Excitons and polaritons in ZnSe

We have performed resonant Brillouin scattering experiments in ZnSe in the [100] and [110] directions. These two directions are needed to obtain the exciton-polariton dispersion curves without ambiguity. Even if only the influence of the isotropic part of the exchange interaction on heavy and light excitons is taken into account, the number of polariton branches is shown to be dependent on the wave-vector direction. However, in ZnSe a three-branch-polariton model ("quasi-isotropic" model) is enough to account for the experimental results. We obtain thus the longitudinal-transverse (triplet) splitting $E_{\mathrm{LT}}=1.45\mathrm{}\pm 0.05$ meV, and the quintuplet-transverse (triplet) splitting $\delta =-0.1\mathrm{}\pm 0.1$ meV, and the masses of heavy and light excitons, $M_h$ and $M_l$, $M_h\mathrm{}\left(100\right)=1.11\mathrm{}\pm {0.1m}_0$, $M_l\mathrm{}\left(100\right)=0.3\mathrm{}\pm {0.05m}_0$, $M_h\mathrm{}\left(110\right)=1.95\mathrm{}\pm {0.1m}_0$, $M_l\mathrm{}\left(110\right)=0.37\mathrm{}\pm {0.05m}_0$ in the free-electron mass unit. These numerical values are little altered if the linear $k$ term is taken into account.