L'interaction d'une impulsion laser intense (~10¹⁴ W /cm²) et de courte durée (femtoseconde) avec un gaz rare induit une polarisation hautement non-linéaire dans le domaine spectral XUV; les harmoniques d'ordre élevés. En raison des propriétés spécifiques du rayonnement harmonique et de ses applications, cette thématique est particulièrement riche et fertile. La production efficace d'harmoniques d'ordres élevés repose à la fois sur la réponse non-linéaire de l'atome unique et un comportement collectif.Le fil directeur des études présentées dans cette thèse est la compréhension et le contrôle de l'accord de phase ou du quasi accord de phase en présence d'une ionisation substantielle du gaz générateur. Dans ce contexte, nous montrons l'importance de la longueur de cohérence sur l'accord de phase en génération d'harmoniques. Nous étudions sa dépendance en fonction de la focalisation du laser, de la pression mais aussi sa dépendance temporelle liée à l'ionisation, effet que nous avons mis en évidence lorsqu'on a cherché à optimiser une double impulsion harmonique. Le travail de développement, sur la station LASERIX, de la source à double impulsion harmonique générée à partir d'un même milieu gazeux et avec un délai picoseconde variable est présenté. Cette source possède un véritable potentiel d'applications scientifiques, injectée dans un milieu amplificateur plasma qu'on appelle laser X, la double impulsion permettra de sonder la réponse temporelle de ce type de milieu. Par ailleurs, des expériences et des simulations menées sur la génération d'harmoniques en propagation guidée visent ainsi à étendre les spectres harmoniques vers les courtes longueurs d'ondes, zone spectrale pour laquelle le laser X à plasmas est émis. Ceci donnera l'accès à une source offrant des caractéristiques complémentaires des lasers X, sources développées en parallèle sur la station LASERIX. / The interaction of an intense laser pulse of short duration with a rare gas induces a highly non-linear polarization in the XUV spectral range: the high order harmonics. Due to the specific properties of the harmonic radiation and its applications, this issue is particularly rich and fertile. The efficient production of high order harmonics is based both on the non-linear response of the single atom and on collective behavior.The principle of the research presented in this thesis is the understanding and control of phase matching or quasi-phase matching in the presence of substantial ionization in the generating gas. In this context, we show the importance of the coherence length on the phase matching in High harmonic generation. We study its dependence on laser focusing, pressure but also its time dependence related to ionization. Moreover, experiments and simulations aim at extending harmonic spectra towards shorter wavelengths, a spectral range for which the X Ray Laser is emitted. This will give access to a source with complementary characteristics as regards to X-ray lasers. This source shall be developed in parallel on the LASERIX station or injected in soft X-ray laser amplifiers.

In the terahertz (THz) band, high-harmonic operations of gyro-devices are attractive for the great potential to reduce the required external magnetic field strength, which is proportional to the oscillation frequency but inversely proportional to the operational harmonics. This thesis focuses on the study of high-harmonic operation of the gyro-devices. Specifically, a high-harmonic large-orbit gyrotron (LOG) and the output system for a gyro-multiplier will be investigated. Firstly, the complex-cavity gyrotron, which is the foundation of the gyro-multiplier scheme, is studied by programming and computer simulation. The computer code and the complex-cavity operation are then verified by three-dimensional (3D) Particle-In-Cell (PIC) simulation. The code can be used as a preliminary design tool of the beam-wave interaction cavity for both the LOG and the gyro-multiplier. Secondly, a high-harmonic LOG operating with the TEm,1,1 modes (m=4-9) has been designed and investigated. It is shown that by proper choice of the operation parameters, selective excitation of oscillation from the fourth to the ninth harmonic of the electron cyclotron resonance can be achieved with kW-level output power from 230 GHz to 465 GHz. By enabling the high-harmonics operation, the required external magnetic field strengths are reduced to the range between 2.6 T to 3.1 T. The parameter variation study is also performed to provide a general conclusion of the high-harmonic LOGs operation. Thirdly, this thesis presents the investigation of a quasi-optical output system for a fourth-harmonic gyro-multiplier in the THz band. A dual-harmonic quasi-optical mode converter (DQMC) and a frequency selective surface (FSS) are employed within this system. It is shown that the optimal design of the DQMC requires a special mode selection in the gyro-multiplier. The high-pass FSS is designed, fabricated and experimentally verified by a THz-TDS system. It is demonstrated that the designed FSS is capable of handling the high-power output from the gyro-multiplier.

