Infoscience

Thesis

III-nitride based microcavities: towards polariton condensation at room temperature

This PhD thesis work describes the study of III-nitride based planar microcavities operating in the strong coupling regime at room temperature. The aim is to use the nonlinear emission properties of such samples in realistic devices. Furthermore these studies allow investigating macroscopic quantum phenomena at room temperature. Strongly coupled semiconductor microcavities are characterized by the non-perturbative coupling between the photon associated to the cavity mode and the semiconductor first excited state, namely the exciton. The quasiparticle resulting from this interaction is called cavity polariton and exhibits a peculiar in plane wave vector (k‖) dispersion. This results from the strong effective mass difference between the cavity photon and the exciton. Consequently, polaritons are subject to trapping in reciprocal space around k‖ = 0. Polaritons are thus characterized by a light effective mass in this trap. Theoretically, this point is favorable for the observation of macroscopic coherent polariton phases at low densities and high temperatures, in particular at room temperature. So far such phenomena have been mainly studied in III-arsenides and II-VI semiconductors. The main outcome is the recent demonstration of polariton Bose-Einstein condensation at cryogenic temperatures. This last point is the main limitation of III-arsenide semiconductors: the strong coupling regime is hardly maintained at room temperatures. This prevents the observation of such macroscopic coherent phases of polaritons in such conditions. In this context, the use of these materials for devices based on such physical phenomena appears strongly compromised. Indeed a potential use would be the realization of low threshold compact coherent light sources, and for such applications, an operation at cryogenic temperatures is not realistic. In this context, the use of III-nitride semiconductors is a very promising approach. Indeed, thanks to better intrinsic material parameters, the strong coupling regime is kept at room temperature. It has thus been observed in microcavities whose quality is much poorer than their III-arsenide or II-VI counterparts. Using high quality planar microcavities is however a prerequisite to observe these nonlinear effects. The aim of this work is to study such samples that can now be obtained thanks to the use of Bragg mirrors made of AlInN layers lattice matched to GaN. The strong coupling regime has been demonstrated at room temperature on several samples grown using this approach through angle resolved photoluminescence and reflectivity measurements. Furthermore, a strong nonlinear emission is observed at room temperature under nonresonant optical excitation for two different samples. This emission exhibits a strong spectral narrowing indicating the onset of an increased temporal coherence. These results are fully consistent with a room temperature polariton lasing picture. With the recent demonstration of the electrical injection of polaritons in III-arsenides, this latter result indicates that the realization of a coherent light source based on cavity polaritons and operating at room temperature is now realistic. In addition to this first major result, numerous properties of this nonlinear emission have been investigated. Thus, some preliminary evidences indicating that the transition is driven by thermodynamics have been obtained. Then some intriguing properties related to the macroscopic polarization buildup have been observed. One of the samples exhibiting this nonlinear emission is indeed characterized by the spontaneous buildup of a random polarization vector for each experimental realization. This result is interpreted in terms of spontaneous symmetry breaking at the transition, which is a fundamental property of phase transition theory. These latter results pave the way to the observation of room temperature Bose-Einstein condensation in the solid state.

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