In this dissertation, I present the results of my research on a prototypical interface of the metal oxide ZnO and the organic semiconductor C₆₀. I establish that the physics at such oxide / organic interfaces is complex and very different from the extensively investigated case of organic semiconductor / metal interfaces. The studies presented in this dissertation were designed to address and improve the understanding of the fundamental physics at such hybrid organic / inorganic interfaces. Using photoemission spectroscopies, I show that metal oxide defect states play an important role in determining the interfacial electronic properties, such as energy level alignment and charge carrier dynamics. In particular, I show that for hybrid interfaces, electronic phenomena are sensitive to the surface electronic structure of the inorganic semiconductor. I also demonstrate applications of photoemission spectroscopies which are unique in that they allow for a direct comparison of ultrafast charge carrier dynamics at the interface and the electronic structure of defect levels. The research presented here focuses on a achieving a significant understanding of the realistic and device relevant C₆₀ / ZnO hybrid interface. I show how the complex surface structure of ZnO can be modified by simple experimental protocols, with direct and dramatic consequences on the interfacial energy level alignment, carrier dynamics and carrier collection and injection efficiencies. As a result of this careful study of the electronic structure and dynamics at the C₆₀ / ZnO interface, a greater understanding of the role of gap states in interface hybridization and charge carrier localization is obtained. This dissertation constitutes a first step in achieving a fundamental understanding of hybrid interfacial electronic properties.

No description available.

In recent years, there has been an intense interest in understanding the microscopic mechanism of laser induced ultrafast magnetization dynamics in picosecond time scales. Magnetization switching on such a time scale has potential to be a significant boost for the data storage industry. It is expected that the writing process will become ~1000 times faster by this technology, compared to existing techniques. Understanding the microscopic mechanisms and controlling the magnetization in such a time scale is of paramount importance at present. In this thesis, laser induced ultrafast magnetization dynamics has been studied for Fe, Co, GdFe, CoMn and Heusler alloys. A multiscale approach has been used, i.e., first-principles density functional theory combined with atomistic spin dynamics utilizing the Landau –Lifshitz-Gilbert equation, along with a three-temperature phenomenological model to obtain the spin temperature. Special attention has been paid to the calculations of exchange interaction and Gilbert damping parameters. These parameters play a crucial role in determining the ultrafast magnetization dynamics under laser fluence of the considered materials. The role of longitudinal and transversal excitations was studied for elemental ferromagnets, such as Fe and Co. A variety of complex temporal behavior of the magnetic properties was observed, which can be understood from the interplay between electron, spin, and lattice subsystems. The very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale was studied. We have shown that the ultrafast thermal switching process can happen above the compensation temperature in GdFe alloys. It is demonstrated that the exchange frustration via Dzyaloshinskii-Moriya interaction between the atomic Gd moments, in Gd rich area of these alloys, leads to Gd demagnetization faster than the Fe sublattice. In addition, we show that Co is a perfect Heisenberg system. Both Co and CoMn alloys have been investigated with respect to ultrafast magnetization dynamics. Also, it is predicted that ultrafast switching process can happen in the Heulser alloys when they are doped with heavy elements. Finally, we studied multiferroic CoCr2O4 and Ca3CoMnO4 systems by using the multiscale approach to study magnetization dynamics. In summary, our approach is able to capture crucial details of ultrafast magnetization dynamics in technologically important materials.

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.

Topological insulators (TI) are a new phase of matter with very exotic electronic properties on their surface. As a direct consequence of the topological order, the surface electrons of TI form bands that cross the Fermi surface odd number of times and are guaranteed to be metallic. They also have a linear energy-momentum dispersion relationship that satisfies the Dirac equation and are therefore called Dirac fermions. The surface Dirac fermions of TI are spin-polarized with the direction of the spin locked to momentum and are immune from certain scatterings. These unique properties of surface electrons provide a platform for utilizing TI in future spin-based electronics and quantum computation. The surface bands of 3D TI can be directly mapped by angle-resolved photoemission spectroscopy (ARPES) and the spin polarization can be determined by spin-resolved ARPES. These types of experiments are the first to establish the 3D topological order, which demonstrates the power of ARPES in probing the surface of strongly spin-orbit coupled materials. Extensive investigation of TI has ranged from understanding the fundamental electronic and lattice structure of various TI compounds to building TI-based devices in search of more exotic particles such as Majorana fermions and magnetic monopoles. Surface-sensitive techniques that can efficiently disentangle the charge and spin degrees of freedom have been crucially important in tackling the multi-faceted problems of TI. In this thesis, I show that laser-based ARPES in combination with a time-of-flight spectrometer is a powerful tool to study the spin structure and charge dynamics of the Dirac fermions on the surface of TI. Chapter 1 gives a brief introduction of TI. Chapter 2 describes the basic principles behind ARPES and time-resolved ARPES (TrARPES). Chapter 3 provides a detailed account of the experimental setup to perform laser-based ARPES and TrARPES. In Chapters 4 and 5, how these two techniques are effectively applied to investigate two unique electronic properties of TI is elaborated. Through these studies, I have obtained a complete mapping of the spin texture of several prototypical topological insulators and have uncovered the cooling mechanism governing the hot surface Dirac fermions. / Physics

No description available.

