  The strong coupling regime in a ZnO microcavity is investigated through room
temperature photoluminescence and reflectivity experiments. The simultaneous
strong coupling of excitons to the cavity mode and the first Bragg mode is
demonstrated at room temperature. The polariton relaxation is followed as a
function of the excitation density. A relaxation bottleneck is evidenced in the
Bragg-mode polariton branch. It is partly broken under strong excitation
density, so that the emission from this branch dominates the one from
cavity-mode polaritons.
