In this thesis, original theoretical and numerical investigations into the interaction of light with excitonic nanostructures are presented, in a bid to demonstrate that excitonic nanostructures are viable alternatives to the use of plasmonic nanostructures where electric field enhancement and confinement are sought. In particular, the field enhancement and confinement around excitonic nanostructures on resonance is shown to be comparable if not in excess of that around noble metal nanoparticles such as gold and silver. These excitonic modes, when set in the context of a core-shell geometry, are shown to offer tunability through nanoparticle design and through the index of the environment. In addition, hybrid `hyperbolic' and `plexcitonic' modes are shown to offer similar properties in metallic-excitonic nanostructures. Altogether, these excitonic and hybrid excitonic modes are shown to have potential in nanophotonic applications.

Les polaritons de cavité sont des quasi-particules, partiellement matière-t partiellement lumière, crées lors du couplage fort d'un exciton et d’un photon de cavité. A une certaine température et densité de particules, les polaritons de cavité peuvent subir une transition de phase de type quasi-Bose-Einstein et condenser dans l'état de plus basse énergie du système; dans ces conditions, la cavité émet de la lumière cohérente et le dispositif associé est appelé laser à polaritons. ZnO est l'un des matériaux les plus adaptés pour la fabrication des lasers à polaritons fonctionnant à température ambiante, en raison de ses excellentes propriétés excitoniques. Cependant, des difficultés techniques ont empêché la réalisation de microcavités à base de ZnO pendant longtemps. Dans cette thèse nous présentons la fabrication de microcavités à base de ZnO par deux approches différentes, ce qui a permis de surmonter les difficultés technologiques existantes et ont permis d'obtenir des figures de mérite avec des valeurs records (pour le facteur de qualité ainsi que pour l’éclatement de de Rabi). Des lasers à polaritons fonctionnant à température ambiante ont été démontré dans les deux cas. Dans la microcavité entièrement hybride, des condensats de polaritons ont été étudiés dans une gamme de désaccord exciton-photon sans précédents, et de basse température à température ambiante; ceci a permis d'obtenir, pour la première fois, un diagramme de phases complet. Cette thèse ouvre la voie à une polaritonique appliquée fonctionnant à température ambiante. / Cavity polaritons are quasi-particles, partially light partially matter, resulting from the strong-coupling of an exciton and a cavity photon. At a certain temperature and particle density, cavity polaritons can go through a quasi-Bose-Einstein phase transition and condense at the lowest energy state of the system; in this situation the cavity emits coherent light and the associated device is termed polariton laser. ZnO is one of the most adapted materials for fabricating room temperature polariton lasers, due to its excellent excitonic properties. However, technical difficulties have been preventing the achievement of ideal ZnO microcavities for a long time. In this thesis we report the fabrication of high quality ZnO microcavities with two different approaches, which overcome the existing technical challenges and allow to achieve a record cavity quality factor and large Rabi splittings. Room temperature polariton lasing has been demonstrated in both cases. In a fully-hybrid ZnO microcavity, polariton condensates were studied within an unprecedented range of exciton-photon detunings, and from low to room temperature. This tunability has enabled to obtain, for the first time, a complete condensation phase diagram for ZnO-based microcavities, wherein the exciton fraction of the polaritons has been tuned between 17% to 96%, corresponding to a modification of the exciton-polariton mass, its lifetime and its interaction constant by 1 order of magnitude. This thesis paves the way for implementing a polariton-based technology operating at room-temperature.

