Infrared reflectivity spectroscopy of electron-phonon interactions

The crucial role of electron-phonon interactions and their signatures in optical conductivity deduced from the analysis of infrared-visible reflection spectra is reviewed. The usefulness of a general phenomenological model able to describe the couplings of phonons, polarons, plasmon, superconducting condensate, is discussed. Among other examples of applications, the problem of the signature of Cooper pair condensation in the superconducting phase of high- T c cuprates is discussed in the framework of London and Mattis-Bardeen models.