Dissertations / Theses on the topic 'Microcavities'

This thesis describes the development and implementation of fibre-coupled, micron-scale optical resonators for the detection and manipulation of neutral atoms. The resonators are intended for integration with atom chips. The latter are microfabricated devices which enable the cooling, trapping, gUiding and manipulation of atoms by means of optical, magnetic and electric fields. The fields are generated in part using micro-fabricated features on the surface of the chips. Optical cavities are among the most important tools in the study of the interactions between light and matter. They allow the observation of fundamental processes in quantum optics, based on the enhanced coupling of atomic transitions to light fields. Our resonators have mode volumes which are two orders of magnitude smaller than those used in typical cavity quantum electrodynamics experiments. Together with their high quality factors, this leads to large enhancement factors, rendering them ideal for the detection and manipulation of atoms on chips. They are scalable and directly fibre-coupled, both of which are qualities of interest for their implementation in quantum information-processing applications. In the thesis, the optical characteristics of the resonators are explained, as well as the basic principles of the interaction of atoms with their light field. The setup used for the test implementation of the devices is presented, together with early experimental results. These include the detection of atoms via their effect on the cavity reflection spectrum, and the detection of enhanced atomic fluorescence into the cavity mode. The thesis concludes with an outlook on further experimentation, possible improvements of the devices themselves, and a view on their integration with existing atom chip technology.

The merging fields of photonics and organic electronics into organic optoelectronics has created a surge of enthusiasm over the possibility of developing low-cost and large-area advanced optoelectronic systems. These applications can combine the best functionalities of both fields, such as tailoring the organic semiconductors by chemical means, engineering the structure in which organic materials are embedded in, are to name a few. These advances have stimulated the excitement over the next generation of optoelectronic systems with enhanced capabilities and low-cost manufacturing processes compared to their inorganic counterparts. Such technology direction is mainly reflected by the high investments towards the aim of developing flexible, and roll-to-roll organic light-emitting diodes and organic solar cells. Interestingly, more sophisticated applications require a deeper understanding of the underlying mechanisms at play that merge concepts from the fields of photonics and organic electronics. Particularly, organic light-emitting diodes (OLEDs) under certain constraints (such as cavity light confinement, strong exciton-photon interaction) exhibit modified spectral emission compared to OLED devices that are not bounded by the same conditions. The introduction of the polariton concept as a quasi-particle, which is part-light and part-matter, has emerged to describe such new physical phenomena caused by this photon-exciton intricate interaction. Polariton physics is well established in inorganic semiconductors were a plethora of physical phenomena have been demonstrated, such as the appearance of Bose-Einstein Condensation or low-threshold laser devices. The later is what has as yet to be demonstrated from the field of solid state physics utilising organic semiconductors. This thesis is focused on the study of the physics and the engineering of organic light-emitting diodes that will aid in the realization of efficient organic polariton LEDs. The main body of work examines various organic semiconductor materials in their ability to reach the strong light and matter interaction regime and, subsequently, to be used in OLEDs as the emissive component. Furthermore, a degradation investigation highlights the issues that affect small-molecule based OLEDs, and finally, the possible pathways for achieving efficient polariton OLEDs are discussed.

In this work the interactions of carriers, electrons and holes, and photons in a semiconductor microcavity are studied in the perturbative and the nonperturbative regimes. In the perturbative regime, modification of the spontaneous emission rate of carriers by a semiconductor microcavity is investigated with 100-nm-thick bulk GaAs. Reabsorption makes the cavity-mode photoluminescence (PL) decay much faster than the square of the carrier density. Here reabsorption distortion is avoided by collecting PL that escapes the microcavity directly without multiple reflections using a ZnSe prism glued to the top mirror. Removal of most of the bottom mirror decreases the true carrier decay rate by only ≈25%, showing that the large enhancements deduced from cavity-mode PL are incorrect. A fully quantum mechanical computation including guided modes corroborates this conclusion. The prism technique could be used to study carrier dynamics and competition between guided and cavity modes in microcavities below and near threshold. In the nonperturbative regime, normal mode coupling (NMC) between the quantum-well excitonic susceptibility and photons is studied. In cw linear experiments, the effects of varying cavity finesse and exciton absorption linewidth and line shape and their contributions to the linewidth of NMC peaks are investigated and compared with the experiments. It is shown that all of the observed experimental features can be explained by a linear dispersion theory model that incorporates the experimental excitonic absorption spectrum of the quantum well. Some nonlinear features of NMC obtained from time-resolved measurements are also studied and discussed.

Semiconductor microcavities have emerged to present abundant opportunities for both device applications and basic quantum optics studies. Here we investigate several aspects of the cw and ultrafast optical response of semiconductor quantum well microcavities. The interaction of a high-finesse semiconductor microcavity mode with a quantum well (QW) exciton leads to normal mode coupling (NMC), where a periodic energy exchange develops between exciton and photon states, appearing as a double peak in the cavity transmission spectrum and a beating in the time resolved signal. The nonlinear saturation of the excitonic NMC leads to a reduction of the modulation depth of the NMC oscillations and corresponding transmission peaks with little change in oscillation period or NMC splitting. This behavior arises from excitonic broadening due to carrier-carrier and polarization scattering without reduction of the oscillator strength. The nonlinear NMC microcavity luminescence exhibits three excitation regimes, from reversible normal mode coupling, through an intermediate double-peaked emission regime, to lasing. The nonlinear PL spectrum is governed by density-dependent changes in both the bare QW emission and in the microcavity transmission. The temporal evolution of the microcavity emission is analogous to the density-dependent behavior, and can be attributed to a time-dependent carrier density which results from a combination of carrier cooling and photon emission. A strong magnetic field applied perpendicular to the plane of a QW confines electrons and holes to Landau orbits in the QW plane, transforming the QW into a quantum dot (QD) whose radius shrinks with increasing magnetic field strength. This strong magnetic confinement enhances the normal mode coupling strength in the microcavity via an increase in exciton oscillator strength. The time-resolved stimulated emission of a QW microcavity which has been transformed to a QD laser by magnetic confinement reveals a fast relaxation which is uninhibited by the magnetic field, indicating the absence of a phonon bottleneck. As a novel manifestation of cavity-modified emission, we demonstrate synchronization of the stimulated emission of a microcavity laser to the electron spin precession in a magnetic field, achieved by modulating the optical gain for the circularly polarized emission via the Larmor precession. The oscillating laser emission is locked to the completely internal electron spin precession clock, and the GHz oscillation frequencies depend only on the magnetic field strength and the QW material parameters.

Cette thèse est consacrée aux propriétés des exciton-polaritons, les particules mixtes formées à partir de la lumière et la matière dans les microcavités de semi-conducteurs dans le régime de couplage fort. D'abord, j'analyse la possibilité de condensation de Bose des exciton- polaritons à température ambiante dans les microcavités de GaN avec les équations de Boltzmann semi-classiques. Puis les effets de polarisation dans le régime d'oscillateur paramétrique sont étudiés avec les équations de Boltzmann semi-classiques avec pseudospin. La deuxième partie de la thèse est consacrée aux propriétés des condensats et modes macrooccupés des exciton-polaritons. Leur polarisation, dispersion des excitations, propagation, localisation et superfluidité sont décrits avec l'équation de Gross-Pitaevskii