Diese Arbeit beschäftigt sich mit der Untersuchung der starken Kopplung und Laseremission in Strukturen, die ZnO, ZnCdO oder organische Moleküle als aktives Material enthalten. Die ZnCdO basierten Vielfachquantengräben erreichen ihre Laserschwelle durch optische Ruckkopplung an streuenden Luftlöchern. Diese Emitter nennt man random laser. Die Dynamik ihrer Emission unter quasi-stationären Bedingungen ist der hier gezeigte Fokus. Hoch reproduzierbare Anregungen werden verwendet um sowohl die Dynamik eines einzelnen Beschusses aber auch die Unterschiede verschiedener Anregungen zu untersuchen. Die experimentellen Daten werden durch numerische Simulation qualitativ reproduziert und mit Methoden der Netzwerktheorie interpretiert. Die verbreitetere optische Rückkopplung durch einen Resonator wird in der Untersuchung des Moleküls L4P und seiner Spiro-derivate benutzt. Zwei identische Braggspiegel umschließen die aktive Schicht aus L4P-SP2, das in eine Polymermatrix eingebettet ist, eine Dicke von 12 Mikrometer hat und in einer einzelnen Mode lasert (schwache Kopplung). Durch Verringerung der aktiven Schicht auf die Hälfte der Resonanzwellenlänge wird das System in den Bereich der starken Kopplung gebracht. Eine Rabi-Aufspaltung von 90 meV wird zu beiden vibronischen Resonanzen beobachtet. Die energetische Position in Resonanz zu ZnO macht dieses Molekül zu einem guten Kandidaten für die Fertigung einer hybriden Mikrokavität im Bereich der starken Kopplung. Dies wurde in einer teilweise epitaktisch gewachsenen Mikrokavität angewandt, die aus einem ZnMgO basierten Braggspiegel und sechs Quantengräben besteht. Darauf folgt eine aufgeschleuderte Schicht von L4P in der Polymermatrix. Der Resonator wird mit einem dielektrischen Spiegel fertiggestellt. Tieftemperatur Reflektion zeigt eine deutlichen ausweichen und eine gleichverteilte Mischung der drei Resonanz im mittleren Polaritonzweig. / This thesis presents the investigation of strong coupling and lasing in structures using ZnO, ZnCdO or organic molecules as active material. The ZnCdO based multi quantum well structures reach the lasing threshold by using scattering at air holes as the optical feedback. Such emitters are called random lasers. The dynamics of their emission under quasi-stationary condition is the point of interest presented. Highly reproducible excitations are used to investigate the single shot dynamics and their shot to shot differences. The experimental data is qualitatively reproduced by numerical simulation and interpreted by means of network theory. The more common optical feedback by a cavity is applied in the investigation of the molecule L4P and its spiro-derivatives. Using two identical SiO2/ZrO2 based Bragg reflectors surrounding an active layer of L4P-SP2 in a polymer matrix of approximately 12 microns thickness reached single mode lasing (weak coupling). Reducing the active layer thickness to half the resonance wavelength pushes the system into the strong coupling regime. Angular resolved reflectivity shows the anticrossing of the tuned cavity resonance to two vibronic transitions of the molecule. The Rabi-splitting to both vibronic resonances reaches around 90 meV. The energetic position in resonance to ZnO makes this molecule a promising candidate for a hybrid inorganic/organic microcavity in the strong coupling regime. This is used in a partially epitaxially grown microcavity composed of a ZnMgO based Bragg reflector (alternating layers of different Mg content) and six quantum wells. This is followed by a spincoated layer of L4P in a polymer matrix. The cavity is finished by a dielectric mirror. Low temperature reflectivity shows a clear anticrossing reaching an equal mixing of all resonances for the middle branch.

Exciton-polaritons are the fundamental excitations arising from the strong coupling of quantum well excitons and cavity photons in semiconductor microcavities. They are compound bosons for which stimulated scattering and macroscopic occupation of single quantum states can occur at sufficiently high densities. One way of creating such polariton condensates is with nonresonant optical pumping. Doing so creates a large density of free- carriers and excitons that strongly interact and blueshift the polariton energy levels. Using spatially patterned nonresonant fields, the polariton potential landscape can be tailored and optically trapped condensates can be created. This thesis shows that the spin properties of polariton condensates are strongly modified by such trapping. Under linearly polarised pumping, helicity can spontaneously develop at a critical occupation, breaking the parity symmetry. This formation of spin-up/spin-down condensates is explained within a Gross-Pitaevskii model which accurately reproduces the influence of electric fields and condensate density. Under elliptically polarised pumping, two phenomena are observed: the formation of condensates with the opposite handedness to the pump and hysteresis of both occupation and spin with respect to pump power. The spatial dependence of these effects highlights the limitations of commonly used models of polariton condensation. Finally, the suitability of patterned optical fields for the creation of polariton lattices is explored. For small chains of condensates, controllable coupling between adjacent spins is demonstrated, with the formation of antiferromagnetic and ferromagnetic domains. The extent of these domains is strongly affected by sample nonuniformity, fundamentally limiting the scalability of these lattices.

