Nonperturbative theory of exciton-phonon resonances in semiconductor absorption

We develop a theory of exciton-phonon sidebands in the absorption spectra of semiconductors. The theorydoes not rely on an ad hoc exciton-phonon picture, but is based on a more fundamental electron-phononHamiltonian, thus avoiding a priori assumptions about excited-state properties. We derive a nonperturbativecompact solution that can be looked upon as the semiconductor version of the textbook absorption formula fora two-level system coupled to phonons. Accompanied by an illustrative numerical example, the importance andusefulness of our approach with respect to practical applications for semiconductors is demonstrated.