Esta tese explora a potencialidade da radiação roveniente de um laser de gás carbônico para iniciar a combustão-modelo de substâncias simples oxigenadas com o bjetivo de estabelecer algumas das reações elementares relevantes ao processo de combustão. Esta tese é baseada no fato de que espécies intermediárias são geradas frequentemente em estados eletrônicos excitados no pulso do laser. O decaimento destas espécies em função do tempo foi observado pelas medidas de emissão por meio de uma fotomultiplicadora acoplada a filtros de interferência de banda estreita e, registrado em um osciloscópio de 100 MHz cuja base de tempo é gatilhada pelo laser de CO2. A evolução temporal do sinal de emissão foi analisada por um modelo cinético que compreende: 1) um processo de de pseudo-primeira ordem resultante por excitação multifotônica vibracional de colisões moduladas, e 2) decaimento através de emissão espontânea, reações químicas ou supressão radiativa. A cinética resultante das curvas experimentais foi obtida por um método de ajuste de curvas por simulação em um microcomputador. Os sistemas químicos estudados nesta tese compreendem o metanol, etanol, n-propanol, n-butanol e o éter dietílico. A eficiência na formação de C2, CH e OH foi estabelecida pela medida da intensidade e o tempo de subida da quimiluminescência determinado de acordo com o procedimento mencionado acima. A análise das curvas de subida e descida foram estudadas com o precursor puro e na presença de Ar, O2, NO, H2, Cl2, CH4 e C2H6, utilizando nestes casos pressões relativamente altas para garantir um número elevado de colisões. Nos sistemas em mistura com ar ou O2 as espécies emissivas CH e OH apresentam um prolongamento do tempo de decaimento, em geral não-exponencial. Ao mesmo tempo a intensidade relativa dos sinais de quimiluminescência atinge um máximo, sendo particularmente significativa para a espécie OH a uma determinada pressão de oxigênio. Misturas de etanol com O2/Ar resultam para a espécie OH num aumento no valor da vida média radiativa, enquanto que em misturas com CO, C2H6 e propileno (C3H6) é observado uma diminuição significativa. No caso de CO este comportamento é assumido como sendo de processos de relaxação, tendo-se para os dois últimos um processo de abstração de hidrogênio por parte da espécie OH. A presença de um inibidor de reações de radicais livres, NO, tem pouco efeito nos valores das constantes das espécies estudadas. Entretanto, a emissão da espécie OH foi totalmente suprimida em misturas do precursor com Cl2. Neste caso a observação de CH3CL por cromatografia em fase gasosa sugere que reações químicas de alguns dos fragmentos primários inibem o adicional bombeamento pelo laser. Um modo particular de se interpretar o possivel mecanismo de formação das espécies transientes, foi desenvolvido através do cálculo teórico RRKM dos principais processos primários aventados para a decomposição por excitação multifotônica vibracional da molécula de etanol. O cálculo RRKM foi modelado para a eliminação de H2O, processo majoritário a pressões baixas, e diferentes processos de cisões que passam a ter importância a intensidades elevadas de laser. O cálculo teórico mostra claramente que a eliminação de H2O é o canal preferido a baixas energias de excitação, sendo rapidamente reposto pelos outros canais a valores maiores de energia. Um efeito similar é observado para a decomposição unimolecular completa em função da pressão, onde a pressões elevadas, os processos de cisão com produção de radicais livres são dominantes como conseqüência da desativação das moléculas com energia mínima antes que sofram reação química. / This thesis explores the use of a CO2 laser to initiate combustion type reactions in a number of simple oxygen containing organic compounds. This method offers a potential tool to isolate and establish some of the elementary reactions responsible for the initial steps of combustion reactions. The core of the thesis is based on the fact that transient chemical species are generated in electronically excited states by the laser pulse. The time dependent behavior of this species has been observed by measuring the emission on a fast photomultiplier provided with narrow band filtres, and recording it on a 100 MHz oscilloscope triggered by the laser pulse. The time evolution of the emission signal has been analyzed by a kinetic model which includes: 1) an ill-defined pseudo-first arder process as a consequence of collisionally modulated multiphoton vibrational excitation, and 2) decay through spontaneous emission, chemical reaction ar radiative quenching. The resulting kinetics were fitted to the experimental curves by computer simulation. The chemical systems covered in this thesis include methanol, ethanol, n-propanol, n-butanol and diethyl ether. Under pulsed CO2 laser radiation, the efficiency of formation of C2, CH and OH have been established by intensity measurements and the rise time for chemiluminescence determined according to the procedure mentioned above. Emission rise times and decays were studied for the neat systems and in the presence of Ar, O2, NO, H2, CH4, and C2H6 at pressures for which collisional processes are important. Emission from the CH and OH species are sustained for longer times in the presence of air or O2 and the decay becomes distincly non-exponential. There is also a significant intensity effect which is particularly noticeable for OH which reaches a maximum at a given oxygen pressure. Mixtures af ethanol with O2/Ar result in longer lifetimes for OH whereas in mixtures with CO, C2H6 and C3H6 a significant decrease is observed. In the case af CO, this behavior is assumed ta be due to relaxation processes, while in the latter cases hydrogen abstraction by OH is presumably responsible for the decrease. The presence of the well known scavenger NO had little effect on the rate constants used to describe the emission process. On the other hand, OH emission was completely quenchend in mixtures of the precursor with Cl2. In this case, the observation of CH3Cl by gas chromatography suggests that chemical reaction of some of the primary dissociation fragments inhibits further laser pumping. An attempt to understand the possible mechanism for the formatiun of the transient species was developed through the use of an RRKM calculation of the primary dissociation processes of ethanol under multiphoton excitation. The RRKM calculations were modelled for the H2O elimination, the main process at low pressures, and different bond scission processes which may become important at higher laser intensities. The theoretical calculation clearly shows that H2O elimination is the preferred pathway at lower excitation energies but is rapidly replaced by the other channels at higher energy contents. A similar effect is observed for the complete unimolecular decomposition as a function of pressure, where at the higher pressures, the bond scission processes yielding free radicals are dominant as a consequence of the deactivation of the molecules with threshold energies before they can undergo chemical reaction.

