The development of the field of the science of ultra-cold matter has opened some exciting possibilities in exploring the quantum-mechanical processes which dominate matter interactions at the sub-microscopic scale. Although methods of cooling atoms are well established, molecular cooling is made difficult by molecules’ additional vibrational and rotational degrees of freedom. It was the goal of the research in this work to approach molecular cooling indirectly, by using broadband shaped-pulse photoassociation for the generation of tightly bound ultracold Rb₂ molecules. The experiments towards this goal conducted by our group included a pumpdecay experiment to observe the generation of ground state singlet or triplet molecules. However, attempts to observe an increase in ground state population have been unsuccessful. A pump-probe study of wavepacket dynamics in the 5s+5p electronic state was conducted in order to determine the appropriate timing for the application of an additional pulse to dump population into the ground state. Although the attempt to observe wavepacket oscillations has been unsuccessful, pump-probe studies have yielded the observation of loosely bound excited state molecules as a result of the photoassociation pulse. These results are promising as a first stage in a fully coherent pump-dump approach to stabilisation into the lowest vibrational ground state. This thesis will provide an introduction and overview to the concerns involved in addressing the problem of molecular cooling and generation. Experimental techniques will be discussed including pulsed laser systems, optical parametric amplifi- cation, and the presentation of an original design for pulse shaping with an acoustooptic modulator. The emphasis of these discussions will be on the principles and operating procedures required for the use of these devices as home-built systems. The thesis will conclude with the results of pump-probe experiments utilising the pulse shaper as a spectral cutting device.

This work is devoted to the investigation of a spin polarization in ferromagnetic semiconductor Ga1-xMnxAs with a broad nominal concentration of manganese ions using one method of the ultrafast laser spectroscopy - the time-resolved Kerr rotation. At first, the experimental setup was optimized for the investigation of the dynamics of spin polarized charge carriers in semiconductors which were photo-generated by circularly polarized laser pulses. It was observed that the measured signal is induced by spin-polarized electrons. Due to a small thickness of the investigated ferromagnetic films the measured signal probably monitored the dynamics of fotogenerated electrons in GaAs substrate. Nevertheless, the measured data show that the electron spin dynamics in the substrate is significantly influenced by proximity effect due to the deposited ferromagnetic layer.

Ces travaux de thèse rentrent dans le cadre de l’étude de la dynamique ultra rapide de l’aimantation. Tout d’abord sont présentés des aspects théoriques, puis les aspects expérimentaux de ces expériences. Pour ce faire, nous avons étudié la réponse d’alliages ferrimagnétiques à composition variables à l’aide d’un dispositif de mesure d’effet Kerr résolu en temps, puis dans une seconde partie, la dynamique de l’aimantation et de la bande de valence du gadolinium épitaxié sur tungstène. Dans ce cadre rentre une étude de l’oxydation de ce matériau, limitant dans le temps les études approfondies. Pour finir, il est mis l’accent sur un phénomène contraignant lors des études de dynamique électronique en photoémission, l’effet de charge-espace. Ceci a pour effet de générer des photoélectrons à partir de métaux, à l’aide d’un processus multiphotonique. Nous proposons dans cette partie un modèle théorique expliquant ce phénomène.Ces travaux sont inscrits dans le cadre du développement du synchrotron SOLEIL, pour permettre le développement du FEMTOSLICING, qui permettra prochainement de mesurer des dynamiques rapides résolues en éléments, à une résolution de l’ordre de la centaine de femtosecondes. / Those thesis works are included in the framework of the study of ultrafast magnetization dynamics. First of all I introduce theoretical aspects, then experimental aspects of this kind of experiments.In this aim, we have studied the answer of ferromagnetic alloys of different compositions with a bench of time resolved magneto optical Kerr effect measurement, then in a second part, the magnetization and valence band dynamics of the epitaxial Gadolinium on tungsten. In this framework, we studied the oxidization of the Gd, which limits in the time the studies. In the end, we focus on a disturbing process that happens during the study of electrons dynamics in photoemission, the space charge effect. This can generate photoelectrons from metals, with a multiphotonic process. We propose in this last part a theoretical model to explain this phenomenon.These works are included in the development of SOLEIL synchrotron facility, in order to allow the development of the FEMTOSLICING, that will next allow to perform element resolved experiments within a time resolution of a hundredth of femtoseconds.

