Rozprawy doktorskie na temat „Strong-matter coupling”

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.

Cette thèse porte sur l'étude du couplage fort lumière-matière pour les transitions intersousbandes. En plaçant une série de puits quantiques dopés dans une microcavité planaire à semiconducteur, les interactions entre excitations intersousbandes et modes photoniques de cavité peuvent être étudiés. Si la fréquence de Rabi, liée à la force du couplage entre photons de cavité et excitations intersousbandes, est supérieure à leur élargissement en fréquence, le régime de couplage fort est atteint. Dans ce régime, les états propres du système sont des superpositions linéaires d'excitations photoniques et électroniques, appelés polaritons de cavité. Dans ce travail, nous démontrons l'implémentation du régime de couplage fort lumière-matière dans un dispositif électrique à semiconducteur. La structure que nous avons réalisée est composée par une structure à cascade quantique en AI0,45Ga0,55As/GaAs contenant un gaz bi-dimensionel d'électrons dans l'état fondamental. Elle est insérée dans une microcavité planaire, basée sur un mode plasmonique et dans laquelle un courant peut être injecté. Le système a été caractérisé d'abord en réflectivité, démontrant que le régime de couplage fort lumière-matière est atteint. Ensuite, des mesures photovoltaïques et d'électro-luminescence ont été effectuées. Les résultats obtenus soulignent l'importance du transport électronique et de l'injection électrique pour les propriétés du système en couplage fort. La possibilité de sonder électriquement des phénomènes de cavité a été démontrée, ainsi que la réalisation du premier dispositif sous pompage électrique, fonctionnant dans le régime de couplage fort lumière-matière en émission
This thesis is focused on the study of the light-matter strong coupling regime for intersubband transitions. A System composed of doped multi-quantum wells inserted in a semiconductor planar microcavity allows the study of the interaction between intersubband excitations and photon cavity modes. If the vacuum Rabi frequency, quantifying the coupling between a cavity photon and an intersubband excitation, exceeds their frequency broadenings, what is referred to as strong coupling regime is achieved. In this regime, the eigenstates of the System are linear superpositions of light and matter excitations and are called cavity polaritons. In this thesis the implementation of the light-matter strong coupling regime in an electrically driven semiconductor device is presented. The structure we have designed is composed of a AI0,45Ga0, 55As/GaAs quantum cascade structure containing a bi-dimensional electron gas in the ground state, inserted in a planar microcavity, based on a plasmon mode, suitable for electrical injection. The System has been first characterized in reflectivity, showing its suitability for the achievement of the light-matter strong coupling regime. Then, photovoltaic and electro-luminescence measurements have been performed. The results obtained have put into evidence the importance of the electrical transport and injection in the properties of the System in strong coupling regime. The possibility of an electrical probe of cavity dynamics has been demonstrated, as well as the realization of the first electrically injected semiconductor device, working in the light-matter strong coupling regime in emission

This thesis presents a theoretical investigation of exciton polaritons in strongly-coupled exciton-photon microcavity systems. Two different systems, a coupled quantum well (CQW) embedded in a planar microcavity and a quantum dot (QD) inside a micropillar cavity, are studied using suitable theoretical models. The exciton-polariton states are calculated and their optical properties are investigated in detail, showing a good agreement with experimental observations. For a CQW structure, the excitonic states in the presence of the electric field applied in the growth direction are calculated by solving the Schrodinger equation in real space. The field dependence of exciton transition energy, binding energy, oscillator strength, lifetime and absorption is studied. The exciton ground state experiences a crossover from direct to indirect state at low electric field. A single state-basis calculation in which only the electron and hole ground states are taken into account is also made and compared with the full accurate calculation model. The polariton effect in a microcavity-embedded CQW is investigated based on the semiclassical theory. The light-matter interaction is treated by solving coupled material and Maxwell's equations. The reflectivity and absorption spectra are calculated for different detunings using the scattering matrix method. When a cavity mode is tuned to an exciton mode (zero detuning), an anticrossing of two polariton modes is observed, showing that the system is in the strong coupling regime. In addition, the fractions of direct exciton, indirect exciton and cavity modes contributed to the polariton states are calculated using the microscopic theory. The resulting polariton state with comparable contributions ofall three components called dipolariton is observed. Finally, the dynamics of the strongly-coupled exciton-cavity system in the QD-micropillar system is studied using the four wave mixing (FWM) theory applied to the Jaynes-Cummings model. Spectrally resolved and time-resolved FWM signals are calculated for different temperatures. Temperature plays the role of the parameter controlling the detuning. The beat periods of the first and second rungs of the JC ladder are also investigated, showing that the second rung has a sqaure- shorter period compared to the beat period of the first rung. To reveal the coherent coupling between two distant QDs, the FWM signals are Fourier-transformed into a two-dimensional frequency domain. It is found that the off-diagonal components in these 2D spectra are nonzero, demonstrating the coherent coupling between isolated QDs. In addition, the phase correction is developed. This procedure is neccessary for a comparison with the experiment which has random phases for different detunings. A quantitative agreement between the prediction and measurement is achieved and demonstrated.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.
Includes bibliographical references (p. 126-133).
J-aggregates of cyanine dyes are the excitonic materials of choice for realizing polariton devices that operate in strong coupling at room temperature. Since the earliest days of cavity QED, there has been a major desire to construct solid state optical devices that operate in the limit where strong light-matter interactions dominate the dynamics. Such devices have been successfully constructed, but their operation is usually limited to cryogenic temperatures, because of the small binding energies for the ,excitonic materials typically used. It has been demonstrated that when J-aggregates are used as the excitonic material, it is possible to achieve strong coupling in solid state even at room temperature. J-aggregates are a unique choice of materials because their central feature, a very large optical transitional dipole, is itself the result of strong coupling amongst monomeric dye elements. The strong coupling amongst dye molecules produces a well-defined cooperative optical transition possessing oscillator strength derived from all of the aggregated monomers that is capable of interacting strongly with the cavity confined electromagnetic field even at room temperature. There are different materials and methods for assembling J-aggregates which are capable of producing strong coupling. This thesis argues in favor of a particular dye and method of assembly which are then thoroughly characterized. With this dye and assembly technique, the first demonstration of electrically pumped polariton emission is reported as is the largest optical absorption coefficient for a solid thin film at room temperature not contained in a full microcavity.
(cont.) This combination is then used to demonstrate strong coupling at room temperature, as characterized by a light-matter coupling strength, Rabi-splitting, that significantly exceeds the dephasing processes competing against the coherence of the interaction. Finally, prospects of this approach for realizing a polariton laser at room temperature are considered, and improved microcavity architectures are demonstrated as a path towards its realization.
by Jonathan Randall Tischler.
Ph.D.

