Infoscience

Thesis

0D microcavity polaritons: trapping light-matter quasiparticles

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.

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