This thesis is devoted to the study of excitations in atomic and polariton Bose-Einstein condensates (BEC). These two specimens are prime examples for equilibrium and non equilibrium BEC. The corresponding condensate wave function of each system satisfies a particular partial differential equation (PDE). These PDEs are discussed in the beginning of this thesis and justified in the context of the quantum many-body problem. For high occupation numbers and when neglecting quantum fluctuations the quantum field operator simplifies to a semiclassical wave. It turns out that the interparticle interactions can be simplified to a single parameter, the scattering length, which gives rise to an effective potential and introduces a nonlinearity to the PDE. In both cases, i.e. equilibrium and non equilibrium, the main model corresponding to the semiclassical wave is the Gross-Pitaevskii equation (GPE), which includes certain mathematical adaptions depending on the physical context of the consideration and the nature of particles/quasiparticles, such as additional complex pumping and growth terms or terms due to motion. In the course of this work I apply a variety of state-of-the-art analytical and numerical tools to gain information about these semiclassical waves. The analytical tools allow e.g. to determine the position of the maximum density of the condensate wave function or to find the critical velocities at which excitations are expected to be generated within the condensate. In addition to analytical considerations I approximate the GPE numerically. This allows to gain the condensate wave function explicitly and is often a convenient tool to study the emergence of excitations in BEC. It is in particular shown that the form of the possible excitations significantly depends on the dimensionality of the considered system. The generated excitations within the BEC include quantum vortices, quantum vortex rings or solitons. In addition multicomponent systems are considered, which enable more complex dynamical scenarios. Under certain conditions imposed on the condensate one obtains dark-bright soliton trains within the condensate wave function. This is shown numerically and analytical expressions are found as well. In the end of this thesis I present results as part of an collaborative effort with a group of experimenters. Here it is shown that the wave function due to a complex GPE fits well with experiments made on polariton condensates, statically and dynamically.

No description available.

We report on the investigation and observation of Bloch surface wave polaritons, resulting from the interaction between excitons in ZnO and a Bloch surface wave supported by a distributed Bragg reflector (DBR), for temperatures up to 430 K. The samples were fabricated using pulsed laser deposition and consist of a DBR made of 6.5 layer pairs of yttrica-stabilized zirconia and Al2O3 with a ZnO surface layer. We measured the reflectivity of transverse electric modes using a SiO2 prism in Kretschmann–Raether configuration, giving access to high in-plane momenta. Whereas the lower polariton branch was clearly observable, the upper polariton branch was not visible, due to the strong absorption in ZnO above the excitonic resonance. By employing a coupled oscillator model for the interaction between the bare surface mode and exciton, we derived a corresponding Rabi splitting between 100–192 meV at 294 K, which decreases with increasing temperature.