We study multi-photon ionization of rare gas atoms and small molecules by infrared femtosecond laser pulses. We demonstrate that ionization is accurately described by a tunnelling model when many infrared photons are absorbed. By measuring photo-electron and photo-ion spectra, we show how the sub-Ångstrom spatial resolution of tunnelling gives information about electron densities in the valence shell of atoms and molecules. The photo-electron and photo-ion momentum distributions are recorded with a velocity map imaging (VMI) spectrometer. We describe a tomographic method for imaging a 3-D momentum distribution of arbitrary symmetry using a 2-D VMI detector. We apply the method to measure the 3-D photo-electron distribution in elliptically polarized light. Using circularly polarized light, we show how the photo-electron momentum distribution can be used to measure the focused laser intensity with high precision. We demonstrate that the gradient of intensities present in a focused femtosecond pulse can be replaced by a single average intensity for a highly nonlinear process like multi-photon ionization. By studying photo-electron angular distributions over a range of laser parameters, we determine experimentally how the photon linear momentum is shared between the photo-electron, photo-ion and light field. We find the photo-electron carries only a portion of the total linear momentum absorbed. In addition we consider how angular momentum is shared in multi-photon ionization, and find the photo-electron receives all of the angular momentum absorbed. Our results demonstrate how optical and material properties influence the photo-electron spectrum in multi-photon ionization. These will have implications for molecular imaging using femtosecond laser pulses, and controlling the initial conditions of laser generated plasmas.

Advancements in spectroscopy rely on the improvement of two fundamental characteristics: spatial and temporal resolutions. High harmonic spectroscopy (HHS) is an emerging technology that promises the capability of studying the fastest processes that exist today: electronic motion with angstrom spatial and attosecond temporal resolution. HHS is based on the process of high harmonic generation (HHG) which arises from the nonlinear interaction between an intense, infrared laser pulse and an atomic or molecular gaseous medium, producing coherent, attosecond-duration bursts of extreme ultraviolet (XUV) light. In order to utilize the attosecond pulses for spectroscopic measurements, it is necessary to improve the conversion efficiency of HHG. Chapter 2 of this thesis describes the improvements we make to the HHG source in order to obtain high XUV photon flux and we report on the nonlinear ionization of atomic systems using these pulses in Chapter 6. In Chapters 3 - 5, we describe several HHG experiments in complex, polyatomic molecules in order to promote the use of HHS as a general spectroscopic tool. Amplitude modulations in high harmonic spectra of complex molecules can be attributed to several types of interference conditions that depend on a system's molecular or electronic structure such as recombination with multiple centres or dynamical interference from multi-orbital contributions to ionization. Our results demonstrate the capability of HHS to extract useful information on molecular and electronic structure from large, polyatomic molecules directly from their high harmonic spectra. Furthermore, we use HHS to investigate the suppression of ionization in complex molecules due to quantum destructive interference during ionization as well as the distinguishability of emitted harmonic spectra from molecular isomers. Chapter 6 explores the study of multi-electron dynamics in complex molecules using XUV multiphoton ionization of atoms and molecules as well as the ionization and fragmentation of C60 which has hundreds of delocalized valence electrons. This thesis also describes the author's role in the design and fabrication of a time-of- flight mass spectrometer (Section 6.1) as well as an HHG detector system (Appendix A).

