Frequency dependence of interband two-photon absorption mechanisms in ZnS and CdI2

Different transition mechanisms related to two-photon absorption (TPA) processes have been investigated in direct (ZnS) and indirect gap (CdI₂) semiconductors. Experimental results show that, in both direct and indirect TPA processes, the dominant transition mechanism depends on the frequency rather than on material parameters. In particular, it is of an allowed-allowed type near the energy gap and, far from this, it becomes of an allowed-forbidden one.