Coherent radiation from thin films

The spontaneous coherent radiation from thin crystal films of n layers is studied. By treating each layer as a unit and by employing the method of multiple-time-scale expansion, exact solutions for such systems, with no restriction on the value λ₀d, are found. Here λ₀ is the characteristic radiation wavelength and d is the average interparticle spacing. In the limit λ₀d1, it is seen that there exist correlated states of the film which are exact analog of Dicke's superradiant states. The decay rate for the superradiant states is roughly enhanced by a factor of n(λ₀d)² compared to the single-atom decay rate γₛ.