  We apply the Dynamical Mean Field Theory to calculate the low-density limit
of the optical conductivity in the Holstein model. This non perturbative
treatment allows to span continuously from the weak-coupling quasi-free
electron behaviour to the strong-coupling small polaron picture. New features
arise in the intermediate coupling regime: i) a noticeable transfer of spectral
weight towards low frequencies occurs at low temperatures, when coherent
processes acquire importance; ii) thermally activated Polaron Interband
Transitions appear at non zero temperature, whose spectral weight has a non
monotonic temperature behaviour. We propose an interpretation of the infrared
active vibrations (IRAV) observed in the optical spectra of the underdoped
superconducting cuprates in terms of small polarons in the intermediate
coupling regime.