En vue de la capture de réearrangements électroniques au sein d’une molécule ou au cours de réactions chimiques il est indispensable de développer un dispositif dont la résolution temporelle est attoseconde (as 1 as = 10−18 s). La voie naturelle est de rechercher des impulsions lumineuses dans cette gamme de durée. Leur fréquence centrale doit alors être dans la gamme UVX et couvrir plusieurs dizaines d’eVs. De plus, ses composantes fréquencielles doivent être synchronisées. Le processus de génération d’harmoniques d’ordre élevé (GHE) dans les gaz remplit ces exigences. Pendant ce processus, une impulsion laser de haute intensité est focalisée dans un jet de gaz, où son champ électrique courbe la barrière de potentiel d’un atome et permet l’ionisation tunnel d’un paquet d’ondes électronique (POE). Entrainé par le champ électrique du laser, le POE accélére et acquiert une énergie cinétique élevée. Dans le cas où il repasse au voisinage du coeur ionique cette énergie cinétique peut être émise sous la forme d’un photon UVX. Ces POE explorent la structure et la dynamique de l’ion dans un schéma d’auto-sonde: le POE émis à un instant donné revient lui même ultérieurement sonder l’ion. Plus précisément ce processus d’autosonde donne accès à la valeur complexe du dipôle de recombinaison moléculaire (DRM), lui-même determiné par les structures nucléaire et électronique de l’ion. Le dipôle de recombinaison, en rayonnant des harmoniques, encode ces caractéristiques dans l’amplitude, la phase et l’état de polarisation de l’émission harmonique. Grâce à la nature cohérente de la GHE nous pouvons mesurer ces trois paramètres.L’objectif de ma thèse de doctorat était double. En mettant en oeuvre des techniques avancées de caractérisation de l’amplitude, de la phase et de la polarisation des harmoniques nous avons dans un premier temps étudié la structure électronique de N2 et l’ionisation tunnel multi-canaux induite par le laser. Nous avons montré les reconstructions des plusieurs orbitals moléculaires et révélé la vibration nucléaire ultra-rapide en fonction des canaux d’ionisations. Dans un deuxième temps nous avons étudié la réflectivité et la dispersion de miroirs UVX à compensation de dérive de fréquence, fabriqués sur mesure. Ces miroirs autorisent la mise en forme temporelle d’une impulsion attoseconde, compriment la durée de l’impulsions où introduisent un TOD. Nous avons aussi proposé un nouveau façonneur d’impulsions. / To capture electronic rearrangements inside a molecule or during chemical reactions, attosecond (as, 1 as =10−18 s) time resolution is needed. To create a light pulse with this duration, the central frequency has to be in the XUV range and cover several tens of eVs. Moreover, the frequency components have to be synchronized. The so called High Harmonic Generation (HHG) in gases well suits this task. During this process a high intensity laser pulse is focused in a gas jet, where its electric field bends the potential barrier of an atom allowing an electron wave packet (EWP) to tunnel ionize. Following the electric field of the laser the EWP gets accelerated, gaining a large kinetic energy that may be released as a high energy (XUV) photon in the event of a re-collision with the ionic core. These recolliding EWP probe the structure and dynamics of the core in a self-probing scheme: the EWP, that is emitted by the molecule at a certain time, probes itself later. More precisely, this ”self-probing” scheme gives access to the complex valued recombination dipole moment (RDM) of the molecule which is determined by both the nuclear and electronic structure. The recombination encodes these characteristics into the spectral amplitude, phase and polarization state of the harmonic radiation emitted by the dipole. Due to the coherent nature of HHG it is possible to measure all these three parameters. Moreover, it is in principle possible through a tomographic procedure to reconstruct the radiating orbital.The objective of my thesis was two-fold. By implementing advanced characterization techniques of the harmonic amplitude, phase and polarization we studied i) the electronic structure of N2 and laser induced multi-channel tunnel ionization. We presented the reconstruction of molecular orbitals and revealed the ionization channel dependent ultrafast nuclear vibration. We also studied ii) the reflectivity and dispersion of recently designed chirped XUV mirrors that can shape the temporal profile of attosecond pulses. With these mirrors we could control the spectral phase over 20 eV and compensate the GDD of the harmonics or introduce a TOD. We also proposed a novel attosecond pulse shaper.

