At the heart of high harmonic generation lies a combination of optical and collision physics entwined by a strong laser field. An electron, initially tunnel-ionized by the field, driven away then back in the continuum, finally recombines back to rest in its initial ground state via a radiative transition. The emitted attosecond (atto=10^-18) XUV light pulse carries all the information (polarization, amplitude and phase) about the photorecombination continuum-to-ground transition dipolar field. Photorecombination is related to the time-reversed photoionization process. In this perspective, high-harmonic spectroscopy extends well-established photoelectron spectroscopy, based on charged particle detection, to a fully coherent one, based on light characterization. The main achievement presented in this thesis is to use high harmonic generation to probe femtosecond (femto=10^-15) chemical dynamics for the first time. Thanks to the coherence imposed by the strong driving laser field, homodyne detection of attosecond pulses from excited molecules undergoing dynamics is achieved, the signal from unexcited molecules acting as the reference local oscillator. First, applying time-resolved high-harmonic spectroscopy to the photodissociation of a diatomic molecule, Br2 to Br + Br, allows us to follow the break of a chemical bond occurring in a few hundreds of femtoseconds. Second, extending it to a triatomic (NO2) lets us observe both the previously unseen (but predicted) early femtosecond conical intersection dynamics followed by the late picosecond statistical photodissociation taking place in the reaction NO2 to NO + O. Another important realization of this thesis is the development of a complementary technique to time-resolved high-harmonic spectroscopy called LAPIN, for Linked Attosecond Phase INterferometry. When combined together, time-resolved high-harmonic spectroscopy and LAPIN give access to the complex photorecombination dipole of aligned excited molecules. These achievements lay the basis for electron recollision tomographic imaging of a chemical reaction with unprecedented angstrom (1 angstrom= 0.1 nanometer) spatial resolution. Other contributions dedicated to the development of attosecond science and the generalization of high-harmonic spectroscopy as a novel, fully coherent molecular spectroscopy will also be presented in this thesis.

At the heart of high harmonic generation lies a combination of optical and collision physics entwined by a strong laser field. An electron, initially tunnel-ionized by the field, driven away then back in the continuum, finally recombines back to rest in its initial ground state via a radiative transition. The emitted attosecond (atto=10^-18) XUV light pulse carries all the information (polarization, amplitude and phase) about the photorecombination continuum-to-ground transition dipolar field. Photorecombination is related to the time-reversed photoionization process. In this perspective, high-harmonic spectroscopy extends well-established photoelectron spectroscopy, based on charged particle detection, to a fully coherent one, based on light characterization. The main achievement presented in this thesis is to use high harmonic generation to probe femtosecond (femto=10^-15) chemical dynamics for the first time. Thanks to the coherence imposed by the strong driving laser field, homodyne detection of attosecond pulses from excited molecules undergoing dynamics is achieved, the signal from unexcited molecules acting as the reference local oscillator. First, applying time-resolved high-harmonic spectroscopy to the photodissociation of a diatomic molecule, Br2 to Br + Br, allows us to follow the break of a chemical bond occurring in a few hundreds of femtoseconds. Second, extending it to a triatomic (NO2) lets us observe both the previously unseen (but predicted) early femtosecond conical intersection dynamics followed by the late picosecond statistical photodissociation taking place in the reaction NO2 to NO + O. Another important realization of this thesis is the development of a complementary technique to time-resolved high-harmonic spectroscopy called LAPIN, for Linked Attosecond Phase INterferometry. When combined together, time-resolved high-harmonic spectroscopy and LAPIN give access to the complex photorecombination dipole of aligned excited molecules. These achievements lay the basis for electron recollision tomographic imaging of a chemical reaction with unprecedented angstrom (1 angstrom= 0.1 nanometer) spatial resolution. Other contributions dedicated to the development of attosecond science and the generalization of high-harmonic spectroscopy as a novel, fully coherent molecular spectroscopy will also be presented in this thesis.

