Defect Induced Changes in the Linear and Non-Linear Optical Properties of ZnO Nanotetrapods

Here, we investigate the origin of green luminescence in ZnO nanotetrapods using nonlinear optical z-scan measurements in tandem with photoluminescence measurements. To this end, we synthesized ZnO nanotetrapods using a chemical vapor deposition (CVD) process, annealed at temperatures below 200 degrees C in O-2, Ar, and Zn metal vapor and measured the ensuing optical properties. Our z-scan measurements show that all samples exhibit an effective three-photon absorption (3PA). Furthermore, the 3PA coefficient varies with the incident laser intensity consistent with the presence of real mid-gap states. Interestingly, the rate of change for the 3PA coefficient is strongly influenced by the annealing conditions. Annealing in Zn vapor (Ar and O-2) leads to an increase (decrease) in green luminescence and the 3PA coefficient, strongly suggesting that the Zn interstitials are involved in the mechanism of green luminescence.