Semiconductor micro- and nano-cavities are excellent platforms for experimental studies of optical cavities, lasing dynamics, and cavity Quantum Electrodynamics (QED). Common materials for such experiments are narrow bandgap semiconductor materials with well-developed epitaxial growth technologies, such as GaAs and InP, among others. Gallium nitride (GaN) and its alloys are industrially viable materials with wide direct bandgaps, low surface re-combination velocities, and large exciton binding energies, offering the possibility of room temperature realization of light-matter interaction. Controlling light-matter interaction is at the heart of nanophotonic research which leads to ultra-low threshold lasing, photonic qubits, and optical strong coupling. Technologically, due to its blue emission, GaN photonic cavities with indium gallium nitride (InGaN) active mediums serve as efficient light sources for the fast growing photonic industry, optical computing and communication networks, display technology, as well as quantum information processing. The main challenges in fabricating high quality GaN cavity are due to its chemical inertness and low material quality as a result of strain-induced defects and threading dislocations. In this dissertation, I examine the designs, novel fabrication processes, and characterizations of high quality factor GaN microdisk and photonic crystal nanobeam cavities with different classes of InGaN active medium, namely quantum dots (QDs), quantum wells (QWs), and fragmented quantum wells (fQWs), for investigating light-matter interaction between cavity and these active media. This dissertation is carefully organized into four chapters. Chapter 1 outlines the background of the research, the materials and growth, and the necessary technique Photoelectrochemical (PEC) etching which is uniquely used to undercut and suspend GaN cavities. Chapter 2 outlines the fabrications, optical experiments, and tuning technique developed for GaN/InGaN microdisks. Microdisks are circular resonant cavities that support whispering gallery modes. Through the use of optimized dry etching and PEC, high quality factor microdisks with relatively small modal volume are fabricated with immediate demonstration of low threshold lasing. On the path to achieving light and matter interactions, irreversible tuning of the cavity mode of p-i-n doped GaN/InGaN microdisks is achieved through photo-excitation in a water environment. Such a technique paves the way for deterministically and spectrally matching the cavity mode to the emitter’s principle emission. Chapter 3 outlines the work done on the high quality GaN photonic crystal nanobeams with InGaN QDs and fQWs. The fragmented nature of the fQW layer has a surprisingly dramatic influence on the lasing threshold. A record low threshold is demonstrated that is an order of magnitude lower in threshold than identical nanobeams with homogeneous QW, and comparable to the best devices in other III-V material systems. As an active medium with greater carrier confinement than quantum wells, and higher carrier capture probability than quantum dots, the fQW active medium, in combination with the nanobeam cavity with ultra-small modal volume and high quality factor, provides an ideal means of probing the limits of light and matter interactions in the nanoscale. Moreover, GaN/InGaN nanopillars are fabricated to isolate a single InGaN QD for understanding its emission properties. Antibunching is observed, demonstrating the quantum nature of the QD emission. Gas tuning is attempted on GaN nanobeams with InGaN QDs to achieve QD-cavity mode coupling and to demonstrate cavity enhanced single photon emission. Last but not least, Chapter 4 concludes the dissertation with summary and future directions.
Engineering and Applied Sciences - Applied Physics

This thesis deals with the use of microcavity resonators for the control of light in organic active materials. In addition to the vertical confinement provided by highly reflecting mirrors in a vertical cavity surface emitting laser (VCSEL), in-plane patterning facilitates additional ways to manipulate the cavity dispersion and enables the observation of novel photonic modes in highly confined systems and an improved performance of organic solid state lasers. Furthermore, organic microcavities are employed for efficient spectrally sensitive photodetection in the near infrared. In microcavities comprising two dielectric distributed Bragg reflectors sandwiching an organic active blend of the matrix molecule Alq3 and the laser dye DCM, optically pumped lasing is investigated, exhibiting a broad spectral tunability over 90 nm due to the large gain bandwith of the laser dye. To directly influence the microcavity dispersion, different interlayers are introduced into the system, facilitating a red-shift of the cavity resonance due to the formation of Tamm-plasmon-polariton states (when using plasmonic Ag interlayers) or an increase of the optical cavity thickness (when using non-absorbing layers such as SiO2). Both concepts are explored and enable strong spectral shifts on the order of 10 meV-100 meV when using interlayers of only few tens of nm in thickness. In order to enhance the optical quality of metal-organic microcavities, the growth of noble metal layers on top of organic films can be improved by the use of diffusion barriers, stopping the diffusion of metal atoms into the organics, and seed layers which provide an improved surface wetting. Both concepts in total lead to an enhancement of the quality factor of such devices by a factor of two. The manipulation of the cavity resonance using different interlayers provides the ability to structure the photon energy landscape in the device plane on the microscale. Using photolithography, photonic wires and dots are fabricated to laterally restrict the photons in potential wells, leading to the observation of discretised energy spectra in two and three dimensions. To facilitate an in-depth investigation, dispersion tomography is utilised and yields the angle resolved emission of multi-dimensionally confined photons in all directions. In metal-organic photonic dots and triangular wedges, such three-dimensional trapping is exploited to reduce parasitic modes, leading to reduced thresholds of an organic microlaser by one order of magnitude. Complex transversal modes are observed in the device emission as a result of the strong lateral confinement that is achieved by such patterning. The manipulation of the photon energy landscape can not only be utilised for enhanced confinement but also for the introduction of photonic lattices. By adding periodic stripes of either Ag or SiO2 into an organic microcavity, an optical Kronig-Penney potential is realised, directly showing the formation of photonic Bloch states in the microcavity dispersion. Utilising a modified Kronig-Penney theory, photons are assigned a polarisation-dependent effective mass, facilitating a quantitative allocation of calculated and observed modes and explaining the emergence of zero and pi-phase coupling of spatially extended supermodes. Finally, by utilising an two-beam excitation geometry, direct control over lasing from multiple discretised states can be exerted, enabling spectral and angular tunability of devices on the microscale. In an alternative concept, a full microcavity stack is deposited onto a periodic grating which couples the waveguided (WG) modes in the active cavity layer to the vertical emission. Coherent interaction between linear WG and parabolic vertical modes is indicated by anti-crossing points where the dispersion of both overlaps. In this hybrid system, novel lasing modes arise not only at the position of the VCSEL parabola apex but also at points of hybridization, showing a drastically enhanced in-plane spatial coherence of at least 50 micrometer. Finally, the concept of organic microcavities is applied towards efficient and spectrally sensitive photodetectors. Making use of the intermolecular charge transfer (CT) state in donor-acceptor blends of organic solar cells, the strong field enhancement of a microcavity is exploited to significantly increase the external quantum efficiency of the initially weak CT absorption at resonance. Consequently, near-infrared photodetection is enabled by cavity-enhanced CT state absorption, leading to devices showing competitive specific detectivities without the need of an external voltage and an EQE above 20% (18% at 950 nm) with a full width at half maximum of significantly below 50 nm. The detectors are shown to be tunable in a broad spectral range via the angular dispersion of the optical microcavity or a thickness variation of the electron and hole transport layers in the solar cell. These findings not only facilitate interesting applications but also enable the direct excitation and observation of the CT state that is integral to the working principles of organic solar cells
Die vorliegende Dissertation beschäftigt sich mit der Kontrolle über Emission und Absorption organischer aktiver Materialien mittels Mikrokavitätsresonatoren. Zusätzlich zum vertikalen Einschluss der Photonen zwischen hochreflektierenden Spiegeln in oberflächenemittierenden Mikrokavitäten (VCSEL, s.o.) werden Strukturierungen in der Bauteilebene hinzugefügt, um eine direkte Manipulation der Photonendispersion zu ermöglichen. Resultierend aus diesen Ergebnissen sind die Beobachtung neuartiger photonischer Moden sowie verbesserte Betriebseigenschaften von organischen Festkörperlasern. Desweiteren wird das Konzept der organischen Mikrokavität zur effizienten und spektral sensitiven Detektion von Nahinfrarot-Photonen angewendet. In Mikrokavitäten aus zwei dielektrischen Bragg-Spiegeln (DBR), welche eine organische aktive Schicht aus dem Matrixmaterial Alq3 und dem Laserfarbstoff DCM einschliessen, wird optisch gepumptes Lasing beobachtet. Dabei ist die Emission spektral über einen weiten Bereich von 90 nm stufenlos einstellbar, was durch die hohe optische Gewinnbandbreite des Laserfarbstoffs ermöglicht wird. Um die Dispersion von Photonen in Mikrokavitäten direkt beeinflussen zu können, werden verschiedene Zwischenschichten in den Laser eingebracht, welche eine Rotverschiebung der Emission nach sich ziehen. In metall-organischen Kavitäten kann dieser Effekt durch die Bildung von Tamm-Plasmon-Polariton Quasiteilchen erklärt werden, die durch die Interaktion der optischen Moden mit den Plasmonen in einer dünnen Silberschicht entstehen. Alternativ werden nichtabsorbierende SiO2-Zwischenschichten eingefügt, welche die optische Kavitätsdicke vergrössern und ähnliche starke Rotverschiebungen der Emission von 10 meV-100 meV nach sich ziehen. Um die optische Qualität metall-organischer Kavitäten zu verbessern, wird das Wachstum der edlen Ag-Schicht auf amorphen organischen Schichten mithilfe von Diffusionsbarrieren und Keimschichten kontrolliert. Die Kombination beider Konzepte ermöglicht eine Verbesserung des Qualitätsfaktors solcher Bauteile um den Faktor 2. Durch die Manipulation der Photonendispersion mithilfe dielektrischer und plasmonischer Zwischenschichten wird eine Strukturierung der photonischen Potentiallandschaft in der Bauteilebene auf Mikrometer-Skala ermöglicht. Mittels Photolithographie werden Photonische Drähte und Punkte hergestellt, welche das Licht auch lateral in Potentialtöpfen einschliessen und zur Beobachtung von diskretisierten Emissionspektren in zwei und drei Dimensionen führen. Um diese Untersuchungen zu erweitern, wird eine tomographische Methode entwickelt, um die winkelaufgelöste Dispersion dieser mehrdimensional eingeschlossenen Photonen in allen Richtungen aufzunehmen. Die Ergebnisse dieser Untersuchung werden in metall-organischen photonischen Punkten und Dreieck-Strukturen ausgenutzt und führen dabei zu einer Verringerung der Laserschwelle von bis zu einer Grössenordnung. Die dabei entstehenden komplexen Transversalmoden sind ein Zeichen für die starke Konzentration des Lichts in solchen Strukturen. Die laterale Strukturierung organischer Mikrokavitäten kann nicht nur für den vollständigen Einschluss von Licht ausgenutzt werden, sondern ermöglicht weiterhin die Beobachtung von photonischen Bandstrukturen in periodischen Gittern. Solch periodische Strukturen bestehend entweder aus Silber oder SiO2 ermöglichen die Realisierung eines optischen Kronig-Penney Potentials in Mikrokavitäten was schlussendlich zur Beobachtung optischer Bloch-Zustände in der Dispersion führt. Durch eine Modifizierung der Kronig-Penney Theorie, bei der unter anderem den Photonen eine polarisationsabhängige effektive Masse zugewiesen wird, ist eine quantitative Berechnung der Modenpositionen in solchen Systemen möglich. In Theorie und experimentellen Untersuchungen wird dabei das Auftreten von 0- oder pi-phasengekoppelten räumlich ausgedehnten Supermoden erklärt. Mithilfe der Anregung durch zwei interferierende Laserstrahlen kann desweiteren eine direkte Kontrolle über die Wellenlänge sowie den Auskopplungswinkel der stimulierten Emission ausgeübt werden. In einem alternativen Konzept der lateralen Strukturierung werden organische Mikrokavitäten auf periodische Gitter aufgedampft, was zu einer kohärenten Kopplung von Wellenleitermoden der aktiven Schicht in die vertikale Emission führt. Diese Moden treten als lineare Dispersion in winkelaufgelösten Spektren auf und zeigen eine direkte Interaktion mit der parabolischen Dispersion der VCSEL-Mode an (Anti-)Kreuzungspunkten. In diesem hybriden System lassen sich neuartige Lasermoden beobachten, welche nicht nur am Scheitelpunkt der Kavitätsparabel auftreten, sondern auch an Punkten, die durch die Hybridisierung beider Systeme entstehen. Diese Kopplung von vertikalen und lateralen Lasermoden zeigt eine drastisch erhöhte Kohärenzlänge von mindestens 50 Mikrometern in der Probenebene. Schließlich wird das Konzept einer organischen Mikrokavität noch in absorbierenden Systemen eingesetzt. Durch das Einbringen einer organischen Solarzelle in eine optische Kavität wird eine starke Erhöhung des Felds im spektralen Bereich des sonst nur schwach absorbierenden intermolekularen Ladungstransferzustands in Donator-Akzeptor Mischschichten ermöglicht. Die Ausnutzung dieses Zustands ermöglicht eine spektral scharfe (Halbwertsbreite deutlich unter 50 nm) Detektion von Nahinfrarotphotonen mit einer externen Quanteneffizienz von über 20% (18% für 950 nm) und einer konkurrenzfähigen spezifischen Detektivität. In weiteren Untersuchungen zeigen sich diese Detektoren als spektral durchstimmbar, zum Einen durch die parabolische Dispersion der Mikrokavität, zum Anderen durch die Variation der Dicken der Elektron- und Lochtransportschichten. Diese Ergebnisse ermöglichen nicht nur interessante Anwendungen, sondern auch die direkte Beobachtung und Anregung des Ladungstransferzustandes, welcher eine zentrale Rolle in der Funktion organischer Solarzellen spielt

