On the physics of multimode polaritons

This thesis is devoted to the study of microcavity polariton systems, in which the strong coupling occurs between more than one exciton or photon modes i.e. multimode polaritons. The first part of this work states the theoretical background of light-matter interaction starting from quantum well excitons to the non-linear regime of interactions between polaritons. We apply this knowledge to the case of GaAs based microcavities with InGaAs/GaAs quantum wells which are experimentally tested. Sample characterization and optimization are also discussed. We first report on the observation of multiple polariton modes, originating from an electronic coupling between quantum wells inside a planar microcavity. When shallow quantum well stacks are placed at the antinodes of a microcavity, we measure a series of anticrossings when the cavity mode energy crosses that of the different excitonic levels. Comparing our experimental results with a coupled oscillator model that includes the electron and hole wave functions allows us to show that the exciton binding energy is affected by the interwell coupling. We then study the non-linear properties of these InGaAs based microcavities, in the search for polariton condensation. We study the effect of Indium content, number of quantum wells, types of quantum well stacks, number of pairs of Bragg mirrors and magnetic field on the non-linearity of the system. In all cases, we measure a single threshold, and a clear signature of the transition from strong to weak coupling regime. We discuss the limiting factors for condensation in our system, namely the cavity losses induced by optical disorder, the light-matter coupling strength, and the saturation of the quantum wells. In the second part of the thesis, we demonstrate the occurrence of spatial multistability using laterally confined polariton modes. We measure a multihysteresis curve of the transmitted intensity when we cycle the excitation power of a blue detuned laser with respect to the polariton modes. At each threshold of the hysteresis loop, we measure a switching of the spatial profile of the transmitted beam, and an energy jump of all the polariton modes. We reproduce all main characteristics of our experiment using a multimode generalization of the Gross-Pitaevskii equations in the exciton photon basis. The mechanism behind the spatial multistability is identified as a repulsive cross-interaction between polaritons in different modes. Following this experiment, we investigate the effect of decoherence on polariton bistability. We demonstrate how the polariton hysteresis loop collapses when increasing either the temperature or the power of a weak non-resonant laser. We explain this effect by the population of an incoherent reservoir that induces dephasing and repulsive interactions on the driven polaritons. All experimental findings are accurately simulated with the excitonic Bloch equations, and indicate that reservoir induced dephasing can be dominant over the reservoir induced energy blue shift. In the final chapter, we present ongoing work on polariton lattices, namely measurements of the band structure for a square, and an hexagonal lattice. We also present preliminary results on the effect of a magnetic field on the band structure of a polariton square lattice. We conclude by discussing a series of future experiments to continue the investigation of multimode polaritons.

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