Frequency Dependence of Interband Two-Photon Absorption Mechanisms in ZnO and ZnS
The transition mechanism changes involved in direct interband two-photon absorption (TPA) processes have been studied in two II–VI compounds:ZnO and ZnS. The experimental results have indicated that the transition mechanism nature changes when the excitation energy is varied. The transitions are of an allowed-allowed type near the energy gap and become of an allowed-forbidden type far from it. In particular, for ZnO when 2ħw-Ey > ~ 400 meV the first experimental evidence of a dominant mechanism of forbidden-forbidden type has been obtained. Furthermore, the spectral behaviour of the TPA coefficient (α(2)) has been well described by a parametric formula containing terms (with different energy dependences) related to the above-mentioned different transition mechanisms.
KeywordsMatrix Element Line Shape Transition Type Spectral Dependence Transition Mechanism
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