The optical absorption of the Fröhlich polaron model is obtained by an approximation-free diagrammatic Monte Carlo method and compared with two new approximate approaches that treat lattice relaxation effects in different ways. We show that: (i) a strong coupling expansion, based on the Franck-Condon principle, well describes the optical conductivity for large coupling strengths ($\alpha >10$); (ii) a memory function formalism with phonon broadened levels reproduces the optical response for weak coupling strengths ($\alpha <6$) taking the dynamic lattice relaxation into account. In the coupling regime $6<\alpha <10$, the optical conductivity is a rapidly changing superposition of both Franck-Condon and dynamic contributions.

• Received 30 January 2006

DOI:https://doi.org/10.1103/PhysRevLett.96.136405