Lasing in a ZnO waveguide: clear evidence of polaritonic gain obtained by monitoring the continuous exciton screening

The stimulated emission of exciton-polaritons was proposed as a means of lowering the lasing threshold because it does not require the dissociation of excitons to obtain an electron-hole plasma, as in a classical semiconductor laser based on population inversion. In this work we propose a method to prove unambiguously the polaritonic nature of lasing by combining experimental measurements with a model accounting for the permittivity change as a function of the carrier density. To do so we use angle resolved photoluminescence to observe the lasing at cryogenic temperature from a polariton mode in a zinc oxide waveguide structure, and to monitor the continuous shift of the polaritonic dispersion towards a photonic dispersion as the optical intensity of the pump is increased (up to 20 times the one at threshold). This shift is reproduced thanks to a model taking into account the reduction of the oscillator strength and the renormalization of the bandgap due to the screening of the electrostatic interaction between electrons and holes. Furthermore, the measurement of the carriers lifetime at optical intensities in the order of those at which the polariton lasing occurs enables us to estimate the carrier density, confirming it is lower than the corresponding Mott density for zinc oxide reported in the literature

Laser emission with excitonic gain in a ZnO planar microcavity

The lasing operation of a ZnO planar microcavity under optical pumping is demonstrated from T=80 K to 300 K. At the laser threshold, the cavity switches from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process

Experimental observation of strong light-matter coupling in ZnO microcavities: influence of large excitonic absorption

We present experimental observation of the strong light-matter coupling regime in ZnO bulk microcavities grown on silicon. Angle resolved reflectivity measurements, corroborated by transfer-matrix simulations, show that Rabi splittings in the order of 70 meV are achieved even for low finesse cavities. The impact of the large excitonic absorption, which enables a ZnO bulk-like behavior to be observed even in the strong coupling regime, is illustrated both experimentally and theoretically by considering cavities with increasing thickness

Etude experimentale par microscopie electronique et optique et viscoelasticite d'une transition de phase dans un cristal liquide lyotrope

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Zigzag disclination in uniaxial nematic phases : study in capillary tubes

We have observed zigzag disclination lines in uniaxial thermotropic and lyotropic nematic liquid crystals. Our samples are placed in round and flat capillary tubes. This confined geometry and the instability of the lines under three-dimensional perturbations are responsible for the observed shape.Nous avons observé des lignes de disinclinaison en zigzag dans des nématiques uniaxes thermotropes et lyotropes. Nos échantillons sont préparés dans des tubes capillaires ronds et rectangulaires. Cette géométrie confinée et l'instabilité des lignes vis-à-vis de perturbations tridimensionnelles sont responsables de la forme observée

Structures exotiques en nanophotonique, théorie et approche numérique

Dans la perspective d un contrôle ultime de la lumière, les arrivées récentes des cristaux photoniques et des métamatériaux constituent des avancées majeures. Ces matériaux nano-structurés présentant des propriétés optiques inédites nous ouvrent tout un champ de possibilités encore inexploré. En particulier, des milieux d indice effectif négatif sont rendus concevables. L objectif de cette thèse est d étudier d un point de vue électromagnétique, à l aide d outils analytiques et numériques, le comportement de la lumière dans ces structures exotiques. Nous nous penchons sur les boucles de lumières, qui sont des modes localisés d une structure multi-couches, émergeant du couplage contra-directionnel entre deux guides distincts. Une analyse physique est proposée et un modèle basé sur la théorie des modes couplés est développé. Cela nous permet de décrire avec précision l excitation d une boucle de lumière par une source lumineuse, et d envisager des applications pour la mise en forme de faisceau. Dans une seconde partie, nous étudions des cristaux photoniques unidimensionnels formés par une alternance de milieux d indices positif et négatif. Lorsque l indice moyen est nul, une bande interdite aux propriétés nouvelles apparait. Nous montrons que le caractère dispersif des milieux transforme des pics étroits de transmission en larges bandes. Pour caractériser la propagation d un faisceau dans un tel cristal, nous développons et validons alors un modèle qui nous permet de démontrer le potentiel en matière de mise en forme de faisceau (auto-collimation, focalisation).In the perspective of an ultimate control of light, the recent arrivals of photonic crystals and metamaterials are major advances. These nano-structured materials with unusual optical properties are opening a whole range of possibilities still unexplored. In particular, negative index media have became conceivable. The aim of this thesis is to study, with an electromagnetic point of view, the behavior of light in these exotic structures, using analytical and numerical tools. We study the light wheels, which are localized modes of a multi-layer structure, emerging from the contra-directional coupling between two separate waveguides. A physical analysis is proposed and a model, based on the coupled mode theory, is developed. This allows us to accurately describe the excitation of a light wheel by a source, and to consider applications for beam shaping. In a second part, we study one-dimensional photonic crystals combining positive and negative index layers whose the average index is equal to zero. A band gap, called zero-n gap, appears and presents new properties that we detail. Index dispersion is shown to broaden the resonant frequencies creating then a conduction band lying inside the zero-n gap. Self-collimation and focusing effects are in addition demonstrated in zero-average index crystals supporting defect modes. This beam shaping is explained in the framework of a beam propagation model by introducing an harmonic average index parameter.CLERMONT FD-Bib.électronique (631139902) / SudocSudocFranceF

Ellipsométrie spectroscopique de composés à base de nitrures

International audienc

Spectroscopy of a Bulk GaN Microcavity Grown on Si(111)

We report the experimental observation of the exciton–photon strong coupling regime in a GaN microcavity. The structure has been grown by molecular beam epitaxy on a Si(111) substrate. The upper mirror is a SiO 2 /Si 3 N 4 dielectric mirror and the silicon substrate acts as the bottom one. Angle resolved reflectivity and photoluminescence experiments have allowed to demonstrate the exciton–photon strong coupling regime, characterized by a Rabi splitting of 31 meV at 5 K. From the modeling of experiments, the oscillator strengths of excitons A and B are evaluated and compared to the values previously published. Then, the design of the bulk microcavity is optimized in order to maintain the strong coupling regime at room temperature; our calculations predict a Rabi splitting of 33 meV at 300 K in this case. A second kind of structure based on GaN/AlGaN quantum wells is also proposed, leading to an expected splitting of 19 meV at 300 K