Doctor of Philosophy / Department of Physics / Artem Rudenko / Carlos Trallero-Herrero / Ultrafast laser systems (pulse durations 10-100 femtoseconds) allow for the practical production of intense fields (≥ 10¹⁴ W/cm²) in a table-top, laboratory setup. The development of this technology has opened the door to studying the interaction of intense laser fields with atoms, molecules, and solid media. These experiments revealed a wealth of dynamics and interplay between the field, ion, and the freed electron, which has led to the production of first attosecond pulses and opened the field of attosecond science. The dynamics of the electron in an intense laser field are fundamental to strong- field phenomena such as higher-order harmonic generation, high energy above threshold ionization, and non-sequential double ionization. As the electron can be strongly accelerated by the instantaneous field, the dynamics depend on both the field's amplitude and wavelength. The latter dependence comes from the fact that the period of the field increases with wavelength. Thus, the electron is accelerated for a longer time and the energy gained is proportional to the wavelength squared. Recent evidence supports the claim that the electron- field interaction at longer wavelengths must include the contribution of the magnetic field and/or the radiation pressure of the field, adding to the wealth of effects associated with strong- field interactions. This thesis explores several routes towards fulfilling gaps in our understanding of the wavelength-scaling of strong- field interactions. I first demonstrate several important developments that reduce the complexity of generating non-sinusoidal, light transient waveforms in the near-infrared, opening the ability to tailor waveforms for more control on strong- field interactions. Next, I demonstrate the development of a strong- field, femtosecond source at wavelengths from 5 micrometers to 9 micrometers. To date, this is the first few-cycle, strong- field (≥ 10¹⁴ W/cm²) source in the long-wave infrared. An important advantage to this design is the wavelength tunability, which provides a control knob for understanding strong- field interactions across a broad wavelength range. Afterwards, I present applications of wavelength tunable sources in strong- field absorption in semiconductors. Specifically, I measure the absorbance of a strong laser field in gallium arsenide as a function of laser polarization, which varies the density of states available to the electron. This is performed for four laser wavelengths spanning 1.2 micrometers to 2.4 micrometers. With these absorbance measurements, we can compare the dependence of the photoexcitation rate on several parameters and compare it to theory. We find that the change in absorbance with density of states deviates from theoretical predictions as the photon order for the photoexcitation increases from two to three. This could be attributed to the field modifying the energy-momentum relationship of the conduction band. To conclude the thesis, I present simulations on a recent experimentally demonstrated technique for amplifying few-cycle electric fields. Due to the difficulty in making these sources, the model I developed includes many of the parameters involved in designing the system. This simulation can be used to plan design criteria, such as nonlinear crystal thickness, for peak performance of the amplification process.

Doctor of Philosophy / Department of Physics / Carlos A. Trallero / Ultra-fast spectroscopy has become a common tool for understanding the structure and dynamics of atoms and molecules, as evidenced by the award of the 1999 Nobel Prize in Chemistry to Ahmed H. Zewail for his pioneering work in femtochemistry. The use of shorter and more energetic laser pulses have given rise to high intensity table-top light sources in the visible and infrared which have pushed spectroscopic measurements of atomic and molecular systems into the strong-field limit. Within this limit, there are unique phenomena that are still not well understood. Many of such phenomena involve a photoionization step. For three decades, there has been a steady investigation of the single ionization of atomic systems in the strong-field regime both experimentally and theoretically. The investigation of the ionization of more complex molecular systems is of great interest presently and will help with the understanding of ultra-fast spectroscopy as a whole. In this thesis, we explore the single ionization of molecules in the presence of a strong electric field. In particular, we study molecular isomer pairs, molecules that are the same elementally, but different structurally. The main goal of this work is to compare the ionization yields of these similar molecular pairs as a function of intensity and gain some insight into what differences caused by their structure contribute to how they ionize in the strong-field limit. Through our studies we explore a wavelength dependence of the photoionization yield in order to move from the multi-photon regime of ionization to the tunneling regime with increasing wavelength. Also, in contrast to our strong-field studies, we investigate isomeric molecules in the weak-field limit through single photon absorption by measuring the total ionization yield as a function of photon energy. Our findings shed light on the complexities of photoionization in both the strong- and weak-field limits and will serve as examples for the continued understanding of single ionization both experimentally and theoretically.

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).