The general goal of this dissertation is to provide a comprehensive description of the limitations of established theories on bound electronic nonlinearities in direct-gap semiconductors by performing various experiments on wide and narrow bandgap semiconductors along with developing theoretical models. Nondegenerate two-photon absorption (2PA) is studied in several semiconductors showing orders of magnitude enhancement over the degenerate counterpart. In addition, three-photon absorption (3PA) is studied in ZnSe and other semiconductors and a new theory using a Kane 4-band model is developed which fits new data well. Finally, the narrow gap semiconductor InSb is studied with regard to multiphoton absorption, free-carrier nonlinearities and decay mechanisms. The non-degenerate two-photon absorption was investigated in several direct-gap semiconductors with picosecond and femtosecond pulses. Large enhancements in 2PA were demonstrated when employing highly non-degenerate photon pairs and the results were shown to be consistent to a simple 2-parabolic band theory based on a "dressed" state approach. The nonlinear refractive index induced in such configurations was also calculated and possible implications of such extreme behavior are discussed. A large number of measurements of 3PA were taken at multiple wavelengths and in several semiconductors. The subsequent analysis has shown that simple 2-band model calculations (based on either perturbative or tunneling approaches) do not adequately describe the experimental trends. A more comprehensive model, based on Kane’s 4-band theory was developed and we calculate three-photon spectra for zincblende structures within the perturbative iv framework. We have confirmed the results of our calculations performing a series of Z-scans in semiconductors ZnSe and ZnS, yielding complete experimental three-photon spectra. A systematic approach based on using a large variety of pulse durations was needed to quantify the wealth of nonlinear optical processes in InSb, accessible in the mid-infrared range. Femtosecond pulses provided a lower limit to measurements of the instantaneous effects (absorptive and refractive), while picosecond pulses allowed further characterization of the freecarrier effects, including population dynamics in the high density regime (Auger effects). The model developed permitted us to verify the temperature dependence of free-carrier absorption recently predicted, and to successfully model optical limiting data with longer, nanosecond pulses.

Nonlinear absorption of Indium Antimonide (InSb) has been studied for many years, yet due to the complexity of absorption mechanisms and experimental difficulties in the infrared, this is still a subject of research. Although measurements have been made in the past, a consistent model that worked for both picosecond and nanosecond pulse widths had not been demonstrated. In this project, temperature dependent two-photon (2PA) and free carrier absorption (FCA) spectra of InSb are measured using femtosecond, picosecond, and nanosecond IR sources. The 2PA spectrum is measured at room temperature with femtosecond pulses, and the temperature dependence of 2PA and FCA is measured at 10.6µm using a nanosecond CO2 laser giving results consistent with the temperature dependent measurements at several wavelengths made with a tunable picosecond system. Measurements over this substantial range of pulse widths give results for FCA and 2PA consistent with a recent theoretical model for FCA. While the FCA cross section has been generally accepted in the past to be a constant for the temperatures and wavelengths used in this study, this model predicts that it varies significantly with temperature as well as wavelength. Additionally, the results for 2PA are consistent with the band gap scaling (Eg-3 ) predicted by a simple two parabolic band model. Using nanosecond pulses from a CO2 laser enables the recombination rates to be determined through nonlinear transmittance measurements. Three-photon absorption is also observed in InSb for photon energies below the 2PA band edge. Prior to this work, data on three-photon absorption (3PA) in semiconductors was scarce and most experiments were performed over narrow spectral ranges, v making comparison to the available theoretical models difficult. There was also disagreement between the theoretical results generated by different models, primarily in the spectral behavior. Therefore, we studied the band gap scaling and spectra of 3PA in several semiconductors by the Z-scan technique. The 3PA coefficient is found to vary as (Eg-7 ), as predicted by the scaling rules of simple two parabolic band models. The spectral behavior, which is considerably more complex than for 2PA, is found to agree well with a recently published theory based on a fourband model.

No description available.

