The nonlinear optical effects such as second harmonic generation and two-photon emission in ZnO nanocrystals have been studied using microscopic time-resolved and time-integrated emission spectroscopy. The two-photon emission efficiency is proportional to the emission from the defect level. The stimulated transition of the carriers from the conduction band-edge to the defect states significantly influences the efficiency of the two photon process. The nonlinear emission characteristics depend significantly on the origin of the emission and the power dependence is significantly different from the surface emission compared to that emitted within the bulk of the nano-crystal. The two photon emission efficiency generated within the bulk semiconductor reduces at higher pump photon densities due to the saturation of the defect level states populated by stimulated emission. The two-photon emission from the surface of the ZnO nanocrystals increases and is then followed by a saturation due to the increase in the intensity. Unlike the two-photon emission from the bulk of the crystal, the surface emission does not show any drop in the efficiency at higher pump intensities.
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