With more atoms in a system, coupling between quantum states complicates the system dynamics. We shine intense laser pulses on three systems with increasing complexity: a molecule, a dimer, and a solid. For single molecules, a 400 nm photon excites NO_2 and initiates a dissociation process. We probe the dynamics using a strong laser pulse to ionize the molecule, and detect the resulting electrons and ions. The evolution of the NO-O molecular bond was directly measured in our experiment. For dimers, a laser pulse removes three electrons from (CO)_2. The dimer breaks up into C^+, O^+ and CO^+. Compared to a monomer, CO^{2+} in the dimer has a new prompt dissociation pathway that produces fragments with higher kinetic energy. Calculation shows that the Coulomb field of the neighboring CO^+ modifies the electronic state of the dimer, giving rise to a prompt channel. Coupling between different charge state configurations results in a new dimer electronic state, which leads to dissociation with higher kinetic energy. For solids, coupling among many atoms creates bands and a bandgap that plays the role of the ionization potential and reduces the threshold for electron-hole pair generation. Thus, solids are a good medium for high-order harmonic generation at the high repetition rates needed for frequency combs. We generate up to the 7th harmonic in silicon and zinc oxide with femtosecond pulses from a thulium fiber laser.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22244号 / 理博第4558号 / 新制||理||1655(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 阪部 周二, 准教授 橋田 昌樹, 教授 鶴 剛 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Rapid progress in ultra-intense laser technology has resulted in intensity levels surpassing 1022 W/cm2, reaching the highest possible density of electromagnetic energy amongst all controlled sources available in the laboratory. During recent decades, fast growth in available intensity has stimulated numerous studies based on the use of high intensity lasers as a unique tool for the initiation of nonlinear behavior in various basic systems: first molecules and atoms, then plasma resulting from the ionization of gases and solids, and, finally, pure vacuum. Apart from their fundamental importance, these studies reveal various mechanisms for the conversion of a laser pulse's energy into other forms, opening up new possibilities for generating beams of energetic particles and radiation with tailored properties. In particular, the cheapness and compactness of laser based sources of energetic protons are expected to make a revolution in medicine and industry.   In this thesis we study nonlinear phenomena in the process of laser radiation interacting with plasmas of ionized targets. We develop advanced numerical tools and use them for the simulation of laser-plasma interactions in various configurations relating to both current and proposed experiments. Phenomenological analysis of numerical results helps us to reveal several new effects, understand the physics behind them and develop related theoretical models capable of making general conclusions and predictions. We develop target designs to use studied effects for charged particle acceleration and for the generation of attosecond pulses of unprecedented intensity. Finally, we analyze prospects for experimental activity at the upcoming international high intensity laser facilities and uncover a basic effect of anomalous radiative trapping, which opens up new possibilities for fundamental science.

The interaction of intense laser pulses with nanometer scale atomic clusters has been an active area of study since the advent of amplified femtosecond lasers. In the case of infrared irradiation of noble gas clusters, direct field-driven ionization results in the ejection of energetic electrons, high ion charge states, and Coulomb explosion of the ion core of the clusters. These processes result from electron motion driven by the cluster potential and the large ponderomotive potential of the laser field. When extreme ultraviolet (XUV) pulses interact with clusters, the mechanisms responsible for the infrared response are 'turned off' because the ponderomotive potential is very small. We have conducted cluster experiments at 38nm using focused XUV pulses produced by high harmonic generation with a 15TW Ti:Sapphire laser. We measured the charge states and kinetic energy spectra of ions produced in the interaction, and observe substantial ion population up to Xe⁵⁺, with a small number of Xe⁶⁺-Xe⁸⁺ ions produced by collisional ionization by hot plasma electrons. The ion kinetic energy spectrum indicates a hydrodynamic expansion at an ion temperature of 8eV. This is in stark contrast to intense infrared/cluster interactions, where clusters are stripped of electrons to a large degree and expand by Coulomb forces, resulting in far higher ion kinetic energy for similar degrees of ionization. / text

