In this thesis a powerful and practical method for characterizing ultrashort pulses in space and time is described (called SEA TADPOLE). First we focus on measuring pulses that are spatially uniform but very complicated in time or frequency. We demonstrate and verify that SEA TADPOLE can measure temporal features as small as 30 femtoseconds over durations as long as 14 picoseconds. The spectral resolution of this device is carefully studied and we demonstrate that for certain pulses, we achieve spectral super resolution. We also develop and test an algorithm for measuring polarization shaped pulses with SEA TADPOLE. Our simple interferometer can even be used to measure the spatiotemporal electric field of ultrashort pulses at a focus. This is because SEA TADPOLE samples the field with an optical fiber which has a small core size. Therefore this fiber can be used to spatially sample the beam, so that the temporal electric field can be measured at every position to obtain E(x, y, z, t). The single mode fiber can be replaced with an NSOM (Near Field Scanning Optical Microscopy) fiber so that spatial resolution as low as 500nm (and possibly lower) can be achieved. Using this device we make the first direct measurements of the compete field of focusing ultrashort pulses. These measurement can be viewed as "snap shots" in flight of the focusing pulse. Also, for the first time, we have observed some of the interesting distortions that have been predicted for focusing ultrashort pulses such as the "forerunner" pulse, radially varying group delay dispersion, and the Bessel-like X-shaped pulse. We have also made the first direct measurements of the electric field of Bessel X-pulses and their propagation invariance is demonstrated. We also use SEA TADPOLE to study the "boundary wave pulses" which are due to diffraction.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 440-459).

Carbon fibre reinforced polymer (CFRP) composites have found a wide range of applications in the aerospace, marine, sports and automotive industries owing to their lightweight and acceptable mechanical properties compared to the commonly used metallic materials. The currently dominating method of machining CFRP is by mechanical means that has found many problems including extensive tool wear, fibre pull-out and delamination. Lasers as non-contact tools have been widely applied for cutting and drilling materials. However, machining of CFRP composites using lasers can be challenging due to inhomogeneity in the material properties and structures, which can lead to thermal damage such as charring, heat affected zones (HAZs), resin recession and delamination. In previous studies, Nd:YAG, diode pumped solid state (DPSS), CO2 (continuous wave), disk and fibre lasers were used in machining CFRP composites and the control of damage such as the size of heat affected zones (HAZ) and achieving comparable material removal rate with the mechanical processes remain a challenge. Most reported work showed a typical heat affected zone of 0.2-1.2 mm. The availability of short pulsed transversely excited atmospheric (TEA) CO2 lasers and ultra-short laser pulse sources such as picosecond lasers make it possible to improve the laser machining quality of CFRP materials. In this research, the machining of CFRP composites using a microsecond pulsed TEA CO2 laser, a state of the art high power picosecond laser and a 1 kW single mode fibre laser system was investigated. The yielded heat affected zone was less than < 25 µm for the TEA CO2 and the picosecond laser machining, although the material removal rate was low. Additionally, it has been shown that the pulsed fibre laser improved the machining quality compared to that with the continuous mode. A potential application of the fibre laser for composite repair and remanufacturing was investigated. The interactions between picosecond laser beam and CFRP composite were studied in more detail including understanding the self-limiting effect in single and multiple parallel tracks drilling/machining through both experimental and theoretical studies. Furthermore, a sequential laser and mechanical drilling of CFRP was investigated to improve the machining rate. The work performed in this PhD was driven by aerospace industry needs, with the collaboration of Rolls-Royce plc and BAE Systems as industrial partners.

