coupling regime. We also note that this splitting follows expected dependence. Although we consider the cavity mode that is in nearly perfect resonance with a frequency corresponding to the maximum of the Frank-Condon distribution (Fig. ), clearly the upper polariton has a higher population as it becomes more evident at high molecular concentrations. This can be explained by the material dispersion of the cavity mode (the linear transmission shown as black line in Fig. ), which is not symmetric for the frequencies above and below the fundamental frequency.