Phonon‐Assisted Two‐Photon Absorption in the Reststrahl Region of Excitons in Alkali Halides

Abstract

The two-photon spectrum of RbI is investigated in the energy region of the lowest IS exciton. A significant increase of the two-photon signal is observed when crossing the transverse energy and scanning through the excitonic reststrahl region. This phenomenon is explained by a LO-phononassisted absorption process based on the Frohlich exciton-phonon coupling. Evaluating the two-photon excitation within a two-band model allows a quantitative analysis, which shows good agreement with the experimental data. Besides a scaling factor, the only fit parameter is the exciton Bohr radius that is found in reasonable agreement with the Wannier exciton model.