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Advantages and remaining issues of state-of-the-art m-plane freestanding GaN (FS-GaN) substrates grown by halide vapor phase epitaxy (HVPE) for m-plane InGaN epitaxial film growth by metalorganic vapor phase epitaxy are described. Because of the low threading dislocation and basal-plane stacking fault densities, improved quantum efficiency and short radiative lifetime are achieved for the near-band-edge emission of 200–250 nm thick m-plane pseudomorphic InGaN epilayers. As the surface flatness is greatly improved, the In-incorporation efficiency is lower than the cases for conventional c-plane growth and m-plane growths on due to the reduction in the area of inclined and tilted planes. Sub-micrometer-wide zonary patterns parallel to the c-axis and 2 μm long axis figure-of-eight patterns parallel to the a-axis are clearly visualized in the monochromatic cathodoluminescence (CL) intensity images.

Spatio-time-resolvedcathodoluminescence (STRCL) spectroscopy isimplemented to assess the local carrier dynamics in a 70 nm-thick, very low threading dislocation (TD) density, pseudomorphic m-plane In GaN epilayer grown on a freestanding GaN substrate by metalorganic vapor phase epitaxy. Although TDs or stacking faults are absent, sub-micrometer-wide zonary patterns parallel to the c-axis and 2 um-long-axis figure-of-8 patterns parallel to the a-axis are clearly visualized in the monochromatic cathodoluminescence intensity images. Because the STRCL measurement reveals very little spatial variation of low-temperature radiative lifetime, the considerable peak energy variation is interpreted to originate from nonidentical In-incorporation efficiency for the growing surfaces exhibiting various miscut angles. The figure-of-8 patterns are ascribed to originate from the anisotropic,severe m-plane tilt mosaic along the a-axis of the GaNsubstrate,and the zonary patterns may originate from the m-plane tilt mosaic along the c-axis

We investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320 K range. We first show that the effective lifetime of QW excitons s increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing s depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs.

Quantized vortices appear in quantum gases at the breakdown of superfluidity. In liquid helium and cold atomic gases, they have been indentified as the quantum counterpart of turbulence in classical fluids. In the solid state, composite light-matter bosons known as exciton polaritons have enabled studies of non-equilibrium quantum gases and superfluidity. However, there has been no experimental evidence of hydrodynamic nucleation of polariton vortices so far. Here we report the experimental study of a polariton fluid flowing past an obstacle and the observation of nucleation of quantized vortex pairs in the wake of the obstacle. We image the nucleation mechanism and track the motion of the vortices along the flow. The nucleation conditions are established in terms of local fluid density and velocity measured on the obstacle perimeter. The experimental results are successfully reproduced by numerical simulations based on the resolution of the Gross-Pitaevskii equation.

Microcavity exciton-polaritons are half-matter half-light quasiparticles with outstanding properties. This offers a great way to access and control fundamental excitations in a solid by simple optical means. Therefore, apart from their importance in fundamental physical research, microcavity polaritons play an important role in various application proposals in the field of all-optical and quantum computing. This thesis is dedicated to the study of confined zero-dimensional microcavity exciton-polaritons. Confinement in all three dimensions is obtained in our case through shallow optical traps created by small local elevations on top of the spacer layer of a planar microcavity sample. This results in a discrete spectrum of confined polariton states, which co-exist with the two-dimensional polaritons in the planar region of the sample. First we investigate the confined polariton states through resonant continuous wave optical excitation. We demonstrate different ways to manipulate the wave function of the trapped zero-dimensional polaritons by optical means. The experimental results are explained and reproduced in the frame of a theoretical model based on the Gross-Pitaevskii equations. Essential for the manipulation is a phase locking effect which locks the phase of the coherent polariton state to the phase of the laser at the excitation point in the reciprocal space. We investigate as well the spatial dynamics of confined polaritons after resonant optical excitation. The experiments are performed in the linear, as well as in the nonlinear regime. In the linear regime, when exciting a small number of well separated states, the observed dynamics are explained in terms of interference between the excited states. In the case where the energy separation becomes equal or smaller than the linewidth of the confined states, the observed dynamics are better explained in terms of ballistically propagating two-dimensional polaritons. In the nonlinear regime we evidence the role of the energy blue shift and parametric scattering on the spatial dynamics. We evidence as well a spin anisotropy of the nonlinear polariton interactions. Further we show how the nonlinear and anisotropic polariton interactions give rise to a spin dependent bistability in our system of confined polaritons. Based on this bistability we demonstrate an all-optical spin flip-flop with outstanding performance. The results are explained with a theoretical model, which shows that the flip-flop operations are based on nonlinear losses between the spin-up and spin-down polariton population. The demonstrated operation could lead to the creation of polariton based high speed all-optical RAM devices for optical computing.

