In dieser Arbeit wird die ultraschnelle Dynamik von Leitungsbandelektronen in Halbleitermaterialien mit Hilfe nichtlinearer Terahertz-Spektroskopie erforscht. Insbesondere wird n-dotiertes Galliumarsenid bei mittleren Dotierdichten zwischen 10^(16) cm^(-3) und 10^(17) cm^(-3) untersucht. Für die Erzeugung intensiever THz Strahlung wurde eine neuartige Quelle entwickelt, die THz Transienten mit nur einer Oszillationsperiode und maximalen Feldamplituden von mehr als 400 kV/cm liefert. Diese THz-Quelle benutzt ultrakurze optische Laserpulse aus einem Ti:Saphir Oszillator. Zusätzlich wurde ein neuartiger zwei-Farben Anrege-Abtast Experimentierplatz aufgebaut, der zweidimensionale, zeitaufgelöste Messungen im mittleren und fernen Infrarotbereich ermöglicht. Feldionisation flacher, neutraler Störstellen im Galliumarsenid-Gitter mittels intensiver, ultrakurzer THz Impulse und die anschliessende kohärente, strahlende Rekombination von Elektronen in die Störstellen-Grundzustände bei Raumtemperatur wird gezeigt. Der superradiante Zerfall der nichtlinearen Polarisation führt zur Abstrahlung eines kohärenten Signals mit Lebensdauern von über einer Pikosekunde. Solche nichtlinearen Signale, die 10-fache Lebensdauern im Vergleich zum linearen Fall aufweisen, wurden in dieser Arbeit zum ersten Mal gemessen. Bei niedrigen Temperaturen und THz Feldstärken unter 5 kV/cm werden Rabi-Oszillationen an Übergängen in flachen Störstellen demonstriert. Zum ersten Mal konnte die polare Elektron-LO-Phonon Wechselwirkung im quantenkinetischen Regime direkt gemessen werden. Die quasi-instantane Beschleunigung von Leitungsbandelektronen im polaren Galliumarsenid-Gitter und die anschließende Messung der Transmission im mittleren Infrarot-Bereich, zeigen eine Modulation der Transmission entlang der Anrege-Abtast Verzögerung mit der Frequenz des LO Phonons. Diese Oszillation ist ein direktes Maß der relativen Phase zwischen der Elektronenbewegung und der umgebenden Phonon Wolke. Quantenkinetische Modellrechnungen reproduzieren vollständig die beobachteten Effekte. / In this thesis, the ultrafast dynamics of conduction band electrons in semiconductors are investigated by nonlinear terahertz (THz) spectroscopy. In particular, n-doped gallium arsenide samples with doping concentrations in the range of 10^16cm^(-3) to 10^17 cm^(-3) are studied. A novel source for the generation of intense THz radiation is developed which yields single-cycle THz transients with field amplitudes of more then 400 kV/cm. The THz source uses ultrashort optical laser pulses provided by a Ti:sapphire oscillator. In addition, a two-color THz-pump mid-infrared-probe setup is implemented, which allows for two-dimensional time-resolved experiments in the far-infrared wavelength range. Field ionization of neutral shallow donors in gallium arsenide with intense, ultrashort THz pulses and subsequent coherent radiative recombination of electrons to impurity ground states is observed at room temperature. The superradiant decay of the nonlinear polarization results in the emission of a coherent signal with picosecond lifetimes. Such nonlinear signals, which exhibit a lifetime ten times longer than in the linear regime are observed for the first time. At low temperatures and THz field strengths below 5 kV/cm, Rabi flopping on shallow donor transitions is demonstrated. For the first time, the polar electron-LO phonon interaction is directly measured in the quantum kinetic transport regime. Quasi-instantaneous acceleration of conduction band electrons in the polar gallium arsenide lattice by the electric field of intense THz pulses and subsequent probing of the mid-infrared transmission reveals a modulation of the transmission along the THz-mid-infrared delay coordinate with the frequency of the LO phonon. These modulations directly display the relative phase between the electron motion and its surrounding virtual phonon cloud. Quantum kinetic model calculations fully account for the observed phenomena.

