Strong Coupling of Exciton-Polaritons in a Bulk Ga N Planar Waveguide: Quantifying the Coupling Strength

We investigate the demonstration and quantification of the strong coupling between excitons and guided photons in a GaN slab waveguide. The dispersions of waveguide polaritons are measured from T=6 to 300 K 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 overestimates 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±10 meV for fundamental transverse-electric mode; the experimental ellipsometry-based model leads to smaller values of 55±6 meV. We evidence that, for waveguides including active layers with large oscillator strengths, as required for room-temperature polaritonic devices, a strong bending of the modes' dispersion is not necessarily the signature of the strong coupling, which requires for its reliable assessment a precise analysis of the material dielectric susceptibility.