Les polaritons inter-sous-bandes, observés pour la première fois il y a une quinzaine d’années, sont des quasi-particules dont de nombreuses propriétés restent encore à découvrir. La recherche dans ce domaine se focalise actuellement sur la réalisation de condensats de Bose-Einstein. Une telle découverte pourrait révolutionner l’optoélectronique du moyen infra-rouge jusqu’au THz ouvrant la voie à l’instauration de nouveaux concepts de sources lumineuses,de détecteurs ou de systèmes logiques en couplage fort. Dans cette quête, le choix de la cavité résonnante est critique. Dans ce manuscrit nous proposons d’utiliser des cavités métal-isolant-métal (M-I-M) avec un réseau dispersif sur le métal supérieur. Ce type de cavité,conservant un confinement élevé entre les deux plans métalliques, offre de nombreuses possibilités d’ajustement de la résonance de cavité : via la géométrie de la cavité ( épaisseur de la cavité, période et recouvrement du réseau) ainsi que par le couplage de la lumière avec la cavité (vecteur d’onde incident). Les cavités M-I-M dispersives ouvrent donc un nouveau champ d’exploration des polaritons inter-sous-bande. Dans un premier temps nous avons introduit ces cavités dans le domaine du THz afin d’étudier les phénomènes de relaxation polariton-polariton. Un système expérimental dédié à cette exploration a été conçu pour mesurer la réflectivité des polaritons THz avec une fine résolution en angle. Dans une second temps, des capteurs moyen infrarouge en couplage fort avec une cavité M-I-M dispersive ont été conçus, fabriqués et mesurés dans le but d’explorer la génération de photo-courant à partir de polaritons et d’utiliser le couplage fort pour dissocier l’ énergie de détection de l’énergie d’activation. Cette seconde étude s’inscrit dans l’objectif de pompage électrique des polaritons ISB. Parallèlement à l’étude des polaritons, nous avons également participé au développement de techniques(interféromètre Gires-Tournois et revêtement anti-réflection) pour compresser les impulsions optiques de lasers à cascade quantique THz
After fifteen years of intersubband polaritons development some of the peculiar properties of these quasi-particles are still unexplored. A deeper comprehension of the polaritons is needed to access their fundamental properties and assess their applicative potential as efficient emitters or detectors in the mid-infrared and THz.In this manuscript we used Metal-Insulator-Metal (MI-M) cavities with a top metal periodic grating as a platform to deepen the understanding of ISB polaritons.The advantages of M-I-M are twofold : first they confine the TM00 mode, second the dispersion of the cavity -over a large set of in-plane wave-vectors- offers various experimental configurations to observe the polaritons in both reflection and photo-current. We reexamined the properties of ISB polaritons in the mid-infrared and in the THz using these resonators. In the first part, we explore the implementation of dispersive M-I-M cavities with THz intersubband transitions. In the THz domain, the scattering mechanisms of the THz ISB polaritons need to be understood. The dispersive cavity is a major asset to study these mechanisms because it provides more degrees of freedom to the system. For this purpose, we fabricated a new experimental set-up to measure the polariton dispersion at liquid Helium temperature. After the characterization of the polaritons in reflectivity, a pump-probe experiment was performed on the polaritonic devices. The second part of this manuscript presents the implementation of M-I-M dispersive cavities with abound-to-quasi-bound quantum well infrared photo detector designed to detect in strong coupling. Beyond electrical probing of the polaritons, the strong coupling can disentangle the frequency of detection from the thermal activation energy and reduce the dark current at a given frequency. In parallel to the exploration of THz polaritons, we developed two techniques (Gires-Tournois Interferometer and Anti-reflection coating) in order to shorten the pulses of THz quantum cascade lasers with metal-metal waveguides

Cette thèse présente une étude exploratoire de plusieurs aspects du couplage fort lumière-matière dans des matériaux moléculaires. Différentes propriétés héritées d’un tel couplage sont démontrées, ouvrant de nombreuses possibilités d’applications, allant du transfert d’énergie à la génération de signaux optiques non-linéaires et à l’élaboration de réseaux polaritoniques chiraux. Au travers des thématiques abordées, l’idée d’un couplage lumière-matière entrant en compétition avec les différentes fréquences de dissipation des molécules se révèle être cruciale. Ainsi, la prédominance du couplage cohérent au champ électromagnétique apparaît comme un moyen de modifier les propriétés quantiques des états moléculaires, ouvrant la voie à une nouvelle chimie des matériaux en cavité
This thesis presents an exploratory study of several aspects of strong light-matter coupling in molecular materials. Different properties inherited from such a coupling are demonstrated, opening the way to numerous applications, ranging from energy transfer to the generation of non-linear optical signals and to the development of chiral polaritonic networks. Through the topics covered, the idea of a light-matter coupling strength competing with the different frequencies of relaxation of the molecules proves to be crucial. Thus, the predominance of the coherent coupling to the electromagnetic field appears as a new mean of modifying the quantum properties of molecular systems, opening the way to a new chemistry of materials in optical cavities