The present Ph.D. thesis consists in a series of experiments carried out in the Laboratory of Quantum Optoelectronics under the direction of professor Benoît Deveaud-Plédran between April 2006 and April 2010. We study the effect of lateral confinement on the dynamics of microcavity polaritons according to two important subjects. On the one hand, we study the polariton relaxation, on the other hand, we study the role of the spin in polariton mutual interactions. We first introduce microcavity polaritons, their optical properties and present the polariton pseudospin formalism. In our sample, polaritons are trapped through their photonic component in cylindrical extensions of the cavity length called mesas. Polariton relaxation is studied in the linear and nonlinear regimes. In the linear regime, we demonstrate that polariton interactions with acoustic phonons are enhanced under lateral confinement. Thermalization, which is forbidden in planar microcavities, is facilitated by confinement, and is very efficient in small diameter mesas. We develop for the first time a comprehensive model of polariton relaxation dynamics under confinement, and highlight the role of polariton states with large exciton content. This work will be profitable to the design of future samples dedicated to the study of Bose-Einstein condensation. In the nonlinear regime, we study the impact of polariton-polariton collisions on the spatial dynamics of microcavity polaritons. In the low-density regime, when the mesa is excited in a coherent superposition of the three lowest energy states, the polariton dynamics is characterized by dipole oscillations. In the high-density regime, we observe a continuous damping of the dipole oscillation. This is due to multiple parametric scattering processes, which redistribute the energy to the benefit of the ground state. Moreover, we demonstrate that the collisional damping is more efficient for collisions between polaritons of the same spin than between polaritons of opposite spins. We investigate in detail the influence of polariton spin in the interactions between polaritons. We first consider the case of planar polaritons. We observe that optical bistability is strongly reduced, or even suppressed, when the system contains polaritons of opposite spins. This results from a pairing of polaritons of opposite spins to form biexciton states. We demonstrate the polarization control of the different optical instability regimes. We then study the case of confined polaritons. As the system is cleaner (we work with a single polariton energy level), we achieve full optical control over the spinor interactions. We demonstrate the first realization of multi-valued switching with a coherent spin ensemble in the solid state. This is a very important step in the research on spin manipulation for the development of spin optoelectronic devices. Finally, we show the influence of the polariton spin on the coupling between planar and confined polaritons and on the control of the polariton fluid dynamics. All our experiments are confirmed by theoretical models. We propose perspectives for the next studies on the dynamics of polaritons and on the coherent spin manipulation in microcavities.

Self-assembled quantum-dots (QDs) represent a distributed ensemble of zero dimensional structures, each presenting a near-singular density of states. The QD size, shape, areal density and optical properties depend on growth parameters, such as growth temperature, growth rate and amount of InAs deposed. QDs are very suitable for optoelectronic device applications as during the epitaxial QD growth the phase transition relieves the strain elastically without introducing defects. Embedded in the active medium of a semiconductor laser diode QD's unique properties lead to improved and often novel characteristics as compared to bulk or quantum-well (QW) devices. In particular In(Ga)As QD lasers on GaAs substrates are of largest interest as the spectral range of their emission wavelength reaches further into the infrared than that of QW lasers of the same material system. The emission wavelength of strained InGaAs QW lasers is limited to about 1150nm, whereas in In(Ga)As QD lasers an emission wavelength beyond 1.3µm is feasible. Such kind of QD lasers performed – as recently reported – ultralow threshold current densities combined with decreased temperature sensitivity, relatively high modulation bandwith, low chirp, which has been already predicted many years ago. Beside quantum dot's application in semiconductor lasers, it has been suggested to use QDs also for the optical active region in so-called semiconductor optical amplifier (SOA). So far, experimentally it was possible to demonstrate in QD SOAs a much faster gain recovery time due to more effective carrier re-filling from excited states than in quantum well structures. Also a smaller noise figure ratio compared to QWs was predicted in QD SOA, but has not been experimentally demonstrated yet. However, two major constraints related to the limit of the high-frequency modulation of QD lasers and the QD SOA's polarization sensitivity are required to be first resolved, before it is possible to implement both technologies replacing QW devices in optical broad telecommunications devices. The polarization properties of semiconductor quantum dot/-dash lasers and amplifiers have been addressed in this PhD thesis. Within this research work – studying systematically several quantum dot and -dash heterostructure of different material families – two major goals have been identified: (1) characterization of the optical polarization properties of quantum dot/-dash structures embedded in the active region of semiconductor optical amplifiers and (2) realization of polarization-insensitive quantum dot based SOAs. Moreover, this thesis focuses on the understanding of the underlying physics being responsible for the QD's polarization and the dependence of the optical gain on the QD's aspect ratio and compositional material contrast. In terms of the experimental work broad area laser devices have been fabricated by applying different clean room processing methods like photolithography, metal deposition, wet and plasma etching. Through a subsequent device characterization using optoelectronic measurement techniques the polarization characteristics of dots and dashes have been accessed. From broad area laser device characterization – measuring the polarization-resolved edge-emitted electroluminescence (EL) – the polarization properties of standard Stranski-Krastanov (SK), closely- and columnar stacked InAs/GaAs quantum dots as well of closely-stacked InAs/InP quantum dots and columnar-stacked InAs/InP quantum dashes – in total 32 devices – have been evaluated. The comparison of the transversal magnetic (TM) versus -electric (TE) integrated EL signal for above heterostructures provided very promising results in order to achieve polarization-independent QD SOA: Starting from standard SK QDs – emitting dominantly in TE mode – it was possible first to enhance the TM EL signal compared to TE by epitaxial shape-engineering of the dot/dash structure of the QD's aspect ratio and the compositional material contrast in columnar-stacked InAs/GaAs (InAs/InP) QDs (QDashes), and then inverting the polarization direction from dominant TE towards dominant TM. The latter polarization transition has been also confirmed by photovoltage spectroscopy (PVS), and as well from room-temperature TM-lasing demonstration – both carried out by our research partner from the Cardiff University, Wales (UK). Introducing the well-known segment contact method for gain measurements, which is based on the analysis of the amplified spontaneous emission (ASE) generated by cavities of different length, it was possible to access both the TM and TE gain of such columnar-stacked InAs/GaAs quantum dots. The work was motivated and funded by the European Union project "Zero Order Dimension based Industrial components Applied to teleCommunications" (ZODIAC) within the sixth framework program. Very fruitful collaborations in order to exchange scientific ideas/know-how and achieving joint results have been established in a pan-European and multicultural inspired environment between four industrial partners – Alcatel Thales III-V Lab (France), Bookham (GB), Innolume – GmbH (Germany), Nanoplus Nanosystems and Technologies-GmbH (Germany) – and three national research institutes – Laboratory for Photonics and Nanostructures (France), The Tyndall National Institute (Irland), Saint-Petersburg Physics and Technology Centre for Research and Education of the Russian Academy of Science (Russia) – and three Universities: École Polytechnique Fédérale de Lausanne (Switzerland), Politechnika Wroclawska (Poland), Würzburg University (Germany). Besides this European ZODIAC project it was possible to establish with the Cardiff University, Wales (UK) a very constructive collaboration.

