Two-photon and three-photon absorption in ZnO nanocrystals embedded in Al₂O₃ matrix influenced by defect states

The broadband nonlinear absorption in ZnO nanocrystals embedded in Al₂O₃ matrix was investigated by Z-scan and pump-probe techniques from 400 nm to 800 nm. The effective two-photon absorption and three-photon absorption coefficients were determined to be ∼1.1×10³  cm/GW at 400 nm and ∼1.1×10^-1  cm³/GW² at 800 nm, respectively, which are at least two orders of magnitude greater than that in ZnO bulk crystal. It may be attributed to the defect-states-mediated multiphoton absorption process, which was proofed by comparison experiments with different densities of interfacial defect states. The corresponding lifetimes for the intraband relaxation, defect-states trapping, and interband recombination processes were measured by femtosecond transient absorption measurements as τ1 ∼ 1  ps, τ2 ∼ 13  ps, and τ3 ∼ 350  ps, respectively.