Surface plasmon polaritons are non radiative modes which exist at the interface between a dielectric and a metal. They can confine light at sub-wavelength scales. However, their propagation is restricted by the intrinsic losses of the metal which imply a rapid absorption of the mode. The aim of this thesis is the study of the coupling of surface plasmons in metallo-dielectric planar structures. Obtaining the properties of the modes implies the extension of the solutions to the complex plane of propagation constants. The method used consists in determining the poles of the scattering matrix by means of Cauchy's integrals. The first solution to solve the problem of propagation of the surface plasmons consists in coupling these modes to one another. In a symmetric medium, when the thickness of the metallic film becomes thin enough, the coupling between the plasmon modes which exist on each side becomes possible. One of the coupled modes which is created, the so-called long range surface plasmon, has a bigger propagation length than the usual plasmon whereas the other coupled mode, named short range surface plasmon, has a smaller propagation length. We present a configuration which allows the excitation of the long range surface plasmon without the short range mode with a metallic layer deposited on a perfect electric conductor substrate. This excitation can be done in air and allows applications, such as the detection and the characterisation of molecules. Then, we present the coupling between dielectric waveguides, and, in particular, the coupled-mode theory in the case of the transverse magnetic polarisation. We consider also the case of PT symmetric structure. The last part of this work presents the demonstration of the strong coupling regime between a surface plasmon and a guided mode. We demonstrate an increase of the propagation length of the hybrid surface plasmon, which still has the confinement of a surface mode. A linear gain is added in the different layers of the structure. When the gain is added in the layer between both coupled modes allows an enhancement of the propagation lengths of the modes, and more precisely of the hybrid surface plasmon mode, which can propagate at the millimeter scale.

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.

Cette thèse contribue à la compréhension fondamentale du phénomène de couplage fort de la lumière avec des molécules organiques en mettant en œuvre de nouveaux systèmes et de nouvelles techniques, afin d'étudier les modifications de propriétés de molécules couplées à des résonances photoniques. Nous présentons des techniques de nanofabrication avancées pour la création de grands réseaux de trous sur des métaux et de cavités de Fabry-Pérot (FP) nanofluidiques. Ces systèmes sont ensuite utilisés pour étudier, sous régime de couplage fort, les modifications des propriétés de surface et de volume de molécules organiques en phase solide et liquide. En particulier les transitions électroniques de molécules du colorant cyanine en solution liquide ont été couplées à des modes photoniques résonants de cavités FP nanofluidiques spécialement conçues. Leur couplage fort a conduit à une amélioration du rendement quantique d'émission, mettant en évidence la nature radiative des états polaritoniques
This thesis contributes to the fundamental understanding of the phenomenon of strong coupling of light with organic molecules by implementing new systems and techniques in order to investigate property modifications of molecules coupled with photonic resonances. State-of-the-art nanofabrication techniques for the formation of large hole-array gratings in metals and nanofluidic Fabry-Perot (FP) cavities are presented. These systems were then invested to study, under strong coupling, surface and bulk properties modifications of organic molecules in the solid and liquid phase. In particular, electronic transitions of cyanine dye molecules in liquid solutions were coupled to resonant photonic modes of specially designed nanofluidic FP cavities. Their strong coupling has led to an enhancement of the emission quantum yield, highlighting the radiative nature of the associated polaritonic states

Bose–Einstein condensates (BEC) of exciton–polaritons represent a successful platform for studies of macroscopic quantum physics at elevated temperatures in a solid-state device. Despite the number of breakthroughs in both experiment and theory, there are still some gaps in our understanding of this part-light part-matter system. The difficulties in our interpretation of the system’s behavior arise from the inherent non-equilibrium nature of exciton–polaritons and their coupling with a reservoir of thermal excitons. This optically induced reservoir creates a repulsive potential and serves as a gain medium or source for exciton–polaritons, thus creating a complex-valued potential. In this Thesis, I summarize my PhD work on trapping, controlling, and manipulating exciton–polariton condensates using this complex potential. Chapter 1 of this Thesis introduces the topic of exciton-polariton condensation, the experimental and modeling techniques used in my work, as well as methods for potential landscape engineering for exciton–polaritons. Chapter 2 presents experiments on trapping the condensate in a one-dimensional array of photonic traps, and controlling the population of different energy states in the band-gap structure by applying a spatially structured pump. Chapter 3 demonstrates how the implementation of the finely controlled, fully optically-induced potentials allows us to finely tune the energy and linewidth of the condensate and elucidate its non-Hermitian nature through observation of non-Hermitian spectral degeneracies. Chapter 4 presents a detailed study of the condensation process in the presence of thermal reservoir, which is inherent in optically-induced trapping. Using an ultra-high-Q microcavity, we image single realizations of condensation with unprecedented detail, and observe filamentation of the condensate, which is a direct consequence of reservoir depletion. Chapter 5 presents further work performed in this “single-shot” regime, where we drive the condensate into the high-density regime, and, assisted by the reservoir depletion, observe a homogeneous profile characteristic of the Thomas–Fermi limit. Furthermore, the spectrum of the high-density condensate shows signatures of crossover from BEC to the Bardeen–Cooper–Schrieffer state, which represents a starting point for future studies beyond the scope of this Thesis.