No description available.

Les matériaux quantiques ont récemment introduit en physique de la matière condensée pour unifier tous les matériaux dans lesquels les fortes corrélations électroniques gouvernent les propriétés physiques du système (e.g. les isolants de Mott) et les matériaux dont les propriétés électroniques sont déterminées par la géométrie de la fonction d’onde (e.g. matériaux de Dirac). Ces matériaux montrent des propriétés émergentes résultantes de l’intrication de différents degrés de libertés : la charge, le spin et le moment orbital, donnant lieu aux propriétés topologiques des électrons. L’étude de ces interactions et des compétitions entre les degrés de liberté pertinents nécessite l’utilisation de techniques pompe-sonde ultra-rapides. Particulièrement, les pulses laser femtosecondes interagissent uniquement avec les électrons pour les placer dans un état hors-équilibre décrit par des distributions de type non Fermi-Dirac. La dynamique subséquente implique de nombreux processus, avec un temps de relaxation relié aux constantes de couplage. De plus, dans les techniques résolues en temps, la lumière peut agir comme un paramètre externe, différent des paramètres thermodynamiques, pour explorer le diagramme de phase. Cela nous donne l’opportunité de stabiliser de nouveaux états inaccessibles par des chemins thermiques quasi-adiabatiques ou de manipuler les propriétés physiques des systèmes.Dans cette thèse, nous avons réalisé différentes expériences dans le but d’étudier les propriétés à l’équilibre et hors équilibre de deux matériaux corrélés: BaCo₁₋ₓNiₓS₂ et (V₁₋ₓMₓ)₂O₃.La première partie de ce projet a été dédiée principalement à l’étude de BaNiS₂, le précurseur métallique de la transition de Mott dans BaCo₁₋ₓNiₓS₂ . En utilisant l’ARPES, nous avons étudié la structure de bandes électroniques de BaNiS₂ dans toute la zone de Brillouin. L’expérience, combinée avec des calculs théoriques, révèle un nouveau type de cône de Dirac bidimensionel à caractère orbitalaire d et induit par les corrélations. Le croisement des bandes est protégé par les symétries particulières de la structure cristalline. Nous avons aussi mesuré la structure de bandes de l’isolant de Mott BaCoS₂ dans ses phases magnétique et non magnétiques.Dans la seconde partie, nous avons étudié la dynamique électronique hors équilibre de BaNiS₂ et (V₁₋ₓMx)₂O₃. Grâce à des mesures tr-ARPES et tr-Réflectivité, nous avons observé une renormalisation non thermique et ultra-rapide du cône de Dirac dans BaNiS₂. Ce phénomène est purement provoqué par les excitations électroniques et est stabilisé par l’intéraction entre les électrons et les phonons. De plus, en utilisant différentes techniques pompe-sonde (tr-XRD basé sur XFEL et tr-Réflectivité) nous avons aussi exploré des phases hors-équilibre du matériau prototype de Mott-Hubbard (V₁₋ₓMx)₂O₃ appartenant à différentes parties de son diagramme de phase. Nos résultats montrent une phase transitoire non thermique se développant immédiatement après la photoexcitation ultra-rapide et durant quelques picosecondes dans les phases métallique et isolantes. Cette phase transitoire est accompagné par une distorsion structural qui correspond à un durcissement du réseau et est marqué par un “blue shift” du mode phononique A₁g. Nos résultats soulignent l’importance du remplissage des orbitales aussi bien que des effets important des forts couplages électron-réseau sélectifs dans les matériaux fortement corrélés. / Quantum materials is a new term in condensed matter physics that unifies all materials in which strong electronic correlation governs physical properties of the system (e.g. Mott insulators) and materials whose electronic properties are determined by the geometry of the electronic wave function (e.g. Dirac materials). These materials show emergent properties– that is, properties that only appear by intricate interactions among many degrees of freedom, such as charge, spin and orbital, giving rise to topological properties of electrons. The study of these interactions and competitions between the relevant degrees of freedom demands applying ultrafast pump-probe techniques. Particularly, femtosecond laser pulses act only on the electrons and set them to an out-of-equilibrium