Polariton dispersion law in periodic-Bragg and near-Bragg multiple quantum well structures

The structure of the polariton spectrum is analyzed for periodic multiple quantum well structures with periods at or close to the Bragg resonance condition at the wavelength of the exciton resonance. The results obtained used to discuss recent reflection and luminescent experiments by M. H\"ubner et al. [Phys. Rev. Lett. 83, 2841 (1999)] that were carried out with long multiple quantum well structures. It is argued that the discussion of quantum well structures with a large number of wells is more appropriate in terms of normal modes of infinite periodic structures rather than in terms of super- and subradiant modes.