In the last decade, a large number of studies have been devoted to the peculiarities of correlated physics found in the quasi-two-dimensional square lattice iridium oxides. It was shown that this 5d family of transition metal oxides has strong structural and electronic similarities to the famous 3d family of copper oxides. Moreover, a delicate interplay of on-site spin-orbit coupling, Coulomb repulsion and crystalline electric field interactions is expected to drive various exotic quantum states. Many theoretical proposals were made in the last decade including the prediction of possible superconductivity in square-lattice iridates emerging as a sister system to high-Tc cuprates, which however met only limited experimental confirmation. One can, therefore, raise a general question: To what extent is the low-energy physics of the quasi-two-dimensional square-lattice iridium oxides different from other transition metal oxides including cuprates? In this thesis we investigate some of the effects which are usually neglected in studies on iridates, focusing on quasi-two-dimensional square-lattice iridates such as Sr2IrO4 or Ba2IrO4. In particular, we discuss the role of the electron-phonon coupling in the form of Jahn-Teller interaction, electron-hole asymmetry introduced by the strong correlations and some effects of coupling scheme chosen to calculate multiplet structure for materials with strong on-site spin-orbit coupling. Thus, firstly, we study the role of phonons, which is almost always neglected in Sr2IrO4, and discuss the manifestation of Jahn-Teller effect in the recent data obtained on Sr2IrO4 with the help of resonant inelastic x-ray scattering. When strong spin-orbit coupling removes orbital degeneracy, it would at the same time appear to render the Jahn-Teller mechanism ineffective. We show that, while the Jahn-Teller effect does indeed not affect the antiferromagnetically ordered ground state, it leads to distinctive signatures in the spin-orbit exciton. Second, we focus on charge excitations and determine the motion of a charge (hole or electron) added to the Mott insulating, antiferromagnetic ground-state of square-lattice iridates. We show that correlation effects, calculated within the self-consistent Born approximation, render the hole and electron case very different. An added electron forms a spin-polaron, which closely resembles the well-known cuprates, but the situation of a removed electron is far more complex. Many-body configurations form that can be either singlets and triplets, which strongly affects the hole motion. This not only has important ramifications for the interpretation of angle-resolved photoemission spectroscopy and inverse photoemission spectroscopy experiments of square lattice iridates, but also demonstrates that the correlation physics in electron- and hole-doped iridates is fundamentally different. We then discuss the application of this model to the calculation of scanning tunneling spectroscopy data. We show that using scanning tunneling spectroscopy one can directly probe the quasiparticle excitations in Sr2IrO4: ladder spectrum on the positive bias side and multiplet structure of the polaron on the negative bias side. We discuss in detail the ladder spectrum and show its relevance for Sr2IrO4 which is in general described by more complicated extended t-J -like model. Theoretical calculation reveals that on the negative bias side the internal degree of freedom of the charge excitation introduces strong dispersive hopping channels encaving ladder-like features. Finally, we discuss how the choice of the coupling scheme to calculate multiplet structure can affect the theoretical calculation of angle-resolved photoemission spectroscopy and scanning tunnelling spectroscopy spectral functions.

Ce travail porte sur la réalisation de dispositifs quantiques fonctionnant en régime de couplage fort entre une excitation d'un gaz d'électrons dans un puits quantique semiconducteur et un mode de cavité dans le moyen infra- rouge. Les quasi-particules issues de ce couplage lumière-matière sont appelées "polaritons intersousbande". La première partie de ce manuscrit est consacrée à l'étude d'un dis- positif électroluminescent basé sur une structure à cascade quantique in- sérée dans une microcavité planaire. Par une analyse détaillée des spectres d'électroluminescence à différents voltages, je démontre que les états de po- laritons sont peuplés de façon résonante, à une énergie qui dépend du voltage appliqué à la structure. Les résultats expérimentaux sont analysés et in- terprétés à l'aide d'un modèle reliant les spectres d'électroluminescence aux propriétés de l'injecteur de la structure à cascade. Pour augmenter la sélectivité de l'injection et observer ainsi une exaltation de l'émission spontanée, un nouveau type de cavité est développé dans la sec- onde partie de ce travail. Il s'agit d'une cavité basée sur un confinement plas- monique, dans laquelle la lumière est confinée entre deux plans métalliques, dans une épaisseur très inférieure à la longueur d'onde. Le miroir supérieur est façonné en réseau. L'influence des différents paramètres du réseau est étudiée et deux régimes sont mis en évidence: un régime où le mode de cavité se couple à un mode de plasmon de surface et un régime où le mode de cavité ne présente pas de dispersion en énergie. En insérant des puits quantiques dopés dans une cavité de ce deuxième type, les régimes de couplage fort puis de couplage ultra-fort lumière-matière sont démontrés jusqu'à température ambiante. La valeur importante du dédoublement de Rabi et la forte densité d'états polaritoniques obtenues dans ce type de cavité en font un candidat très prometteur pour la réalisation de dispositifs électroluminescents infrarouges de grande efficacité radiative et fonctionnant sans inversion de population.