Gallium Nitride (GaN) and its ternary alloys with aluminium and indium have met a growing interest in the last decade. These semiconductors have a large direct bandgap and can be doped with either silicon (Si) for n-type and magnesium (Mg) for p-type layers. Consequently, they are attractive targets for the fabrication of electrically driven light-emitting diodes, laser diodes and photodetectors, operating in the UV and visible range. Nitride semiconductors are also robust ans stable at high temperatures, suggesting their use in high frequency – high power electronic devices. The research on the III-nitride semiconductors is still in its infancy and, although several devices have been demonstrated and commercialized, there are still many issues to be solved. In particular, the III-nitride materials suffer from the lack of an adequate lattice-matched substrate for epitaxial growth, the high density of defects and dislocations, the poor quality of the p-type layers (low hole concentration and mobility), localization effects in indium containing layers, chemical inertness, etc… Also, due to the material quality issues, some electrical and optical parameters are only approximatively known. AlInN alloys have not attracted much attention compared to the InGaN and AlGaN ternary alloys, due to their unstable growth conditions. High-quality AlInN layers were deposited by metalorganic vapor-phase epitaxy (MOVPE). AlInN with an indium content of 18% is lattice-matched to GaN. It has a bandgap energy of about 4.35 eV and a refractive index contrast to GaN of 7% at 450 nm. It is shown that the optical, electrical and structural characteristic of epitaxial AlInN layers start degrading above a critical thickness of 200 nm, due to an ongoing pollution of the MOVPE reactor by products of the reaction. The distributed Bragg reflector (DBR) is an essential building block for advanced optoelectronic devices such as vertical-cavity surface emitting lasers (VCSELs). The introduction of the lattice-matched AlInN quarter-layers allows to grow thick DBR structures, with limited strain issues. It is used with GaN quarter-layers for visible reflectors and AlGaN quarter-layers for UV reflectors Optimizing the growth conditions of AlInN layers leads to the realization of DBRs with reflectivity above 99 % in the blue wavelength range (410-450 nm) as well as in the UV wavelength range (345 nm). The novel AlInN/GaN DBR is introduced in various microcavity LED (MCLED) structures. The basic features of the MCLED devices are presented, as well as optical simulations obtained using the transfer matrix representation (TMR) formalism, coupled with a dipole approximation for source terms. Electrical characterizations on these devices show that an intra-cavity contact (ICC) scheme should be used to achieve a sufficient electron injection in the active region. The external quantum efficiency of MCLED devices with thick AlInN/GaN DBRs (12 pairs) were lower than expected, due to the degradation of the crystalline quality of the active layers caused by the pollution of the reactor. Another requirement for the realization of a nitride VCSEL is the optimization of the electrical injection of carriers in the optical cavity. Assuming that circular metallic contacts are used, the carriers should spread laterally and be injected in the active region under the top DBR. This is generally achieved by the combination of a current spreading cap layer, which helps for the lateral injection of the carriers, and a current confinement layer, which inhibit the injection of the carriers in the active region outside of the vertical cavity. For the III-nitride structures, several methods are available for the lateral injection: n+/p+ tunnel junctions, doped-AlGa(In)N/GaN superlattices, semi-transparent electrodes, etc… A novel technique is developed to oxidize selectively surface or buried AlInN layers and form a stable, semi-insulating compound, with a refractive index of about 1.8. A detailed description of the technique is presented. It can be applied to form current confinement layers in VCSELs, but also layers for high reflectivity distributed Bragg reflectors (DBRs), layers for improved light confinement in planar waveguides or insulating buffer layers for field effect transistors (FETs). Also, oxidized AlInN could be used as a sacrificial layer for the preparation of free-standing GaN substrates. In this work, the technique was applied to form a current confinement aperture in a LED device. A buried AlInN layer was oxidized laterally, leaving a small 5 µm aperture in a 28 µm × 28 µm mesa structure. Optical and electrical confinement are demonstrated. With the availability of a high-reflectivity lattice-matched nitride DBR and a technique to produce small current apertures, both based on the new AlInN material, the realization of an electrically injected VCSEL becomes a realistic goal. An ideal structure is realized and discussed.