Understanding the electronic structure and dynamics of cyclic organic molecules is becoming increasingly the subject of investigations from different perspectives due to their unique chemical and physical properties. Since they are largely involved in the biochemistry of living organisms, studies on this class of compounds are also valuable to understand biologically relevant complex systems. Compared to other techniques, high-order harmonic generation (HHG) has been increasingly considered as a powerful spectroscopic tool with Angstrom spatial and attosecond temporal resolutions. This thesis demonstrates that high-order harmonic spectroscopy is capable of providing structural and dynamical information on the electronic systems of representative cyclic organic molecules comprising randomly oriented five-membered or six-membered rings. The first part of this thesis shows that the HHG from these molecules is sensitive to their aromatic character, which results from the de-localized pi electrons, and can potentially be a useful qualitative measure of aromaticity. We show that the advantage of utilizing HHG in this direction stems from the result that only pi molecular orbitals, associated with aromatcity, are responsible for the HHG emission in aromatic systems. The capability of HHG to distinguish cyclic isomers is demonstrated in the case of xylene molecules. Supported by numerical calculations, differences in the isomers are attributed to both tunnel ionization and photorecombination, the first and last steps of HHG. These results enable further HHG-based time-resolved studies of the dynamics associated with isomeric effects that these molecules exhibit. The present work also challenges the well-established prediction that strong field ionization from a molecular orbital is suppressed along nodal planes, where the electron density is zero. In fact, our study shows that considerable tunnel ionization in some cyclic molecules can occur near or along nodal planes. This unusual ionization is reported to have its signature on the quantitative and qualitative dependence of harmonic yield on laser ellipticity. The high symmetry displayed by the cyclic molecule, 1,4 cyclohexadiene, is shown to leave its imprints on the HHG in the form of structural interferences even if the target is randomly oriented. Two-color HHG from this molecule also indicates that hole dynamics could be involved in the generation process. A general study on high harmonic spectroscopy of the Cooper minimum in molecules is also reported. The presence of this minimum could affect the interpretation of harmonics spectra in any molecule containing S or Cl atoms. The molecular environment is shown to influence the position of this spectral modulation.

Many-body quantum dynamics is a challenging problem that has induced the development of many different computational techniques. One powerful technique is the multiconfiguration time-dependent Hartree-Fock (MCTDHF) method. This method allows proper consideration of electronic correlation with much less computational overhead compared to other similar methods. In this work, we present our implementation of the MCTDHF method on a non-uniform cylindrical grid. With the one-body limit of our code, we studied the controversial topic of tunneling delay, and showed that our results agree with one recent experiment while disagreeing with another. Using the fully correlated version of the code, we demonstrated the ability of MCTDHF to address correlation by calculating the ground state ionization energies of a few strongly correlated systems.

Cette thèse s’intéresse à la photo-ionisation de systèmes atomiques et moléculaires en phase gazeuse à l’aide d’harmoniques d’ordre élevé, un rayonnement cohérent dans le domaine de l’extrême ultraviolet (10-100 eV) sous la forme de trains d’impulsions attosecondes (1 as = 10-18 s). Dans un premier temps, les dynamiques électroniques au cours de l’auto-ionisation de gaz rares sont étudiées par interférométrie électronique. L’auto-ionisation résulte de l’interférence entre un chemin d’ionisation direct et un chemin résonant pour lequel l’atome reste transitoirement piégé dans un état excité.L’amplitude de la transition associée à ces processus est accessible via des expériences de photo-ionisation dans le domaine spectral (sur synchrotron par exemple), mais ce n’est pas le cas de la phase qui est pourtant essentielle à la compréhension de la dynamique électronique.Nous avons développé plusieurs méthodes interférométriques afin de mesurer la phase spectrale associée aux transitions électroniques vers des résonances de Fano dans les gaz rares.A partir des informations dans le domaine spectral, nous avons reconstruit pour la première fois la dynamique d'auto-ionisation ultra-rapide dans le domaine temporel et observé les interférences électroniques donnant lieu au profil de raie asymétrique. Dans un second temps, la photo-ionisation de molécules de NO est étudiée dans le référentiel moléculaire et utilisée comme un polarimètre afin de caractériser complètement l’état de polarisation du rayonnement harmonique, et en particulier de distinguer la partie du rayonnement polarisée circulairement d’une éventuelle partie dépolarisée. Nous présentons les résultats des mesures de polarimétrie moléculaire dans le cas de la génération d’harmoniques par un champ à deux couleurs polarisées circulairement en sens opposé. Ces études, complétées par des simulations numériques, permettent de proposer des conditions optimales de génération de rayonnement harmonique polarisé circulairement et contribuent à ouvrir la voie vers des études de dichroïsme circulaire ultrarapide dans la matière. / In this work, photoionzation of atomic and molecular species in the gas phase is investigated with high-harmonic radiation. In a first part, electronic dynamics in the autoionization process of rare gases in studied with electron interferometry. This method gives access to the spectral phase of the transition to the autoionizing state, and allows there construction of the entire autoionization dynamics. The ultrafast electronic dynamics, as well as the build-up of the celebrated asymmetric Fano profile, are observed experimentally for the first time. In a second part, photoionization of NO molecules in the molecular frame is used as a polarimeter to completeley characterize the polarization state of high-harmonics. In particular, this method can address the challenging disentanglement of the circular and unpolarized components of the light. The experimental results, completed by numerical simulations, allow defining optimal generation conditions of fully circularly-polarized harmonics for advanced studies of ultrafast dichroisms in matte

