Physics of Polariton Condensates in GaN-based Planar Microcavities

Since its prediction in 1996 by Imamoğlu and coworkers, the use of a non-equilibrium polariton condensate to produce an intense coherent light source referred to as a polariton laser has attracted a lot of interest in the whole physics community as it should allow the realization of ultralow threshold coherent light-emitting devices due to the release of the Bernard-Duraffourg condition. Excitons-polaritons, admixed particles resulting from the strong coupling between a cavity photon and an exciton, are the eigenmodes of a strongly coupled microcavity and exhibit a very light effective mass at the center of the Brillouin zone (105 times lighter that a free electron) inherited from the cavity photon. In the present work, we are interested in III-nitride based microcavities embedding GaN quantum wells in the active region. Thanks to the stability of the excitons at room temperature in this system and a large oscillator strength, polariton condensation has been observed up to 340K under optical excitation, paving the way toward the realization of the first electrically injected polariton laser. The goal of the present study is to provide a detailed analysis of the system properties accounting for nitride specificities, to describe the mechanisms leading to the formation of polariton condensates and to give the key elements for the optimization of devices relying on polariton nonlinearities. For this purpose, a Fourier-imaging setup allowing for the simultaneous monitoring of real space and far-field energy dispersions was carefully designed to operate in the UV spectral range in order to probe the sample emission at various temperatures. The first main result of this thesis is the establishment of the complete polariton phase diagram of our multiple quantum well-based GaN microcavity, which provides a comprehensive tool to favor or inhibit the condensation threshold by adjusting the microcavity parameters. The condensation is shown to be governed either by the kinetics or by the thermodynamics depending on the strength of the interactions. As polaritons are half-light, half-matter particles, the mechanisms leading to the nonlinear threshold are totally different from those of a conventional semiconductor laser. In particular, the possibility to tune the interactions in the system by changing the photonic fraction of the polaritons or the lattice temperature allows discriminating between different relaxation regimes. Then the spin of the polariton condensate is discussed. It is shown that the dimensionality of the system plays a major role in the polarization state of the emitted light. In particular above threshold, for a bulk microcavity, the polarization is randomly oriented whereas for a GaN multiple quantum well based microcavity, the polarization is pinned by the system anisotropy originating from the static disorder. With increasing pumping power, a depinning of the polarization is observed resulting in a progressive decrease in the polarization degree of the emitted light. These two results are well accounted for by a stochastic model of the condensate formation taking into account the in-plane anisotropy caused by the stationary photonic disorder, the self-induced Larmor precession of the condensate pseudospin and the interplay between energy and polarization relaxation rates. In the last part of this work, the case of nonpolar m-plane GaN based microcavities is addressed. In these structures, the optical axis lies in the plane of the cavity leading to a twofold anisotropy: the birefringence is responsible for the anisotropy of the cavity mode and the distribution of the exciton oscillator strength causes different coupling constants between light and matter along the two orthogonal directions. In such structures different selection rules and optical constants for light polarization perpendicular and parallel to the optical axis can lead to the coexistence of weak and strong coupling regimes with a transition to nonlinear emissions.

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