<p> </p> <p>High-speed flows are important for defense, national security, and transportation applications and generate harsh environments where simplifying assumptions such as the ideal gas law are not valid due to nonequilibrium and chemistry effects. These flows are difficult and expensive to replicate experimentally, so the development and improvement of high-speed vehicles often relies on high-fidelity computational fluid dynamics (CFD) models. The successful modeling of complicated phenomena, such as heat transfer in a turbulent boundary layer, relies on validation by experimental data taken with high spatiotemporal resolution, precision, and accuracy. Precise experimental measurement of temperature, an important thermodynamic property for CFD models, is difficult with physical probes which are typically slow and perturb the flow. Instead, hybrid femtosecond/picosecond (fs/ps) coherent anti-Stokes Raman scattering (CARS) allows for non-intrusive, spatially-resolved, collision-free thermometry at kHz repetition rates with high precision and accuracy. </p> <p>The goal of this thesis is to advance hybrid fs/ps CARS for extension to high-speed flows, with particular improvements to the spatial extent, probe characteristics, and precision of the technique. A novel method for multipoint measurements in a simple and effective optical arrangement is demonstrated, enabling single-shot and averaged measurements of temperature and O₂/N₂ concentration along a linear array of probe volumes. The generation of a variable-pulsewidth probe beam by a ps slicer, electro-optic modulator, fiber amplifier, and custom narrowband amplifier system is used for improved signal-to-noise ratios at low pressure. Simultaneous CARS thermometry and femtosecond laser electronic excitation tagging (FLEET) velocimetry are performed in the freestream of Mach 3 and Mach 4 nitrogen flows. These measurements reveal the need to quantify and establish the ultimate precision of the hybrid fs/ps CARS technique. Sources of uncertainty in hybrid fs/ps CARS thermometry are determined through a theoretical uncertainty analysis and the predicted precision of the technique is confirmed experimentally in room temperature nitrogen. Benchtop measurements in a supersonic nozzle are used to indicate spatial and temporal simultaneity between FLEET and CARS measurements and hybrid fs/ps CARS thermometry is performed in a high-speed, low temperature flow.</p>

Depuis une vingtaine d’années, la physique attoseconde, via le phénomène de génération d’harmoniques d’ordres élevés (HHG), a permis de nombreuses avancées dans la compréhension des phénomènes de dynamique ultra-rapide. Les lasers femtoseconde émettant des impulsions de fortes énergies et de durées de quelques cycles optiques sont les outils indispensables à cette physique. De plus, la phase entre la porteuse et l’enveloppe (CEP) des impulsions doit être contrôlée. Récemment les lasers basés sur les fibres dopées ytterbium ont permis de transposer les expériences d’HHG à haute cadence. La stabilisation de la CEP pour ce type de systèmes constitue la brique manquante au développement de sources à haute cadence pleinement compatibles avec ces applications. Cette thèse a été consacrée à la stabilisation CEP d’un laser à fibre dopée ytterbium pour une application à la génération de rayonnement cohérent dans l’XUV à fort flux de photon. Dans la première partie nous présentons l’architecture d’une source à un taux de répétition de 100 kHz stable en CEP émettant des impulsions de 30 microjoules et 96 fs. Ce système constitue une preuve de principe pour les futures sources haute énergie. La stabilisation de CEP est assurée par une architecture hybride composée d’un injecteur stabilisé passivement suivi d’un amplificateur de puissance stabilisé activement. Un bruit résiduel de CEP inférieur à 400 mrad est obtenu dans différentes configurations, de la mesure courte durée (1 s) tir à tir jusqu’à la mesure sur une heure de fonctionnement. Dans la seconde partie nous présentons la mise au point d’une ligne HHG XUV optimisée à 13 nm sur les paramètres d’un laser à fibre, pour des applications à l’imagerie par diffraction cohérente. / In the last two decades, attosecond physics, based on the high harmonic generation (HHG) phenomenon, has allowed a better understanding of ultrafast dynamics in the microcosm. High-energy few-cycles carrier-envelope phase (CEP) stabilized sources are the main enabling tools for this physics. Recently, temporally compressed Ytterbium-doped fiber amplifiers have been successfully used as high XUV photon flux HHG drivers. CEP stabilization of these sources would ensure their full compatibility with attoscience. The thesis is devoted to the CEP stabilization of a high repetition rate Yb-doped fiber femtosecond source, for high XUV photon flux beamline applications. In the first part, we present the architecture of such a source at 100 kHz repetition rate delivering 30 microjoules 96 fs CEP-stable pulses. It constitutes a test bench for future energy-scaled few-cycle sources. The CEP stabilization is ensured in a hybrid architecture including a passively stabilized frontend followed by an actively stabilized power amplifier. A residual CEP noise <400 mrad is measured using various setups, including a shot-to-shot measurement over 1 s and a long-term stability over 1 h. In the second part, we discuss the design of a high flux HHG beamline optimized for a future generation of fiber-based driver at 13 nm for applications to coherent diffraction imaging.

