Couplage fort exciton-photon pour une boîte quantique de GaAs en microdisque

This work is related to Quantum Electrodynamics in semiconductor systems, which is a currently a very active field. I present one of the first experimental demonstrations of the strong coupling regime (SCR) for a single quantum dot inserted in a microcavity. The manuscript shows the progression toward the SCR observation with in-depth studies of the electronic and optical properties of natural GaAs quantum dots. It is organized in 6 chapters. The first chapter recalls the necessary conditions for SCR observation, highlighting the most promising systems for the realisation of the SCR at the beginning of 2003. The candidate system was determined to be a combination of : (i) a GaAs/AlGaAs interface quantum dot exciton, because of its great oscillator strength and ;(ii) a microdisk for the microcavity, because of the high quality factor of the whispering gallery modes.As well as the above criteria, there are some requirements on the linewidth of the exciton emission and of the cavity mode. Indeed, the transition between weak coupling and strong coupling occurs when the Rabi splitting becomes greater than the mean linewidth of the emitter and the cavity mode. These severe constraints led to the detailed studies on the electronic and optical properties of the quantum dot emitter, described in chapters II, III and IV.Chapter II presents the GaAs quantum dot samples. Structural analysis and optical characterisations in photoluminescence (PL) allow to distinguish two types of samples : a « small » quantum dot sample and a « large » quantum dot sample, as compared to the 2D exciton Bohr radius.Chapter III concerns the coupling strength between the GaAs quantum dot excitons and the electromagnetic field. This work was essential as although considerable work has been previously published on InAs/AlGaAs quantum dot exciton characteristics, little is known regarding GaAs natural quantum dots. The main result of this chapter is the possibility of “engineering” the excitonic oscillator strength using the lateral quantum dot size. As the characteristic lifetimes of capture and recombination in these quantum dots are similar, the interpretation of the PL kinetics required modelling of the emission dynamics. Simple models, coupled to complementary experiments such as measurements of the photon emission statistics, lead to a good description of the light/matter coupling. I show that « large » quantum dots present a greater oscillator strength compared to smaller ones, and are for this reason a better candidate for the observation of the SCR if inserted in a microdisk. In addition to large oscillator strengths, GaAs quantum dots must exhibit an excitonic linewidth narrow enough to observe the SCR. I show theoretically and experimentally in chapter IV the dependence of the exciton-phonon coupling with the confinement of the exciton, which governs the spectral linewidths and their evolution with temperature. It is shown that large quantum dots present a reduced spectral broadening with temperature as compared to small quantum dots, further enforcing their advantages as a candidate for SCR observation.In chapter V, I present the fabrication process of GaAs/AlGaAs microdisks. I also present a new technique for the realisation of AlOx microdisks, which show some advantages over traditional microdisks. At the end of this chapter, I show that in both types of microdisks, whispering gallery modes exhibit high quality factors (Q>10000) and small effective volumes, which satisfy the criteria for SCR observation. Chapter VI is the outcome of the extensive studies presented in the previous chapters. The exciton-photon SCR in a GaAs quantum dot within a microdisk is demonstrated for the first time. Spectral tuning of the exciton and the cavity mode with temperature showed the characteristic SCR anticrossing. A Rabi splitting twice as large as each individual linewidth is demonstrating, corresponding to a clear achievement of the SCR.In conclusion, this work has lead to the observation of strong quantum optical coupling effects in 0D semiconductor nanostructures. It opens up a rich research path in Cavity Quantum Electrodynamics, with implications for Quantum Information Science as well as for fundamental optics.Lorsqu'un émetteur est placé dans une cavité, il existe deux régimes de couplage lumière-matière : dans le régime dit de couplage faible, la cavité a pour effet de modifier le taux d'émission spontanée de l'émetteur. Cet effet perturbateur de la cavité est connu sous le nom d'effet Purcell. Dans le régime dit de couplage fort, l'interaction dipolaire électrique n'est plus perturbative ; les états-propres du système couplé sont des états mixtes lumière-matière. Dans le domaine temporel, ce couplage se traduit par le fait que l'émission spontanée devient réversible : le photon émis spontanément par l'émetteur dans le mode de cavité peut à nouveau être ré-absorbé par l'émetteur, puis ré-émis,...donnant ainsi lieu à un cycle d'oscillations de Rabi. Dans le domaine spectral, le couplage se manifeste par une levée de dégénerescence (ou doublet de Rabi) lorsqu'émetteur et mode de cavité sont mis en résonance. L'objet de cette thèse est la démonstration expérimentale du couplage fort entre un exciton confiné par une boîte quantique naturelle de GaAs et un mode de galerie d'un microdisque semi-conducteur.Les paramètres-clefs pour atteindre ce régime sont, pour ce qui est de l'émetteur, sa force d'oscillateur ainsi que sa largeur spectrale, gouvernée par l'interaction avec l'environnement. Un chapitre est consacré à chacune de ces 2 notions-clefs. Concernant la cavité, les 2 figures de mérite pertinentes pour le renforcement de l'interaction lumière-matière sont le facteur de qualité et le volume modal. Nous présentons la réalisation technologique et la caractérisation des microdisques de GaAs (sur air et sur AlOx) les plus prometteurs en terme de facteur de qualité et volume modal.Enfin, nous présentons la première démonstration expérimentale du régime de couplage fort pour une boîte quantique naturelle de GaAs en microdisque