This thesis concerns the fabrication and study of strongly coupled organic microcavities containing a series of different boron-dipyrromethene (BODIPY) fluorescent dyes dispersed in an optically inert polystyrene matrix. The photophysics of the different BODIPY dyes are first studied and it is shown that they are promising materials for polariton condensation. DBR-DBR microcavities containing thin films of dye/polystyrene blends are then investigated under angular white-light reflectivity and CW laser excitation; measurements that show that they can enter the strong coupling regime. Polaritons in such high quality factor structures are shown to undergo a phase transition when excited with a high density pulsed excitation, forming a polariton condensate. Power dependent and interferometry measurements are used to identify the condensation threshold and the spatial coherence length of the polariton condensate. Lower Q-factor microcavities, comprised of two silver mirrors are fabricated, containing two different BODIPY dyes. Energy transfer between the molecules is engineered using two different processes; (1) direct short-range dipole-dipole coupling between the molecules, and (2) polariton-mediated energy transfer. We assess the efficiency of the energy transfer by quantifying the polariton population density along each polariton branch following laser excitation. It is concluded that short-range (< 3 nm) energy transfer induced by dipole-dipole coupling is more efficient compared to long-range (60 nm) polariton-mediated energy transfer, although the long-range process is estimated to transfer up to 87% of states to the lower-polariton branch. The generation of anti-Stokes polariton fluorescence is studied in low Q-factor metallic cavities following resonant excitation at the bottom of the lower polariton branch. Here, it is concluded that thermal energy in the system provides the excess of energy needed for emission of photons having higher energy than that of the initial laser excitation. Using temperature dependent and time resolved measurements it is concluded that polaritons return to the exciton reservoir by optically pumping a molecule in a vibrationally excited ground state. The exciton created then emits fluorescence that populates polariton states with an energy higher than the laser energy resulting in anti-Stokes polariton fluorescence. We believe such systems will be of significant interest in exploring laser-cooling phenomena in solid-state systems.

This thesis presents work undertaken with cold rubidium atoms interacting with an optical microcavity. The optical microcavity used is unique in its design, being formed between an optical fibre and silicon micromirror. This allows direct optical access to the cavity mode, whilst the use of microfabrication techniques in the design means that elements of the system are inherently scalable. In addition, the parameters of the system are such that a single atom has a substantial impact on the cavity field. In this system, two types of signal arise from the atoms' interaction with the cavity field; a `reflection' signal and a `fluorescence' signal. A theoretical description for these signals is presented, followed by experiments which characterise the signals under a variety of experimental conditions. The thesis then explores two areas: the use of the microcavity signals for atom detection and the investigation of how higher atom numbers and, as a result, a larger cooperative interaction between the atoms and the cavity field, impacts the signals. First, the use of these signals to detect an effective single atom and individual atoms whilst falling and trapped is explored. The effectiveness of detection is parameterised in terms of detection confidence and signal to noise ratio, detection fidelity and dynamic range. In the second part of this thesis, the effect of higher atom numbers on the reflection and fluorescence signals is investigated. A method for increasing the atom number is presented, alongside experiments investigating the impact on the measured signals. This is followed by experiments which explore the dispersive nature of the atom-cavity interaction by measuring the excitation spectrum of the system in reflection and fluorescence. In doing so, it is shown that, for weak coupling, these two signals are manifestly different.

The research work presented in this thesis is focused on the study of the optoelectronic coupling between the intersubband excitation in a two-dimensional electron gas (2DEG) and the resonant photonic mode of a planar semiconductor microcavity, in which the 2DEGs are embedded. When a generic electronic excitation interacts resonantly with a discrete cavity mode, a strong-coupling regime arises if the interaction strength of the electron-photon system (vacuum-field Rabi energy) is larger than the damping rates. This condition has been demonstrated in diverse research fields: from atomic physics to organic/semiconductor excitons coupled to a planar microcavity, to superconductor qubits coupled to microwave transmission lines. In semiconductor physics, the strong coupling results in the formation of quasi-particles termed cavity polaritons, which are the linear superposition of light and matter excitations. In 2003, the strong coupling of intersubband transitions in doped quantum wells with confined photons, and the corresponding formation of `intersubband cavity polaritons', were experimentally observed up to room temperature. In contrast to other strongly coupled systems, intersubband microcavities are more appealing due to the unique possibility of externally controlling light-matter interaction. The manipulation of polariton coupling hinges on the principle that the intensity of intersubband absorption in the active region can be controlled either through the carrier density modulation or by altering the oscillator strength of the transition. Owing to the large oscillator strength and relatively low-energy of the transition, in intersubband microcavities the vacuum-field Rabi splitting can be a significant fraction of the intersubband transition energy. Such a regime of light-matter interaction was predicted theoretically and termed as the `ultrastrong coupling regime'. The investigation of the optoelectronic coupling is here conducted in two different directions: (i) exploring suitable means for the external manipulation of intersubband cavity polaritons, (ii) realizing the conditions for observing the ultrastrong coupling regime of light-matter interaction. The devices employed in the investigation are multiple quantum well active structures embedded in intersubband microcavities - based either on dielectric mirrors or on plasmon mode resonators. The results presented in this thesis contain various experimental realizations of the external control of polariton coupling in a solid-state device, with unprecedented modulation depth and speed. Moreover the first experimental observation of the ultrastrong coupling of light-matter interaction is also reported. These are fundamental steps towards the generation of the photon pairs from vacuum fluctuations in a quantum electrodynamical scheme analogous to the well known dynamic Casimir effect, which is yet to be realized experimentally.

