Interband two-photon absorption mechanisms in direct and indirect GAP materials

Abstract

Interband transition mechanisms involved in direct and indirect two-photon absorption (TPA) processes have been investigated in ZnO, CuCl, Bi4Ge3O12, AgCl and CdI2. It has been shown that the transition mechanism nature changes in both direct and indirect processes when the excitation energy is varied. In particular, the transitions are of the allowed-allowed type near the energy gap and become of the allowed-forbidden one far from it. Furthermore, for direct TPA processes, it has been pointed out that the third possible mechanism, i.e., the forbidden-forbidden one, becomes dominant for energies well above the direct gap.