state inexplicable by the Fermi-Dirac distribution. The ensuing dynamics involves various processes and the rate at which the relaxation occurs is related to the coupling constants. Moreover, in time-resolved pump-probe techniques light can act as an additional external parameter to change of the phase diagram – different from thermodynamic parameters. It gives us the opportunity of stabilizing new states inaccessible by quasi-adiabatic thermal pathways or eventually manipulating the physical properties of the systems.In this thesis, we performed different experiments in order to study the equilibrium and out-of-equilibrium properties of two correlated compounds: BaCo₁₋ₓNiₓS₂ and (V₁₋ₓMₓ)₂O₃.The first part of the project was mainly devoted to the study of BaNiS₂ that is the metallic precursor of the Mott transition in BaCo₁₋ₓNiₓS₂. By applying ARPES, we studied the electronic band structure of BaNiS₂ in its entire Brillouin zone. These results combined with some theoretical calculations give evidence of a novel correlation-induced and two-dimensional Dirac cone with d-orbital character. The band crossing is protected by the specific symmetries of the crystal structure. We also investigated the electronic band structure of the Mott insulator BaCoS₂ in its magnetic and nonmagnetic phases.In the second part, we studied the out-of-equilibrium electron dynamics of BaNiS₂ and (V₁₋ₓMx)₂O₃. By means of tr-ARPES and tr-reflectivity measurements, we observed an ultrafast and non-thermal renormalization of the Dirac cone in BaNiS₂ . This phenomenon is purely provoked by the electronic excitation and is stabilized by the interplay between the electrons and phonons. Moreover, by applying various pump-probe techniques (XFEL-based tr-XRD and tr-Reflectivity) we also explored the out-of-equilibrium phases of the prototype Mott-Hubbard material (V₁₋ₓMx)₂O₃ in different parts of its phase diagram. Our results show a transient non-thermal phase developing immediately after ultrafast photoexcitation and lasting few picoseconds in both metallic and insulating phases. This transient phase is followed by a structural distortion that corresponds to a lattice hardening and is marked by a “blue shift” of the A₁g phonon mode. These results underline the importance of the orbital filling as well as the strong effect of the selective electron-lattice coupling in the strongly correlated materials.

In dieser Arbeit wird Terahertzspektroskopie (THz) eingesetzt, um nichtlineare niederfrequente Anregungen von kondensierter Materie zu untersuchen. Insbesondere die Anwendung zweidimensionaler (2D) THz-Spektroskopie ermöglicht es, verschiedene Beiträge zu nichtlinearen Signalen zu entflechten. Zunächst wird die nichtlineare polaronische Antwort solvatisierter Elektronen und umliegenden Lösungsmittelmolekülen in der polaren Flüssigkeit Isopropanol erforscht. Solvatisierte Elektronen werden durch Multiphotonen-Ionisation erzeugt. Longitudinale Polaronoszillationen mit THz-Frequenzen werden während der ultraschnellen Lokalisierung der Elektronen impulsiv angeregt. Die Störung solcher Polaronschwingungen mit einem externen THz-Impuls führt zu nichtlinearen Änderungen der transversalen Polaron-Polarisierbarkeit, die sich in deutlichen Änderungen der Oszillationsphase zeigen. Darüber hinaus wird die Erzeugung monozyklischer THz-Impulse in asymmetrischen Halbleiter-Quantentrögen bei resonanter Intersubband-Anregung im Mittelinfraroten (MIR) demonstriert. Die zeitliche Form des emittierten elektrischen THz-Feldes wird durch die Steuerung der Impulsdauer und des elektrischen Feldes der MIR Impulse verändert. Phasenaufgelöste 2D-MIR-Experimente bestätigen, dass die THz-Emission vorrangig auf einen nichtlinearen Verschiebungsstrom bei Femtosekunden-Intersubband-Anregung zurückzuführen ist. Der Einfluss von Intra- und Interbandströmen auf Symmetrieeigenschaften wird in 2D-THz-Experimenten an Wismut demonstriert. Nichtperturbative langwellige Anregung von Ladungsträgern nahe der L-Punkte führt zu einer anisotropen Ladungsträgerverteilung, die sich in einer hexagonalen Winkelabhängigkeit der pump-induzierten THz Transmission manifestiert. Eine damit einhergehende Symmetrieverringerung für bestimmte elektrische Feldpolarisationen erlaubt die