This thesis describes the fabrication and characterisation of double dielectric mirror GaN-based microcavities (MCs) along with investigation of the properties of various materials required for them, including GaN on AlInN, AlGaN, AlInGaN, and the use of FS-GaN substrate. Several processing routes for MC fabrication are detailed, with characterisation measurements after each step and for completed structures. Strong coupling between an exciton and a photon was observed for some approaches. The structures were grown by MOVPE and MBE on FS-GaN, sapphire and silicon substrates. Microcavities were fabricated using various techniques for substrate removal in order to access the back-side of active region for deposition of the bottom mirror. The finalised structures were characterised by optical spectroscopy. The structures grown on silicon resulted in the first observation of SC in transmission measurements for III-nitrides. High quality factors were observed from MCs grown on FS-GaN and on GaN-on-sapphire templates. These approaches open the way to improved structural quality of the active region, resulting from the use of substrates with low TDD.

This thesis presents the design, fabrication and experimental analysis of ZnSe based microcavities. Semiconductor microcavities are micro-structures in which the exciton ground state of a semiconductor is coupled to a photonic mode of an optical cavity. The strong light matter coupling mixes the character of excitons and photons, giving rise to the lower and upper cavity polaritons, quasiparticles with an unusual dispersion due to the extreme mass contrast between the composite exciton and photon. In particular, the dispersion of the lower polariton forms a dip around the lowest energy state with zero in-plane momentum. In this dip, which can be seen as a trap in momentum space, the polaritons are efficiently isolated from dephasing mechanisms involving phonons. The features of these quasiparticles promise a variety of applications, for instance lasing without inversion and micro-optical parametric amplifiers, and an environment to study fundamental physics, such as Bose-Einstein condensation in the solid state. By overcoming the longstanding fabrication problems of ZnSe-based microcavities, the enlarged exciton binding energy in combination with the use of highly reflective dielectric mirrors makes this material system ideally suited to the realisation of polariton-based devices operating at room temperature. An epitaxial liftoff technology is developed that relies on the high etch selectively between the ZnSe heterostructure and a novel II-VI release layer, MgS. Three hybrid microcavities are fabricated with the liftoff technique and spectroscopically characterised. Angle resolved transmission experiments reveal strong hybridization of the ZnSe/Zn0:9Cd0:1Se quantum well excitons and cavity photons in a fixed microcavity. A completely length tunable microcavity is presented and shown to exhibit similar dispersion as for the fixed microcavity, with the addition of evidencing the cavity polariton bottleneck effect. The nonlinear optical features are discussed. Photoluminescence data is presented that evidences the first observation of the build up of cavity polaritons at the edge of the momentum space trap in the lower polariton branch, the bottleneck effect, in a ZnSe based microcavity. Finally, lasing at room temperature in the blue spectral region is presented for a metal/dielectric hybrid microcavity.

Two projects are presented in this thesis. The first project is an investigation into linewidth-narrowing phenomena of intersubband cavity polaritons (ICPs) in a microcavity/multi-quantum well sample through angle-resolved laser spectroscopy. Strong coupling of the vacuum field of the microcavity, and the intersubband transition (ISBT) of the multi-quantum well led to vacuum Rabi splitting of 12.4 meV of the ICP modes at room temperature in the absorbance spectra. The linewidths of the ICPs were found to be substantially narrowed (4.2 meV, at room temperature) at the anticrossing point, narrower than the bare ISBT and empty microcavity linewidths at room temperature (6.2 and 6 meV, respectively, at room temperature), and narrower than existing theory predicted. The same effect was observed at cryogenic temperatures. Narrowing was explained by the light effective mass of the ICP, rendering the ICP unaffected by interface roughness scattering of the multi-quantum well. The measurement of the narrow linewidths was made possible by the superior angular resolution of laser spectroscopy compared to previously-used thermal light sources. The second project consisted of the development of a Q switched Er 3+ ,Cr 3+ :YSGG laser, of 3 μm wavelength, with 76 mJ fundamental mode pulses in free-running mode. Q switching of the laser was investigated in order to produce short (<400 ns) laser pulses, using two 'slow' Q switch methods. The first, a rotating mirror Q switch, was found to produce pulses of ~10 mJ, with an optimum mirror rotation rate of 300 rotations per second. The second Q switch method, a 'polygon chopper Q switch' produced single pulses of energy ~4 mJ, using a rotating polygon as an optical chopper. From this Q switch, it was deduced that the longest Q switching time possible for Er 3+ ,Cr 3+ :YSGG, with any method of Q switching, was ~30 μs for single pulse operation, and ~80 μs for multiple pulses.

This thesis describes the experimental investigations into the transverse mode structure of nearly hemispherical microcavities. The nearly hemispherical niicrocavity structures are fabricated electrocemically through a template of self assembled latex spheres. Controlling the electrochemical parameters, such as the electrochemical solution and electrode potential. allows a wide range of uearly henuspherical rnicrocavities to be realised. The spatial intensity profiles arid resonant frequeucies of the transverse modes of nearly hemispherical nucrocavities are measured experimentally for a wide range of cavity lengths amid mirror curvatures. The experimental mode profiles are radially symmetric Gauss-Laguerre modes, but do not display the frequency degeneracies typical of large scale optical cavities. The nearly hemispherical mnicrocavity samples are compared to investigate how the cavity parameters. such as cavity length and mirror curvature, affect the experimental spatial intensity profiles and resonant frequencies of the transverse modes. Higher order modes are observed despite the fact that they are forbidden due to the symmetrical coupling geometry. rrhe symmetry breaking is shown to be produced by the surface roughness of the curved nnrror. The frequency degeneracy lifting which occurs in the nearly hemispherical niicrocavity structures can he explained and modelled by considering non-parabolic elements in the cavity. A nmnher of mathematical models for the cavity propagation are developed based on paraxial theory. rrliese models are analysed and the predictions made from the models are compared with the experimental profiles and frequencies. The basic agreement between theory and experinient shows that the paraxial theory is able to model the cavity modes. However, the spectrum and the mnode profiles are cpnte sensitive functions of the geometry of the cavity amid the surface roughness of the cavity mirrors. The nearly hemispherical mnicrocavities are structures which offer a new fabrication technique allowing inexpensive and a ummconmplicated method of fabrication. An important feature of the nearly hemispherical microcavities is the tunablity, and the ease in which this can be achieved. The structures are also empty, and this will allow them, in the future, to be easily filled with functional optical nmaterials such as liquid crystals.

Semiconductor microcavities represent a rich playground for the investigation and exploitation of fundamental light{matter interaction as well as opto{electronic devices. Due to strong interaction of confined photons with electronic excitations new quasiparticles are formed, known as exciton{polaritons. These new eigenstates play a key role in a various number of intriguing effects like Bose-Einstein condensation and superfluidity due to their light-matter duality which unifies at the same time small effective mass and strong inter-particles interaction. Meanwhile, research achievements in the study of organic light emitting diodes and organic trasistors combined with strong advancements of the fabrication technologies has propelled the organic photonic and electronic field. In the present thesis the physics of organic microcavities is explored with particular attention at the limiting factors which prevent from the observation of cooperative non-linear phenomena. Such structural and material issues are addressed by following new engineeristic approaches. The inclusion of different organic dyes in the cavity active region is demonstrated to enhance polariton population density by direct intracavity pumping or either provide new efficient channels for particles relaxation. Inspired by a similar design, an hybrid organic{inorganic microcavity which exploits coupling of organic with inorganic quantum well excitons (Frenkel and Wannier{Mott) in a light emitting diode scheme is presented. Within this system, the optical cavity mode simultaneously couples to both excitonic transitions for the formation of mixed polariton states. The new bosonic eigenstates which arise from photon{mediated hybridiazation of Frenkel and Wannier-Mott excitons are predicted to exhibit large radius, small saturation density and large oscillator strength. Results from the optical characterization enlighten the enhancement of nonlinear properties of such hybrid polaritons while observation of strong coupling regime under electrical injection suggests the possibility for an effective exploitation of such unique polaritonic features in a electro-optic device.