I report on the effect of a moderate excess electron population on nonlinearities in modulation-doped CdTe quantum wells. I show that the electron population does not qualitatively affect the nature of correlations between excitons. In this respect, I bring strong evidence of the existence of unbound and bound (stable) two-exciton states in the presence of electrons and charged excitons (trions). In time-resolved pump and probe experiment, they lead to the observation of electromagnetically induced transparency and optical Stark shift of the exciton resonance. Rabi flopping of excitons within a sea of electrons is also clearly evidenced through ac Stark splitting and gain without inversion. The quantum coherence is more robust to electron induced dephasing than what would have been expected. I demonstrate third and higher-order exciton correlations in the presence of electrons, which manifest, for increasing exciton densities, through the blue-shift of counter-polarized exciton resonance, the red-shift of the biexciton resonance and the exciton to biexciton crossover. I also evidence correlated behavior of excitons and trions under excitation which manifests itself by crossed trion-exciton effects. I observe a wealth of phenomena encompassing bleaching, crossed bleaching, induced-absorption and energy shifts of the resonances. Significant differences are found between the nonlinear optical effects induced by an exciton and a trion population. Electron scattering with electron and exciton is shown to strongly broaden the high energy tail of exciton and trion resonance lineshapes. Variations of electron density result in a clear modification of both resonance lineshapes. The dynamics of the formation of trions is also studied. I propose a two-channel mechanism for their formation; they are formed through bi- and tri-molecular processes. This implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is applied to a time-resolved photoluminescence experiment performed on a very high quality InGaAs quantum well sample, in which the photoluminescence contributions at the energy of the trion, exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor is explained.

Polaritons are hybrid quasi-particles made of photons and excitons. They represent the fundamental electronic excitations of an insulator interacting with the electromagnetic field. In planar semiconductor microcavities, polaritons are confined along one spatial direction and can be described as two-dimensional quasi-particles possessing a very light effective mass of the order of 10-5 times the free electron mass. Their spatial confinement in the two lateral directions is a challenging task because of the large group velocity. On the other hand, spatial trapping would be beneficial for the long sought phenomenon of polariton Bose-Einstein Condensation, as it proved to be for BEC of alkali atoms. Here, we report on the first realization of a spatial trap for polaritons through local variations of a microcavity thickness, within micrometer sized circular regions. Due to their very light mass, confined polaritons display quantized energy levels that are delocalized in reciprocal space, already when trapped within a spatial region of several microns. We characterize these levels using angle-resolved emission spectroscopy and in comparison with a theoretical model. This novel structure proves very effective in producing spatial confinement and opens the way for the realization of mesoscopic devices showing quantum collective phenomena.

In the ever-growing world of optical telecommunications. Semiconductor Optical Amplifiers (SOAs) are called to play an increasingly important role. Their attractive performances both in the linear and non-linear regime along with their reduced cost and small size compared to Erbium-doped Fiber Amplifiers (EDFAs) render them extremely appealing for metro and area optical networks, where cost and size are of utmost importance. We have carried out a comprehensive study of several amplifiers (from standard and gain clamped to new amplifiers with distributed electrodes for non-uniform current injection) with novel configurations by means of two independent experimental setups providing complementary information on both the static carrier distribution along the active laver of the devices (spatially resolved spontaneous emission collection and analysis) and their transient response (cross gain modulation with 150fs of temporal resolution). One of these new configurations, called Optical Speed-up at Transparency (OSAT), was proposed by our group in 2000 and consists of a high power assist MN' beam injected near the amplifier transparency. As we shall demonstrate, this scheme copiously enhances the performances of SOAs on all critical issues: acceleration of the response of the amplifier reducing its gain recovery time, larger saturation output power allowing for an extended operation in a linear regime and reduced noise figure. One striking outcome of the OSAT principle further revealing its full potential emerges when applied to a gain clamped amplifier. These devices exhibit improved amplification stability through the presence of an internal lasing mode offset with respect to gain maximum but suffer from relaxation oscillations arising from the coupling between the internal lasing mode and carrier density just like semiconductor lasers do. These disturbing oscillations fade away once the assist light is ted into the amplifier. entailing an ultrafast (13ps of record gain recovery time) and highly stable device. Our experimental findings regarding carrier distribution and dynamic behaviour are sustained by a powerful simulation code solving the position dependent rate equation for the carrier density coupled to the propagation equations for all optical signals present within the device including amplified spontaneous emission. The future of semiconductor lasers and amplifiers has been enlightened by the appearance of brand-new devices based on Quantum Dot structures. We have carried numerical and experimental study of the inherent relaxation oscillations in the time-domain in a two-state lasing QD laser revealing an extremely slow modulation response with a period of the order of ins. Although this low dimensionality technology is still tender, it definitely holds the key to the evolution of ultimate all-optical telecommunications networks.