L'optoacoustique picoseconde est une technique qui permet de sonder la matière à l'échelle submicronique. L'objectif de ce travail est la synthèse d'ondes de cisaillement de fréquences GHz dans des couches minces isotropes pour l'analyse des propriétés transverses de la matière. C'est grâce à la diffraction du champ acoustique à l'intérieur du transducteur que ces ondes de cisaillement sont synthétisées. Nous exposons dans une première partie le développement des outils théoriques nécessaires à l'analyse de la répartition spatiale du champ acoustique généré lors de l'interaction laser-matière. Les diagrammes de directivité ainsi établis permettent de prévoir les directions de rayonnement privilégiées. Par une méthode de post traitement adaptée,nous synthétisons ensuite des ondes transverses dans ces directions particulières. Lorsque le transducteur est chargé par un milieu dont on cherche à connaître les propriétés transverses, les ondes de cisaillement synthétisées sont réfléchies à l'interface entre le transducteur et ce milieu. L'analyse du coefficient de réflexion permet alors de déterminer les propriétés mécaniques du milieu à sonder. Nous illustrons cette méthode numériquement et expérimentalement dans le cas d'une interface titane/glycérol. / Picosecond optoacoustics is a technique that can probe properties of matter on a submicronscale. The aim of this work is to synthesize shear waves at GHz frequencies in a thin isotropiclayer in order to analyse its shear properties. Transverse waves are synthesized using diffraction.In the first part we develop theoretical tools in order to analyse the spatial repartition of theacoustic field generated by the laser-matter interaction. Directivity patterns predict the particulardirections for which amplitude of the shear waves is higher. Thus, we synthesized shearwaves in these particular directions using a dedicated post processing method. To investigateshear properties of a film lying on the sample, we study the reection of shear waves betweenthe transducer and the lying film. Starting with either simulated or experimental recordedwaveforms, we demonstrate the method in the specific case of the titanium/glycerol interface.

Cette thèse s’intéresse à l’endommagement laser d’optiques réflectives en régime sub-picoseconde. Ces composants optiques, réalisés à partir d’empilements sub-micrométriques de matériaux diélectriques, sont couramment utilisés dans les installations laser de puissance telles que le laser PETAL. Ce dernier, construit sur le centre du CEA-CESTA en France, a été conçu pour délivrer des impulsions de 500 fs et de quelques kJ à une longueur d’onde de 1053 nm, dans le but d’atteindre des puissances supérieures à 6PW. Dans ce type d’installations de puissance, afin de limiter l’accumulation d’effets non-linéaires liés à la propagation d’impulsions intenses, des optiques réflectives en fin de chaîne se substituent aux optiques en transmission. Les composants optiques illuminés par les plus fortes densités de puissance sont des réseaux de compression, des miroirs et une parabole qui servent respectivement à la compression temporelle des impulsions, au transport du faisceau ainsi qu’à sa focalisation. Le phénomène d'endommagement laser est le paramètre principal qui limite la puissance accessible par ces lasers. Il est nécessaire de comprendre et de maîtriser ce phénomène afin de fiabiliser une installation laser et également d’améliorer ses performances. Trois axes d’études ont donc été explorés. Le premier concerne la conception d’optiques réflectives. Des définitions de structures sont recherchées afin d’obtenir de hautes efficacités de diffraction dans le cas des réseaux et des forts coefficients de réflexion dans le cas des miroirs, tout en diminuant le renforcement du champ électrique dans les structures, l’une des causes de l’endommagement laser. Le second axe d’étude porte sur la mise en place d’une métrologie fine de l’endommagement laser avec le développement de nouvelles procédures de test. Elles amènent de nouveaux points de vue pour la qualification de la résistance au flux laser des optiques. En dernier lieu, nous étudions le phénomène de croissance des dommages lorsqu’ils sont soumis à plusieurs irradiations laser. L’évolution de la surface des dommages lors de séquences de croissance est observée, quantifiée et comparée à un modèle numérique. Cela nous permet de mieux comprendre la phénoménologie de la croissance des dommages en régime sub-picoseconde. A terme, elles permettront de développer des modèles de prédiction de l’endommagement et des outils pour l’amélioration des optiques utilisées sur chaîne laser. / In this thesis, laser-induced damage phenomenon of reflective components is investigated in the sub-picosecond regime. These components, made of stacks of dielectric materials, are widely used in powerful laser facilities such as PETAL laser. PETAL laser has been built at the CEA-CESTA in France to deliver multi-kJ/500fs pulses at 1053nm and reach a power higher than 6PW. For this kind of laser systems, reflective components are commonly used instead of optics operating in transmission to limit the accumulation of non-linear phase along the beam propagation due to the high intensities. Optical components irradiated by the highest power densities are the pulse compression gratings, transport mirrors and the focusing parabola, located at the end of the laser chain. Nowadays, laser-induced damage is the main factor that limits the overall performances of powerful laser systems. This manuscript presents three study axes to better understand and control damage phenomenon. The first one concerns the conception of reflective optics for the petawatt applications. The design of new structures has been investigated to reach high diffraction efficiencies in the case of pulse compression gratings and a high reflectivity in the case of mirrors, while reducing the Electric-field enhancement which is one of the causes of the laser-induced damage. The second axis deals with the development of a precise damage metrology with new testing tools which brings new perspectives and a new viewpoint for the assessment of the laser resistance of optical components. Finally, the third axis concerns the study the damage growth after several irradiations in the sub-picosecond regime. The evolution of the damage area during growth sequences is observed and compared to numerical simulations. It enables to improve the understanding in the growth phenomenon. In the end, these studies will allow to develop predictive models of the laser-induced damage and new tools for the conception of reflective optics for petawatt applications.