L'interaction d'un laser ultra-intense et ultra-court avec la matière donne naissance à une grande variété de processus issus du couplage des ondes électromagnétiques associées au laser avec les modes du plasma. Ce couplage hautement non-linéaire excite des phénomènes plasmas collectifs capables de produire des champs intenses pouvant atteindre le TV/m. Ces champs ouvrent la possibilité de réaliser des accélérateurs de particules compacts, aussi bien d'électrons que d'ions. Des sources laser-plasma d'ions de plusieurs dizaines de MeV ont été démontré au début des années 2000 et de nombreux mécanismes ont été suggérés depuis afin d'en améliorer les propriétés. Historiquement, les sources d'ions par laser ont été obtenues à partir de cibles solides dîtes sur-denses. L’innovation sur les cibles a été un moteur majeur de l’amélioration de ces sources. Dans la continuité de cette dynamique, l’utilisation de cibles gazeuses a été proposé afin d’alléger les contraintes de contraste laser et de taux de répétition. De récentes démonstrations expérimentales sont venus renforcer l’intérêt pour ces cibles, dîtes sous-denses ou proche critiques, dont la valeur est propice à la propagation, à l’absorption du laser et à la création de structures accélératrices que sont les chocs plasmas et les vortex magnétiques. Les travaux présentés dans cette thèse constituent une exploration expérimentale des paramètres plasmas nécessaires à l’accélération d’ions dans des cibles gazeuses de densité proche-critique. Pour la première fois ces régimes sont explorés avec un laser ultra-intense femtoseconde de 150TW. Une partie des travaux a été consacrée à la réalisation d’une cible innovante, adaptée aux contraintes de densité et de gradients plasma requises par ces régimes. Suivent, les travaux expérimentaux décrivant la propagation du laser et l’accélération d’électrons dans des cibles proche-critiques. Enfin une dernière partie décrit la production d’un faisceau d’atome issue d’une source d’ion laser. / Interaction of ultra-intense, ultra-short laser with matter gives rise to a wealth of phenomena, due to the coupling between the electromagnetic field and the plasma. The non-linear coupling excites collective plasma processes able to sustain intense electric fields up to 1TV/m. This property spurred early interest in laser accelerator as compact, next-generation source of accelerated electrons and ions. Laser-driven ion source of several MeV was demonstrated in early 2000 an various mechanisms had been suggest to improve the their properties. These first ion sources have been obtained on solid targets, called “overdense”. Target innovation has driven the improvement of these sources. In the continuity of this dynamic, new gaseous targets had been proposed in order to relax the constraints that solid targets impose on laser contrast and repetition rate. Recent experimental demonstrations of monoenergetic ion acceleration in gas renew the interest in such targets, called under-dense or near-critical because of their intermediate densities. At near-critical density the laser can propagate, but undergoes significant absorbtion, giving rise to the accelerating structures of plasma shocks and magnetic vortex.The work presented in this thesis is an experimental exploration of the plasma conditions required to drive ion acceleration in gaseous near-critical target. For the first time, these regimes are explored with an ultra-intense, femtosecond laser of 150TW. A part of this work has been dedicated to the design of an innovative gas target, suited for plasma density and gradient constraints set by these regimes. Then the experimental works describe laser propagation and electron acceleration in near-critical targets. Finally the last part report the efficient production of an atomic beam from a laser-driven ion source.