This thesis work contains an experimental study of many-body and cooperative effects in quantum wells. Many-body effects prove important for an understanding of the physical and specifically optical properties of semiconductors. The Coulomb interaction between electrons profoundly modifies the electronic states and the light-matter interaction, leading to a number of density-dependent effects that can not be accounted for in the one-electron picture which is the starting point of most textbooks on the subject. The simplest of these modifications is the exciton, the bound state of one electron and one hole which leads to the emergence of two-particle states energetically below the continuous band electron-hole states. Optical excitation in the band continuum is possibly followed by the formation of excitons upon release of the excess energy to the lattice vibrations. Chapter 5 contains an experimental analysis of the dynamics of this process. A high quality single In0.05Ga0.95As/GaAs quantum well sample allows us to separately study the temporal evolution of the continuum photoluminescence on the one hand and the excitonic photoluminescence on the other. The results indicate a rather fast formation process, leading to the establishment of a species interconversion equilibrium between free pairs and bound pairs at higher density. However, at the lowest excitation density, we found evidence for a non-equilibrium species repartition. At high electron-hole pair density, the two-particle bound exciton state disappears due to the screening of the Coulomb interaction by the surrounding carriers as well as the blocking of the constituting states according to the Pauli exclusion principle. The transition from an excitonic population to a free carrier plasma is referred to as excitonic Mott transition. Chapter 6 reports an experimental investigation of the repercussions of this transition on the optical spectra, performed on the In0.05Ga0.95As/GaAs quantum well of chapter 5. The time-resolved spectra do not show an abrupt change of photoluminescence intensity or spectral form when transforming gradually from an exciton dominated spectrum to free plasma photoluminescence, indicating a rather gradual transition. The separate visibility of continuum and excitonic PL contributions below the transition density reveals the total absence of exciton binding energy variations, indicating a very low exciton ionization degree in contrast to theoretical work on the subject. Another high-density effect arising from the particle interactions is the energetic lowering of the electronic states due to screening and correlation, denoted as band gap renormalization (BGR). Chapter 7 presents measurements of BGR using a single GaAs/Al0.25Ga0.75As quantum well of 60 Å width. We obtain good agreement with calculations of the carrier self energy within the framework of the random phase approximation. The last part of this thesis (chapter 8) reports an experimental test of the possibility of cooperative spontaneous emission, often denoted as superradiance, from a high-density electron-hole plasma. This effect has been extensively studied in optically excited atomic gases, where it leads under certain conditions to the reemission of the stored energy in a single coherent radiation burst instead of the slow exponential decay according to the spontaneous radiation of a single atom. We find no evidence for the occurrence of this effect in our samples, probably due to the fast incoherent polarization decay in the carrier plasma.

After the observation of Bose-Einstein condensation in the solid state in 2006 and the demonstration on its superfluid character in 2009 the interest for the demonstration of the underlying elementary excitations – the Bogoliubov excitations – was growing. We dedicate this PhD thesis to the experimental study of Bogoliubov excitations in an coherent polaritons gas. This thesis initially introduces microcavity polaritons, which are quasiparticles composed by semiconductor excitons and microcavity photons. Afterwards we expose the Bogoliubov theory, which predicts sound-like excitations for a polariton quantum fluid. Thus, the excitations are no longer given by single polaritons but by collective excitations. In order to probe the Bogoliubov excitations, a heterodyne four-wave mixing (FWM) technique is used. Its theory and the experimental technique is described in this thesis. The main part of this thesis reports on the experimental study of Bogoliubov excitations in a polariton quantum fluid. First, the Bogoliubov character of the coherent polariton gas is evidenced through the observation of its normal and ghost branch emission resonances in the FWM spectrum. We then demonstrate the Bogoliubov transformation at the transition from single-particle excitation to collective excitation by increasing the polariton density. By probing the dispersion of the polariton quantum fluid, we evidence the expected linear behavior of the Bogoliubov excitations. Here, we also investigate the influence of dissipation on the Bogoliubov excitations of the coherent polariton gas. It yields a asymmetry between the Bogoliubov normal and ghost branch. However we demonstrate that, despite their dissipative character, microcavity polariton gases can be qualitatively described in the frame of Bogoliubov theory of quantum fluids. We then introduce the powerful technique of 2-dimensional Fourier transformation spectroscopy in order to analyze the different fundamental processes which are causing a FWM emission. Finally, we study the effect of polarization on the Bogoliubov excitations. The experimental findings, which are presented in this thesis, are confirmed by the theoretical predictions based on Gross-Pitaevskii calculations.

Self-assembled quantum-dots (QDs) represent a distributed ensemble of zero dimensional structures with a near-singular density of states. Implemented as the active medium in a diode laser their unique properties lead to improved and often novel characteristics as compared to bulk or quantum-well (QW) devices. In particular In(Ga)As QD lasers on GaAs substrates are of largest interest as the spectral range of their emission wavelength reaches further into the infrared than that of QW lasers of the same material system. The emission wavelength of strained InGaAs QW lasers is limited to about 1150nm, whereas in In(Ga)As QD lasers an emission wavelength beyond 1.3µm is feasible. In the recent past ultralow threshold current densities combined with decreased temperature sensitivity, high modulation bandwidth and low chirp were reported on these 1.3µm QD lasers - as predicted many years ago. The differential efficiency of these devices is to date lower than expected. There are, however, difficulties associated with the random shape and size distribution of the QDs, and a strongly reduced density of states. The latter facilitates a low transparency current and consequently a low threshold current. But it is also accompanied by a smaller modal gain than it is usual in a QW laser, which has severe consequences for both the technical feasibility and the functionality of such a device. Particularly in applications demanding laser structures with larger total loss - e.g. for short cavity lengths and without high reflection coatings at the facets - the modal gain that can be obtained from a single layer of QDs is insufficient. One major goal of this thesis has been to realize In(Ga)As QD lasers emitting at 1.3µm. To achieve this, various QD growth structures have been analyzed in photoluminescence experiments, in order to optimize the QD properties with respect to emission wavelength and material gain. In addition comparative studies with other material systems emitting at 1.3µm have been conducted, e.g. GaInNAs QWs. In this context the major advantages and drawbacks of QDs appeared very clearly - the much better efficiency and the much lower density of states. Further on the QD efficiency has been tested in various light emitting diode (LED) structures, in order to optimize the design of the laser structures. During the characterization of the first fabricated broad-area QD lasers a novel and intriguing behavior became apparent: under certain conditions the devices operate at two well separated wavelengths, corresponding to a ground-state and an excited-state transition. In order to understand this behavior a comprehensive numerical model based on a rate equation system has been developed. The effect of simultaneous two-state lasing has then been attributed to incomplete clamping of the excited-state population above ground-state lasing threshold. The exact behavior of the energy level populations during lasing operation have been studied further in spontaneous emission spectra detected from a µm-sized hole on the top contact of the devices. In a very fruitful collaboration with Opto+, Alcatel CIT (Marcoussis, France) the dynamical properties of single transverse-mode lasers based on the same device structure have been investigated. Also here the QD lasers showed a different behavior as compared to QW devices: (1) The linewidth enhancement factor increases dramatically above threshold. (2) The modulation performance is degraded when the second lasing line appears. (3) Indications for the absence of relaxation oscillations on the ground state transition have been found. To a certain extent the experimentally found behavior could be explained in the framework of the rate equation model. For example the increase of the linewidth enhancement factor is well explained by an increasingly asymmetric gain spectrum due to the increase of excited state population above threshold. Finally quantum dot lasers including an intracavity quantum dot absorber section have been fabricated and characterized. In such a device the total loss can be manipulated by changing the bias on the absorber section. The basic idea was to achieve an efficient wavelength switch between the lasing lines of the ground state energy transition and the excited state energy transition, by electrically manipulating the total loss. This effect could not be demonstrated. The characteristics of this device could be qualitatively described in a traveling-wave rate equation model. This model also predicts the occurrence of self-sustained pulses in this device - a behavior that could not be investigated experimentally during this thesis.