Doctor of Philosophy / Department of Physics / Itzhak Ben-Itzhak and Charles Lewis Cocke / A commercially available modern laser can emit over 10^15 photons within a time window of a few tens of femtoseconds (10^-15 second), which can be focused into a spot size of about 10 um, resulting in a peak intensity above 10^14 W/cm^2. This paves the way for table-top strong field physics studies such as above threshold ionization (ATI), non-sequential double ionization (NSDI), high order harmonic generation (HHG), etc.. Among these strong laser-matter interactions, high order harmonic generation, which combines many photons of the fundamental laser field into a single photon, offers a unique way to generate light sources in the vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) region. High order harmonic photons are emitted within a short time window from a few tens of femtoseconds down to a few hundreds of attoseconds (10^-18 second). This highly coherent nature of HHG allows it to be synchronized with an infrared (IR) laser pulse, and the pump-probe technique can be adopted to study ultrafast dynamic processes in a quantum system. The major work of this thesis is to develop a table-top VUV(EUV) light source based on HHG, and use it to study dynamic processes in atoms and small molecules with the VUV(EUV)-pump IR-probe method. A Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) apparatus is used for momentum imaging of the interaction products. Two types of high harmonic pump pulses are generated and applied for pump-probe studies. The first one consists of several harmonics forming a short attosecond pulse train (APT) in the EUV regime (around 40 eV). We demonstrate that, (1) the auto-ionization process triggered by the EUV in cation carbon-monoxide and oxygen molecules can be modified by scanning the EUV-IR delay, (2) the phase information of quantum trajectories in bifurcated high harmonics can be extracted by performing an EUV-IR cross-correlation experiment, thus disclosing the macroscopic quantum control in HHG. The second type of high harmonic source implemented in this work is a single harmonic in the VUV regime (around 15 eV) filtered out from a monochromator. Experiments on D_2 molecules have been conducted using the 9th or the 11th harmonic as the pump pulse. Novel dissociative ionization pathways via highly excited states of D_2 have been revealed, thus suggesting potential applications for time-resolved studies and control of photochemistry processes.

La diffraction cohérente est une technique étonnante par sa simplicité expérimentale : une source XUV cohérente illumine un échantillon unique, isolé, et la figure de diffraction de l’objet est enregistrée sur une caméra CCD. Une inversion de la figure de diffraction à une image dans l’espace réel est possible grâce à une approche basée sur des algorithmes itératifs. Les techniques d’holographie par transformée de Fourier, pour lesquelles une référence est placée à proximité de l’objet que l’on veut imager, permettent-elles la reconstruction directe de l’image, même lorsque la qualité des données expérimentales est moindre. Nous disposons dans notre laboratoire d’une source compacte XUV suffisamment intense pour réaliser ce type d’expérience. Les impulsions XUV ultrabrèves (femtoseconde à attoseconde) sont produites en sélectionnant les harmoniques d’ordre élevé d’un laser infra-rouge femtoseconde focalisé dans une cellule de gaz rare. Nous avons récemment démontré la possibilité d’utiliser cette source pour l’imagerie par diffraction cohérente avec une résolution spatiale de 78 nm. De plus, nous avons démontré expérimentalement une technique d’holographie avec référence étendue, et obtenu une résolution de 110 nm en simple tir (soit un temps d’intégration de 20 femtosecondes). Une perception d’un objet en trois dimensions nous donne une meilleure compréhension de celui-ci. A l’échelle nanométrique, les techniques d’imagerie 3D sont issues de techniques tomographiques autour de la microscopie électronique. Cependant, les nombreuses prises de vue nécessaires (sous des angles différents) rendent ces techniques caduques lors de l’étude résolue en temps de phénomènes irréversibles sur des échantillons non reproductibles. Dans ce contexte, le but de ma thèse est d’étendre les techniques d’imagerie 2D à une perception 3D d’objets nanométriques (physiques, biologiques), tout en préservant l’aspect ultrarapide. Le développement d’une nouvelle technique d’imagerie cohérent 3D en seul vue, l’ankylographie, proposée par le professeur J. Miao de UCLA [Raines et al., Nature 2010] a été effectué. Cette technique