Résumé
Linear dichroism experiments show that exciton-polariton ground states are split into a linearly polarized doublet. At normal incidence and zero detuning this splitting does not exceed 10 mu eV for the upper polariton branch and 3 mu eV for the lower one. For both branches the splitting decreases with positive and negative detuning. (c) 2007 Elsevier Ltd. All rights reserved.