Optical signatures of exciton-polarons from diagrammatic Monte Carlo

We study the interplay of electron-electron and electron-phonon interactions in the course of electron-hole bound state formation for gapped solid state systems. Adapting the essentially approximation-free diagrammatic Monte Carlo method for the calculation of the optical response, we discuss the absorption of light in correlated electron-phonon systems for the whole interaction and phonon frequency regimes. The spectral function obtained by analytical continuation from the imaginary-time current-current correlation function demonstrates the dressing of excitons by a phonon cloud when the coupling the lattice degrees of freedom becomes increasingly important, where notable differences show up between the adiabatic and anti-adiabatic cases.