<p dir="ltr">Some of the simplest molecules that are found in abundance in nature, like oxygen, nitrogen, carbon dioxide and water can be playgrounds for complex quantum mechanical phenomenon. Although we can calculate their static properties, like binding energies, equilibrium geometries and ionization/decay rates with extraordinary precision, their dynamics offer new avenues for exploration. Although analytical techniques have been successfully applied in studying single-particle and many-particle systems, few-particle systems like simple molecules are still best understood through a combination of numerical calculations and experimental work. However, the small size of these molecules endows them with dynamics that occur on timescales of a few picoseconds to a few attoseconds, making their experimental study challenging. The overarching goal of this work is the study of such ‘ultrafast’ dynamics in excited state molecules/atoms, by developing and demonstrating novel optical probes of quantum dynamics.</p><p dir="ltr">One way to probe ultrafast dynamics in molecules is by measuring their nonlinear optical response. Such a measurement can potentially track the evolution of the symmetries of excited molecules, shedding light on their transient dynamics. We start chapter 1 with a brief discussion of the formalism behind nonlinear optical spectroscopy. Direct measurement of ultrafast (and ultraweak) optical pulses is discussed as a useful probe of nonlinear processes. After presenting preliminary results on direct electric field reconstruction, experimental work on measuring emitted nonlinear electric fields from impulsively aligned molecules is discussed. In such an experiment, however, contributions from both aligned and unaligned molecules are present, and new experimental capabilities had to be developed to disentangle and measure the ultraweak signal from aligned molecules. Following a detailed discussion of the developed measurement capabilities, results from experiments done on aligned carbon dioxide and nitrogen molecules are discussed.</p><p dir="ltr">Unlike solids, where electronic states can be excited with visible/UV light, binding energies in isolated atoms/molecules are on the order of electron-volts (eVs), and they need vacuum-ultraviolet (VUV) extreme-ultraviolet (EUV) light to excite electronically. Polyatomic molecules, like ethylene, when excited to an electronic state with VUV light, often relax back to the ground state by redistributing energy to their internal degrees of freedom non-adiabatically. These relaxation pathways are important in many chemical and biological systems, and control the yield of chemical reactions ranging from elementary reactions involving few atoms to large biomolecules such as DNA and proteins. For instance, in the photochemical reaction of the protein Rhodopsin, considered to be the primary event in human vision. In chapter 2 we discuss progress made towards extending nonlinear response measurements to study ultrafast dynamics in electronically excited molecules, using a high-harmonic VUV source. Details about the design of the high-harmonic generation beamline, and preliminary experimental data are presented. In chapter 3 we discuss preliminary theoretical work on the development of an EUV entangled-photon source, using two-photon emission from the metastable 2s state in neutral Helium. Such a source, if demonstrated, can possibly even extended to the zeptosecond regime in the future.</p>

The main topic of this thesis is the development of novel diagnostics for the characterisation of infrared femtosecond and extreme-ultraviolet (XUV) attosecond pulses. High-resolution interferometric methods are applied to high harmonic radiation, both to measure the properties of the XUV light and to relate this information to the physics of the fundamental generation process. To do so, a complete high harmonic beamline has been built and optimised to enable the observation of strong signatures of the macroscopic response of the medium. The distinct spatial characteristics of long and short trajectories are studied, as well as the interference between them. An interferometric measurement allows the extraction of the atomic dipole phase, which gives direct access to the sub-cycle electron dynamics. A major focus of this thesis is on the development of a novel method which simultaneously characterises two independent electric fields as a function of any degree of freedom in which it is possible to shear one of the beams. Since each field alternately takes the role of the reference to retrieve the other field, this technique is referred to as mutual interferometric characterisation of electric-fields (MICE). One of the key features of MICE is that no sheared but otherwise identical replica of the test pulse needs to be generated, which is a typical requirement of self-referencing techniques. Furthermore, no a priori information is needed for the reconstruction. The strength and the wide applicability of MICE are demonstrated using two fundamentally different examples. First, the temporal pulse profiles of two infrared femtosecond pulses are simultaneously reconstructed in a single laser shot. In the second demonstration, the MICE approach is used to simultaneously reconstruct the wavefronts of two high harmonic beams. Having this new technique at hand, the phase properties of the different quantum trajectories are compared. All pulse characterisation techniques implicitly assume full coherence of the beam. This, however, is often not the case in practice, in particular when dealing with complex XUV light sources. Here the standard characterisation techniques fail to provide an accurate description of the electric field. Instead, the electric field must be seen as a statistical mixture of different contributions to the overall field. Here an interferometric experiment is first proposed and then performed involving multiple lateral shears to measure the two-point correlation function of high harmonic radiation. This directly provides information about the existence and the magnitude of partial coherence of high harmonics.