Dans les microcavités semiconductrices, les polaritons excitoniques sont obtenus à partir du couplage fort entre l'exciton et le photon. Le régime de laser à polaritons à température ambiante, première étape vers le condensat de Bose-Einstein (BEC), a été atteint dans des microcavités ZnO et nous avons dans cette thèse étudié les propriétés des condensats de polaritons. Nous avons réalisé la spectroscopie linéaire et déterminé les propriétés spatiales de nouvelles microcavités ZnO sur substrat Si structuré en mesa, de haut facteur de qualité Q. Plusieurs géométries de génération de condensats de polariton ont été mises en oeuvre et comparées. Nous avons également mesuré, au travers d'expériences d'imagerie 2D en champ proche et en champ lointain, et modélisé, en résolvant l'équation de Gross-Pitaevskii, la propagation des condensats. Nous avons ainsi pu décrire les phénomènes mis en jeu dans la propagation des condensats à 80 et 300K pour une excitation fortement focalisée par rapport à une excitation étendue à 2D. Ces travaux posent les bases de dispositifs polaritoniques à 300K dans lesquels les condensats seront façonnés et contrôlés
In semiconductors microcavities, exciton-polaritons arise from the strong coupling between excitons and photons. The polariton laser at room temperature, which is the first step to Bose-Einstein condensation (BEC), has been achieved in ZnO microcavities and the study of polariton condensates is the main issue of this work. We have studied the linear spectroscopy and measured the spatial properties of new high-Q ZnO microcavities grown on a patterned Si-substrate. Many generation geometries have been set up and compared to control the shape of polariton condensates. We have also measured and simulated polariton condensates propagation, using respectively 2D imaging experiments in near-field and far-field and by resolving the Gross-Pitaevskii equation. Then we were able to describe the variety of phenomena involved in the condensates propagation at 80 and 300 K for a tightly focused excitation compared to a wide 2D excitation. Those experiments pave the way for the development of polariton devices operating at 300 K in which polariton condensates can be patterned and controlled

Over the last few years, the available computing power allows us to have a deeper insight into photonics components than we ever had before. In this thesis we use the finite element method (FEM) to explore the behavior of the waves in 2D planar microcavities. We demonstrate the tunability of the cavity over a wide range of frequencies taking into account both the thermo-mechanical and the thermo-optical effect. Geometry and material choices are done so that the latter is predominant. We also demonstrate an odd mode disappearing phenomenon reported here for the first time as far as we know. Using this knowledge, we design two structures with these remarkable properties.

One of the devices will be used as micro-sized solid-state dye laser with Rhodamine 6G as the active medium and SU-8 polymer as a cavity material in sizes that have never been reached before. This opens new opportunities not only for future implementation for “labs-on-a-chip” (LOC) but also for a higher integration density of optical communication systems. The second device is a wavefront shaper creating plane waves from a point source performing the functions of beam shaper and beam splitter with plane wave as the output result.

After an introduction to FEM and comparison with a rival algorithm, some issues related to FEM in electromagnetic simulation are resolved and explained. Finally, some fabrication techniques with feature sizes <100 nm, such as electron beam lithography (EBL) and nano-imprint lithography (NIL), are described and compared with other lithographic techniques.

In the past decades, the miniaturization in optics led to new devices with structural sizes in the range of the light wavelength, where the photonic modes are con- fined and the number of states is limited. In the smallest microcavities, i.e. micrometer sized optical resonators, the propagation of only one mode is permitted that is simultaneously amplified internally. This particularly strong enhancement of the electric field is directly related to the quality factor of the cavity. By introducing an optical dipole into a high-Q microcavity, the spontaneous emission is amplified at the cavity mode frequency enabling stimulated emission in an inverted system. Although some of theses cavity e®ects can only be understood by quantum elec- trodynamic theory, most mechanisms are accessible by classical and semi-classical approaches. In this thesis, one-dimensional planar microcavities with quality factors up to 4500 have been fabricated by physical vapor deposition of dielectric thin films and organic active materials. A new cavity design based on anisotropic dielectric mirrors grown by oblique angle deposition microcavities with two energetically shifted orthogonally polarized modes is presented. The application of these anisotropic structures for terahertz di®erence signal generation is demonstrated in spectrally and time resolved transmission experiments, where optical beats with repetition rates in the terahertz range are observed. Optically pumped organic vertical cavity surface emitting lasers (VCSELs) have been realized by applying an organic solid state laser compound and high reflectance distributed Bragg reflectors. These lasers combine a very low laser threshold with small beam divergence and good stability. A transfer of the anisotropic design towards an organic VCSEL results in the generation of two perpendicularly polarized laser modes with a splitting adjustable by the fabrication conditions. The observation of an oscillation of two laser modes in a photomixing experiment proves a phase coupling mechanism. This demonstrates the potential of the anisotropic cavity design for a passive or active component in a terahertz radiation source or frequency generator. Furthermore, microcavities with two and three coupled resonators are investigated. By the application of time-resolved transmission experiments, spatial oscil- lations of the internal electric field - photonic Bloch oscillations - are successfully demonstrated. In combination with the anisotropic microcavities, this is a second concept for the modulation of transmitted light with terahertz frequencies. All experiments are accompanied by numerical or analytical models. Transmission experiments of continuously incident light and single laser pulses are compared with transfer matrix simulations and Fourier transform based approaches. For the modeling of emission experiments, a plane wave expansion method is successfully used. For the analysis of the organic VCSEL dynamics, we apply a set of rate equations that explains the gain switching process.

Placing single emitters inside optical cavities provides a way of drastically modifying their interaction with the electric field. Such systems can be used to test our fundamental understanding of quantum electrodynamics and to develop devices that exploit this new found knowledge. This thesis describes work towards coupling single dye molecules to optical fibre microcavities at cryogenic temperatures. I present the design of an oven growth chamber and develop a method to grow cosublimated crystals of anthracene doped with dibenzoterrylene. I then describe the characterisation of these samples using a confocal microscope setup both at ambient and cryogenic temperatures. The samples grown show bright, stable, single emitters in a defined orientation in the anthracene crystals. By varying the growth parameters of the homebuilt crystal growing chamber, we can control the density of dibenzoterrylene molecules and grow at an optimum density to use these crystals in conjunction with optical fibre microcavities. I show how optical fibre microcavities have been developed that can be cooled to the liquid helium temperatures required to take advantage of the lifetime-limited emission from single dibenzoterrylene molecules, that only occurs below 4K. I also present cavity quantum electrodynamic simulations of the cavities, showing what we expect to see when we take reflection spectra of single molecules coupled to these cavities. In addition, I describe the optical setup that has been developed to take these measurements. I conclude with proposed improvements to the cavity setup that will enable these reflection spectra to be more easily taken. This will allow these molecule-cavity systems to be used as infrared single photon sources for quantum optics experiments.

Cavity polaritons are quasiparticles formed when a photon con ned within a cavity interacts with an elementary excitation in a semiconductor that is called exciton. Under the right conditions, cavity polaritons form a macroscopic condensate in the ground state. This condensate decays through the cavity mirrors, thus providing coherent light-emission: a phenomenon termed polariton lasing. The threshold for polariton lasing can be signi cantly lower than that required for conventional lasing. Large exciton binding energies are an essential requirement to obtain polariton lasing at room temperature. Group III nitrides and ZnO are the only inorganic semiconductors possessing Wannier-Mott exciton binding energies above 25 meV, the room-temperature thermal energy. In contrast, Frenkel excitons in organic semiconductors possess binding energies of 1 eV and are thus highly stable at room temperature. This thesis consists of two parts. The first part concerns the fabrication and optical characterisation of samples consisting of an ultra-smooth GaN membrane encapsulated in an all-dielectric (SiO2/Ta2O5) distributed Bragg reflector (DBR) microcavity. By utilising the selective photo-electro-chemical (PEC) etching of an InGaN sacri cial layer, GaN membranes 200 nm thick are produced and introduced between DBRs. The second part is devoted to the demonstration of a room-temperature organic polariton condensate. The studied samples consist of a thermally evaporated 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl) fluorene (TDAF) thin film enclosed within an all-dielectric microcavity, consisting of SiO2 and Ta2O5 pairs. In both GaN and organic systems, the strong coupling for various detunings is demonstrated by performing angle-resolved reflectivity and photoluminescence (PL) measurements. On reaching threshold, the nonlinear increase in the PL is blueshifted with respect to low power emission, and is accompanied by a simultaneous reduction in the linewidth, marking the onset of polariton lasing at room-temperature. In the organic microcavities particularly, the condensate formed above threshold is linearly polarised and exhibits o -diagonal long-range order with a spatial coherence that is dependent on the pump shape. Moreover, the ambipolar electrical characteristics of this organic semiconductor and the high electron mobility of GaN suggest both materials as promising candidates for direct electrical injection.

