THEORY OF EXCITON-POLARITONS IN SEMICONDUCTORS WITH NEARLY DEGENERATEEXCITON LEVELS

We show that the phenomenological approach to polariton theory is just ified by a microscopic quantum mechanical treatment, obtaining a set o f oscillating polarization densities. This approach makes it possible to maintain the dipole approximation in the calculation of matrix elem ents without neglecting macroscopic variation of the radiation field i n the crystal, leading to correct wave vector dependence of the coeffi cients of the quantum Hamiltonian. As a result correct diagonalization is achieved without imposing any sum rule. Finally, polariton states are expressed in terms of photon and exciton states in the case of two nearly degenerate exciton levels, different components are singled ou t and numerical results for ZnSe are presented.