Résumé
We report the observation of confined modes in the mixed photon/exciton-like branches of exciton–polaritons propagating along the growth axis of a 700 nm-thick GaN film, deposited by Molecular Beam Epitaxy on bulk GaN substrates. The energies of the confined modes are in agreement with a simple modelling of quantized wavevectors in the thin layer, including the proper four-branch dispersion relation of polaritons in the wurtzite GaN. Such an observation results from the large coherence length of the exciton–polariton in the epilayer, testifying to the very good crystalline quality of the material.