Interactions of exciton-polaritons in semiconductor microcavities and the resulting polarisation dynamics are investigated theoretically. Within the coboson framework of polariton-polariton scattering, it is shown that the matrix element of direct Coulomb scattering is proportional to the transferred momentum, q, cubed in the limit of small q. In the same limit, the magnitude of superexchange/exchange interactions can be considered constant. These results are applied to the elastic circle geometry, where a system of equations describing the steady-state pseudospin components is derived. It is shown, that for this geometry, polariton-polariton scattering can account for the generation of circularly/linearly polarised final states from linearly/circularly polarised initial states, depolarisation and the generation of spin currents. In the low density regime polaritons are good bosons and the dynamics of polariton Bose-Einstein condensation (BEC) are investigated. A stochastic model is derived, resulting in a Langevin type equation describing the time dynamics of the condensate spinor order parameter. The build up in condensate polarisation degree is shown to evidence macroscopic ground state population, while the stochastic choice of polarisation vector evidences the symmetry breaking nature of the phase transition. The decrease of polarisation degree above threshold is demonstrated to be a consequence of polariton-polariton interactions, a result which is complemented by recent experimental work. The stochastic model is extended to include Josephson coupling of spatially separate condensates. The coupling results in polarisation and phase correlations between the condensates, explaining the polarisation locking and spatial coherence seen experimentally. Finally, the effect of polarisation pinning by local effective fields is examined.

Semiconductor microcavities offer the possibility to strongly confine light in a small cavity volume. Here the light interacts strongly with the electronic excitations of the quantum wells, which are embedded in the cavity, giving rise to a new kind of quasiparticle called exciton-polaritons or polaritons. These polaritons are the superposition of a photon and an exciton and inherit a light effective mass from the photon part and strong inter-particle interactions from the exciton part. Polaritons have extremely rich physics such as Bose-Einstein condensation (BEC) and super fluidity, to name a few. Thanks to their spin properties and fast dynamics polaritons could have potential applications in ultrafast optoelectronics such as optical switches. Under certain conditions the strong coupling does not sustain and the Eigenstates of the system change to the uncoupled cavity and exciton mode, which is called the weak coupling regime. In this thesis non-linear effects in the strong and in the weak coupling regime are investigated. In particular a crossover between a photon and a polariton laser is observed. Distribution functions and the dynamic behaviour of the long-range coherence confirms great similarities with BEC and exhibit the transition between two coherent states. Following these observations we study the spinor nature of polaritons and photons. In single shot experiments the spontaneous symmetry breaking at the phase transition to a coherent state was shown. In a nearly isotropic system the phase of the order parameter was chosen spontaneously and showed strong variations from shot to shot. This phenomenon which was once identified as the smoking gun for BEC was observed in a polariton and a photon laser. The spinor nature of polariton condensates was further exploited by studying the transport of spin by a propagating polariton condensate. Whilst travelling through the sample the spin experiences the optical spin-Hall effect and coherently precesses around an effective magnetic field. We observe up to four complete revolutions of the pseodospin around the effective magnetic field and the formation of a spin pattern that extends to 300 microns.

Nous avons obtenu des angles de rotation Faraday (RF) allant jusqu'à 19° par orientation optique d'un gaz d'électrons dans GaAs de type n inclus dans une microcavité (Q=19000), sans champ magnétique. Cette forte rotation est obtenue en raison des multiples allers-retours de la lumière dans la cavité. Nous avons également démontré la commutation optique rapide de la RF à l'échelle sub-microseconde en échantillonnant le signal de RF sous excitation impulsionnelle mono-coup. De la dépolarisation de la RF en champ magnétique transverse, nous avons déduit un temps de relaxation de spin de 160 ns. Le concept de section efficace de RF, coefficient de proportionnalité entre l'angle RF, la densité de spin électronique, et le chemin parcouru, a été introduit. La section efficace de RF, qui définit l'efficacité du gaz d'électrons à produire une RF, a été estimée quantitativement, et comparée avec la théorie. Nous avons également démontré la possibilité de mesurer de manière non destructive l'aimantation nucléaire dans GaAs-n, via la RF amplifiée par la cavité. Contrairement aux méthodes existantes, cette détection ne nécessite pas la présence d'électrons hors équilibre. Par cette technique nous avons étudié la dynamique de spin nucléaire dans GaAs-n avec différents dopages. Contrairement à ce qu'on pourrait attendre, le déclin de la RF nucléaire est complexe et consiste en deux composantes ayant des temps de relaxation très différents. Deux effets à l'origine de la RF nucléaire sont identifiés: le splitting de spin de la bande de conduction, et la polarisation en spin des électrons localisés, tous deux induits par le champ Overhauser. Le premier effet domine la RF nucléaire dans les deux échantillons étudiés, tandis que la RF induite par les électrons localisés n'a été observée que dans l'échantillon métallique
We obtained Faraday rotation (FR) up to 19° by using optical orientation of electron gas in n-doped bulk GaAs confined in a microcavity (Q=19000), in the absence of magnetic field. This strong rotation is achieved because the light makes multiple round trips in the microcavity. We also demonstrated fast optical switching of FR in sub-microsecond time scale by sampling the FR in a one-shot experiment under pulsed excitation. From the depolarization of FR by a transverse magnetic field, we deduce electron spin relaxation time of about 160 ns. A concept of FR cross-section as a proportionality coefficient between FR angle, electron spin density and optical path is introduced. This FR cross-section which defines the efficiency of spin polarized electrons in producing FR was estimated quantitatively and compared with theory. We also demonstrated non-destructive measurement of nuclear magnetization in n-GaAs via cavity enhanced FR. In contrast with the existing optical methods, this detection scheme does not require the presence of detrimental out-of-equilibrium electrons. Using this technique, we studied nuclear spin dynamics in n-GaAs with different doping concentrations. Contrary to simple expectation, the nuclear FR is found to be complex, and consists of two components with vastly different time constants. Two effects at the origin of FR have been identified: the conduction band spin splitting and the localized electron spin polairzation both induced by the Overhauser field. The first effect dominates the FR in both studied samples, while the FR induced by the localized electrons has been observed only in the metallic sample

Semiconducting nanocrystals in open-access microcavities are promising systems in which enhanced light-matter interactions lead to quantum effects such as the modulation of the spontaneous emission process and exciton-polariton formation. In this thesis I present improvements of the open cavity platform which serves to confine the electromagnetic field with mode volumes down to the λ³ regime and demonstrate results in both the weak and strong coupling regimes of cavity quantum electrodynamics with a range of different low-dimensional materials. I report cavity fabrication details allowing a peak finesse of 5 × 10⁴ and advanced photonic structures such as coupled cavities in the open cavity geometry. By incorporating two-dimensional materials and nanoplatelets in the cavity I demonstrate the strong coupling regime of light-matter interaction with the formation of exciton-polaritons, quasi-particles composed of both photon and exciton, at room temperature. In the perturbative weak coupling regime I show pronounced modulation of the single-photon emission from CdSe/ZnS quantum dots and the two-dimensional material WSe₂ and demonstrate Purcell enhancement of the spontaneous emission rate by factors of 2 at room temperature and 8 at low temperature. The findings presented in this thesis pave the way to establish open microcavities as a platform for a wide range of applications in nanophotonics and quantum information technologies.