Anregung von Zonenrand-Phononen, welche sich in in oszillierenden Signalen in der nichtlinearen 2D-THz-Antwort manifestieren. / This thesis exploits techniques of terahertz (THz) spectroscopy to investigate nonlinear low-frequency excitations of condensed matter. In particular, application of two-dimensional (2D) THz spectroscopy allows to disentangle different nonlinear signal contributions. The nonlinear polaronic response of solvated electrons and their surrounding solvent molecules in the polar liquid isopronal is studied. Solvated electrons are generated via multiphoton ionization. Longitudinal polaron oscillations with THz frequencies are impulsively excited during the ultrafast localization of the electrons. Perturbation of such polaron oscillations with an external THz pulse induces nonlinear changes of the transverse polaron polarizability, reflected in distinct modifications to the oscillation phase as mapped in 2D-THz experiments. Further, the generation of mono-cycle THz pulses from asymmetric semiconductor quantum wells upon resonant intersubband excitation in the mid-infrared (MIR) range is demonstrated. The temporal shape of the emitted THz electric field is modified by controlling pulse duration and peak electric field of the MIR driving pulses. Phase-resolved 2D-MIR experiments confirm that the THz emission is predominantly due to a nonlinear shift current generated upon femtosecond intersubband excitation. The influence of combined intra- and interband currents on symmetry properties, which opens novel quantum pathways for phonon excitation in narrow-band-gap materials, is demonstrated by 2D-THz experiments on bismuth. Nonperturbative long-wavelength excitation of charge carriers close to the L points leads to an anisotropic carrier distribution, reflected in a six-fold azimuthal angular dependence of the pump-induced change of THz transmission. A concomitant symmetry reduction for certain electric-field polarizations allows for the excitation of phonons at the zone boundary which are reflected in oscillatory signals in the nonlinear 2D-THz response.

<p dir="ltr">In the realm of two-dimensional (2D) materials, monolayer (ML) transition metal dichalcogenides (TMDCs) have gained significant interest due to their direct bandgap transition, high carrier mobility, strong light-matter interaction, and robust spin and valley degrees of freedom, starkly contrasting their bulk counterparts. Owing to their large surface-to-volume ratio, the integration of ML TMDCs with other various 2D semiconductors and microcavities offers opportunities to study fundamental photo-physics processes at the heterointerfaces, paving the way for implementation of next-generation devices.</p><p dir="ltr">Chapter 1 provides a concise introduction to 2D materials, particularly TMDCs, and their fascinating optical and electronic properties. It examines the role of excitons in 2D materials, and the impact of energy transfer (ET) and charge transfer (CT) on exciton’s properties in TMDC through the construction of 2D van der Waals (vdW) heterostructures and coupling with optical microcavities. This chapter also delves into the potential enhancement of TMDCs’ optical properties by integrating 2D hybrid lead halide perovskites and ultra-thin three-dimensional (3D) halide perovskites with TMDCs. Furthermore, it sets the general context for light-matter interaction, another form of ET, considering both weak and strong coupling regimes.</p><p dir="ltr">Chapter 2 outlines the optical techniques employed to gather data for this work. A focus is placed on ultrafast optical techniques like transient absorption spectroscopy, which allow for direct probing and analysis of ET and CT dynamics at the heterointerface.