permet de reconstruire l’image 3D d’un échantillon d’après une unique image de diffraction. Son principe basique est de retrouver la profondeur d’un objet 3D par l’interférence constructive longitudinale. Cependant, cette technique d’imagerie cohérent 3D est plus exigeante en termes de qualité de données expérimentales comme en moyen informatique d’analyse et d’inversion. L’autre idée en imagerie 3D est de mimer la vision humaine en utilisant deux faisceaux X cohérents arrivant simultanément sur l’échantillon mais avec un petit angle. Dans ce schéma, on utilise des références à coté de l’objet mire (holographie) pour améliorer le rapport signal sur bruit dans la figure de diffraction (soit hologramme). On recueille ensuite deux hologrammes sur le même détecteur. L’inversion Fourier de chacun des hologrammes forme deux images issues d’une vision différente de l’objet. La parallaxe est ainsi réalisée. La reconstruction stéréo de l’objet est effectuée numériquement. Enfin, des applications de démonstration seront envisagées après ma thèse. Il s’agit d’imager des objets biologiques (nanoplanktons déjà collectés et préparés au CEA). Et nous nous intéresserons également à l’étude du mouvement 3D d’objets nanométriques (azo-polymères) sur des temps ultracourts. Une autre application importante sera d’étudier la transition de phase ultra-rapide tel que le nano-domaine magnétique où des phénomnes de désaimantation induite par des impulsion femtoseconde ont lieu. / Coherent diffraction is an amazing art by its experimental simplicity: a coherent XUV source illuminates a single, isolated sample, and the diffraction pattern of the object is recorded by a CCD camera. An inversion of the diffraction pattern to an image in real space is possible through an approach based on iterative algorithms. The techniques for Fourier transform holography, for which reference is placed near the object to be imaged, allow the direct reconstruction of the image, even when the quality of the experimental data is worse. We have a laboratory sufficiently intense compact XUV source for this type of experience. The ultrashort XUV pulses (from femtosecond to attosecond) are produced by selecting high order harmonics of a femtosecond infrared laser which is focused into a cell of rare gas. We recently demonstrated the feasibility of using this source for coherent diffraction imaging with a spatial resolution of 78 nm. Furthermore, we demonstrated experimentally a holographic technique with extended reference and obtained a resolution of 110 nm in single shot (i.e. an integration time of 20 femtoseconds). A perception of an object in three dimensions gives us a better understanding thereof. A nanoscale 3D imaging techniques are from tomographic techniques of electron microscopy. However, many shots required (from different angles) make these techniques obsolete during the study time-resolved irreversible phenomena on non-reproducible samples. In this context, the aim of my thesis is to extend the 2D imaging techniques for 3D perception of nanoscale (physical, biological ) objects, while preserving the ultrafast appearance. The development of a new technology of 3D coherent imaging in single view, named ‘ankylography’, proposed by Professor Miao J. UCLA [Raines et al., Nature 2010] was made in progress. This technique allows reconstructing a 3D image of the sample after a single diffraction image. Its basic principle is to find the depth of a 3D object by the longitudinal constructive interference. However, this technique is more requested in both the quality of experimental data and the computer hardware and analysis. The other idea for 3D imaging is to imitate human vision using two coherent beams X arriving simultaneously on the sample but with a small angle. In this scheme, we use references near the target object (i.e. holography) to improve the signal to noise ratio in the diffraction pattern (hologram). Two holograms are then collected on the same detector. The inverse Fourier of each hologram forms two images from different views of the object. Parallax is thus produced. The stereo reconstruction of the object is performed by computer. Finally, the demonstration of applications will be considered after my thesis. This imaging of biological objects (such as nanoplanktons already collected and prepared CEA). And we are also interested in the study of 3D nanoscale objects (azo-polymers) movement on ultrashort time. Furthermore, another important application will be to study the ultra-fast phase transition such as nano-magnetic field where demagnetization phenomena induced by femtosecond pulse occurs.