Les deux grands défis actuels pour l'optoélectronique térahertz (THz) sont d'une part, le besoin de miniaturiser les sources de rayonnement térahertz, et d'autre part, la nécessité d'améliorer leurs performances actuelles. Parmi les sources de rayonnement térahertz existantes, le laser à cascade quantique (QCL) est à ce jour le meilleur candidat pour remplir ces critères. Afin d'y parvenir, il faut cependant apporter des solutions aux verrous qui limitent la miniaturisation des QCLs THz. Le premier est d'ordre fondamental, et tient au fait que les dimensions des cavités photoniques usuelles sont soumises à la limite de diffraction. Le second verrou provient du fait que la recherche de compacité des sources se traduit généralement par la détérioration de leur puissance optique de sortie et de la directionnalité du faisceau laser. Une nouvelle famille de résonateurs THz métal - semiconducteur - métal (M-SC-M) est présentée de façon théorique et expérimentale. Ces dispositifs, inspirés des oscillateurs électroniques LC, ont permis d'atteindre un volume effectif record Veff=LxLyLz/λeff=5.10−6, où Lx,y,z sont les dimensions de la cavité et λeff est la longueur d'onde de résonance dans le cœur du résonateur (GaAs). Ces résonateurs hybrides photoniques-électroniques ont la particularité d'être libérés de la limite de diffraction dans les trois dimensions spatiales, et bénéficient pour la première fois de toutes les fonctionnalités habituellement réservées aux dispositifs électroniques. Une application aux polaritons inter-sousbandes THz a permis d'obtenir des résultats à l'état de l'art, démontrant d'une part que ces résonateurs hybrides conservent leurs propriétés photoniques, et d'autre part qu'ils permettent un couplage lumière-matière fort. En parallèle de ce travail, la faisabilité d'un QCL THz avec une région active extrêmement fine est démontrée expérimentalement. Une étude systématique des caractéristiques du laser en fonction de l'épaisseur de la région active (Lz) a permis la réduction de Lz=10 μm (≈λeff/2,7) jusqu'à la valeur record de Lz=1,75 μm (≈ λeff/13) dans une cavité Fabry-Pérot M-SC-M. Malgré l'augmentation des pertes optiques, l'effet laser est obtenu au-dessus de la température de l'azote liquide (78 K) pour la région active la plus fine. Ces résultats sont très encourageants pour le développement de régions actives plus performantes, et permettent d'envisager le développement de micro-cavités lasers avec des volumes effectifs extrêmement sub-longueur d'onde. Les perspectives de ce travail de thèse s'étendent de l'électrodynamique quantique en cavité au nanolaser. Les applications potentielles varient énormément en fonction de la configuration des résonateurs hybrides. Ils peuvent être utilisés comme des éléments passifs pour la détection, ou encore comme des éléments actifs tels que des antennes. Enfin, l'utilisation d'une région active fine en combinaison avec un résonateur hybride devrait permettre d'obtenir un QCL THz ultra-compact libéré de la limite de diffraction, tout en introduisant pour la première fois la possibilité d'accorder la fréquence du laser en adaptant l'impédance complexe équivalente de la combinaison d'éléments LC.

La condensation de Bose-Einstein est prédite par Einstein en 1925 pour des particules indiscernables de spin entier, les bosons. Il s'agit d'une transition de phase vers un état quantique de cohérence macroscopique, dont la température critique dépend directement de la masse des particules. Ce n'est qu'en 1995 qu'un condensat a pu être formé en phase gazeuse en refroidissant des atomes alcalins à la température ultra-basse de 10−6 degré Kelvin, provoquant ainsi une explosion d'activités de recherche dans le monde sur le sujet. Concernant la phase solide, les excitons dans les semi-conducteurs sont
considérés comme le candidat le plus prometteur pour la condensation de Bose-Einstein. En e_et leur masse est cent mille fois plus légère que celle des atomes alcalins, ce qui devrait permettre leur condensation
à une température voisine du degré Kelvin. Cependant malgré de nombreuses études depuis une trentaine d'années, aucune preuve convaincante de l'existence de condensat d'excitons n'avait été apportée à
ce jour. Récemment l'attention s'est portée sur les polaritons dans les microcavités semi-conductrices contenant des puits quantiques. Une microcavité semi-conductrice à puits quantiques est une hétérostructure
photonique destinée à exalter l'interaction matière-rayonnement entre les excitons con_nés dans le puits quantique et les photons con_nés dans la microcavité. Lorsque l'énergie de ces photons coïncide avec
celle des excitons, la microcavité peut entrer dans le régime de couplage fort d'oscillations de Rabi. Les nouveaux états propres du système (microcavité-puits quantique) sont appelés polaritons qui sont des états
mixtes exciton-photon. Par leur nature photonique, ces bosons possèdent une masse dix mille fois plus légère que celle des excitons, un avantage certain pour l'étude de la condensation de Bose-Einstein.
Nous avons observé l'occupation massive de l'état fondamental du polariton, qui se développe à partir d'un nuage de polaritons thermalisés à une température de (16-20) K. La formation du condensat est accompagn
ée par l'apparition spontanée de la cohérence temporelle et de la cohérence spatiale à longue portée, ainsi qu'une forte polarisation linéaire. La transition d'un état thermique à un état quantique est démontrée par des mesures de la fonction de corrélation d'ordre 2 en fonction de la densité des polaritons. L'ensemble de ces observations constitue la première évidence de la condensation de Bose-Einstein en phase solide.

This thesis presents details of x-ray and neutron scattering experiments used to probe quantum magnets with strong spin-orbit interaction. The first of these systems are the three-dimensional iridate compounds, in which the three-fold co-ordination of IrO₆ octahedra has been theoretically hypothesized to stabilize anisotropic exchange between Ir⁴⁺ ions. This novel interaction between these spin-orbital entangled, J_eff=1/2 moments is described by a Hamiltonian first proposed by Kitaev, and would be the first physical realization of this Hamiltonian in a condensed matter system. This thesis details the determination of the structure of a new polytype within these compounds, the 'stripyhoneycomb' γ-Li₂IrO₃. Furthermore, through resonant magnetic x-ray diffraction experiments on single crystals of β-Li₂IrO₃ and γ-Li₂IrO₃, an incommensurate, non-coplanar structure with counter-rotating moments is found. The counter-rotating moment structure is a rather counter-intuitive result, as it is not stabilizied by Heisenberg exchange between magnetic sites, however, the Kitaev exchange naturally accounts for this feature. As such, these experiments reveal, for the first time, systems which exhibit dominant Kitaev interactions. The ordering wavevector of both β- and γ-Li₂IrO₃ polytypes are found to be identical, suggesting that the same magnetic interactions are responsible for stabilizing magnetic order in both materials, despite their different lattice topologies. Following this, the spinel FeSc₂S₄ is considered. Here, despite the presence of strong exchange between Fe^{2+,/sup>, and the fact that these ions sit in a Jahn-Teller active environment, the system does not order in the spin or orbital degrees of freedom. A 'spin-orbital singlet' has been theoretically proposed to describe the groundstate of this system, and here inelastic neutron scattering (INS) is used to probe the resulting triplon excitations. This allows determination of microscopic parameters in the single ion and exchange Hamiltonians, and moreover experiments in external magnetic field reveal the true spin-and-orbital nature of these triplon excitations. Finally, Ba₃CoSb₂O₉, a physical realization of the canonical triangular antiferromagnet model is explored with INS and the high energy excitations from the 120 degree magnetic structure are found to display significant differences from those calculated by linear spin wave theory, suggesting the presence of quantum dynamics not captured in the 1/S linear spin wave expansion.}