</p><p dir="ltr">Photoinduced interfacial CT plays a critical role in the field of energy conversion involving vdW heterostructures constructed by inorganic nanostructures and organic materials. However, the control of atomic-scale stacking configurations to modulate charge separation at interfaces remains challenging. Chapter 3 aims to illustrate tunability of interfacial charge separation in a Type-II heterojunction between ML-WS₂ and an organic semiconducting molecule by rational design of relative stacking configurations using 2D perovskites as scaffoldings. This chapter investigates how different molecular stacking, face-to-face versus face-to-edge, affects CT at the heterointerface. Our findings reveal that the CT process heavily depends on the relative stacking configurations at the organic-TMDCs heterointerface, with charge separation being notably slowed down for face-to-edge configuration compared to face-to-face configuration. These investigations open new opportunities for designing efficient charge separation processes in energy conversion applications by judiciously engineering interfaces between organic and inorganic semiconductors, using 2D perovskites as scaffolds.</p><p dir="ltr">Though TMDCs’ large surface-to-volume ratios make them excellent platforms for studying interfacial properties, the presence of bulky ligands on the surface of 2D perovskite poses a challenge, impeding direct interfacial coupling in their heterostructures. Chapter 4 details the fabrication of ML-WS₂ and ultra-thin CH₃NH₃PbX₃ (MAPbX₃, X=Br, I) heterostructures with tunable energy levels, to study the dynamics of CT and ET at these hybrid interfaces. Notably, heterojunctions of WS₂ with pure MAPbBr₃ and MAPbI₃ were elucidated as Type-I and Type-II respectively, using photoluminescence (PL) and time-resolved photoluminescence (TR-PL) measurements. Transit absorption (TA) spectroscopy investigations unambiguously revealed a rapid ET facilitated by CT in the WS₂/MAPbBr₃ heterostructure, with a time constant of ~20 ps, and a predominantly CT in the WS₂/MAPbI₃ heterostructure with a time constant of ~50 femtosecond (fs). The successful interfacing of low-dimensional perovskites with an extensive array of traditional 2D materials such as TMDCs opens up possibilities for novel optoelectronic properties and applications within the field of 2D material systems. Furthermore, the ultrafast and efficient ET and CT processes hold promise for the creation of advanced energy conversion devices.</p><p dir="ltr">In the last chapter, we successfully fabricated a ML-WS₂ in conjunction with a silver (Ag) nanoparticle (NP) array. Our findings affirmed a weak light-matter coupling between ML-WS₂ and the Ag NP array, as evidenced by angle-resolved photoluminescence spectroscopy. Furthermore, an enhancement in the bright exciton emission from ML-WS₂ was observed at reduced temperatures. The analysis of PL enhancement factor at varying temperatures suggested that an upper bound of the enhancement factor for the bright exciton could reach ~51 or even higher at 7 K, given the imperfect uniformity of the electric filed generated around the NPs. This discovery carries significant implications for the manipulation of excitons in TMDCs and expands their potential applications in the field of optoelectronics.</p>

In elektrochemischen Zellen vollziehen sich die Haupteaktionen in der Regel an Oberflächen von Elektroden und Katalysatoren und deren Elektrolytgrenzflächen, wodurch Änderungen dort die Effizienz der Zelle stark beeinflussen können. Diese Arbeit behandelt elementare Prozesse an solchen Ober- und Grenzflächen, die die Bandstruktur und damit möglicherweise auch die Reaktivität des Systems verändern. Mit Zwei-Photonen-Photoelektronenspektroskopie (2PPE) untersuche ich solche Prozesse in drei Modellsystemen für Elektrodenoberflächen beziehungsweise Elektrolyt/Elektroden-Grenzflächen: ZnO wird als Material für die photoelektrochemische Wasserspaltung diskutiert. In zeitaufgelösten 2PPE-Spektren beobachte ich Oszillationen des Dipols der (10-10)-Oberfläche, die bislang unbekannten kohärenten Oberflächenphononen zuzuordnen sind. Ich diskutiere ihre Erzeugung und entwickle eine Methode, um ultraschnelle Änderungen des Oberflächendipols anhand der Intensität des Sekundärelektronenschwanzes eines 2PPE Spektrums zu quantifizieren. An der D2O/ZnO(10-10)-Grenzfläche untersuche ich mehrere Effekte der Wasseradsorption, wie Veränderungen der Austrittsarbeit und der kohärenten Oberflächenphononen. Anders als in früheren Studien stelle ich keine Oberflächenmetallisierung durch Wasseradsorption fest. Auch gibt es keinen klaren Hinweis auf Elektronensolvatisierung, wie sie an Wasser/Metall-Grenzflächen zu beobachten ist. An der DMSO/Cu(111)-Grenzfläche, einem