With increasing global energy demand and diminishing fossil fuel supplies, the development of clean and affordable renewable energy technology is more important than ever. Photovoltaic devices harvest the sun’s energy to produce electricity and produce very little pollution compared to nonrenewable sources. In order to make these devices affordable, however, technological advances are required. In this dissertation a novel photovoltaic device architecture that is designed to enhance sunlight-to-electricity conversion efficiency of photovoltaics is proposed and demonstrated. The increase in efficiency arises due to enhancement of the internal quantum efficiency of photoexcitation in the semiconductor absorber. In other words, the probability that the absorption of a single photon will produce two or more electron-hole pairs, instead of just one, is increased. This occurs through the process of impact ionization, by which a highly excited charge carrier (via absorption of a high energy photon) relaxes by excitation of a second electron-hole pair. The result is an increased photocurrent, and efficiency, of the photovoltaic device. Using thin films of ZnS on Si substrates, we demonstrate that the probability of impact ionization is enhanced at the (unbiased) heterojunction between these layers. The magnitude of enhancement depends on material properties, including crystallinity of the ZnS film as well as concentration of oxygen (impurity) at the interface. Thin films of ZnSe on Si substrates do not exhibit heterojunction-assisted impact ionization, but they do display promising characteristics that make them an intriguing system for future work. The same is true for ZnS/Si materials fabricated by O2-free chemical bath deposition. For the analysis of plain Si as well as ZnS/Si and ZnSe/Si heterostructures, we employ a novel pump-probe transient transmission and reflection spectroscopy technique. A method is demonstrated for using this technique to quantify internal quantum efficiency as well as interface recombination velocity in each of these materials. In bulk silicon, a free carrier absorption cross section that depends on free carrier concentration (above 1018 cm-3) is observed and the relationship is quantified. This dissertation includes unpublished and previously published co-authored material.

Ce travail de thèse se concentre sur la caractérisation et la correction de la gigue temporelle de lasers ultrabrefs, c'est-à-dire avec une durée d'impulsion sub-picoseconde, pompés par diode et avec un milieu à gain dopé à l'Ytterbium. Ce type de source laser a bénéficié d'un développement industriel rapide depuis le début des années 2000 et a trouvé de très nombreuses applications, notamment dans le domaine scientifique pour des expériences résolues en temps (FELs, accélérateurs de particules, OPAs...). Dans ce type d’applications, la réduction de la gigue des impulsions permet d’accéder à de meilleures résolutions temporelles et permet d’observer des phénomènes physiques extrêmement brefs.Dans ce travail, la gigue d'oscillateurs lasers ultrabrefs à verrouillage de mode passif a été corrigée et caractérisée en générant un signal d'erreur par photo-détection directe d'une part et par corrélation croisée optique d'autre part. La correction de la gigue temporelle par corrélation croisée optique permet d'obtenir une gigue résiduelle inférieure à 5fs, de l'ordre de la durée d'un cycle optique, avec des oscillateurs compacts et industriels. Une gigue résiduelle de ce niveau permet d’accéder à des expériences résolues en temps exigeantes en termes de stabilités temporelles