The generation of high-order harmonics opened an era of attosecond science wherein coherent light bursts are used to probe dynamic processes in matter with a time resolution short enough to resolve the motions of electrons. It enabled the development of extreme ultraviolet (XUV) and X-ray table-top sources with both temporal and spatial coherence, which provides the ability to shape the temporal and spatial structure of the XUV pulses. Scientists developed techniques to control and measure the temporal structure high harmonic emissions. These techniques exploited control of the driving laser pulse in the time domain and facilitated development of more advanced high-harmonic based XUV sources that have greatly impacted ultrafast measurements. In this thesis, I apply techniques to control and measure the spatial structure of high harmonic emissions, and discuss the underlying physics and potential applications of the interaction between spatially structured laser beams and materials. This study exploits the spatial degree of freedom in strong field interaction, which has not been given as much attention as the temporal degree of freedom. I use liquid crystal devices to shape the wave front of a fundamental laser beam to a vortex structure, then imprint this structured wave front onto XUV beams through high harmonic generation. This method provides an alternative to special XUV optics, which can manipulate the wave front of XUV radiation by all optical means. This result also reveals the conservation of orbital angular momentum in this extreme nonlinear wave mixing process. In addition to shaping the wave front, shaping the polarization of the driving beam also allows generation of circularly polarized the XUV radiation using a high harmonic source. This thesis also highlights the interplay between shaping the wave front and polarization in the high harmonic generation process. The topology of the structured beam can be maintained through this extreme nonlinear interaction due to the spin selection rules and spin-orbit conservation. Moreover, this thesis demonstrates an approach to integrate a vector beam into a broadband ultrafast light source and overcome the bandwidth limitation of mode converters. We use this approach to generate a few-cycle structured beam. In the future, this beam will be used to generate a strong ultrafast magnetic impulse in gas and solid targets by driving currents in a loop, which is a valuable tool for the future of magnetic metrology. The novel properties of structured laser beams discussed in this thesis expanded the capabilities of high harmonic based XUV sources and have opened a new field to explore this additional degree of freedom in strong field interactions.

Molecules are traditionally treated quantum mechanically using the Born-Oppenheimer formalism. In this formalism, different electronic states of the molecule are treated independently. However, most photo-initiated phenomena occurring in nature are driven by the couplings between different electronic states in both isolated molecules and molecular aggregates, and therefore occur beyond the Born-Oppenheimer formalism. These couplings are relevant in reactions relating to the perception of vision in the human eye, the oxidative damage and repair of DNA, the harvesting of light in photosynthesis, and the transfer of charge across large chains of molecules. While these reaction dynamics have traditionally been studied with visible and ultraviolet spectroscopy, attosecond XUV pulses formed through the process of high harmonic generation form a perfect tool for probing coupled electronic dynamics in molecules. In this thesis, I will present our work in using ultrafast, XUV spectroscopy to study these dynamics in molecules of increasing complexity. We begin by probing the relaxation dynamics of superexcited states in diatomic O₂. These states can relax via two types of electronic couplings, either through autoionization or neutral dissociation. We find that our pump-probe scheme can disentangle the two relaxation mechanisms and independently measure their contributing lifetimes. Next, we present our work in observing a coherent electron hole wavepacket initiated by the ionization of polyatomic CO₂ near a conical intersection. The electron-nuclear couplings near the conical intersection drive the electron hole between different orbital configurations. We find that we can not only measure the lifetime of quantum coherence in the electron hole wavepacket, but also control its evolution with a strong, infrared probing field. Finally, we propose an experiment to observe the migration of an electron hole across iodobenzene on the few-femtosecond timescale. We present experimental modifications made to the high harmonic generation set-up in order to probe this ultrafast and elusive charge migration. These results demonstrate the potential of ultrafast, XUV spectroscopy in probing the inner-workings of electronic couplings occurring in nature.

Extreme ultraviolet (XUV) frequency combs exhibit promise for enabling high-precision spectroscopic measurements of myriad chemical species for the first time. Coherent XUV radiation can be generated through high harmonic generation (HHG) in femtosecond enhancement cavities. HHG efficiency is limited by nonlinear phase shifts induced by residual intracavity plasma. The goal of this work is to gain insight regarding plasma dynamics in order to allay the detrimental effects of plasma interactions. Our approach is to conduct simulations of cavity pump-probe experiments by probing with higher-order transverse modes. We propose methods for estimating spatial plasma profiles, gas jet velocities, and the plasma recombination coefficient based on measurements of plasma-induced phase shifts. Beam distortion due to plasma interaction is analyzed and used as another reference for plasma dynamics.