Modellsystem der Elektrolyt/Kathoden-Grenzfläche in Metall-Luft-Batterien, bestimme ich die elementaren Schritte der Sauerstoffreduktion. Im DMSO werden kleine Polaronen ultraschnell gebildet und zum Teil in Oberflächendefekten eingefangen. Die Lebensdauer dieser gefangenen Elektronen kann mehrere Sekunden betragen. Sie reagieren mit co-adsorbiertem O2, nachdem es in das DMSO diffundiert ist, zu O2-. Die Modellierung der Diffusion liefert eine Abschätzung des Elektroden-Reaktanten-Abstandes für Elektronentransfer in DMSO. / In electrochemical cells, the main reactions usually proceed at the surfaces of electrodes and catalysts and their interfaces with the electrolyte. Hence, changes there can have a huge impact on the efficiency of the cell. This thesis concerns elementary processes at such surfaces and interfaces, which affect the electronic band structure and, thus, potentially the reactivity of the surface. Using two-photon photoelectron spectroscopy (2PPE), I investigate such processes in three model systems for electrode surfaces and electrolyte/electrode interfaces: ZnO is discussed as material for photoelectrochemical water splitting. In time-resolved 2PPE spectra, I observe oscillations of the (10-10) surface dipole, which are assigned to previously unknown coherent surface phonons. I discuss their generation and develop a method to quantify ultrafast surface dipole changes from the intensity of the secondary electron tail of a 2PPE spectrum. At the D2O/ZnO(10-10) interface, I examine several effects of water adsorption, such as changes of the work function and the coherent surface phonons. Unlike in a previous study, I do not observe surface metallization upon water adsorption. Moreover, there is no clear indication of electron solvation as found at water/metal interfaces. At the DMSO/Cu(111) interface, a model system for the electrolyte/cathode interface in metal-air batteries, I determine the elementary steps of superoxide formation. In the DMSO, small polarons are formed and partly trapped in surface defects on an ultrafast time scale. These trapped electrons can persist for several seconds and react with co-adsorbed O2 to from O2-. Modelling the diffusion yields estimates for the electrode-reactant distance for electron transfer in DMSO.

No description available.

This thesis is concerned with the influence of nanoscale boundaries and interfaces upon the electronic processes that occur within the inorganic semiconductors. Inorganic semiconductor nanowires and their blends with semiconducting polymers have been investigated using state-of-the-art ultrafast optical techniques to provide information on the sub-picosecond to nanosecond photoexcitation dynamics in these systems. Chapters 1 and 2 introduce the theory and background behind the work and present a literature review of previous work utilising nanowires in hybrid organic photovoltaic devices, revealing the performances to date. The experimental methods used during the thesis are detailed in Chapter 3. Chapter 4 describes the crucial roles of surface passivation on the ultrafast dynamics of exciton formation in gallium arsenide (GaAs) nanowires. By passivating the surface states of nanowires, exciton formation via the bimolecular conversion of electron-hole plasma can observed over few hundred picoseconds, in-contrast to the fast carrier trapping in 10 ps observed in the uncoated nanowires. Chapter 5 presents a novel method to passivate the surface-states of GaAs nanowires using semiconducting polymer. The carrier lifetime in the nanowires can be strongly enhanced when the ionization potential of the overcoated semiconducting polymer is smaller than the work function of the nanowires and the surface native oxide layers of nanowires are removed. Finally, Chapter 6 shows that the carrier cooling in the type-II wurtzite-zincblend InP nanowires is reduced by order-of magnitude during the spatial charge-transfer across the type-II heterojunction. The works decribed in this thesis reveals the crucial role of surface-states and bulk defects on the carrier dynamics of semiconductor nanowires. In-addition, a novel approach to passivate the surface defect states of nanowires using semiconducting polymers was developed.