The combination of direct laser interference patterning (DLIP) with laser-induced periodic surface structures (LIPSS) enables the fabrication of functional surfaces reported for a wide spectrum of materials. The process throughput is usually increased by applying higher average laser powers. However, this causes heat accumulation impacting the roughness and shape of produced surface patterns. Consequently, the effect of substrate temperature on the topography of fabricated features requires detailed investigations. In this study, steel surfaces were structured with line-like patterns by ps-DLIP at 532 nm. To investigate the influence of substrate temperature on the resulting topography, a heating plate was used to adjust the temperature. Heating to 250°C led to a significant reduction of the produced structure depths, from 2.33 to 1.06 µm. The reduction is associated with the appearance of a different LIPSS type, depending on the grain orientation of the substrates and laser-induced superficial oxidation. This study revealed a strong effect of substrate temperature, which is also to be expected when heat accumulation effects arise from processing surfaces at high average laser power.

This thesis reports an experimental characterization of the ultrafast gain and refractive index dynamics of a novel InAs/InGaAsP/InP quantum-dot (QD) semiconductor optical amplifier (SOA) operating near 1.55-µm wavelengths, assessing its high-speed performance characteristics for the first time. The thesis also studies the influence of the degree of quantum confinement on the dynamics of SOAs by comparing the zero-dimensional (0-D) QD's dynamics to those in 1-D InAs/InAlGaAs/InP quantum-dash (QDash), and 2-D InGaAsP/InGaAsP/InP quantum-well (QW) SOAs, both of which also operate near 1.55-µm wavelengths, and are made with matching or similar materials and structures. The ultrafast (around 1 ps) and long-lived (up to 2 ns) amplitude and phase dynamics of the SOAs are characterized via advanced heterodyne pump-probe measurements with 150-femtosecond resolution. It is found that the QD SOA has an 80-picosecond amplitude, and 110-picosecond phase recovery lifetime in the gain regime, 4-6 times faster than the QDash and QW recovery lifetimes, as well as reduced ultrafast transients, giving it the best properties for high-speed (> 100 Gb/s) all-optical signal processing in the important telecommunications wavelength bands. An impulse response model is developed and used to analyze the dynamics, facilitating a comparison of the gain compression factors, time-resolved linewidth enhancement factors (alpha-factors), and instantaneous dynamic coefficients (two-photon absorption and nonlinear refractive-index coefficients) amongst the three structures. The quantum-dot device is found to have the lowest effective alpha-factor, 2-10, compared to 8-16 in the QW, as well as time-resolved alpha-factors lower than in the QW—promising for reduced-phase-transient operation at high bitrates. Significant differences in the alpha-factors of lasers with the same structure are found, due to the differences between gain changes that are induced optically or through the electrical bias. The relative contributions of stimulated transitions and free-carrier absorption to the total carrier heating dynamics in SOAs of varying dimensionality are also reported for the first time. Examining the QD electroluminescence and linear gain spectra in combination with the carrier dynamics also brings about conclusions on the nature of the quantum confinement, dot energy-level structure, and density of states—aspects of the material that have not been previously well understood.

This thesis reports an experimental characterization of the ultrafast gain and refractive index dynamics of a novel InAs/InGaAsP/InP quantum-dot (QD) semiconductor optical amplifier (SOA) operating near 1.55-µm wavelengths, assessing its high-speed performance characteristics for the first time. The thesis also studies the influence of the degree of quantum confinement on the dynamics of SOAs by comparing the zero-dimensional (0-D) QD's dynamics to those in 1-D InAs/InAlGaAs/InP quantum-dash (QDash), and 2-D InGaAsP/InGaAsP/InP quantum-well (QW) SOAs, both of which also operate near 1.55-µm wavelengths, and are made with matching or similar materials and structures. The ultrafast (around 1 ps) and long-lived (up to 2 ns) amplitude and phase dynamics of the SOAs are characterized via advanced heterodyne pump-probe measurements with 150-femtosecond resolution. It is found that the QD SOA has an 80-picosecond amplitude, and 110-picosecond phase recovery lifetime in the gain regime, 4-6 times faster than the QDash and QW recovery lifetimes, as well as reduced ultrafast transients, giving it the best properties for high-speed (> 100 Gb/s) all-optical signal processing in the important telecommunications wavelength bands. An impulse response model is developed and used to analyze the dynamics, facilitating a comparison of the gain compression factors, time-resolved linewidth enhancement factors (alpha-factors), and instantaneous dynamic coefficients (two-photon absorption and nonlinear refractive-index coefficients) amongst the three structures. The quantum-dot device is found to have the lowest effective alpha-factor, 2-10, compared to 8-16 in the QW, as well as time-resolved alpha-factors lower than in the QW—promising for reduced-phase-transient operation at high bitrates. Significant differences in the alpha-factors of lasers with the same structure are found, due to the differences between gain changes that are induced optically or through the electrical bias. The relative contributions of stimulated transitions and free-carrier absorption to the total carrier heating dynamics in SOAs of varying dimensionality are also reported for the first time. Examining the QD electroluminescence and linear gain spectra in combination with the carrier dynamics also brings about conclusions on the nature of the quantum confinement, dot energy-level structure, and density of states—aspects of the material that have not been previously well understood.