Master of Science / Department of Physics / Brett D. Esry / The two-state field-aligned (1-D) model has been employed to investigate the dissociation dynamics of a hydrogen molecular ion and its isotopes under the Born-Oppenheimer approximation without rotation. The emphasis of this work was on the role of mass during the dynamical dissociation processes and on the laser-induced branching ratios between different photon pathways. Firstly, we have found that scaling the pulse duration of the laser pulse, applied to H[subscript]2+ and D[subscript]2+ , by the square root of the mass ratio of these isotopes will produce similar structure in the nuclear kinetic energy release (KER) spectra. In fact, the similarity of the spectra is enhanced by including some averaging that is necessary for comparison with experiment. For this to occur, the same broad initial vibrational distribution and a short pulse are preferred. Using this scaling idea, it is possible to produce effectively shorter laser pulses by studying heavier isotopes, like D[subscript]2+. Secondly, we have demonstrated analytically and numerically that there is a carrier-envelope phase effect in the total dissociation probability (TDP) of H[subscript]2+, and this effect grows with nuclear mass. We further show that under the same laser conditions, the CEP effect in the asymmetry between breakup channels decreases with mass. Our analytic expressions enhance the idea that CEP effects can be understood as an interference between different n-photon processes. Thirdly, the trends in the dissociation dynamics of H[subscript]2+ and D[subscript]2+ in a 800nm ultra short intense laser field were demonstrated by studying the dissociation branching ratios of multiphoton processes as a function of the laser peak intensity (from 8[times]10[superscript]9 to 10[superscript]14 W/cm[superscript]2) or pulse length (5fs-7.5fs). Based on the two-state approximation, an energy-analysis method (EAM) was employed to separate multiphoton processes. The results show that the one-photon dissociation process dominates over all other photon processes under all the laser conditions applied in the calculations and that the zero-photon process contributes to a surprisingly large fraction of the total dissociation. Two- and three- photon dissociation are weaker processes, but become more and more important as the laser peak intensity and pulse length increases. A two-state Floquet method was used to check the accuracy of the EAM, and good agreement between the two methods was found, demonstrating the reliability of the EAM. In comparison with H[subscript]2+, D[subscript]2+ displays stronger two and three photon branching ratios (above-threshold dissociation - ATD), which can be attributed to the late arrival of D[subscript]2+ to the critical distance for ATD to occur due to its heavier mass. Therefore, this "mass" effect can be used to steer the molecular dissociation pathways.

L'avancement technologique des sources de rayonnement laser est tel qu'il permet désormais l'observation résolue en temps des phénomènes se déroulant dans les atomes et les molécules sous l'effet de champs intenses et de très courte durée. La complexité croissante de ces expériences et des processus auxquels elles s'intéressent suscite plusieurs questions à propos de la dynamique multiélectronique de ces systèmes. Par exemple, dans un travail récent portant sur la molécule de CO2, une technique d'imagerie moléculaire exploitant les interférences dans le signal d'émission des harmoniques élevées a été proposée. Ces interférences qui dépendent fortement de la géométrie moléculaire sont également influencées par divers effets multiélectroniques déterminant l'importance relative des différentes voies d'ionisation possibles de la molécule sondée rendant difficile leur interprétation. Il est donc très important de développer des modèles théoriques suffisamment précis pour pouvoir s'adresser à de telles interrogations. La compréhension du déroulement de ces processus permet d'avoir une meilleure emprise sur ceux-ci et de tirer une juste part de ce type d'expérience. Ainsi, nos développements méthodologiques récents s'inscrivent dans cette ambition de dévoiler la nature des effets multiélectroniques sur la dynamique des molécules polyélectroniques dirigées par un champ laser intense. Le développement complet de ce schéma général multi-configurationnel à champ auto-cohérent dépendant du temps (TDMCSCF) sera présenté et illustré avec quelques résultats préliminaires obtenus pour des systèmes simples. Finalement, on discutera de quelques applications utiles de l'étude de la dynamique électronique, telle que l'imagerie moléculaire dynamique. / Latest advances in laser sources are such that it is now possible to observe, in real-time, phenomena occurring in atoms and molecules under the effect of an intense and ultra-fast external field. The increasing complexity of these experiments and the processes under scrutiny give rise to a number of questions concerning the multielectron dynamics of such systems. For example, in a recent work on CO2 molecule, a molecular imaging technique exploiting interference structures in the high harmonic emission signal was proposed. Such interferences are directly correlated to the molecular geometry but are also influenced by diverse multielectron effects that govern the relative importance of possible ionization channels, making their interpretation difficult. The developement of accurate theoretical models capable of addressing such questionings thus takes a significant importance. The complete understanding of how these processes take place allow one to control them with better ease and to gain meaningful insights on these kinds of experiments. Therefore, our methodology developments are precisely intended to unveil the nature of multielectron effects on the dynamics of polyatomic molecules driven by intense laser fields. The complete development of a general Time-Dependent Multi-Configuration Self-Consistent Field (TDMCSCF) methodology \cite{Nguyen-Dang2007, Nguyen-Dang2009} will be presented and illustrated with preliminary results obtained for some simple molecular systems. Finally, some useful applications of the study of electron dynamics, such as dynamic molecular imaging, will be discussed.