In dieser Arbeit untersuchen wir die ultraschnelle Dynamik von Ladungsträgern und kohärenten Intersubbandpolarisationen in quasi-zweidimensionalen Halbleiternanostrukturen und Halbleiterbauelementen. Insbesondere werden n-Typ modulationsdotierte multiple Quantentöpfe und Quantenkaskadenlaserstrukturen basierend auf dem Materialsystem GaAs/AlGaAs mit der Methode der ultraschnellen Spektroskopie im mittleren Infrarot (3-20 mu) studiert. Ein neuartiger experimenteller Aufbau ist entwickelt worden, der zum ersten Mal das phasen- und amplitudenkontrollierte Formen von ultraschnellen Feldtransienten im mittelinfraroten Spektralbereich erlaubt. Wir untersuchen die Möglichkeit der kohärenten Kontrolle von Intersubbandübergängen. Amplituden- und phasenkonntrollierte Feldtransienten im mittleren Infrarot, die mit unserer neuen Laserquelle erzeugt werden, induzieren resonante Intersubbandanregungen in n-Typ modulationsdotierten GaAs/AlGaAs Quantentöpfen. Die transmittierten elektrischen Feldtransienten werden mit Hilfe des ultraschnellen elektro-optischen Abtastverfahrens gemessen. Unter Anwendung zweier phasengekoppelter Mittinfarotpulse variabler relativer Phase zeigen wir erstmalig die kohärente Kontrolle an linearen Intersubbandpolarisationen mit Dephasierungszeiten unterhalb einer Pikosekunde. Eine Sättigung von mehr als 0.2 wird bei einer Mittinfrarotpulsenergie von nur 1 pJ erreicht. Es wird erstmalig ein direktes, zeitaufgelöstes Experiment an elektrisch betriebenen Quantenkaskadenstrukturen vorgestellt. Diese Untersuchung ermöglicht den Einblick in die Dynamik des Elektronentransports, der mit stationären Methoden nicht meßbar ist. Der ultraschnelle Quantentransport der Elektronen vom Injektor durch die Injektionsbarriere in das obere Lasersubband wird in Femtosekunden-Mittinfrarot-Anreg-Abtast-Experimenten untersucht. Auf diese Weise beobachten wir die ultraschnelle Sättigung und die nachfolgende Wiederherstellung des elektrisch induzierten Gains. Wir beobachten ausgeprägte Gainoszillationen bei angelegtem Vorwärtsstrom und an spektralen Positionen am Gainmaximum. Dies ist ein direkter Beweis für eine kohärente Wellenpaketspropagation vom Injektor in das obere Lasersubband mittels resonantem Tunneln trotz der hohen Ladungsträgerdichte in Quantenkaskadenlasern. Nach der Sättigung ist der elektrisch induzierte Gain bei niedrigen Gitter- und Ladungsträgertemperaturen innerhalb einer Pikosekunde vollständig wiederhergestellt. / In this thesis we investigate the ultrafast dynamics of carriers and coherent intersubband polarizations in quasi-two-dimensional semiconductor nanostructures and devices. In particular, we study n-type modulation doped multiple quantum wells and quantum cascade laser structures based on the GaAs/AlGaAs material system using ultrafast spectroscopy in the mid-infrared spectral range (3-20 mu). A novel experimental setup is developed allowing for the first time the controlled phase and amplitude shaping of ultrafast field transients in the mid-infrared wavelength range. We study the feasibility of coherent nonlinear control of intersubband polarizations. Amplitude and phase-controlled mid-infrared field transients from our new laser source induce resonant intersubband excitations in n-type modulation doped GaAs/AlGaAs quantum wells. The transmitted electric field transients are directly measured by ultrafast electro-optic sampling. We demonstrate for the first time coherent control of linear intersubband polarizations with subpicosecond dephasing times by applying two phase-locked pulses with variable relative phase. A saturation of the intersubband excitation by more than 0.2 is achieved with mid-infrared pulses of only 1 pJ pulse energy. We present for the first time a direct time-resolved experimental study on electrically driven quantum cascade laser structures. These studies provide insight into the dynamics of electron transport, which can not be obtained by stationary measurements. The ultrafast quantum transport of electrons from the injector through the injection barrier into the upper laser subband is investigated in femtosecond mid-infrared pump-probe experiments. In this way we directly monitor the ultrafast saturation and subsequent recovery of electrically induced gain. For forward bias and spectral positions around the gain maximum we observe pronounced gain oscillations. This gives direct evidence for a coherent wave packet motion from the injector into the upper laser subband via resonant tunneling even at the high electron density present in a quantum cascade laser structure. After saturation the electrically induced gain is completely recovered within 1 ps at low lattice and carrier temperatures.