Die Untersuchungen der nichtskalierbaren Eigenschaften von Clustern in dem Größenregime, in dem jedes Atom zählt, zeigten, daß hier neuartige Phänomene und Funktionalität entstehen können. Dadurch motiviert wurden in dieser Arbeit: i) strukturelle und elektronische Eigenschaften sowie die Reaktivität von Metall Clustern, ii) stationäre optische Eigenschaften und iii) zeitabhängige Eigenschaften und optimale Kontrolle von ultraschnellen Prozessen in Edelmetallcluster und in nonstoichiometrischen Natrium-Fluorid Cluster, untersucht. / The study of the nonscalable properties of clusters in the size regime in which each atom counts have shown that fully new phenomena and striking new unexpected properties of small clusters can emerge. In this work three aspects have been addressed: i) the structural and electronic properties and reactivity of metal clusters, ii) stationary optical propertis and iii) real time investigation and control of ultrafast processes in noble metal and in nonstoichiometric sodium fluoride clusters.

Cette thèse présente la dynamique ultrarapide de relaxation de molécules photochromes et des agrégats d'argon en phase gazeuse à l'échelle femtoseconde. Des expériences utilisant la technique « pompe-sonde » ont été menées sur un dispositif utilisant un faisceau moléculaire pulsé couplé à de l'imagerie de vitesse de photoélectron/photoion (VMI) et un spectromètre de masse à temps de vol (TOF-MS). Ces études nous ont permis de caractériser les changements de distribution électronique des différents systèmes en fonction du temps. Par ailleurs une étude théorie/expérience sur la caractérisation de la densité et de la distribution de vitesse au sein d'un faisceau moléculaire pulsé a aussi été réalisée. Dans le cas de la dynamique des dithienyléthènes, nous avons observé des mécanismes de relaxation électronique parallèles. Le paquet d'onde initial se sépare en deux parties distinctes. Une première partie se dirige vers l'état fondamental via une intersection conique, tandis que la deuxième partie reste quelques picosecondes dans l'état excité en oscillant avant de relaxer vers l'état fondamental. Cette étude nous a permis de comprendre la dynamique intrinsèque des différentes molécules étudiées, mais aussi d'étendre le mécanisme de relaxation à toute cette famille de molécules photochromes dans les trois phases dans lesquelles elles sont étudiées. Dans le cas des agrégats d'argon, deux phénomènes ont été observés à différentes échelles de temps. Le premier se produit dans les premières picosecondes et est la relaxation électronique d'un état excitonique à une vitesse d'environ 1 eV.ps⁻¹. Le deuxième phénomène résulte de la localisation de l'excitation sur une paire Ar₂* que nous avons pu observer à partir de 4-5 ps. L'éjection d'atomes d'argon excités a aussi été observée, nous permettant ainsi de connaitre la durée de vie maximale de l'état excitonique délocalisé. Ce travail a permis d'apporter des informations supplémentaires à celles fournies par les études réalisées en phase condensée. Il ouvre donc la voie vers l'étude de systèmes plus complexes tels que les nanoparticules en phase gazeuse. / This PhD thesis investigated the ultrafast dynamics of photochromic molecules and argon clusters in the gas phase at the femtosecond timescale. Pump-probe experiments are performed in a set-up which associates a versatile pulsed molecular beam coupled to a photoelectron/photoion velocity map imager (VMI) and a time-of-flight mass spectrometer (TOF-MS). Theses pump-probe experiments provides the temporal evolution of the electronic distribution for each system of interest. Besides, a modelization has been performed in order to characterize the density and the velocity distribution in the pulsed beam. Regarding the photochromic dithienylethene molecules, parallel electronic relaxation pathways were observed. This contrasts with the observation of sequential relaxation processes in most molecules studied so far. In the present case, the initial wavepacket splits in two parts. One part is driven to the ground state at the femtosecond time scale through a conical intersection, and the second part remains for ps in the excited state and experiences oscillations in a suspended well. This study has shed light into the intrinsic dynamics of the molecules under study and a general relaxation mechanism has been proposed, which applies to the whole family of dithienylethene molecules whatever the state of matter (gas phase or solution) in which they have been investigated. Concerning argon clusters excited at about 14 eV, two behaviors of different time scale have been observed at different time scales. The first one occurs in the first picoseconds of the dynamics. It corresponds to the electronic relaxation of an excitonic state at a rate of 1 eV.ps⁻¹. The second phenomenon corresponds to the localization of the exciton on the excimer Ar₂*. This phenomenon is observed 4-5 ps after the excitation. In this study, we also observed the ejection of excited argon atoms, addressing the lifetime of the delocalized excitonic state. This work provide additional informations compared to those contributed in condensed phase and it pave the way for new studies in gas phase on more complex system such as nanoparicles.