des sources de lumière utilisée, par exemple une expérience d’interaction lumière-matière de type pompe-sonde avec une source laser à électrons libres.Un nouvel instrument basé sur la corrélation statistique des signaux de gigue générés par deux canaux de corrélations croisées optiques a été mis en place. Un tel instrument a permis de caractériser complètement la gigue temporelle d'oscillateurs à des fréquences de Fourier supérieures à la dizaine de kiloHertz lasers ultrabref en atteignant un plancher de bruit sub-zeptoseconde. La gigue additive d'amplificateurs lasers ultrabrefs a également été caractérisée par corrélation croisée optique, cela afin d'acquérir une compréhension globale de la gigue introduite par une chaîne laser ultrabrève amplifiée.Les mesures effectuées ont permis d’analyser les origines physiques de la gigue dans un système laser ultrabref. Ces résultats permettront à terme d’améliorer la correction de la gigue sur les systèmes industriels étudiés ainsi que de développer de nouvelles sources laser ultrabrèves intrinsèquement moins bruyantes. Une telle étude expérimentale pourra également servir de support pour éventuellement enrichir le modèle théorique des perturbations dans un laser ultrabref. / The main scope of this PhD work is to characterize and reduce the timing jitter of optical pulses with sub-picosecond duration, emitted by diode-pumped ultrafsat lasers with Ytterbium doped gain media. Such laser sources have been rapidly enhanced by a high-grade and efficient industrial development, giving access to numerous novel applications, especially in the fields of applied physics and chemistry with time-resolved experiments based on interaction between light and matter. In this latter topic resides a constant need to minimize the laser pulse train timing jitter in order to access physical phenomena with extremely short timescales such as electrons recombination.Two main detection techniques have been studied to characterize the timing jitter from passively modelocked laser oscillators. The first technique is based on direct photo-detection in the microwave domain whilst the second technique makes use of the balanced optical cross-correlation. Residual timing jitter from industrial compact laser oscillators has been reduced below 5fs, close to an optical cycle period. This performance makes the laser oscillators produced by Amplitude-Systemes already suitable for challenging time-resolved pump-probe experiments, in the presence for example of a free electron laser source.Timing jitter analysis has been enhanced by studying the cross-spectrum from jitter signals generated from two balanced optical cross-correlators. This technique allowed the entire characterization of the timing jitter density spectrum from laser oscillators above the ten-kiloHertz Fourier frequency with a sub-zeptosecond noise floor. Additive timing jitter from ultrafast laser amplifiers has also been studied with optical cross-correlation and gives a global comprehension of the timing jitter from a complete high-energy ultrafast laser chain.The collected measurments will be conducive to design a more adapted and performant timing synchronization system for industrial laser sources. A better knowledge of the non-trivial jitter sources, dependent on the laser parameters, will also allow to design intrinsically low-noise new laser sources based on the Ytterbium gain media. The cross-spectrum technique developed could be of use to characterize other laser technologies and to investigate the theoretical perturbation model of ultrafast lasers with a better experimental insight.

