Upconversion luminescence and optical power limiting effect based on two- and three-photon absorption processes of ZnO crystal

Near-infrared to UV and visible upconversion luminescence was observed in single-crystalline ZnO under an 800 nm infrared femtosecond laser irradiation. The optical properties of the crystal reveal that the UV and VIS emission band are due to the exciton transition (D₀X) bound to neutral donors and the deep luminescent centers in ZnO, respectively. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to three-photon sequential band-to-band excitation and the VIS emission belongs to two-photon simultaneous defect-absorption induced luminescence. A saturation phenomenon and polarization-dependent effect are also observed in the upconversion process of ZnO. A very good optical power limiting performance at 800 nm has been demonstrated. The two- and three-photon absorption coefficients of ZnO crystal were measured to be 0.2018 cm GW^-1 and 7.102 × 10^-3 cm³ GW^-2, respectively. The two- and three-photon cross sections were calculated to be 1.189 × 10^-51 cm⁴ s and 1.040 × 10^-80 cm⁶ s², respectively.