Exciton-polaritons are hybrid quasi-particles, eigenstates of the strong coupling regime between light and matter. Their Bose-Einstein condensation has been recently demonstrated, opening the way to several fundamental investigations on the associated phenomenology and topological defects. Major advantages are brought by the unique light-matter mixed nature of polaritons, allowing for example for condensation at high temperatures and direct optical access to the condensate wavefunction. This thesis is devoted to the investigation of polariton condensates, with particular attention to the role of disorder, which represents an intrinsic and pronounced feature of the CdTe-based microcavity samples under study. The experiments are carried on in several configurations and conditions for the condensates, with particular attention to coherence behavior and topological defect formation and stabilization. One-dimensional polariton condensates have been studied in linear disorder valleys where, despite the potential fluctuations, the non-equilibrium processes underlying condensation allow for the phase-locking of single-energy condensates. Even in such conditions, disorder is responsible for the observed pronounced fragmentation of the condensate density, leading to a non-trivial modulation of the spatial coherence. The investigations have been carried further, performing a full quantum tomography of one-dimensional polariton condensates. The reconstruction of the single-particle density matrix of the system allowed to directly test and validate, in a strongly out-of-equilibrium framework, the Penrose-Onsager criterion for Bose-Einstein condensation, which was initially formulated for equilibrium systems. By shaping the non-resonant laser excitation, one-dimensional polariton condensates could be tamed to occur also within the annular geometry. Periodic boundary conditions are there imposed by the closed ring topology. In such setting, upon spontaneous condensation, further ordering occurs in the condensate density in the form of standing-wave patterns. Theoretical insight into the pattern formation are brought by Gross-Pitaevskii modeling, reproducing reasonably well the experiments. The pump shape anisotropy together with disorder are responsible for the pinning into a preferential orientation of the spatial pattern, allowing the observation in our time-integrated experiments. Two-dimensional polariton condensates have also been extensively investigated. The attention was focused onto topological defects formation, dynamics and stabilization within disorder in the spinor condensate framework. Polaritons indeed constitute a two-component spinor condensate. Previous works brought experimental observation of both integer and fractional vorticity, spatially pinned in the disorder potential. Yet, in our system, the formation dynamics of such entities and the inter-relation of integer and fractional vorticity was to be experimentally explored. Ultrafast spectroscopy experiments have been designed and performed, allowing for the study of the dynamics of spinor topological defects. In particular, the dissociation of integer vortices into pairs of fractional vortices was demonstrated. A novel disorder-dependent coupling acting on the spin components of the condensate has been introduced in order to reproduce the observed phenomenology within a generalized spinor Gross-Pitaevskii model. In two-component spinor polariton condensates, integer spin vorticity has also been predicted to occur under specific configurations, yielding formation of spin textures into the condensate polarization structure. Advanced interferometric techniques with polarization sensitivity have been extensively employed to eventually provide the first observation of such entities in polariton fluids. They are spontaneously forming and pinning into disorder potential traps in the sample. The polarization textures have been also directly imaged and their stabilization properties understood in terms of the realistic polarization features of the microcavity sample. The generalized spinor Gross-Pitaevskii model is once more successfully employed to catch the physics of spin vortices formation and stabilization within the landscape of a realistic disorder.