Les systèmes fortement corrélés, pouvant adopter des phases surprenantes de la matière, émergent dans le domaine des atomes ultra-froids ou dans celui de l’électrodynamique quantique en cavité (CQED). Ceux-ci sont au centre d’intenses travaux expérimentaux et théoriques. Dans cette thèse, nous présentons une étude de deux modèles de bosons avec deux ou zéro états internes. Ceux-ci peuvent se déplacer sur un réseau, et sont localement couplés avec des spins 1/2. Notre intérêt réside dans la détermination du diagramme de phase de l’état fondamental de ces systèmes ainsi que de l’étude des propriétés de phase et des transitions entre ces dernières. Nous avons utilisé deux outils : une approximation de champ moyen et des simulations de Monte-Carlo quantique, qui fournit des résultats numériquement exacts. Le premier modèle, appelé modèle de Kondo bosonique sur réseau, s’inscrit dans le contexte des atomes ultra-froids sur réseau. Nous trouvons que sa physique est proche de celle du modèle de Bose-Hubbard, présentant des phases de Mott et superfluide. Le couplage local renforce le caractère isolant et on observe l’émergence de phases magnétiques au travers de couplage direct ou indirect entre bosons et/ou spins. Les effets thermiques, inhérents à tout dispositif expériemental, sont aussi étudiés. Le second modèle s’inscrit dans le domaine de la CQED sur réseau, décrit un régime de couplage ultra-fort entre des photons et des atomes, et est appelé modèle de Rabi sur réseau. Le diagramme de phase présente juste deux phases : une phase cohérente dans laquelle les spins locaux s’ordonnent ferromagnétiquement ainsi qu’une phase incohérente compressible paramagnétique
Strongly correlated systems, where new surprising phases of matter may appear both in the context of ultra-cold atoms and cavity quantum electrodynamics, are the focus of intense experimental and theoritical activity. In this thesis we present a study of two models of bosons with two or zero internal states, that is to say spin-1/2 or spin-0 bosons. These particles can move around a lattice, and they are locally coupled to immobile spins 1/2. Our interest was to determine the ground state phase diagram, study phase properties and quantum phase transitions. We used two methods: an approximate one using a mean field approach and the other using quantum Monte-Carlo simulations, which provides numerically exact results. The first model, namely the bosonic Kondo lattice model, is in the context of ultra-cold atoms in optical lattices. We found that its physics is close to that of the Bose-Hubbard model, exhibiting Mott and superfluid phases. The local coupling strengthens the insulating behaviour of the system and magnetism emerges through indirect or direct coupling between bosons. Thermal effects, inherent in experiments, are also studied. The second model, which is in the context of light-matter interaction, describes a situation of an ultra-strong coupling between spin-0 bosons (photons) and local spins 1/2 (two levels atoms) and is known as the Rabi lattice model. The phase diagram generally consists of only two phases: a coherent phase and a compressible incoherent one. The locals

This publication is dedicated to investigate strong light-matter coupling with excitons in 2D materials. This work starts with an introduction to the fundamentals of excitons in 2D materials, microcavities and strong coupling in chapter 2. The experimental methods used in this work are explained in detail in chapter 3. Chapter 4 covers basic investigations that help to select appropriate materials and cavities for the following experiments. In chapter 5, results on the formation of exciton-polaritons in various materials and cavity designs are presented. Chapter 6 covers studies on the spin-valley properties of exciton-polaritons including effects such as valley polarization, valley coherence and valley-dependent polariton propagation. Finally, the formation of hybrid-polaritons and their condensation are presented in chapter 7
Diese Veröffentlichung beschäftigt sich mit starker Licht-Materie Kopplung mit Exzitonen in 2D Materialien. Dies Arbeit beginnt mit einer Einführung in Exzitonen in 2D Materialien, in Mikrokavitäten und starke Licht-Materie Kopplung (Kapitel 1). Die verwendeten, experimentellen Methoden werden in Kapitel 3 beschrieben. Kapitel 4 deckt Voruntersuchungen ab, die helfen die richtigen Materialien und Mikrokavitäten für die folgenden Experimente auszuwählen. In Kapitel 5 werden die Ergebnisse zur Erzeugung von Exziton-Polaritonen in verschiednen Materialen und Kavitäten gezeigt. Kapitel 6 beschäftigt sich mit Untersuchungen der Spin-Tal Eigenschaften der Exziton-Polaritonen, inkl. Effekte wie Tal Polarisation und Koherenz. Abschließend wird in Kapitel 7 die Erzeugung von Hybrid-Polaritonen und deren Kondensation dargestellt

