We have developed Greenís function formalism in order to calculate the components of the photon propagator in a quantum well with high electronic densities in a transversal magnetic field. The power spectra and the dispersion curves of the sets of collective modes which arise in such a system are discussed for cyclotron frequencies lying in the reststrahlen region of the spectra. The resonance between the cyclotron frequency and the TO and LO modes are also discussed.
