OPTICAL-ABSORPTION IN THE STRONG-COUPLING LIMIT OF ELIASHBERG THEORY

We calculate the optical conductivity of superconductors in the strong -coupling limit. In this anomalous limit the typical energy scale is s et by the coupling energy, and other energy scales such as the energy of the bosons mediating the attraction are negligibly small. We find a universal frequency dependence of the optical absorption which is dom inated by bound states and differs significantly from the weak-couplin g results. A comparison with absorption spectra of superconductors wit h enhanced electron-phonon coupling shows that typical features of the strong-coupling limit are already present at intermediate coupling.