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).

La physique attoseconde est un domaine en pleine expansion, intrinsèquement lié au processus de génération d’harmoniques d’ordre élevé. Cette émission, sous forme d’un train d’impulsions attosecondes ou d’une impulsion attoseconde isolée, constitue une source de lumière dans le domaine spectral extrême-UV (XUV), ultra-brève, cohérente, parfaitement synchrone du champ générateur. Deux thématiques ont été abordées. La première consiste en la caractérisation complète de l’état de polarisation des harmoniques par Polarimétrie Moléculaire en collaboration avec l’ISMO-Orsay. Cette technique est basée sur la mesure de la distribution angulaire des photoélectrons dans le référentiel moléculaire lors de l’ionisation dissociative de la molécule de NO. Nous l’appliquons à trois configurations produisant un rayonnement harmonique polarisé elliptiquement. Nous obtenons ainsi, pour la première fois, la valeur absolue de l’ellipticité harmonique, son signe, ainsi que le taux de dépolarisation.La seconde thématique est la photoionisation attoseconde résonante : nous avons étudié la photoionisation de l’hélium au voisinage de la résonance d'autoionisation 2s2p à 60.15eV, excitée par une impulsion XUV accordable et sondée par une impulsion laser IR en utilisant la technique RABBIT, qui permet la mesure de l’amplitude et de la phase spectrales de la transition résonante à deux photons. Il est ainsi possible de reconstruire dans le domaine temporel, le paquet d'ondes électronique (POE) à 2 photons. Ces mesures ont été complétées par des simulations effectuées par nos collaborateurs à UAM-Madrid et au LCPMR-Paris, qui montrent que, dans nos conditions expérimentales, ce paquet à deux photons est une image fidèle du paquet résonant à un photon. Ceci représente la première reconstruction de la dynamique temporelle d’une résonance non perturbée par le champ laser, avec une résolution attoseconde. / Attosecond physics is an expending field, intrinsically linked to the High Harmonic Generation process. This emission, which can be either an attosecond pulse train or an isolated attosecond pulse, constitutes a light source in the extreme-UV (XUV) spectral domain, coherent, perfectly synchronous of the generating field. Two thematic have been studied. The first one consists in the complete characterization of the harmonic emission through Molecular Polarimetry, in collaboration with ISMO-Orsay. This technique is based on the measurement of the Molecular Frame PhotoElectron Angular Distribution, during the dissociative ionization of NO molecules. We applied this technique to three configurations producing an elliptically polarized light. For the first time, we obtain the absolute value of the ellipticity, its sign and the depolarization rate. The second topic is the resonant attosecond photoionization: we studied the photoionization of helium, close to the 2s2p autoionization resonance at 60.15 eV, excited by a tunable XUV pulse and probed by an IR pulse, using RABBIT technique, enabling the measurement of the spectral amplitude and phase of the two photons resonant transition. From this, we can reconstruct the two-photons electron wave packets (EWP). These measurements have been completed by simulations done by our collaborator from UAM-Madrid and LCPMR-Paris, showing that, in our experimental conditions, this two photons EWP corresponds to the image of the one-photon EWP. This measurement is the first reconstruction of the temporal dynamic of a resonance non-perturbed by a laser field, with an attosecond resolution.

No description available.

A variant of RABBITT pump-probe spectroscopy in which the attosecond pulse train comprises both even and odd harmonics of the fundamental IR probe frequency is explored to measure time-resolved photoelectron emission in systems that exhibit autoionizing states. It is shown that the group delay of both one-photon and two-photon resonant transitions is directly encoded in the energy-resolved photoelectron anisotropy as a function of the pump-probe time-delay. This principle is illustrated for a 1D model with symmetric zero-range potentials that supports both bound states and shape-resonances. The model is studied using both perturbation theory and solving the time-dependent Schodinger equation on a grid. Moreover, we study the case of a realistic atomic system, helium. In both cases, we demonstrate faithful reconstruction of the phase information for resonant photoemission.