The microscopic origin of ultrafast demagnetization, i.e. the quenching of the magnetization of a ferromagnetic metal on a sub-picosecond timescale after laser excitation, is still only incompletely understood, despite a large body of experimental and theoretical work performed since the discovery of the effect more than 15 years ago. Time- and element-resolved x-ray magnetic circular dichroism measurements can provide insight into the microscopic processes behind ultrafast demagnetization as well as its dependence on materials properties. Using the BESSY II Femtoslicing facility, a storage ring based source of 100 fs short soft x-ray pulses, ultrafast magnetization dynamics of ferromagnetic NiFe and GdTb alloys as well as a Au/Ni layered structure were investigated in laser pump – x-ray probe experiments. After laser excitation, the constituents of Ni50Fe50 and Ni80Fe20 exhibit distinctly different time constants of demagnetization, leading to decoupled dynamics, despite the strong exchange interaction that couples the Ni and Fe sublattices under equilibrium conditions. Furthermore, the time constants of demagnetization for Ni and Fe are different in Ni50Fe50 and Ni80Fe20, and also different from the values for the respective pure elements. These variations are explained by taking the magnetic moments of the Ni and Fe sublattices, which are changed from the pure element values due to alloying, as well as the strength of the intersublattice exchange interaction into account. GdTb exhibits demagnetization in two steps, typical for rare earths. The time constant of the second, slower magnetization decay was previously linked to the strength of spin-lattice coupling in pure Gd and Tb, with the stronger, direct spin-lattice coupling in Tb leading to a faster demagnetization. In GdTb, the demagnetization of Gd follows Tb on all timescales. This is due to the opening of an additional channel for the dissipation of spin angular momentum to the lattice, since Gd magnetic moments in the alloy are coupled via indirect exchange interaction to neighboring Tb magnetic moments, which are in turn strongly coupled to the lattice. Time-resolved measurements of the ultrafast demagnetization of a Ni layer buried under a Au cap layer, thick enough to absorb nearly all of the incident pump laser light, showed a somewhat slower but still sub-picosecond demagnetization of the buried Ni layer in Au/Ni compared to a Ni reference sample. Supported by simulations, I conclude that demagnetization can thus be induced by transport of hot electrons excited in the Au layer into the Ni layer, without the need for direct interaction between photons and spins. / Der mikroskopische Ursprung der ultraschnellen Entmagnetisierung, d.h. des Rückgangs der Magnetisierung eines ferromagnetischen Metalls innerhalb einer Pikosekunde nach Laseranregung, ist bisher nur unvollständig verstanden, trotz umfangreicher experimenteller und theoretischer Arbeiten, die seit der Entdeckung des Effekts vor mehr als 15 Jahren durchgeführt wurden. Zeit- und elementaufgelöster Röntgenzirkulardichroismus kann Einblick in die mikroskopischen Prozesse hinter der ultraschnellen Entmagnetisierung sowie deren Materialabhängigkeit gewähren. Am BESSY II Femtoslicing, einer speicherringbasierten Quelle für 100 fs kurze Röntgenpulse, wurde ultraschnelle Magnetisierungsdynamik von ferromagnetischen NiFe- und GdTb-Legierungen sowie einer Au/Ni-Schichtstruktur in Anregungs-Abfrage-Experimenten untersucht. Nach Laseranregung zeigen die Konstituenten von Ni50Fe50 und Ni80Fe20 deutlich unterscheidbares Verhalten und damit entkoppelte Dynamik, trotz starker Austauschkopplung der Ni- und Fe-Untergitter im Gleichgewichtszustand. Weiterhin variieren die Werte der Zeitkonstanten der Entmagnetisierung von Ni und Fe für Ni50Fe50 und Ni80Fe20, und auch für die jeweiligen reinen Elemente. Diese Unterschiede werden durch die magnetischen Momente der Untergitter erklärt, die sich in den Legierungen gegenüber den reinen Elementen ändern, sowie durch die Stärke der Austauschkopplung zwischen den Untergittern. GdTb zeigt Entmagnetisierung in zwei Stufen, was typisch für Seltene Erden ist. Die Zeitkonstante der langsameren zweiten Stufe wurde kürzlich mit der Stärke der Spin-Gitter-Kopplung in reinem Gd und Tb in Verbindung gebracht, wobei die stärkere, direkte Spin-Gitter-Kopplung in Tb zu schnellerer Entmagnetisierung führt. In GdTb folgt die Entmagnetisierung von Gd auf allen Zeitskalen der von Tb. Dies beruht auf einer verstärkten Kopplung der magnetischen Momente von Gd an das Gitter, über die indirekte Austauschkopplung an die Tb-Momente. Dadurch kann Spindrehimpuls schneller an das Gitter abfließen. Zeitaufgelöste Messungen der Entmagnetisierung einer Ni-Schicht unter einer Au-Deckschicht, deren Dicke ausreichend ist um den anregenden Laserpuls praktisch vollständig zu absorbieren, zeigen eine leicht verzögerte aber trotzdem ultraschnelle Entmagnetisierung im Vergleich mit einer Ni-Referenzprobe. Unterstützt durch Simulationen zeigt sich, dass Entmagnetisierung durch den Transport heißer Elektronen von der Au-Deckschicht in die Ni-Schicht ausgelöst wird, ohne dass direkte Wechselwirkung zwischen Photonen und Spins notwendig ist.

