Quantum fluids of waveguide exciton-polariton

Valentin DEVELAY

Exciton-polaritons are bosonic quasi-particles composed of a photon and an exciton, an electron-hole pair in Coulomb interaction, in a strong coupling regime. This coupling requires efficient light confinement, which is provided by waveguides through total internal reflection. Due to their large interactions, these quantum fluids of lights allow the study of polariton condensates at thermal equilibrium, such as Bose-Einstein Condensation (BEC) as well as polariton laser out-of-equilibrium. Unlike conventional semiconductor lasers, polariton lasers do not require population inversion to reach lasing, opening new opportunities in optoelectronics. Additionally, Kerr polaritonic nonlinearity and the coupling of exciton to unconventional photonic states, such as Bound-state-In-the-Continuum (BIC), extend applications of the polaritonic platform to nonlinear and integrated photonics. In this context, this thesis investigates gallium nitride (GaN) polariton waveguide structures via optical spectroscopy. These structures present interesting properties compared to their microcavity counterparts. The key results of this work are: (i) the observation at room-temperature, of a mode-locked polariton laser due to the formation of a bright soliton in a 60µm-long stripe cavity enabled by polaritonic nonlinearity, (ii) the formation of a multisolitonic regime by an asymmetrical pumping of a 40µm-long stripe cavity. Continuous variations of the pump position from the mirrors to the center of the cavity evidence a gradual transition from a single to multisolitonic regime. (iii) The detection of lasing in a GaN doped waveguide cavity through optical and electrical excitation. (iv) The first experimental observation of a BIC-polariton in a GaN photonic crystal waveguide at T=70K, characterized by a darkening at k=0, a quadratic evolution of the intensity dispersion and the formation of a polarization vortex at k=0µm , and its associated condensation in gap-confined states.