In dieser Arbeit werden Untersuchungen an dem transparenten p-Typ-Halbleiter Kupferiodid (CuI) und den ternären Verbindungshalbleitern CuBrxI1-x und AgxCu1-xI vorgestellt. Im ersten Teil der Arbeit liegt der Fokus auf der Untersuchung der dielektrischen Funktion und der optischen Übergänge dieser Materialien. Basierend auf der Kombination von experimentellen Daten und theoretischen Bandstrukturrechnungen werden die beobachteten optischen Übergänge bestimmten elektronischen Übergängen in der Brillouin-Zone zugeordnet. Die optischen Eigenschaften der ternären Legierungen CuBrxI1-x und AgxCu1-xI , insbesondere die exzitonischen Übergangsenergien am Γ-Punkt werden präsentiert. Es wird gezeigt, dass die Bandlückenenergie beider Legierungen ein nichtlineares Verhalten als Funktion der Legierungszusammensetzung aufweist, welches bei tiefen Temperaturen durch einen quadratischen Krümmungsparameter im Bereich von 0.49 eV bis 0.54 eV beschrieben werden kann. Der Ursprung des beobachteten Krümmungsverhaltens wird im Hinblick auf verschiedene Beiträge wie Bindungslängenänderungen und Ladungsumverteilungseffekte diskutiert. Der Einfluss der Legierungszusammensetzung auf die Spin-Bahn-Aufspaltung wird diskutiert, wodurch Rückschlüsse auf die p-d-Hybridisierung der obersten Valenzbänder am Γ-Punkt ermöglicht werden. Es wird gezeigt, dass der Beitrag der metallischen d-Orbitale zum Valenzbandmaximum mit steigendem Br-Gehalt zunimmt, während mit steigendem Ag-Gehalt das Gegenteil der Fall ist. Des Weiteren wird die Temperaturabhängigkeit der Bandlücke für AgxCu1-xI Legierungen in Abhängkeit der Legierungszusammensetzung präsentiert. Es wird gezeigt, dass die Temperaturabhängigkeit der Bandlückenenergie mit steigendem Ag-Gehalt abnimmt, was hauptsächlich auf die Änderung des Beitrags der thermischen Gitterausdehnung zurückzuführen ist. Im zweiten Teil dieser Arbeit werden die bandkantennahen Emissionseigenschaften von CuI Einkristallen vorgestellt. Die Ergebnisse unterstreichen die Notwendigkeit, Exziton-Polariton-Effekte bei der Interpretation der spektralen Linienform der Emissionsspektren und ihrer zeitlichen Entwicklung nach optischer Anregung zu berücksichtigen. Das unterschiedliche transiente Emissionsverhalten der freien und gebundenen Zustände wird durch deren gekoppelte Wechselwirkung erklärt, was eine Abschätzung der Defektdichte im Material ermöglicht. Die Zunahme der Lebensdauer der freien Exziton-Polaritonen mit steigender Temperatur bis etwa 150K deutet auf eine vorwiegend strahlende Rekombination in diesem Temperaturbereich hin, während die Abnahme der Zerfallszeiten bei noch höheren Temperaturen auf eine Zunahme der nicht-strahlenden Rekombinationskanäle hindeutet.:I Introduction 1 II Basics of the Cumulative Part 7 1 Physical properties and concepts . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.1 Copper and silver halides . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2 Dielectric function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3 Exciton-polaritons in semiconductors . . . . . . . . . . . . . . . . . . 17 2 Experimental and computational techniques . . . . . . . . . . . . . . . . . . 21 2.1 Sample preparation techniques . . . . . . . . . . . . . . . . . . . . . . 21 2.2 Structural and electrical characterization . . . . . . . . . . . . . . . . 24 2.3 Optical spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4 Density functional theory . . . . . . . . . . . . . . . . . . . . . . . . . 33 III Cumulative Part 35 [C1] Dielectric function of CuBrxI1-x alloy thin films . . . . . . . . . . . . . . . . . 37 [C2] Epitaxial growth of AgxCu1-xI on Al2O3 (0001) . . . . . . . . . . . . . . . . . 56 [C3] Optical properties of AgxCu1-xI alloy thin films . . . . . . . . . . . . . . . . . 64 [C4] Dynamics of exciton-polariton emission in CuI . . . . . . . . . . . . . . . . . 81 IV Summary and Outlook 95 V Publications 101 1 Publications in the basics of the cumulative part . . . . . . . . . . . . . . . . 103 2 Publications forming the cumulative part with contribution by the author . . 119 3 Further publications with contribution by the author . . . . . . . . . . . . . . 