La chiralité est une propriété géométrique caractérisant les objets qui ne sont pas superposables à leur image dans un miroir. Nos mains en sont un exemple emblématique, puisqu’elles existent sous deux formes différentes droite et gauche. Si la chiralité s'observe à toutes les échelles de l'univers, elle joue un rôle particulièrement important en chimie. Une molécule chirale et son image miroir peuvent réagir différemment avec leur environnement et être thérapeutiques ou toxiques. Ces effets ont évidemment d'immenses répercussions sur le règne animal et végétal. Il apparaît alors clairement qu'il est essentiel d’étudier précisément les dynamiques des réactions chimiques chirales.Dans cette thèse, nous avons étudié les dynamiques ultrarapides de molécules chirales par des sources lasers de durée femtosecondes).($10^{-15}$ s). La chiralité moléculaire étant généralement difficile à détecter, nous avons ici utilisé une technique récente, le dichroïsme circulaire de photoélectrons (PECD) qui permet de générer un signal chiral très important. Nous avons ainsi observé des dynamiques moléculaires ultrarapides jusqu'à l'échelle attoseconde ($10^{-18}$ s), et mis en avant des dynamiques de relaxation et d'ionisation encore jamais observées.Parallèlement à ces études résolues en temps, nous avons développé plusieurs expériences employant une nouvelle source laser Yb fibrée à haute cadence et grande puissance moyenne. Nous avons développé une nouvelle méthode, par extension du PECD, qui nous a permis de mesurer la compositions d'échantillons chiraux rapidement avec une grande précision. Enfin, nous avons développé une ligne de lumière XUV ultrabrève de très haute brillance ($sim 2$ mW). Cette source, couplée à un détecteur de photoélectrons et photoions en coïncidence, servira à étudier les mécanismes de reconnaissance chirale. / Chirality is a geometric property that characterizes objects that cannot be superposed on their mirror image. Our hands are an emblematic example of this, since they exist in two different forms, right and left. While chirality is observed at all scales in the universe, it plays a particularly important role in chemistry. A chiral molecule and its mirror image can react differently with their environment and be therapeutic or toxic. These effects obviously have immense repercussions on the animal and plant kingdom. It then becomes clear that it is essential to study precisely the dynamics of chiral chemical reactions.In this thesis, we studied the ultrafast dynamics of chiral molecules by laser sources of femtosecond duration ($10^{-15}$ s). Molecular chirality is generally difficult to detect, so we have used a recent technique, circular photoelectron dichroism (PECD), to generate a very important chiral signal. We have thus observed ultrafast molecular dynamics at the attosecond scale ($10^{-18}$ s), and highlighted relaxation and ionization dynamics never observed before.In parallel to these time-resolved studies, we have developed several experiments using a new high repetition rate, high mean power Yb fiber laser. We have developed a new method, by extending the PECD, that has allowed us to measure the composition of chiral samples quickly and accurately. Finally, we have developed an ultra-short XUV beamline with very high brightness ($sim 2$ mW). This source, coupled with a photoelectron and photoion coincidence detector, will be used to study chiral recognition mechanisms.