Organic photovoltaic technology is a very promising area of research - potentially able to balance the current energy demand the world requires every day. Organic materials in-herit fascinating properties like tailorable optical properties, adjustable optical bandgap and electrical conductivity, and easy and cheap processing among others. These materials also present short exciton diffusion lengths, low charge mobilities and weak absorption, negatively impacting the short-circuit current (Jsc) and open-circuit voltage (Voc) of organic solar cells and their performance. Light-matter coupling is an exponentially growing field of research, which focuses on the coupling of photonic modes and excitonic states of matter. This interaction results in the formation of hybrid states of matter - polaritons - with interesting new properties. The pioneer work of incorporating this concept in optoelectronic devices has allowed for improved photoconductivity, long-range energy transfer, reduced optical losses, higher harvesting rates and enhanced power conversion efficiencies. The work of this thesis focuses on the simulation, fabrication, and characterization of organic solar cells with a nanostructured layer to optimize its short-circuit current, as well as simpler systems of metallic lattices to study strong coupling between collective plasmonic resonances and organic excitons in standard donor-acceptor blends. Full solar cells devices containing square gold lattices were achieved. The open-circuit voltage and fill factor (FF) retrieved through J-V curves remained roughly unchanged whereas the short-circuit is slightly reduced. Although the investigated solar cells do not show evidence of strong coupling as they were not optimized, these measurements illustrate that it is possible to structure the electrode of the cell with a structure supporting optical modes without degrading the Voc. External quantum efficiency analysis provided an insight on how the gold lattices influence the quantum efficiency spectrum of the solar cells, allowing the absorption edge to be shifted towards lower energies in some cases.
A tecnologia de fotovoltaicos orgânicos é um campo de pesquisa bastante promissor - potencialmente capaz de equilibrar a atual procura energética a nível mundial. Os materiais orgânicos possuem propriedades fascinantes tais como a adaptabilidade de propriedades óticas, ajustabilidade do hiato ótico e condutividade elétrica, apresentando um custo reduzido e facilidade de fabrico, entre outros. Por outro lado, estes materiais também possuem cumprimentos de difusão de excitões bastante curtos, mobilidades de carga baixas e baixa absorção, o que influencia negativamente a corrente de curto-circuito (Jsc) e a tensão de circuito aberto (Voc) de uma célula solar orgânica e, consequentemente, o seu desempenho. A área da física que estuda o coupling entre luz e matéria encontra-se num crescimento exponencial, focando-se principal-mente no acoplamento de modos fotónicos e excitões. Esta interação resulta na formação de estados híbridos de matéria - polaritões - que possuem novas propriedades interessantes. O trabalho pioneiro de incorporar este conceito em dispositivos optoeletrónicos com matrizes nano estruturadas permitiu uma melhoria da fotocondutividade, uma transferência de energia a longo alcance, redução de perdas ópticas, recolhas de cargas mais altas e eficiências de conver-são de energia superiores nestes dispositivos. Este trabalho foca-se em simular, fabricar e caracterizar células solares orgânicas com um elétrodo nano estruturado, bem como sistemas mais simples de matrizes metálicas cujo objetivo é estudar o strong coupling entre ressonâncias coletivas e excitões orgânicos que resultam de misturas orgânicas de camadas doadoras e aceitadoras standard na industria. Neste trabalho foram realizados dispositivos de células solares contendo matrizes de ouro de forma a maximizar a Jsc destas, bem como sistemas mais simples de matrizes metálicas para estudar strong coupling entre estas e modos excitónicos. A tensão de circuito aberto e o fator de forma (FF), extraídos através das curvas J-V, permaneceram praticamente inalterados, enquanto que a corrente de curto-circuito sofreu uma ligeira redução. Apesar das células solares investigadas não apresentarem quaisquer evidência de strong coupling dado que estas não se encontram totalmente otimizadas, estas medições provam que é possível modificar um elétrodo, de forma a que este suporte modos óticos, sem que haja uma degradação da tensão de circuito aberto.A análise da eficiência quântica externa forneceu uma visão sobre como as matrizes de ouro influenciam fortemente a forma da eficiência quântica, permitindo que a fronteira de absorção se desloca-se para energias mais baixas em alguns casos.

Monolayer transition metal dichalcogenide crystals (TMDCs) hold great promise for semiconductor optoelectronics because their bound electron-hole pairs (excitons) are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, their excitons can hybridise with cavity photons to form exciton polaritons (polaritons herein), which inherit useful properties from their constituents. For example, the low effective mass inherited from the photonic component enables polaritons in principle to macroscopically occupy their ground state at room temperature, i.e., to form a polariton condensate. Such a polariton condensate can behave like a superfluid and hence, polariton condensation in TMDCs might be a route towards dissipationless transport of information carriers on a microchip. However, because of the low effective interactions between the excitons in TMDCs, spontaneous formation of a polariton condensate in a single monolayer is challenging and clear evidence was not demonstrated yet. In other material systems it was shown that maximising the polariton lifetimes and increasing their density through strong confinement can help to reach this regime. A prevailing approach to generate polaritons with large lifetimes in conventional semiconductors are all-dielectric planar microcavities, in which polaritons can be spatially confined by patterning the cavity spacer. However, integrating monolayer TMDCs in these high-quality structures remains a challenge since they are notoriously fragile and their excitonic properties are extremely sensitive to many nanofabrication techniques. This thesis presents experimental work performed with the aim to integrate TMDCs in high-quality all-dielectric microcavities and create optimal conditions for driving the system to the regime of bosonic condensation at room temperature. In particular, we focus on monolayer WS2, which has a high exciton quantum yield and displays strong light-matter interaction at room temperature. First, the challenge presented of integrating monolayer WS2 into high-Q microcavities without causing damage to the monolayer is overcome by mechanical assembly. We show that WS2 polaritons in such a microcavity can propagate ballistically over tens of micrometres in the thermal regime, before the onset of condensation, and possess enhanced macroscopic coherence due to strong motional narrowing and weak inter-particle interactions. However, the mechanical assembly process of the microcavities is intrinsically non-scalable. To enable the integration of TMDC monolayers into functional devices on larger scales, we developed a new passivation and protection technology utilising liquid-metal printed, ultrathin Ga2O3 glass. We show that the Ga2O3 film strongly suppresses exciton-exciton annihilation in monolayer WS2, which prohibits large exciton densities in blank TMDC monolayers, and that it provides excellent protection against dielectric material deposition. The latter allows us to integrate the monolayer into all-dielectric environments with conventional deposition techniques while maintaining its high optical performance. Finally, we engineer an effective trapping potential for WS2 polaritons at room temperature by placing a WS2/Ga2O3/WS2 structure with a small top layer inside a microcavity. This design allows us to compare the properties of trapped and free polaritons. Remarkably, the ground state emission and the macroscopic coherence is strongly enhanced when trapped due to efficient energy relaxation, long photon lifetimes and strong motional narrowing. Overall, this thesis provides significant insights into the properties and dynamics of free and trapped WS2 polaritons in the thermal regime at room temperature, which can guide future work towards demonstrating unambiguous signatures of polariton condensation in this novel material class.

