Radiative width of collective modes in a resonant Bragg reflector: the validity limit of superradiant mode

The validity limit of the concept of the “superradiant mode” of a resonant Bragg reflector consisting of multiple (N) quantum wells has been theoretically studied according to the nonlocal formalism developed by the authors. The radiative shift and width have been treated from first principle with its dependence on frequency, which describes the natural evolution, from the superradiant to the photonic gap behavior, of the reflectivity in both frequency and time domains. The criterion for the upper limit of the speed-up of radiative decay is given for various systems including crystals of Mössbauer nuclei.