Optical signatures of a fully dark exciton condensate

We propose optical means to reveal the presence of a dark exciton condensate that does not yield any photoluminescence at all. We show that (i) the dark exciton density can be obtained from the blueshift of the excitonic absorption line induced by dark excitons; (ii) the polarization of the dark condensate can be deduced from the blueshift dependence on probe photon polarization and also from Faraday effect, linearly polarized dark excitons leaving unaffected the polarization plane of an unabsorbed photon beam. These effects result from carrier exchanges between dark and bright excitons.Comment: 5 pages, 4 figure

Effects of fermion exchanges on the polarization of exciton condensates

Exchange processes are responsible for the stability of elementary boson condensates with respect to their possible fragmentation. This remains true for composite bosons when single fermion exchanges are included but spin degrees of freedom are ignored. We here show that their inclusion can produce a "spin-fragmentation" of a condensate of dark excitons, i.e., an unpolarized condensate with equal amount of dark excitons with spins (+2) and (-2). Quite surprisingly, for spatially indirect excitons of semiconductor bilayers, we predict that the condensate polarization can switch from unpolarized to fully polarized, depending on the distance between the layers confining electrons and holes. Remarkably, the threshold distance associated to this switching lies in the regime where experiments are nowadays carried out.Comment: 5 pages, 1 figur

Questionner le digital labor par le prisme des émotions : le capitalisme affectif comme métadispositif ?

International audienceCette communication dévoile un projet de recherche interrogeant la place des affects dans l'économie numérique, avec l'hypothèse que les stratégies mises en place par les plateformes web servent d'indicateur à un procès plus général appelé capitalisme affectif. Notre cadre théorique emprunte au digital labor le concept de travailleur cognitif auquel nous rattachons les enjeux relevant de l'exploitation des données personnelles. Nous nous associons également au champ de la communication affective quand il s'agit de modéliser le méta-dispositif. Pour ce faire, nous proposons dans cet article une analyse technique puis économique des plateformes web nous permettant de construire nos hypothèses et de lister les éléments méthodologiques à déployer pour les valider

Questionner le digital labor par le prisme des émotions : le capitalisme affectif comme métadispositif ?

International audienceCette communication dévoile un projet de recherche interrogeant la place des affects dans l'économie numérique, avec l'hypothèse que les stratégies mises en place par les plateformes web servent d'indicateur à un procès plus général appelé capitalisme affectif. Notre cadre théorique emprunte au digital labor le concept de travailleur cognitif auquel nous rattachons les enjeux relevant de l'exploitation des données personnelles. Nous nous associons également au champ de la communication affective quand il s'agit de modéliser le méta-dispositif. Pour ce faire, nous proposons dans cet article une analyse technique puis économique des plateformes web nous permettant de construire nos hypothèses et de lister les éléments méthodologiques à déployer pour les valider

Spectroscopic Signatures for the Dark Bose-Einstein Condensation of Spatially Indirect Excitons

We study semiconductor excitons confined in an electrostatic trap of a GaAs bilayer heterostructure. We evidence that optically bright excitonic states are strongly depleted while cooling to sub-Kelvin temperatures. In return, the other accessible and optically dark states become macroscopically occupied so that the overall exciton population in the trap is conserved. These combined behaviours constitute the spectroscopic signature for the mostly dark Bose-Einstein condensation of excitons, which in our experiments is restricted to a dilute regime within a narrow range of densities, below a critical temperature of about 1K.Comment: 7 pages and 5 figure

Controlling the charge environment of single quantum dots in a photonic-crystal cavity

We demonstrate that the presence of charge around a semiconductor quantum dot (QD) strongly affects its optical properties and produces non-resonant coupling to the modes of a microcavity. We first show that, besides (multi)exciton lines, a QD generates a spectrally broad emission which efficiently couples to cavity modes. Its temporal dynamics shows that it is related to the Coulomb interaction between the QD (multi)excitons and carriers in the adjacent wetting layer. This mechanism can be suppressed by the application of an electric field, making the QD closer to an ideal two-level system.Comment: 12 pages, 4 figure

From Strong to Weak Coupling Regime in a Single GaN Microwire up to Room Temperature

Large bandgap semiconductor microwires constitute a very advantageous alternative to planar microcavities in the context of room temperature strong coupling regime between exciton and light. In this work we demonstrate that in a GaN microwire, the strong coupling regime is achieved up to room temperature with a large Rabi splitting of 125 meV never achieved before in a Nitride-based photonic nanostructure. The demonstration relies on a method which doesn't require any knowledge \'a priori on the photonic eigenmodes energy in the microwire, i.e. the details of the microwire cross-section shape. Moreover, using a heavily doped segment within the same microwire, we confirm experimentally that free excitons provide the oscillator strength for this strong coupling regime. The measured Rabi splitting to linewidth ratio of 15 matches state of the art planar Nitride-based microcavities, in spite of a much simpler design and a less demanding fabrication process. These results show that GaN microwires constitute a simpler and promising system to achieve electrically pumped lasing in the strong coupling regime.Comment: 14 pages, 4 figure