Investigations of soft matter using ultrashort high intensity pulses have been made possible through the advent of X-ray free-electrons lasers. The last decade has seen the development of a new type of protein crystallography where femtosecond dynamics can be studied, and single particle imaging with atomic resolution is on the horizon. The pulses are so intense that any sample quickly turns into a plasma. This thesis studies the ultrafast transition from soft matter to warm dense matter, and the implications for structural determination of proteins.                    We use non-thermal plasma simulations to predict ultrafast structural and electron dynamics. Changes in atomic form factors due to the electronic state, and displacement as a function of temperature, are used to predict Bragg signal intensity in protein nanocrystals. The damage processes started by the pulse will gate the diffracted signal within the pulse duration, suggesting that long pulses are useful to study protein structure. This illustrates diffraction-before-destruction in crystallography. The effect from a varying temporal photon distribution within a pulse is also investigated. A well-defined initial front determines the quality of the diffracted signal. At lower intensities, the temporal shape of the X-ray pulse will affect the overall signal strength; at high intensities the signal level will be strongly dependent on the resolution. Water is routinely used to deliver biological samples into the X-ray beam. Structural dynamics in water exposed to intense X-rays were investigated with simulations and experiments. Using pulses of different duration, we found that non-thermal heating will affect the water structure on a time scale longer than 25 fs but shorter than 75 fs. Modeling suggests that a loss of long-range coordination of the solvation shells accounts for the observed decrease in scattering signal. The feasibility of using X-ray emission from plasma as an indicator for hits in serial diffraction experiments is studied. Specific line emission from sulfur at high X-ray energies is suitable for distinguishing spectral features from proteins, compared to emission from delivery liquids. We find that plasma emission continues long after the femtosecond pulse has ended, suggesting that spectrum-during-destruction could reveal information complementary to diffraction.

<p>Ultrafast laser-based diagnostic techniques are powerful tools for the detailed understanding of highly dynamic combustion chemistry and physics. The ultrashort pulses provide unprecedented temporal resolution along with high peak power for broad spectral range−ideal for nonlinear signal generation at high repetition rate−with applications including next-generation combustors for gas turbines, plasma-assisted combustion, hypersonic flows and rotating detonation engines. The current work focuses on advancing (i) femtosecond (fs) two-photon laser-induced fluorescence, and (ii) hybrid femtosecond/picosecond vibrational and rotational coherent anti-Stokes Raman scattering (fs/ps RCARS and VCARS) to higher pressures for the first time. </p><p>Quantitative single-laser-shot kHz-rate concentration measurements of key atomic (O-atom) and molecular (CO) species is presented using femtosecond two-photon laser-induced fluorescence (TP-LIF) for a range of equivalence ratios and pressures in diffusion flames. A multitude of signal-interfering sources and loss mechanisms−relevant to high-pressure fs TP-LIF applications−are also quantified up to 20 atm to ensure high accuracy. The pressure scaling of interferences take into account degradation, attenuation and wave-front distortion of the excitation laser pulse; collisional quenching and pressure dependent transition line-broadening and shifting; photolytic interferences; multi-photon ionization; stimulated emission; and radiation trapping. </p><p>Hybrid fs/ps VCARS of N₂ is reported for interference-free temperature measurement at 1300-2300 K in high-pressure, laminar diffusion flames up to 10 atm. A time asymmetric probe pulse allowed for detection of spectrally resolved CARS signals at probe delays as early as ~200-300 fs while being independent of collisions for the full range of pressures and temperatures. Limits of collisional independence, accuracy and precision of the measurement is explored at various probe-pulse delays, pressures and temperatures. </p><p> </p><p>Additionally, a novel all diode-pumped Nd:YAG amplifier design is presented for generation of time-synchronized ps-probe pulses for hybrid fs/ps RCARS of N₂. High-energy, nearly transform-limited, single-mode, chirp-free ps probe-pulses are generated at variable pulsewidths. The detailed architecture and characterization of the laser is presented. kHz-rate RCARS thermometry is presented up to 2400 K. Excellent spatial, spectral, and temporal beam quality allowed for fitting the theoretical spectra with a simple Gaussian model for the probe pulse with temperature accuracies of 1-2%. </p> <p><br></p>