The bone marrow (BM) niche provides hematopoietic stem (HSC) and progenitor cells with many exogenous cues that tightly regulate homeostasis. These cues orchestrate cellular decisions, which are difficult to dissect and analyze in vivo. This thesis introduces a novel in vitro platform that permits systematic studies of BM-relevant factors that regulate homeostasis. Specifically, the role of 3D patterned adhesion ligands and soluble cytokines were studied in a combinatorial fashion. Analysis of human HSC differentiation and proliferation at both population and single cell level showed synergistic and antagonistic effects of adhesion- and cytokine-related signals. Those effects were dependent on the cytokine concentration and the distribution and number of adhesion ligands. The aim of this thesis was to model the in vivo bone marrow with its porous 3D structure and different sized niche compartments using a microcavity culture carrier. The developed culture system presented extracellular matrix (ECM) adhesion ligands to the HSCs in various defined dimensions ranging from single- to multi-cell capacity. The 3D open well geometry of the microcavity carriers also allowed HSCs to freely explore different scenarios including homing, migration, adhesion, or suspension. Furthermore, the developed setup offered straightforward accessibility to analytical methods like cytometry and quantitative microscopy. Single cell analysis of adherent HSCs showed decreased DNA synthesis and higher levels of stem cell marker expression within single cell microcavities under low cytokine conditions . This effect was reflected in a decline of proliferation and differentiation with decreasing microcavity size. When the cytokine concentration was increased2 beyond physiological levels the inhibitory effect on proliferation and differentiation due to single-cell-microcavity adherence was diminished. This result highlighted the fine balance between adhesion related and soluble cues regulating HSC fate. Within small microcavities more adhesion related receptors were engaged due to the 3D character of the culture carrier compared to multi-cell wells or conventional 2D cell culture plates. This study demonstrated that adhesion-related signal activation leads to reduced proliferation and differentiation. This geometry-based effect could be reversed by increased cytokine supplementation in the culture media. For plane substrates, HSCs attachment to fibronectin or heparin initiated early cell cycle entry compared to non-adherent cells during the initial 24h. Cytokine supplemented media favored integrin activation that induced fast adhesion, ultimately leading to early cell cycle activation. However, after prolonged cell culture the system balanced itself with a lower cycling rate of adherent versus non-adherent HSCs. Furthermore, HSCs within the 3-dimensionality of the microcavities cycled less than 2D adherent cells. These findings additionally supported the above stated idea of limited HSC proliferation as a consequence of more adhesion-related signals overwriting cytokine driven expansion. To complement the various in vitro studies, an in vivo repopulation study was performed. Cultured HSCs derived from single cell microcavities outperformed freshly isolated HSCs in a competitive repopulation assay, indicating that carefully engineered substrates are capable of preserving stem cell potential. Overall the reported findings provide a promising in vitro culture strategy that allows the stem cell field to gain a better understanding of the impact of distinct exogenous signals on human HSCs, which discloses new concepts for the wide scientific community working towards tissue engineering and regenerative medicine
Die Homöostase der Hämatopoietischen Stamm- und Vorläuferzellen (HSC) in der Knochenmark Nische wird von einer Vielzahl exogener Faktoren gezielt reguliert. Diese Faktoren orchestrieren intrazelluläre Vorgänge, deren in vivo Analyse kompliziert ist. Die vorliegende These widmet sich einem neuen biotechnologischen Ansatz, der systematische Studien von Knochenmark-relevanten Faktoren ermöglicht. Im Speziellen wurde die Rolle 3D-präsentierter Zell Adhäsionsliganden in Kombination mit verschiedenen Konzentrationen löslicher Zytokine untersucht. Die Auswertung der Proliferation und Differenzierung von humanen HSC auf Einzelzell- und Populationsebene offenbarte die synergistischen und antagonistischen Effekte von Adhäsions- und Zytokinsignalen in ihrer Abhängigkeit von der Verteilung und der Anzahl von Adhäsionsliganden sowie der Zytokinkonzentration. Um die poröse Struktur des Knochenmarks in vivo-ähnlich darzustellen, wurde eine Zellkultur Plattform mit Mikrokavitäten verschiedenster Dimensionen von Multi- bis Einzelzellgröße entwickelt und mit Molekülen der extrazellulären Matrix beschichtet. Die Vorteile dieser Plattform liegen in der offenen 3D-Geometrie dieses mikrokavitäten Kultursystems, die den Zellen ermöglichte verschiedene Wachstumsbedingungen bezüglich Homing, Migration, Adhäsion oder Suspension frei zu erkunden. Das leicht zugängliche Setup eignete sich zudem hervorragend für die zytometrische Analyse der Zellen oder die quantitative Mikroskopie. Die Einzelzellanalyse adhärenter HSC ergab eine Reduktion von DNA Synthese und eine höhere Expression von Stammzelloberflächenfaktoren innerhalb der Einzelzell-Mikrokavitäten bei niedrigen Zytokinkonzentrationen . Dieser Effekt spiegelte sich auch auf Populationsebene in verminderter Proliferation und Differenzierung mit abnehmender Größe der Mikrokavitäten wider. Wurde die Zytokinkonzentration jedoch weit über physiologische Bedingungen erhöht, verminderte sich der Effekt (reduzierte DNA Synthese und höhere Stammzellfaktorexpression) beschrieben für die Einzelzellmikrokavitäten. Dieses Ergebnis verdeutlicht die empfindliche intrazelluläre Balance, vermittelt durch Adhäsionsignale und löslichen Faktoren, die das Verhalten von HSCs regulieren. Aufgrund des 3D-Charakters des Zellkulturträgers wurden innerhalb kleiner Mikrokavitäten mehr Adhäsionsrezeptoren ringsum die Zelle aktiviert. Dieser Vorteil gegenüber den Multizellkavitäten oder der herkömmlichen 2D–Zellkultur ermöglichte eine hohe Anzahl adhäsionsvermittelter Signale mit entsprechend höherer Proliferations-inhibitorischer Wirkung. Je höher die Konzentration der Zytokine war, desto stärker erfolgte die Stimulation der Proliferation und Differenzierung. Auf 2D Substraten, initiierte Adhäsion zu Fibronektin und Heparin innerhalb der ersten 24h einen frühen Zell-Zyklus-Start im Gegensatz zu nicht adhärenten Zellen. Die Zytokine im Zellmedium förderten die Integrin Aktivierung, was zu einer schnellen Zelladhäsion führte. Die Adhäsionsrezeptoren wiederum kooperieren mit Zytokinrezeptoren im Zellinneren und begünstigten damit einen zeitigeren Zell-Zyklus- Start. Allerdings stellte sich danach ein Gleichgewicht im Kultursystem ein, wobei weniger adhärente Zellen als nicht-adhärente Zellen den Zellzyklus durchliefen. Des Weiteren war die Zellzyklusrate innerhalb von 3D Mikrokavitäten niedriger verglichen mit herkömmlichen 2D Substraten. Diese Ergebnisse bestätigen ferner obenstehende These, dass Zytokin-induzierte Zellexpansion durch erhöhte Zelladhäsions-vermittelte Signale überschrieben wird. Um die in vitro Studien zu komplettieren wurde ein in vivo Repopulationsversuch durchgeführt. HSC kultiviert auf Einzel-Zell-Mikrokavitäten übertrafen frisch isolierte Konkurrenz-Zellen in einem kompetitiven Repopulationsversuch. Dieses erste Ergebnis zeigt, dass sich der Zellgröße entsprechende Biomaterialien für die erfolgreiche Stammzell-Kultur eignen. Die Ergebnisse dieser Arbeit bieten eine vielversprechende in vitro Zellkulturstrategie, die ein besseres Verständnis der Einflüsse von exogenen Signalen auf HSC erlaubt und damit eine Grundlage für neue Erkenntnisse in Richtung erfolgreicheres Tissue Engineering und klinische Anwendungen im Bereich der regenerativen Medizin bildet

