Direct and indirect two-photon processes in semiconductors

Summary The expressions describing direct and indirect two-photon absorption in crystals are given. They are valid both near and far from the energy gap. A perturbative approach through two different band models is adopted. The effects of the nonparabolicity and the degeneracy of the energy bands are considered. The numerical results are compared with the other theories and with recent experimental data in ZnO and AgCl. It is shown that the dominant transition mechanisms are of the allowed-allowed type near and far from the gap for both direct and indirect processes. Permanent address: Physics and Mathematics Department, Faculty of Engineering, Ain Shams University, Cairo, Egypt.