Strong coupling of exciton-polaritons in a bulk GaN planar waveguide: quantifiying the Rabi splitting.

We investigate the demonstration of the strong coupling between excitons and guided photons in a GaN slab waveguide. The dispersions of waveguide polaritons are measured from T=6K to 300K through gratings. They are carefully analyzed within four models based on different assumptions, in order to assess the strong coupling regime. We prove that the guided photons and excitons are strongly coupled at all investigated temperatures, with a small $(11 \%)$ dependence on the temperature. However the values of the Rabi splitting strongly vary among the four models: the "coupled oscillator" model over-estimates the coupling; the analytical "Elliott-Tanguy" model precisely describes the dielectric susceptibility of GaN near the excitonic transition, leading to a Rabi splitting equal to $82 \pm 10 \ meV$ for TE0 modes; the experimental ellipsometry-based model leads to smaller values of $55 \pm 6 \ meV.$ When evaluating the exciton-photon coupling strength, we evidence the importance of a careful understanding of the dielectric susceptibility of the active layers with a large oscillator strength that are used for room-temperature polariton devices.