Optical absorption from a nondegenerate polaron gas

We apply the dynamical mean field theory to calculate the low-density limit of the optical conductivity in the Holstein model. This nonperturbative treatment allows continuous spanning from the weak-coupling quasi-free-electron behavior to the strong-coupling small polaron picture. Different features arise in the intermediate coupling regime: (i) a noticeable transfer of spectral weight toward low frequencies occurs at low temperatures, when coherent processes acquire importance; (ii) thermally activated polaron interband transitions appear at nonzero temperature, whose spectral weight has a nonmonotonic temperature dependence. We propose an interpretation of the infrared active vibrations observed in the optical spectra of underdoped superconducting cuprates in terms of small polarons in the intermediate coupling regime.