The nonlinear properties of multi-mode InP and Si planar photonic crystal microcavities are investigated in experiments relevant to integrated schemes for classical and quantum optical information processing. Normally incident, short laser pulses are used to coherently initialize the relative phase and amplitudes of two modes of a single-missing-hole InP microcavity. The two modes are orthogonally polarized, and separated by less than the bandwidth of the ~130 fs excitation pulses. The relative amplitudes of the two modes can be controlled by adjusting the polarization and the centre frequency of the excitation beam. Cross-polarized detection of the resonantly scattered light reveals a well-defined relative phase between the modes that is characteristic of their coherence. When the short-pulse excitation is used to coherently excite two modes in a three-hole line-defect (L3) InP microcavity, second-order harmonic radiation is observed due to the interactions of the resonant fields with the second-order nonlinear susceptibility (χ⁽²⁾) of the host InP slab. Second-harmonic and sum-frequency generated signals are observed due to the intra- and inter-mode nonlinear mixing of the microcavity fields. When a separate non-resonant pulse is focussed onto an InP microcavity, sum-frequency light is generated conditional to the resonant mode population of the microcavity. The conditionally generated signals can be tuned by tuning the frequency of the non-resonant pulse. All of the results can be explained with reference to the bulk χ⁽²⁾ properties of the InP slab. While the transient, multi-mode response of the microcavities is harnessed with the short-pulse technique, a continuous wave excitation laser exploits the local-field enhancement intrinsic to these wavelength-scale microcavities. A single-mode InP L3-microcavity with Q = 3,800 is pumped on resonance with a CW laser, and the 2D pattern of far-field second-harmonic radiation is directly imaged. The second-harmonic light is enhanced by 1000 times compared to non-resonant excitation, demonstrating integrated low-power frequency generation. The spatial pattern of the radiation is consistent with simulations based on the bulk χ⁽²⁾ tensor, and reveals the importance of scattering and material absorption of the harmonic light. Ultrafast, all-optical switching is demonstrated in a Si microcavity with a single Q = 35, 000 resonant mode. The mode is resonantly excited with a weak probe pulse, and a non-resonant 200 pJ pump pulse with a precisely controlled time delay is used to inject free-carriers above the silicon bandgap. The free-carrier dispersion shifts the mode frequency by 9 line-widths, and broadens its width by a factor of 4. When the excited mode is perturbed while it is ringing down, coherent oscillations in the spectra are observed which can be explained in terms of a model of an instantaneously perturbed harmonic oscillator, The implications for frequency conversion and for the generation of squeezed optical states are considered.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

In this thesis, polariton dynamics in GaAs semiconductor microcavities are investigated. Insertion of layers of active material within a Fabry-Perot resonator leads to strong coupling between excitons and photons, giving rise to new eigenmodes called excitonpolaritons. Bose-Einstein condensation, the ability for massive occupation of a quantum state is considered a fascinating property of polaritons, due to their bosonic character. A full study of polariton condensation in 2D and 0D microcavities is included in this thesis. Formation of a ground state condensation in a planar cavity is resolved by studying the spatial, angular, coherence, energy and transient dynamics of polariton photoluminescence, as well as the transition from the weak- to the strong-coupling regime in the time-domain. The role of longitudinal optical phonons in the relaxation dynamics is also investigated. Encouraging experimental results confirm the efficiency of this mechanism towards the formation of a ground state condensate. Polariton condensation in 0D GaAs quantum well microcavities is facilitated by etching the 2D semiconductor microcavity sample into pillars, removing the wavevector conservation. The spontaneous formation of a condensate in ground and non-ground states in 0D microcavities is investigated experimentally. A complete kinematic model that satisfactorily describes the spectral and temporal behaviour of polaritons in 0D structures completes this study. Finally, the observation of the all optical spin Hall effect, the separation of spin polarised carriers in real and momentum space, in a pure photonic cavity is included in this thesis. Experimental findings suggest that the excitonic contribution in similar observations in the strong coupling regime with polaritons acting as spin carriers, is not essential for the observation of the anisotropic polarisation flux.

This thesis presents experimental analysis of polariton dynamics in semiconductor umicrocavities. A microcavity is a monolithic structure composed of two distribnted Bragg reflectors which are separated by a layer of active material. Strong coupling between excitons residing in the active layer and photons confined within the cavity leads to new eigenstates of the system, called microcavity exciton-polaritons. The dynamics of these qnasi-bosons are stndied using a range of optical spectroscopy techniques. It has been shown previously tlmat resonant injection of polaritons nsing a continuous wave laser allows the rnicrocavity to operate as an optical parametric oscillator. A full study of the recovery dynanucs of a transiently destabilized nucrocavity optical parametric oscillator is made in this thesis. Destabilization was achieved by optically injecting surplus polaritonms into a systeum that had reached equilibrium. The dynamics of the scattering processes is theoretically described using a rate equation model. Bose condensation and polariton lasing have recently been demonstrated at liquid hehum temperatures. In this thesis, we use a hybrid bulk gallium nitride nuicrocavity to demonstrate the operation of the first room temperature polariton laser. Polarisation measurements also show spontamieous symmetry breaking, implying observation of the first room tenmperature Bose-Einstein condensate.

We conclusively explain the different lasing mode energies in ZnO nano- and microcavities observed by us and reported in literature. The limited penetration depth of usually used excitation lasers results in an inhomogeneous spatial gain region depending on the structure size and geometry. Hence, weakly or even nonexcited areas remain present after excitation, where modes are instantaneously suppressed by excitonic absorption. We compare the effects for ZnO microwires, nanowires, and tetrapod-like structures at room temperature and demonstrate that the corresponding mode selective effect is most pronounced for whispering-gallery modes in microwires with a hexagonal cross section. Furthermore, the absorptive lasing mode suppression will be demonstrated by correlating the spot size of the excitation laser and the lasing mode characteristic of a single ZnO nanowire.

El silicio es un material de suma importancia en microelectronica y en fotonica. Las propiedades semiconductoras del silicio estan detras de los conceptos que gobiernan el funcionamiento de la mayoría de los dispositivos electronicos como los diodos y los transistores. El concepto de integracion ha permitido procesar dispositivos muy pequeños, llegando a alcanzar un tamaño nanometrico. El alto indice de refraccion del silicio permite confinar la luz en estructuras de tamaño micrometrico. Este es el caso de dispositivos fotonicos tales como las guias de onda y las cavidades. Usualmente, tanto los dispositivos fotonicos como los electronicos estan basados en la tecnologia planar, es decir poseen una topologia plana, siendo esto una fuente de perdidas. Es bien conocido que las cavidades esfericas confinan la luz con mas eficiencia que las cavidades planares. Esta tesis trata sobre el desarrollo de un nuevo tipo de microparticulas esfericas que llamamos Coloides de Silicio. Debido a su forma esferica, su alto indice de refraccion y su suave superficie, estas particulas funcionan como microcavidades opticas con modos resonantes bien definidos en el infrarrojo cercano. La tesis reporta sobre la sintesis, y las propiedades estructurales y opticas de los coloides de silicio con diametro entre 0.5 y 3.5 micrometros. Los coloides de silicio pueden facilitar el desarrollo de microcavidades de alto factor de calidad con alta eficiencia de confinamiento de la luz, y permitir la integracion de dispositivos electronicos y fotonicos tales como una union p-n en una sola particula coloidal. Esta tesis reporta tambien sobre los coloides de silicio como elementos integrantes de las Esponjas Fotonicas, las cuales estan formadas por una red desordenada de microesferas de silicio de diferentes tamaños, e interaccionan con la luz fuertemente en un ancho rango de longitudes de onda.
Tymczenko, MK. (2010). Photonic microcavities and photonic sponges based on silicon colloids [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/8425
Palancia

Nonlinear optical phenomena in both semiconductor microcavities and sodium vapor are investigated. Systems displaying atomic and excitonic coupling are studied in detail in order to unravel underlying physical principles. Possible applications for these systems are evaluated where appropriate. Normal-mode coupling (NMC) in a semiconductor microcavity is achieved when a narrow-linewidth quantum well exciton resonance is coincident with the cavity mode of a high-finesse microcavity. This interaction leads to a double-peaked spectrum in either transmission, reflection, or photoluminescence (PL). The high-quality microcavities studied here at liquid-Helium temperatures reveal intensity dependent behavior that has not previously been observed. Nonlinear saturation of the exciton leads to a spectral broadening of the excitonic absorption without a significant loss in oscillator strength. This results in a reduction of the two transmission peaks with almost no change in the splitting between the peaks. Such behavior is easily explained using phenomenological nonlinear dispersion theory. In this nonlinear regime, the luminescence from the microcavity shows a gradual transition from the nonperturbative regime of NMC to lasing with increasing excitation. The observed behavior is explained by density-dependent changes in both the transmission of the microcavity and the bare-exciton emission, rather than a boson-condensation of excitons which has been previously proposed. An intermediate-finesse microcavity also modifies the emission distribution from a bulk-semiconductor at room temperature. Angularly resolved emission spectra and quantum efficiency measurements show the PL is strongly dependent on the reflectivity of the microcavity. Unfortunately, the enhancement in the decay rate of excitons due to high-reflectivity mirrors seen previously by our group does not result in an increased quantum efficiency. High-gain optical-wavefront amplification in atomic sodium vapor is demonstrated via both the three-photon effect and stimulated Raman scattering (SRS). In both cases, single-pass weak-field gain of nearly 400 is achieved with only 800 mW of pump power near 589 nm. In the case of SRS, phase preservation of the amplified wavefront, which is necessary in adaptive imaging applications, is also demonstrated.