This thesis explores the properties of the transmission of light through subwavelength slits in metal films. Theoretical and experimental studies on the transmission through slits in metals are presented. In addition, the potential use of these slits for different applications is investigated. The theoretical aspect concentrates on the study of the transmission of light through slit arrays. It is observed that high transmission is due to the coupling of two modes: one whose properties depend on the slit geometry and the other one whose properties depend on the grating periodicity. To determine the exact nature of these hybrid modes an analytical model is derived. The simplified model gives accurate analytical expressions for the transmission and also for the dispersion relations of the hybrid modes responsible for high transmission. The dispersion relations give a clear picture of the role played by the different resonances in the transmission process. We find that the high transmission modes are hybrid in nature between that of a Fabry-Pérot mode and a surface plasmon polariton mode. These hybrid modes have either symmetric or antisymmetric profiles. These findings are important as they clarify the nature of the modes responsible for high transmission, and therefore can be useful as a design tool for metal gratings for various applications. The light transmission through single slits, slit and groove structures and slit arrays is optically characterized. Also, slit and groove structures covered by a thin dielectric layer are fabricated. With this type of structure, transverse electric (TE) polarized dielectric waveguide modes can be excited inside the dielectric film, which strongly increases the TE polarized transmission as compared to conventional slit and groove structures, which only gives high transmission for transverse magnetic (TM) polarization. These structures demonstrate an efficient way to enhance light transmission through subwavelength apertures as both TE and TM polarization can be efficiently transmitted. The transmission measurements for the different types of structures are compared with theory. This gives an idea of the validity of the theoretical tools considered, which is necessary to address the potential of subwavelength slits in metals for applications. Taking advantage of the theoretical and experimental work developed in this thesis, the use of slits in metals is considered for three different optical devices: biosensors, light emitting diodes and image sensors. In the case of biosensors, slit arrays whose optical transmission is highly sensitive to refractive index changes could be useful to design compact and portable sensors. Our work show that a high sensitivity to refractive index changes can be obtained in the case of suspended slit arrays. As an example, theoretically an adsorption of a 1.6 nm thick layer of bovine serum albumin on a slit array could provide a 3% intensity change. Slit arrays are also investigated for light emitting diodes in order to obtain transparent metal contacts. It is shown that light transmission through slit arrays on a high refractive index substrate, characteristic of light emitting diode, can be very efficient. Light can be extracted even for angles of incident larger than the critical angle due to grating coupling. Preliminary calculations show that a metal contact with arrays of slit can give twice the extraction efficiency of a planar GaP-air interface. Finally, slit arrays and slit and groove structures have been integrated as a post process onto pixels of image sensors. In the case of a pixel with a slit array, a polarization extinction ratio higher than 200 over a bandwidth of 200 nm is measured, which means that slit arrays can be used as very efficient micro-polarizers in view of polarization imaging. In the case of pixels with slit and groove structures, measurements show that the slit and groove structure could improve by a factor of eight the signal measured on the pixel. Such structure could be used to replace current optical systems or to improve device performances by increasing the signal to noise ratio and allowing polarization and colour selectivity.

The exceptional performance of self-assembled Quantum Dot (QD) materials renders them extremely appealing for their use as optical communications devices. As lasers, they feature reduced and temperature independent threshold current and proper emission wavelength at the fiber telecommunication windows. These characteristics, together with the low linewidth enhancement factor and broad spectrum, make QD materials extremely attractive for application as light emitters or amplifiers. There exist, nevertheless, several unclear issues which prevent QDs from conquering the new generation of optoelectronic devices. Their differential efficiency is lower than expected. The output power of QD lasers is lower than that of their quantum well counterpart. Still, it is their dynamics which has incited the majority of studies. The modulation bandwidth of these devices seems to be limited by the relaxation of carriers from the upper energetic layers to the low levels within the dot. Besides, the electron-hole interaction is widely unknown, the extent of the electron-hole Coulombic attraction is not yet established. Throughout this thesis I present a theoretical and experimental study of the gain and phase dynamics of quantum dot lasers. I explain the appearance of different decay times observed in pump and probe experiments in QD amplifiers as a result of the different electron and hole relaxation times, by means of an electron-hole rate-equation model. The ultrafast hole relaxation first leads to an ultrafast recovery of the gain, which is then followed by electron relaxation and, on the nanosecond timescale, radiative and non-radiative recombinations. The phase dynamics is slower and is affected by thermal redistribution of carriers within the dot. Our results corroborate with spectral measurements of the dephasing and gain in QD amplifiers. Additionally, our work is compared with existing pump and probe results. Exploiting the capacity of QD lasers to emit at two different wavelengths corresponding to the ground state (GS) and excited state (ES), I present a theoretical study of the QD dynamics, based on a linearization of the QD rate-equations. The results predict the existence of single oscillation frequency of GS and ES, meaning that both states are highly coupled. In order to verify our theory, we perform two kinds of experiments. By modulating these lasers at high frequency, we measure separately the dynamics of GS and ES. However, in contradiction to our theory, two different modulation frequencies are found. Additional temporally-resolved measurements of the laser dynamics reveal a surprising effect. By injecting a sub-bandgap pump in an InAs/InGaAs QD laser, the emitted photons are depleted. Through additional transmission and photocurrent measurements, we relate this observation with carrier photoexcitation, which was so far only theoretically addressed. The role of carrier photoexcitation in our experimental laser dynamics is further supported by a rate-equation model. Impelled by this finding, we study the effect of carrier photoexcitation in the static and dynamic characteristics. We find that carrier photoexcitation reduces the efficiency of QD lasers, which is one of the major QD handicaps, and depletes the GS lasing after the ES threshold, as observed experimentally. Moreover, by adding carrier photoexcitation to our linearization of the rate-equations, we find that the theory predicts the appearance of two lasing resonance frequencies, in agreement with our previous experimental results. Additionally, we deal with the improvement of carrier relaxation. In tunnel injection devices, carriers are given an additional path towards the ground state of the dot by growing a quantum well layer close to the QD active plane. Through the quantum-mechanical tunneling effect, carriers relax from the nearby quantum well layer to the QDs, which speeds up relaxation. We aim at the increase of the modulation bandwidth while keeping the good performances quantum dot lasers have exhibited, such as low and temperature insensitive threshold current and proper emission wavelength. In the final part of this work, we present dynamical measurements of 1.5 µm InAs/InP tunnel injection and non-tunnel injection QD lasers, which display remarkable static characteristics. After proving with static measurements that tunnel injection is actually taking place in these structures, we show several dynamic measurements. Pump and probe measurements on QD devices show that the tunnel injection samples exhibit a slightly faster relaxation time than the non-tunnel injection samples used as reference, meaning that relaxation time is improved with tunnel injection. However, by probing the device with an ultrafast pump no improvement of the dynamic characteristics is observed. These results confirm that the laser dynamic properties of InP QD lasers, both standard and tunnel-injection designs, are actually not limited by relaxation of carriers. We point towards the size distribution of these quantum dash-like structures as the limiting factor of the modulation frequency.