Bulk laser-induced breakdown and self-focusing in single samples of fused quartz and NaCl were examined using picosecond optical pulses at 1.0 ym and 0.5 ym. The results of three separate but related experiments are reported. First the nonlinear index of refraction, n2, of each of the test materials is measured near the respective damage thresholds of the samples. The values of 1*2 were determined by detecting beam distortions in the far field, transmitted laser beam profile caused by the irradiance dependent index of refraction. The experimental traces were compared to theoretical beam profiles generated by a nonlinear propagation code and n2 was extracted from the resulting fits.

The picosecond optical response of germanium is investigated by performing excitation-probe experiments on a thin, intrinsic-germanium wafer maintained at 135 K. The results of three distinct experiments are reported: (1) the transmission of a single pulse is measured as a function of irradiance, (2) the probe transmission is measured at a fixed time after excitation as a function of the excitation energy, and (3) the transmission of a probe pulse is monitored as a function of time after excitation. These experiments employ 10-picosecond laser pulses at 1.06 um and Stokes-shifted pulses at 1.55-um.

L'interaction de particules énergétiques avec les résultats de l'eau dans l'excitation et l'ionisation des molécules d'eau. Le processus d'ionisation se rapporte à la génération de l'excès d'électrons détachés de leurs molécules parentes et laissant derrière le trou positive (notée H₂O•⁺). Cela se produit sur le calendrier d'une transition électronique ~ 10⁻¹⁵s. Les processus chimiques plus anciens de H₂O•⁺ et l'excès d'électrons vers autre question suivie de l'eau en vrac ionisants restent encore peu par rapport connu et constitue un sujet difficile dans la chimie de rayonnement. Dans ma thèse, les techniques de radiolyse d'impulsions picoseconde ont été utilisés pour observer la cinétique de la SO₄•⁻, H₂PO₄• dans de l'acide sulfurique très concentré et solutions d'acide phosphorique sur une large gamme de concentrations (de 1 mol L⁻¹ à l'acide pur). Les résultats expérimentaux montrent clairement que le radical secondaire de sulfurique (SO₄•⁻) et de l'acide phosphorique (H₂PO₄•) peuvent être formés par l'intermédiaire de deux mécanismes : détachement d'électrons direct par l'impulsion d'électrons (7 ps) et le transfert d'électrons ultra-rapide des solutés au radical cation de l'eau H₂O•⁺. La réactivité des espèces oxydantes fortes, H₂O•⁺ vers les solutés dans des solutions aqueuses très concentrées est quantitativement démontré. / The interaction of energetic particles with water results in the excitation and ionization of water molecules. The ionization process refers to the generation of the excess electrons detached from their parent molecules and leaving behind the positive hole (denoted as H₂O•⁺). This occurs on the timescale of an electronic transition ~10⁻¹⁵ s. The earliest chemical processes of H₂O•⁺ and excess electron towards other matter followed water ionizing in bulk still remain relative little known and constitute a challenging subject in radiation chemistry. In my thesis, picosecond pulse radiolysis techniques were used to observe the kinetics of the SO₄•⁻, H₂PO₄• in highly concentrated sulfuric acid and phosphoric acid solutions over a large range of concentrations (from 1 mol L⁻¹ to neat acid). The experimental results showed clearly that the secondary radical of sulfuric (SO₄•⁻) and phosphoric acid (H₂PO₄•) can be formed via two mechanisms: direct electron detachment by the electron pulse (7 ps) and ultrafast electron transfer from the solutes to the radical cation of water H₂O•⁺. The reactivity of the strongest oxidizing species, H₂O•⁺ towards the solutes in highly concentrated aqueous solutions is quantitatively demonstrated.

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