<p> 3D printing has established itself as a critical tool for manufacturing in all areas. It has evolved from a purely rapid prototyping technique into a feasible process for large-scale processing. A wide variety of 3D printing processes exist across an extreme range of size, from meters to nanometers. Much of the current technological advances come from pushing fabrication techniques to smaller and smaller scales. For 3D printing this has led to the rise of two-photon polymerization, a direct laser writing process with submicron structuring capabilities. Two-photon polymerization has proven its worth as a nanoscale 3D fabrication technique but is often considered slow and expensive, two undesirable qualities for high throughput manufacturing. Parallelization methods such as projection lithography are potential solutions to increasing the throughput capabilities of two-photon polymerization 3D printing. Additionally, the drive for further reducing the print size has inspired printing resolution enhancing strategies in two-photon polymerization printing by processes such as stimulated emission depletion (STED) and other STED-inspired pathways. This work will explore avenues for improving two-photon polymerization printing throughput and resolution.</p> <p> First, a two-photon polymerization printing system is constructed with a secondary laser for controlling polymerization inhibition. Through a STED process, a 65 nm wide printed line feature was achieved. Alongside this, a characterization and verification methodology for choosing new photoinitiator molecules for similar inhibition lithography processes is presented. Through implementation of tests such as Z-scan, fluorescence depletion, ultrafast transient spectroscopy and UV-Vis absorption and fluorescence measurements a promising new photoinitiator with 5-factor improvement in printing efficiency is found. </p> <p> Second, a projection lithography scheme is developed for rapid two-photon 3D printing. A digital micro-mirror device (DMD) is utilized for dynamic pattern generation and the effects of its dispersion properties are considered. Through a spatiotemporal focusing process, continuous 3D printing is achieved at vertical prints speeds of 1 mm s-1. Simulations performed representing this rapid printing process indicate a ~1 µm layer print feature size for large areas of exposure. Comparably, a printed vertical feature size of ~ 1 µm was achieved. Lateral feature sizes ~200 nm were also demonstrated in fabrication. A variety of complex 3D structures are printed for demonstration of the spatiotemporal focusing projection lithography process including millimeter scale objects with micrometer scale 3D features.</p> <p> Finally, resolution enhancing strategies are implemented into the continuous, projection two-photon lithography technique. An investigation of the inhibition properties of a variety of photoinitiator systems for inhibiting polymerization achieved with low repetition rate laser exposure is presented. A planar polymerization inhibiting region is generated by creating a light sheet propagating perpendicularly to the projection printing plane. </p>

Δύο από τα πιο γνωστά μη γραμμικά οπτικά φαινόμενα που λαμβάνουν χώρα κατά την ακτινοβόληση επιφανειών χρυσού με ισχυρούς παλμούς laser είναι η γένεση πολλαπλών αρμονικών και η πολυφωτονική φωτοηλεκτρική εκπομπή. Οι διεργασίες αυτές αποτελούν το αντικείμενο της παρούσας διδακτορικής διατριβής. Κατ' αρχήν η προσπάθεια επικεντρώνεται στην ανάπτυξη ρεαλιστικών μοντέλων περιγραφής των συγκεκριμένων φαινομένων. Εν συνεχεία πραγματοποιούνται αριθμητικοί υπολογισμοί με στόχο την αναπαραγωγή πρόσφατων πειραματικών δεδομένων μέσω της οποίας προτείνονται νέες ιδέες αξιοποίησης των εν λόγω διεργασιών. / Multiple harmonic generation and multiphoton photoelecton emission are the most important nonlinear optical phenomena that take place when a metal surface is illuminated with intense laser pulses. The main objective of this work is the development of realistic theoretical models for these processes. Numerical calculations for several pulse widths are also presented. Through these calculations the validity of the models is checked and new experimental perspectives are proposed.