After the first demonstration of Bose Einstein condensation in the solid state in 2006 and the establishment of exciton polariton condensates in the wider scientific community, an intense interest has been attracted by this phenomenon at both theoretical and experimental level. It is the object of this thesis to follow on from the demonstration of polariton condensation and to study more in detail different aspects of fundamental importance related to the polariton condensation. This thesis initially presents the basic concepts for excitons, polaritons and condensates in and out of equilibrium. The following part focuses on the presentation of the experimental methods. The thesis is separated in two conceptual parts, one for the experiments under continuous wave excitation and one that was dedicated to the dynamical properties of polariton condensates. The appearance of a single condensate despite the presence of disorder is initially presented with the study of the mode synchronization of neighboring condensates. Then the coexistence of multiple condensates is demonstrated, a phenomenon that had previously passed unperceived due to the noise of the excitation laser. The use of a low noise excitation source reveals spectral characteristics that were "hidden" in the previously broad spectra and a detailed experimental analysis is performed in order to study the full real and momentum space distribution of the individual condensates and their long range spatial coherence. Interferometric measurements which give access to the long range spatial coherence and the condensate phase, revealed the existence of pinned singularities. These were further investigated and were identified as the commonly known singly quantized vortices, here shown for the first time in a polariton condensate. Up to this point most of the studies demonstrated that the polarization of polariton condensates was preferentially linearly polarized. A more detailed analysis that was pursued in this thesis showed that the degree of linear polarization is not necessarily high but there are regions on the sample where it is rather poor. In certain areas where we do not observe a lift of degeneracy in polarization, the polariton condensates have to be considered as spinor objects. This unique property permitted the first observation of the celebrated case of fractional vortices which constitute the elementary excitations of spinor superfluids. In the second part of the thesis the work is focused on the dynamics of polariton condensates. Beginning with the dynamics of singly quantized vortices, it is shown that vortices present traceable in time trajectories from their nucleation positions all the way to their pinning locations. Finally taking advantage of the inherent disorder of the sample, a naturally occurring coupled double well system is investigated. The observation of oscillating polariton currents between the two wells was demonstrated. This phenomenon is analogous to the well known Josephson oscillations. Most -if not all- of the studied phenomena are a complex interplay of the disorder, the driven-dissipative character of the polariton condensates and the nonlinearities occurring in the system. A detailed theoretical analysis that supports the observed experimental findings is finally provided.

Polaritons are half-matter half-light quasiparticles, arising, in a two-dimensional semiconductor microcavity, from the strong-coupling between an exciton (an elementary electronic excitation of a crystal) and a photon. This thesis presents the fabrication of polariton confining structures, their characterization and the study of the linear and non-linear optical properties of the confined polaritons. Thanks to their bosonic character, to their extremely light effective mass and to the peculiar shape of their dispersion curve, polaritons were proven to accumulate in their ground state to form a Bose-Einstein condensate in a CdTe based sample, at a high temperature of the order of 20 Kelvin. No such effect was observed in GaAs materials, who offer a less disordered environment, where we developed a method to fabricate traps of any shape and size. The latter should facilitate the condensation of polaritons by lowering the density thresholds, and allow us to manipulate the condensate. Thanks to the strong-coupling regime, it is possible to confine polaritons either through their photon or through their exciton part. We thus fabricated two-dimensional microcavities with local thickness variations, confining the cavity photon along its two free dimensions. We were able to perform this through high-quality molecular beam epitaxy (MBE) growth, accompanied by a controlled processing of the sample. We measured the anticrossing behaviors characteristic of the strong-coupling regime in zero and two dimensions. As the confining structures have sizes of the order of the micron, we could image the confined polaritons' wave functions in the real and reciprocal (momentum) spaces, and tried to understand how the transition between confined (0D) and extended (2D) polariton modes occurs. We also gave first evidences of the interaction between the two and zero-dimensional structures, and of the polariton trapping from one to the other. We then studied the nonlinear optical properties of this new object, performing two different kinds of experiments: a study of the response of the system to a non-resonant excitation, in order to probe the formation of a condensed phase. Collective electronic excitations were created, at energies far higher than the modes which are of interest for us. We observed the effect of high densities in the system and evidenced Coulomb interaction. We then observed the cross-over from strong to weak-coupling regime, and the onset of lasing in the weakly coupled system. a study of the response of the system to a resonant excitation in order to probe parametric effects between the discrete states. In this configuration a number of polaritons are intentionally created in a given state. We observed various nonlinear behaviors as a function of the created population, which may be interpreted as effects of Coulomb interaction, or indications of bistable behaviors in the system. We were nevertheless not able to discriminate. We give some potential applications in the field of single or correlated photon emission. Although industrial applications may not be in the short-term agenda, it should be possible to take advantages of this original type of structures for research and development applications. We finally give some experimental perspectives, which may help deepen the observations shown and the interpretations proposed here, and should allow to work towards the fabrication of new samples, where BEC of polaritons is observed and controlled, as well as parametric oscillations between various confined states.