Direct and indirect two-photon absorption in Ge within the effective mass approximation

Abstract

We have calculated the two photon absorption coefficient for Ge within the effective mass approximation for the direct as well as the indirect gap region. By including all the possible contributions to the nonlinear absorption based on the Ge band structure, we found excellent agreement with the recent experimental report on the two photon absorption in Ge. It is found that even though the direct transition is stronger than the indirect process, a tail at the low end of the energy spectrum appears, and the ultimate limit for the two photon absorption is half of the indirect gap energy.