Résumé
Exciton-photon coupling in a waveguide geometry is attracting more and more interest in the field of polaritonics. Waveguide polaritons are obtained when the exciton and the guided mode of a photonic waveguide are brought in the strong coupling regime, in strong analogy with cavity polaritons in Fabry-Perot resonators. One of the first demonstrations of waveguide polaritons was reported in a waveguide embedding InGaAs/GaAs quantum wells, with a Rabi splitting of 6 meV [Walker et al., APL 102, 012109 (2013) and Nat. Comm. 6, 8317 (2015)], leading to the observation of ultra-low power temporal solitons at T=10K. Nitride materials offer the possibility to investigate polariton physics up to room temperature thanks to the large binding energy and large oscillator strength of the excitons in GaN. The first demonstration of strong coupling in nitrides waveguides was based on InGaN multiple quantum wells, with a Rabi splitting of 63 meV at 4K, and a strong coupling observed up to T=100K [Ciers et al., Phys. Rev. Applied 7, 034019 (2015)]. In the meantime, we have achieved polariton lasing in a waveguide geometry for the first time in another wide bandgap material, ZnO [Jamadi et al., Arxiv:1708.00501], up to room-temperature.Here we report the enhancement of the Rabi splitting in a bulk GaN slab waveguide, compared to the case of multiple quantum wells. The Rabi splitting in a monomode GaN waveguide reaches 140meV, and slightly decreases to 100 meV at T=300K. To measure it, we have fabricated diffraction gratings on the waveguides and performed polarization-resolved Fourier-space imaging of the TE and TM modes dispersion. Besides, through Fourier-space imaging and spectral filtering of the photoluminescence image we have measured the polaritons propagation length as a function of the detuning: at T=10K it amounts to 10µm at zero detuning and 30µm at more negative detuning (= -Rabi); most importantly, similar propagation lengths are observed up to room temperature.The increased Rabi splitting and large propagation lengths pave the way for the demonstration of polariton lasers at room temperature in a GaN waveguide, and for a control of the nonlinear processes in polariton fluids of light.