saturation of the sample, and therefore leads to an increase in the population transfer, which is determined to be ~10% at a chirp rate of ~0.1 MHz ns-1. The final chapter introduces the first use of a cw-QCL for population transfer within a molecular beam. The excited state is probed via resonantly enhanced multiphoton ionisation time of flight (REMPI-TOF) spectroscopy and a velocity selected population transfer of ~8 % is achieved. The effects of laser intensity, molecular beam carrier gas and laser linewidth on the population transfer are investigated. This thesis concludes with a discussion of some potential extensions to the work presented in this final chapter.