Superradiance of excitons

The theory of superradiance of excitons in thin crystal films is presented. While the broken crystal symmetry in the direction normal to the film plane renders the radiative decay of the exciton possible, the remaining crystal symmetry on the film leads to phase coherence between the exciton and the photon fields everywhere on the film. Consequently, a trading between the degrees of freedom of the radiation field and those of the atoms is shown to be responsible for the predicted superradiative enhancement of the exciton decay by a factor ( lambda /d) 2 >>1 in the optical region.