The purpose of this thesis is to advance the understanding of strong-field QED processes in short laser pulses. The processes of non-linear one-photon and two-photon Compton scattering are studied, that is the scattering of photons in the interaction of relativistic electrons with ultra-short high-intensity laser pulses. These investigations are done in view of the present and next generation of ultra-high intensity optical lasers which are supposed to achieve unprecedented intensities of the order of 10^24 W/cm^2 and beyond, with pulse lengths in the order of some femtoseconds. The ultra-high laser intensity requires a non-perturbative description of the interaction of charged particles with the laser field to allow for multi-photon interactions, which is beyond the usual perturbative expansion of QED organized in powers of the fine structure constant. This is achieved in strong-field QED by employing the Furry picture and non-perturbative solutions of the Dirac equation in the presence of a background laser field as initial and final state wave functions, as well as the laser dressed Dirac-Volkov propagator. The primary objective is a realistic description of scattering processes with regard to the finite laser pulse duration beyond the common approximation of infinite plane waves, which is made necessary by the ultra-short pulse length of modern high-intensity lasers. Non-linear finite size effects are identified, which are a result of the interplay between the ultra-high intensity and the ultra-short pulse length. In particular, the frequency spectra and azimuthal photon emission spectra are studied emphasizing the differences between pulsed and infinite laser fields. The proper description of the finite temporal duration of the laser pulse leads to a regularization of unphysical infinities (due to the infinite plane-wave description) of the laser-dressed Dirac-Volkov propagator and in the second-order strong-field process of two-photon Compton scattering. An enhancement of the two-photon process is found in strong laser pulses as compared to the corresponding weak-field process in perturbative QED.

Les lasers de haute puissance permettent d'atteindre des intensités très importantes (jusqu'à 10²²W.cm⁻²). Parvenir à ce niveau d'intensité nécessite de concentrer une quantité modérée d'énergie (de l'ordre du joule) dans un temps très court (de l'ordre de la dizaine de femtosecondes) sur une surface réduite (de l'ordre du μm²). Ces faisceaux sont donc ultra-courts et focalisés à l'aide d'une optique à grande ouverture. Ces caractéristiques signifient que leur diamètre avant focalisation est grand et leur largeur spectrale est importante. Pour cette raison, ces faisceaux sont à même de présenter des distorsions spatio-spectrales (ou couplages spatio-temporels). Après focalisation, ces distorsions ont pour effet une diminution drastique de l'intensité pic. Ceci est d'autant plus vrai que le système laser est puissant et donc que son diamètre et sa largeur spectrale sont grands. En dépit de cet effet néfaste, les couplages spatio-temporels présentent aussi un intérêt lorsqu'ils sont maitrisés. On peut en effet introduire des couplages spatio-temporels de faible amplitude à des fins expérimentales. Dans les années 1990 et 2000, un effort important a été fourni pour permettre la caractérisation et l'optimisation du profil temporel des lasers femtoseconde. Dans le même temps, des solutions d'optique adaptative ont été développées pour contrôler le profil spatial des faisceaux ultra-intenses et obtenir la meilleure tache focale possible. Les systèmes laser de haute-puissance actuels sont maintenant caractérisés et optimisés indépendamment par ces deux types de diagnostics. Par essence, cette approche est aveugle aux couplages spatio-temporels. Seule une caractérisation spatio-temporelle permettrait de mesurer ces distorsions. Il existait déjà des méthodes de caractérisation spatio-temporelle avant le début de cette thèse. Aucun de ces dispositifs n'avait cependant été adapté à la mesure de faisceaux ultra-intenses. Lors de cette thèse, nous avons développé une nouvelle technique de caractérisation spatio-temporelle appelée TERMITES. Cette technique est basée sur un schéma de spectroscopie par transformée de Fourier auto-référencée. TERMITES nous a permis d'effectuer la première caractérisation spatio-temporelle totale d'un laser 100 TW (le laser UHI-100 du CEA Saclay). Les distorsions spatio-temporelles détectées à l'aide de ces mesures ont confirmé la nécessité d'une généralisation de la métrologie spatio-temporelle des lasers de haute puissance. / High power laser make it possible to reach very high intensities (up to 10²²W.cm⁻²). In order to get to this level of intensity, a moderate quantity of energy (on the order of the Joule) is concentrated in a very short time (on the order of tens of femtoseconds) onto a small surface (on the order of 1 μm²). These beams are therefore ultra-short and focused with a high aperture optic. These features mean that their diameter prior to focus is large and their spectral width is big. As a result, these beams are subject to spatio-spectral distorsions (of spatio-temporal couplings). After focus, these distorsions induce a dramatic reduction of the peak intensity. This situation is all the more true when the laser is more intense and its diameter and spectral width are therefore bigger. Despite their detrimental effects, spatio-temporal couplings can be of great interest when controlled. One can indeed introduce weak spatio-temporal couplings for experimental purposes. In the 1990s and 2000s, a big effort was put in order to characterize dans optimize the temporal profile of femtosecond lasers. Meanwhile, adaptative optics solutions were developed to control the spatial profil of ultra intense laser beams and provide the best focal spot achievable. By nature, this approach is blind to spatio-temporal couplings. Measuring these distorsions requires a spatio-temporal characterization. Before the start of this Phd thesis, spatio-temporal characterization methods already existed. Although none of these devices were ever adapted to the measurement of ultra-intense laser beams. During this Phd Thesis, we developped a new spatio-temporal characterization technique which we called TERMITES. This technique is based on a self-referenced Fourier transform spectroscopy scheme. TERMITES made it possible for us to perform the first total spatio-temporal characterization of a 100 TW laser (UHI-100 at CEA Saclay, France). The detection of spatio-temporal distorsions with the help of these measurements confirmed the need for a generalization of spatio-temporal characterization of ultra-high power lasers.

La présente thèse s’inscrit dans le cadre du développement des lasers X-UV générés en régime collisionnel transitoire et a pour objet principal d’étudier l’influence de la mise en forme temporelle des impulsions laser de haute puissance sur l’efficacité de génération de ces sources. Mon travail essentiellement expérimental a consisté à étudier de nouveaux schémas de pompage mettant en oeuvre différents dispositifs permettant de produire des préimpulsions et/ou un piédestal d’ASE au sein de la chaîne laser pilote. Dans ce manuscrit, je présente ces dispositifs et montre l’influence des différents paramètres laser sur l’efficacité de production du laser X-UV. L’étude expérimentale met tout d’abord en évidence une augmentation significative de l’énergie et de la durée de vie dela source laser X-UV en présence d’une préimpulsion.Dans ce contexte, un dispositif a été expérimenté permettant de générer au sein d’un unique faisceau laser, les deux principales impulsions précédées de la pré-impulsion, tout en gardant un contrôle sur leurs caractéristiques spectro-temporelles.Une approche alternative a été expérimentée dans laquelle un laser annexe « Q-switch » à bas coût est utilisé pour générer un plasma peu dense avecde faibles gradients de densité. Ce dernier dispositif a montré d’excellentes performances sur une large plage de longueur d’onde, et a été utilisé pour réaliser une expérience d’injection d’harmoniques d’ordre élevé, générées sur la voies econdaire à partir d’une cellule de gaz d’Argon.Une amélioration notable des caractéristiques spatiales et de la cohérence temporelle du laserX-UV a pu être observée. / The thesis fits within the framework ofsoft x-ray lasers (SXRL) development and has formain objective to study the influence of the temporal shaping of ultra-intense laser pulses, on the efficiency of SXRL generation. My thesiswork consisted in studying, designing and calibrating new pumping schemes through various devices based on the prepulse generation and/or an amount of ASE within the laser driver.In this manuscript, I study their influence on the SXRL generation efficiency by highlighting the optimization parameters such as the delay and the energy ratio between pulses, or the duration of each of them. The experimental study highlights first of all the influence of a prepulse on the SXRL generation efficiency. In the same framework, a device was experimented, allowing to generate within a single laser beam two mainpulses preceded by a prepulse, while maintaining a control over their spectro-temporalcharacteristics.An alternative approach was experimented in which an additional low cost « Q-Switch » lase rwas used to produce a under dense plasma presenting smooth electronic density gradients.This last device has showed excellent performances on a wide wavelength range andhas been used to perfom an experiment of highorder harmonic seeding generated from an Argongas cell on the secondary LASERIX beamline. A noteworthy improvement of the spatial characteristics and the temporal coherence of theSXRL have been observed.