Collective strong coupling in a lossy optical cavity

We observe vacuum Rabi splitting in a lossy nearly confocal cavity indicating the strong coupling regime, despite a weak single-atom single-mode coupling. Strong collective interaction manifests itself in the typical N(1/2)-dependence of the normal mode splitting on the number of atoms N. The TEM(00)-mode coupling parameters are (g, kappa, gamma) = 2 pi x (0.12, 0.8, 2.6) MHz and up to (1.33 +/- 0.08) x 10(5) cesium atoms were loaded into the mode volume.