The dynamical Schwinger effect refers to the creation of electron-positron pairs by a time dependent, spatially homogeneous electric field. It probes the fundamentals of quantum electrodynamics and is sought to be verified with upcoming high-intensity laser installations. In the dynamically assisted Schwinger effect, the pair yield is increased by orders of magnitude through the combination of fields of different field strength and frequency scales. In this thesis we, investigate both processes in the framework of a quantum kinetic equation. We are especially interested in what amplification factors can be achieved by the assisting field and whether intermediately large mode occupation numbers have a physical significance and could serve as a verification of the Schwinger effect. For the latter goal, we couple our system to a quantized radiation field that serves as a secondary (photon) probe and study its spectrum, as the afterglow of the dynamical Schwinger effect. / Als dynamischen Schwingereffekt bezeichnet man die Erzeugung von Elektron-Positron-Paaren durch ein zeitabhängiges, räumlich homogenes elektrisches Feld. Er dient zur Untersuchung der Grundlagen der Quantenelektrodynamik und es wird versucht, ihn an zukünftigen Hochintensitäts-Laseranlagen zu verifizieren. Bei dem dynamisch assistierten Schwingereffekt wird die Paarausbeute durch eine Kombination von Feldern mit unterschiedlichen Feldstärken- und Frequenzskalen um mehrere Größenordnungen erhöht. In dieser Dissertation untersuchen wir beide Prozesse im Rahmen einer quantenkinetischen Gleichung. Wir interessieren uns besonders dafür, welche Verstärkungsfaktoren durch das assistierende Feld erreicht werden können und ob intermediär große Modenbesetzungszahlen physikalische Signifikanz haben und als Verifizierung des Schwingereffekts dienen können. Für zweiteres Ziel koppeln wir unser System an ein quantisiertes Strahlungsfeld, das als sekundäres (Photonen-) Signal dient, und untersuchen dessen Spektrum, das Nachglühen des dynamischen Schwingereffekts.

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This thesis outlines the high intensity tabletop attosecond extreme ultraviolet laser source at the Institute for the Frontier of Attosecond Science and Technology Laboratory. First, a unique Ti:Sapphire chirped pulse amplifier laser system that delivers 14 fs pulses with 300 mJ energy at a 10 Hz repetition rate was designed and built. The broadband spectrum extending from 700 nm to 900 nm was obtained by seeding a two stage Ti:Sapphire chirped pulse power amplifier with mJ-level white light pulses from a gas filled hollow core fiber. It is the highest energy level ever achieved by a broadband pulse in a chirped pulse amplifier up to the current date. Second, using this laser as a driving laser source, the generalized double optical gating method is employed to generate isolated attosecond pulses. Detailed gate width analysis of the ellipticity dependent pulse were performed. Calculation of electron light interaction dynamics on the atomic level was carried out to demonstrate the mechanism of isolated pulse generation. Third, a complete diagnostic apparatus was built to extract and analyze the generated attosecond pulse in spectral domain. The result confirms that an extreme ultraviolet super continuum supporting 230 as isolated attosecond pulses at 35 eV was generated using the generalized double optical gating technique. The extreme ultraviolet pulse energy was ∼100 nJ at the exit of the argon gas target.

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