Les modèles numériques personnalisables d'organes du corps humain offrent un formidable potentiel pour évaluer le risque lésionnel dans les domaines de la sécurité des transports, du médical ou du sport. Suivant les applications, différents niveaux de détails peuvent être nécessaires. En particulier, lorsque le comportement mécanique des tissus biologiques doit être finement reproduit, les modèles de comportement doivent intégrer des considérations sur la structure du tissu, et simuler les mécanismes suivant lesquels il réagit à un chargement mécanique. Le travail de thèse présenté ici s'est focalisé sur la capsule de foie, notamment sur ses propriétés microstructurales et mécaniques, afin d'identifier les hypothèses importantes à intégrer dans la construction d'un modèle constitutif de tissu fibreux basé sur la microstructure. La méthodologie expérimentale a été mise en place afin de caractériser le comportement mécanique de ce tissu, en lien avec l'organisation de sa microstructure. Des essais de traction uniaxiale et de gonflement sous microscope confocal biphotonique ont été développés, pour observer l'évolution de la microstructure sous chargement. Des déformations macroscopiques ont été mesurées, et une méthode de mesure de champs de déformations locaux a été développée pour quantifier l'état de déformation du réseau de fibres. La réorganisation du réseau de fibre de collagène a également été quantifiée. L'analyse des liens existant entre les grandeurs mesurées à l'échelle macroscopique et ces phénomènes microscopiques est proposée, pour préciser les hypothèses à adopter dans les modèles permettant de passer de l'échelle des fibres au comportement global du tissu / Customized human body models offer a great potential to assess the injury risks in the fields of transport safety, surgery or sport. Various detail levels can then be needed, according to the targeted application. In particular, when the mechanical behavior of biological tissues needs to be accurately reproduced, numerical models have to include information about the structure of the tissue, and model the mechanisms of the response to mechanical loading. The work presented here focuses on the microstructural and mechanical characterization of the human liver capsule, in order to identify the important hypotheses that need to be included in a fibrous tissue constitutive model, based on microstructure. Thus, an experimental methodology has been developed to identify the mechanical behavior of this particular tissue, related with its microstructural organization. Uniaxial tensile tests, as well as bulge tests under a multiphoton confocal microscope have been performed, to observe the microstructure evolution during loading. Macroscopic strain has been assessed, and a method to measure local strain fields has been developed, to quantify the strain state of the fibrous network. The reorganization of the collagen fibers network has also been quantified. An analysis of the links between the measured macroscopic parameters and the microscopic phenomena is given. Therefore, the hypotheses that need to be included in constitutive models are highlighted, with particular consideration given to the affine transformation hypothesis which allows to link the fibers behavior to the global response of the tissue

<p>Laser-induced plasmas since their discovery in the 1960’s have found numerous applications in laboratories and industries. Their uses range from soft ionization source in mass spectroscopy, development of compact particle accelerator, and X-ray and deep UV radiation sources to diagnostic techniques such as laser-induced breakdown spectroscopy and laser electronic excitation tagging. In addition, the laser-induced plasma is important for studying of various nonlinear effects at beam propagation, such as laser pulse filamentation.</p> <p>This work deals with two challenging aspects associated with laser-induced plasmas. First is the study of Multi-Photon Ionization (MPI) as a fundamental first step in high-energy laser-matter interaction critical for understanding of the mechanism of plasma formation. The second is application of laser induced plasma for diagnostics of combustion systems.</p> <p>Numerous attempts to determine the basic physical constants of MPI process in direct experiments, namely photoionization rates and cross-sections of the MPI, were made; however, no reliable data was available until now, and the spread in the literature values often reached 2–3 orders of magnitude. This work presents the use of microwave scattering in quasi-Rayleigh regime off the electrons in the laser-induced plasma as method to measure the total number of electrons created due to the photoionization process and subsequently determine the cross-sections and rates of MPI. Experiments were done in air,<i> O₂, Xe, Ar, N₂, Kr</i>, and <i>CO</i> at room temperature and atmospheric pressure and femtosecond-laser pulse at 800 nm wavelength was utilized. Rayleigh microwave scattering (RMS) technique was used to obtain temporally resolved measurements of the electron numbers created by the laser. Numbers of electrons in the range 3 × 10⁸–3 × 10¹² were produced by the laser pulse energies 100–700 <i>μ</i>J and corresponding electron number densities down to about 10¹⁴ cm^-3 in the center of laser-induced spark were observed. After the laser pulse, plasma decayed on the time scale from 1 to 40 ns depending on the gas type and governed by two competing processes, namely, the creation of new electrons from ionization of the metastable atoms and loss of the electrons due to dissociative recombination and attachment to oxygen. </p> <p>Diagnostics of combustion at high pressures are challenging due to increased collisional quenching and associated loss of acquired signal. In this work, resonance enhanced multiphoton photon ionization (REMPI) in conjunction with measurement of generated electrons by RMS technique were used to develop diagnostics method for measuring concentration of a component in gaseous mixture at elected pressure. Specifically, the REMPI-RMS diagnostics was developed and tested in the measurements of number density of carbon monoxide (<i>CO</i>) in mixtures with nitrogen (<i>N₂</i>) at pressures up to 5 bars. Number of REMPI-induced electrons scaled linearly with <i>CO</i> number density up to about 5×10¹⁸ cm^-3 independently of buffer gas pressure up to 5 bar, and this linear scaling region can be readily used for diagnostics purposes. Higher <i>CO</i> number densities were associated laser beam energy loss while travelling through the gaseous mixture. Four (4) energy level model of <i>CO</i> molecule was developed and direct measurements of the laser pulse energy absorbed in the two-photon process during the passage through the <i>CO</i>/<i>N₂</i> mixture were conducted in order to analyze the observed trends of number of REMPI-generated electrons with <i>CO</i> number density and laser energy.</p>