La technique d’optique ultra-rapide pompe-sonde, qui repose sur l’emploi de lasers à impulsion ultracourte(femtoseconde), permet de déclencher et étudier des processus ultrarapides dans la matière. L’acoustique picoseconde concerne pour sa part l’étude des processus de génération et détection de phonons acoustiques haute fréquence ainsi quel’analyse des nanomatériaux avec ces phonons (nanoéchographie). Les travaux de recherche de cette thèse avaient pourbut l’étude des couplages électronphonon acoustique dans le matériau ferroélectrique BiFeO3 par acoustique ultrarapide. Nous avons pu mettre en évidence que selon l’orientation du cristal photoexcité, l’émission des phonons acoustiques cohérents longitudinaux (LA) et transverses (TA) pouvait être modulée. De manière spectaculaire, nous avons purévéler un couplage électron-phonon acoustique transverse très efficace comme cela n’avait jamais été observé jusqu’alors dans les métaux, semiconducteurs ou nanostructures artificielles. Une étude détaillée indique que le mécanismepiézoélectrique inverse semble être le moteur de ce couplage électron-phonon (Lejman et al, Nature Communications, 2014). Dans une seconde partie, nous avons montré que BFO, ainsi qu’un autre ferroélectrique biréfringent LiNbO3 (LNO), peuvent être utilisés pour la conversion de mode ultra-rapide par processus acousto-optique (manipulation de la polarisation de la lumière à l’échelle de la picoseconde avec des phonons acoustiques). Cet effet, jamais mis enévidence jusqu’alors dans le domaine GHz, pourrait potentiellement être exploité dans de nouveaux dispositifs photoniques/phononiques pour des modulations acousto-optiques à haute cadence. / Ultrafast optical pump-probe technique, by exploiting ultrashort laser pulses (femtosecond), allows to initiate and monitor ultrafast processes in matter. Picosecond acoustics is a research field that focuses on the generation and detection mechanisms of high frequency coherent acoustic phonons in different media, as well as on their application in testing of nanomaterials and nanostructures. This PhDs research project was devoted to study of electron-acoustic phonon coupling in ferroelectric BiFeO3 (bismuth ferrite, BFO) by ultrafast acoustics. We have evidenced that depending on the BFO crystal orientation it was possible to tune the coherent phonons spectrum with in particular variable amplitude of longitudinal (LA) and transverse (TA) acoustic modes. In some grains with particular crystallographic orientations much stronger TA than LA signal was observed. Spectacularly, we have revealed an efficient coupling between electron and transverse acousticphonon. Such high ratio never reported before in any metal, semiconductor or nanostructure before, can be principally attributed to the photoinduced inverse piezoelectric effect (Lejman et al Nature Communications 2014). In a second part, we have shown that BFO as well as another birefringent ferroelectric LiNbO3 (LNO) can be used for ultrafast acousto-optic modeconversion (manipulation of light polarization at the picosecond time scale with coherent acoustic phonons). This effect, never reported at GHz up to now, can be potentially applied in photonics for ultrafast manipulation of light polarization bycoherent acoustic phonons in next generation photonic/phononic devices.

Continuing advances in laser and photonic technology has seen the development of lasers with increasing power and increasingly short pulsewidths, which have become available over an increasing range of wavelengths. As the availability of laser sources grow, so do their applications. To make better use of this improving technology, understanding and controlling laser propagation in free space is critical, as is understanding the interaction between laser light and matter. The need to better control the light obtained from increasingly advanced laser sources leads to the emergence of beam engineering, the systematic understanding and control of light through refractive media and free space. Beam engineering enables control over the beam shape, energy and spectral composition during propagation, which can be achieved through a variety of means. In this dissertation, several methods of beam engineering are investigated. These methods enable improved control over the shape and propagation of laser light. Laser-matter interaction is also investigated, as it provides both a means to control the propagation of pulsed laser light through the atmosphere, and provides a means to generation remote sources of radiation.