This thesis deals with exciton relaxation processes in thin polycrystalline films and matrix-isolated molecules of the perylene derivatives PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide). Using femtosecond pump-probe spectroscopy, transient absorption spectra, excitonic relaxation in the lowest excited state subsequent to excitation, and exciton-exciton interaction and annihilation at high excitation densities have been addressed. Transient absorption spectroscopy in the range 1.2eV-2.6eV has been applied to thin polycrystalline films of PTCDA and MePTCDI and to solid solutions of PTCDA and MePTCDI molecules (monomers) in a SiO2 matrix. We are able to ascribe the respective signal contributions to ground state bleaching, stimulated emission, and excited state absorption. Both systems exhibit broad excited-state absorption features below 2.0eV, with dominant peaks between 1.8eV and 2.0eV. The monomer spectra can be consistently explained by the results of quantum-chemical calculations on single molecules, and the respective experimental polarization anisotropies for the two major transitions agree with the calculated polarizations. Dimer calculations allow to qualitatively understand the trends visible in the experimental results from monomers to thin films. The broad excited state absorption band between 1.8eV and 2.0eV allows to probe the population dynamics in the first excited state of thin films. We show that excitons created at the Gamma point relax towards the border of the Brillouin zone on a 100fs time scale in both systems. Excitonic relaxation is accelerated by increase of temperature and/or excitation density, which is attributed to stimulated phonon emission during relaxation in k-space. Lower and upper limits of the intraband relaxation time constants are 25fs (resolution limit) and 250fs (100fs) for PTCDA (MePTCDI). These values agree with the upper limit for the intraband relaxation time of 10ps, evaluated from time-resolved luminescence measurements. While the luminescence anisotropy is in full accordance with the predictions made by a luminescence anisotropy model being consistent with the exciton model of Davydov-split states, the pump-probe anisotropy calls for an explanation beyond the models presently available. At excitation densities 10^(19)cm^(-3), the major de-excitation mechanism for the relaxed excitons is exciton-exciton annihilation, resulting in a strongly reduced exciton life time. Three different models for the microscopic behavior have been tested: a diffusion-limited annihilation model in both three and one dimensions (with diffusion constant D as fit parameter) as well as a long-range single-step Förster-type annihilation model (with Förster radius RF as fit parameter). For PTCDA, the latter two, being structurally equivalent, allow to fit a set of multiexponential decay curves for multiple initial exciton densities with high precision. In contrast, the three-dimensional diffusion-limited model is clearly inferior. For all three models, we extract annihilation rates, diffusion constants and diffusion lengths (or Förster radii), for both room and liquid helium temperature. Temperature dependence and orders of magnitude of the obtained parameters D or RF correspond to the expectations. For MePTCDI, the 1D and the Förster model are in good agreement for a smaller interval of excitation densities. For a initial exciton densities higher than 5 x 10^(19)cm^(-3), the 3D model performs significantly better than the other two.