Two-photon resonance assisted huge nonlinear refraction and absorption in ZnO thin films

Abstract

Optical nonlinearities of ZnO thin films, made by laser deposition, were investigated by the Z-scan method using a mode-locked femtosecond Ti:sapphire laser. The measured bound-electron nonlinear index of refraction γ and the two-photon absorption coefficient β at near-IR wavelengths show an enormous enhancement compared with measurements on bulk ZnO at 532 nm. The results reveal that two-photon resonance to the band edge and exciton energy level is responsible for the nonlinear absorption and that the free carrier induced the optical nonlinearity. With the excitation wavelength operated between 810 to 840 nm, a negative β value is measured due to the saturation of linear absorption of the defect states. Finally, we compared the values of β from the closed aperture Z-scan data (by considering the multi-photon absorption induced thermal nonlinearity) with those obtained from the open aperture Z-scan data. The results show that nonlinear refraction in the near-IR region is dominated by the bound-electron and free-carrier effect, although the thermal optical nonlinearity cannot be completely ignored.