Special frequencies in reflection spectra of Bragg multiple quantum well   structures
  We have studied theoretically optical reflection spectra from the Bragg multiple quantum well structures. We give an analytical explanation of the presence of two special frequencies in the spectra at which the reflection coefficient weakly depends on the quantum well number. The influence of the exciton nonradiative damping on the reflection spectra has been analyzed. It has been shown that allowance for the dielectric contrast gives rise to the third special frequency at which the contributions to the reflectivity related to the dielectric contrast and the exciton resonance mutually compensate one another. 
M.M. Voronov, E.L. Ivchenko, A.N. Poddubny, V.V. Chaldyshev