Doctor of Philosophy / Department of Physics / Itzhak Ben-Itzhak / The use of ultrafast lasers allows one to study and even control quantum mechanical systems on their natural timescales. Our aim is to study the fragmentation of small molecules in strong laser fields as a means to gain understanding of molecular dynamics and light-matter interactions. Our research group has utilized fast, positively charged molecular ion beams as targets to study and control fragmentation by strong laser fields. This approach allows for detection of all molecular fragments including neutrals, and a coincidence three-dimensional momentum imaging technique is used to characterize the fragmentation. A natural extension of these types of studies is to expand the types of molecular systems that can be studied, from positively charged molecules to neutral and negatively charged molecules. To that end, the primary technical development of this dissertation involved the generation and use of fast, negatively charged molecular beams. Using fast molecular anion beams as targets allows for the study of fragmentation in which all fragments are neutral. As a demonstration, we employ this capability to study F2- dissociation and photodetachment. The dissociation pathways are identified and used to evaluate the initial vibrational population of the F2- beam. The role of dissociation in photodetachment is also explored, and we find that it competes with other dissociative (F+F) and non-dissociative (F2) photodetachment mechanisms. Also highlighted are studies of fragmentation of LiO-, in which the dissociation into Li+O- fragments provides information about the structure of Li O-, including the bond dissociation energy, which was found to be larger than values based on theory. Studies of the autodetachment lifetimes of Li O- were also performed using a pump-probe technique. Additional experimental advancements have made successful pump-probe studies of the ionization of HD+ and Ar2+ possible. Enhancement in the ionization of dissociating HD+ and Ar2+ was observed at surprisingly large internuclear separation where the fragments are expected to behave like separate atoms. The analysis methods used to quantify this enhancement are also described. Finally, the production of excited Rydberg D* fragments from D2 molecules was studied utilizing a state-selective detection method. The carrier-envelope phase dependence of D* formation was found to depend on the range of excited final states of the atomic fragments. We also measured the excited state population of the D* fragments. Together, the studies presented in this work provide new information about fragmentation of positive, negative, and neutral molecules in strong laser fields, and the experimental developments serve as building blocks for future studies that will lead to a better understanding of molecular dynamics.

Epsilon-near-zero materials are an emerging class of nanophotonic materials which engender electromagnetic field enhancement and small phase variation due to their approximate zero permittivity. These quasi-static fields facilitate a number of unique optical properties such as supercoupling, subwavelength confinement, and enhanced light-matter interactions, which has made epsilon-near-zero media a rapidly expanding field of optical physics. Contemporary methods of realizing a system with zero permittivity rely on microwave cavities/waveguides or complex metal-dielectric metamaterials; however, both techniques require advanced fabrication and their operational wavelength is fixed relative to their geometric and optical parameters. It remains an open and substantial challenge to realize an epsilon-near-zero material at pertinent wavelengths, particularly near- and mid-infrared, with tunable/dynamic properties. The focus of this thesis is the exploration of transparent conducting oxides for the development of epsilon-near-zero nanophotonic phenomena and applications. Transparent conducting oxides have an inherent low permittivity, in addition to simple fabrication and tunable optical properties, making them exceptionally promising. Application of transparent conducting oxide films for highly confined modes, nonlinear/ultrafast optics, and strongly coupled systems are discussed.

Optical cell manipulation allows precise and non-invasive exploration of mammalian cell function and physiology for medical applications. Plants, however, represent a vital component of the Earth's ecosystem and the knowledge gained from using optical tools to study plant cells can help to understand and manipulate useful agricultural and ecological traits. This thesis explores the potential of several biophotonic techniques in plant cells and tissue. Laser-mediated introduction of nucleic acids and other membrane impermeable molecules into mammalian cells is an important biophotonic technique. Optical injection presents a tool to deliver dyes and drugs for diagnostics and therapy of single cells in a sterile and interactive manner. Using femtosecond laser pulses increases the tunability of multiphoton effects and confines the damage volume, providing sub-cellular precision and high viability. Extending current femtosecond photoporation knowledge to plant cells could have sociological and environmental benefits, but presents different challenges to mammalian cells. The effects of varying optical and biological parameters on optical injection of a model plant cell line were investigated. A reconfigurable optical system was designed to allow easy switching between different spatial modes and pulse durations. Varying the medium osmolarity and optoinjectant size and type affected optoinjection efficacy, allowing optimisation of optical delivery of relevant biomolecules into plant cells. Advanced optical microscopy techniques that allow imaging beyond the diffraction limit have transformed biological studies. An ultimate goal is to merge several biophotonic techniques, creating a plant cell workstation. A step towards this was demonstrated by incorporating a fibre-based optical trap into a commercial super-resolution microscope for manipulation of cells and organelles under super-resolution. As proof-of-concept, the system was used to optically induce and quantify an immunosynapse. The capacity of the super-resolution microscope to resolve structure in plant organelles in aberrating plant tissue was critically evaluated.