This thesis proposes a partition of optical states into radiative and non-radiative parts when an emitter is proximal to resonant absorbing nanostructures. The conventional partition is valid only for the weak coupling regime (Purcell regime), and the proposed partition is significant to understand spontaneous emission in the (moderate or) strong coupling regime of an emitter and plasmonic metal nanostructures. It highlights and explains the anomalous large increase in the spontaneous emission of photons from emitters placed near fully absorbing plasmonic nanoparticles (< 10 nm in dimensions) that do not scatter light. Further, this work also explains the origins of the large gains observed in surface-enhanced-Raman-spectroscopy (SERS). In SERS, a rough metal surface (or metal nanopore) effects a near-field enhancement of the incident radiation exciting the proximal molecule by factors up to 10^5. But the radiation emitted from the molecule is predicted to be largely dissipated by the metal, making the observed large gains of emission anomalous in conventional theory. This remarkable divergence of SERS from theoretical predictions has been widening for four decades, during which the reported SERS enhancements have grown from 10^4 to 10^{14}. The first objective of this work was to establish the divergence of multiple independent experimental observations from the theory, using quantitative evaluations of an emitter coupled to metal nanostructures. The second part involved a study of collective spontaneous emission from multiple emitters coupled to metal nanoparticles; the study was possible due to a computational method developed earlier for solving such problems. This established that collective modes of emission from many emitters are not the source of this divergence of theory from the observations. Later, we proposed a theory for a modified partition of optical states into the radiative and non-radiative (absorbing) parts, which is also valid for the strong-coupling regime of an emitter and absorbing matter. Note that the effects of a weak coupling, also known as the Purcell effect, can be recast as the quantum interference of the classical paths of a photon. We invoked the quantum interference of additional paths involved in the strong coupling regime of the emitter and a metal nanostructure. These additional non-classical paths of the photon arise due to the possible re-absorption of the photon by the emitter, from the excited metal nanostructure. This modified partition of optical states was shown to predict the experimental observations well. Finally, the proposed theory was also incorporated into the models of collective emission, and this allowed us to elucidate the coherence of these classical and non-classical paths in bulk materials dispersed with extremely small metal nanoparticles. To conclude this work, we also studied the decoherence of this effect with variations in the number of emitters and metal particles, and the role of finite sizes of emitters on the strengths of coupling and this resulting effect. Our work that further establishes the proposed theory using a first principles microscopic model of non-local interactions will be reported elsewhere.

In the last decade, a large number of studies have been devoted to the peculiarities of correlated physics found in the quasi-two-dimensional square lattice iridium oxides. It was shown that this 5d family of transition metal oxides has strong structural and electronic similarities to the famous 3d family of copper oxides. Moreover, a delicate interplay of on-site spin-orbit coupling, Coulomb repulsion and crystalline electric field interactions is expected to drive various exotic quantum states. Many theoretical proposals were made in the last decade including the prediction of possible superconductivity in square-lattice iridates emerging as a sister system to high-Tc cuprates, which however met only limited experimental confirmation. One can, therefore, raise a general question: To what extent is the low-energy physics of the quasi-two-dimensional square-lattice iridium oxides different from other transition metal oxides including cuprates? In this thesis we investigate some of the effects which are usually neglected in studies on iridates, focusing on quasi-two-dimensional square-lattice iridates such as Sr2IrO4 or Ba2IrO4. In particular, we discuss the role of the electron-phonon coupling in the form of Jahn-Teller interaction, electron-hole asymmetry introduced by the strong correlations and some effects of coupling scheme chosen to calculate multiplet structure for materials with strong on-site spin-orbit coupling. Thus, firstly, we study the role of phonons, which is almost always neglected in Sr2IrO4, and discuss the manifestation of Jahn-Teller effect in the recent data obtained on Sr2IrO4 with the help of resonant inelastic x-ray scattering. When strong spin-orbit coupling removes orbital degeneracy, it would at the same time appear to render the Jahn-Teller mechanism ineffective. We show that, while the Jahn-Teller effect does indeed not affect the antiferromagnetically ordered ground state, it leads to distinctive signatures in the spin-orbit exciton. Second, we focus on charge excitations and determine the motion of a charge (hole or electron) added to the Mott insulating, antiferromagnetic ground-state of square-lattice iridates. We show that correlation effects, calculated within the self-consistent Born approximation, render the hole and electron case very different. An added electron forms a spin-polaron, which closely resembles the well-known cuprates, but the situation of a removed electron is far more complex. Many-body configurations form that can be either singlets and triplets, which strongly affects the hole motion. This not only has important ramifications for the interpretation of angle-resolved photoemission spectroscopy and inverse photoemission spectroscopy experiments of square lattice iridates, but also demonstrates that the correlation physics in electron- and hole-doped iridates is fundamentally different. We then discuss the application of this model to the calculation of scanning tunneling spectroscopy data. We show that using scanning tunneling spectroscopy one can directly probe the quasiparticle excitations in Sr2IrO4: ladder spectrum on the positive bias side and multiplet structure of the polaron on the negative bias side. We discuss in detail the ladder spectrum and show its relevance for Sr2IrO4 which is in general described by more complicated extended t-J -like model. Theoretical calculation reveals that on the negative bias side the internal degree of freedom of the charge excitation introduces strong dispersive hopping channels encaving ladder-like features. Finally, we discuss how the choice of the coupling scheme to calculate multiplet structure can affect the theoretical calculation of angle-resolved photoemission spectroscopy and scanning tunnelling spectroscopy spectral functions.