120 List of Abbreviations 121 Author Contributions 123 Zusammenfassung nach §11 (4) der Promotionsordnung der Fakultät für Physik und Geowissenschaften der Universität Leipzig 129 / The present work presents investigations on the transparent p-type semiconductor copper iodide (CuI) and the related ternary compound semiconductors CuBrxI1-x and AgxCu1-xI . The first part of the thesis focuses on the study of the dielectric function and related optical transitions of this family of materials. Based on the combination of the experimental data and first principles band structure calculations the observed optical transitions are assigned to specific electronic transitions in the Brillouin zone. The optical properties of ternary CuBrxI1-x and AgxCu1-xI alloys and in particular the excitonic transition energies at the Γ-point as a function of the alloy composition are discussed in detail. It is shown that the bandgap energy of both alloys exhibits non-linear behavior as a function of alloy composition, which can be described by a quadratic bowing parameter in the range of 0.49 eV and 0.54 eV at low temperatures. The origin of the observed bowing behavior is discussed in terms of various physical and chemical contributions, such as changes in bond lengths and charge redistribution effects. The influence of the alloy composition on the spin-orbit splitting energy Δ0 is also discussed in detail, allowing conclusions to be draw down about the p-d hybridization of the top valence bands at the Γ-point. It is shown that the contribution of the metal d orbitals to the valence band maximum increases with increasing Br-content, while the opposite is true for increasing Ag incorporation. In addition to the transition energies at low temperatures, the temperature dependence of the bandgap for AgxCu1-xI alloys as a function of alloy composition is presented. It is shown that the temperature dependent energy shift of the bandgap decreases with increasing Ag content, which is mainly due to the change in the contribution of lattice thermal expansion. In the second part of this work, the properties of the near-band edge emission of CuI single crystals are presented. The obtained results highlight the importance of considering the exciton-polariton effects when interpreting the spectral line shape of the emission peaks as well as their temporal behavior after an optical excitation. The different transient decay characteristics of the free and bound states are explained by their coupled interaction, allowing an approximation of the defect density in the material. With increasing temperature, an increase in the lifetime of the free exciton polaritons was observed up to ca. 150K, indicating mainly radiative recombination in this temperature range. The observed decrease in the decay time at higher temperatures indicates a rapid increase in non-radiative recombination channels.:I Introduction 1 II Basics of the Cumulative Part 7 1 Physical properties and concepts . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.1 Copper and silver halides . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2 Dielectric function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3 Exciton-polaritons in semiconductors . . . . . . . . . . . . . . . . . . 17 2 Experimental and computational techniques . . . . . . . . . . . . . . . . . . 21 2.1 Sample preparation techniques . . . . . . . . . . . . . . . . . . . . . . 21 2.2 Structural and electrical characterization . . . . . . . . . . . . . . . . 24 2.3 Optical spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4 Density functional theory . . . . . . . . . . . . . . . . . . . . . . . . . 33 III Cumulative Part 35 [C1] Dielectric function of CuBrxI1-x alloy thin films . . . . . . . . . . . . . . . . . 37 [C2] Epitaxial growth of AgxCu1-xI on Al2O3 (0001) . . . . . . . . . . . . . . . . . 56 [C3] Optical properties of AgxCu1-xI alloy thin films . . . . . . . . . . . . . . . . . 64 [C4] Dynamics of exciton-polariton emission in CuI . . . . . . . . . . . . . . . . . 81 IV Summary and Outlook 95 V Publications 101 1 Publications in the basics of the cumulative part . . . . . . . . . . . . . . . . 103 2 Publications forming the cumulative part with contribution by the author . . 119 3 Further publications with contribution by the author . . . . . . . . . . . . . . 120 List of Abbreviations 121 Author Contributions 123 Zusammenfassung nach §11 (4) der Promotionsordnung der Fakultät für Physik und Geowissenschaften der Universität Leipzig 129

Les excitons-polaritons sont des particules mixtes de lumière et de matière. Ils peuvent être le futur des applications optoélectoniques, en vertu de leur réponse optique non-linéaire qui est extrêment forte. Cette thèse est consacrée aux effets non-linéaires et aux applications variées des excitons-polaritons dans les nanostructures à base de semi-conducteurs. Les microcavités planaires et les polaritons 2D sont étudiés dans les premiers chapitres, alors que le dernier chapitre est consacré à l'étude du système de boites quantiques en cavité (polaritons 0D). L'oscillateur paramétrique et la bi stabilité sont le sujet de la première partie de la thèse. Une approche mathématique intermédiaire entre les approches semi-classiques et purement cohérente est présentée. L'impact des fluctuations proches du seuil de bi stabilité est étudié. La deuxième partie est consacrée à la présentation de différentes applications basées sur les propriétés des polaritons. Un laser à polaritons basé sur une cavité de ZnO est modélisé et les résultats soulignent les avantages de l'utilisation de ce matériau pour la réalisation de ce type d'application à température ambiante. La structure de spin particulière des polaritons est par la suite utilisée pour proposer deux nouvelles applications. La première est un analogue optique du transistor de spin pour les électrons, appelé transistor Datta et Das. La deuxième propose d'utiliser le comportement chaotique d'une jonction Josephson polaritonique afin d'implémenter un système de crytage chaotique d'un signal. Le dernier chapitre est consacré aux polaritons 0D. Nous montrons comment la réalisation du régime de couplage fort permet de réaliser une source de photons intriqués basée sur le déclin du bi exciton dans une boite quantique en résolvant un certain nombre de difficultés par rapport au système constitué d'une boite quantique simple.

In this work we studied the effects of coherent laser in a system formed by a quantum dot (QD) in a nanocavity. Initially we investigated the case where the energy of an éxciton are in resonance with the cavity mode and we vary the frequency of the laser. During our simulation we have used pulsed and continuos laser. We consider only the éxciton state of the QD, and in situations where both the laser and the cavity mode are close to resonance, allowing us to use the rotating wave approximation. The QD was treated as a two-level system and the cavity containing a maximum of four photons. For this study we used the Jaynes-Cummings model with an extra term related to the external pulse, which was used to manipulate quantum states. This was done with the help of diagonalization of the approximated Hamiltonian and searching for anticrossings in its energy spectrum. Then we use a continuous laser and the technique of calculating the average occupational each state to do a better mapping of the parameter values required to make the manipulation of quantum states. We found that the system can be manipulated using the process of two or more photons, and the energies of these photons could be estimated using a mapping procedure developed in this dissertation. / Neste trabalho estudamos os efeitos da incidência de um laser coerente num sistema formado por um ponto quântico (PQ) no interior de uma nanocavidade. Inicialmente investigamos o caso em que a energia de um éxciton formado no PQ está em ressonância com um modo da cavidade e variamos a frequência do laser incidente. Usamos laser contínuo e pulsado em nossas simulações. O ponto quântico foi tratado como um sistema de dois níveis e consideramos que a cavidade contém no máximo quatro fótons. Para este estudo foi utilizado o modelo de Jaynes-Cummings com acréscimo de um termo relacionado com o pulso externo usado para manipular os estados quânticos, que foi feito com o auxílio da diagonalização do Hamiltoniano aproximado e procurando por anticruzamentos no seu espectro de energia. Consideramos apenas o estado de éxciton no PQ, e situações em que tanto o laser como o modo da cavidade estão próximos à energia de ressonância, o que nos permitiu usar a aproximação de ondas girantes. Utilizamos um laser contínuo e calculamos a média ocupacional de cada estado para fazer um melhor mapeamento dos valores dos parâmetros necessário para realizar a manipulação dos estados quânticos. Encontramos que o sistema pode ser manipulado usando processo de dois ou mais fótons, sendo que as energias destes fótons podem ser estimadas usando um procedimento de mapeamento desenvolvido nesta dissertação. / Mestre em Física