Nonlinear Fluorescence Of Zinc Oxide Material And Non-linear Lasing Study

The wide band gap semiconductor ZnO has drawn great interest recently for its potential applications in the short-wavelength devices owing to its wide band gap and high exciton binding energy. The successes of fabrication of ZnO nanostructures in the last ten years make ZnO very attractive to design the nanoscaled opto-electronical devices for integrated optics. Optical pumped ultraviolet nano-lasers have been demonstrated based on ZnO nanostructures at room temperature. Nonlinear optical properties of ZnO open new doors to design ZnO-based optical devices. In this thesis, we focus on the nonlinear optical response of ZnO materials with different morphologies and investigate the multi-photon process in these materials. The multi-photon absorption induced photoluminescence, stimulated emission in ZnO bulk crystal, microtubes, and nanowires were investigated, and time-resolved photoluminescence were preformed to research the mechanisms of these emission behaviors.The main creative and innovative research results are given below.1. With pumping of 527 nm nanosecond lasers, we observed the ultraviolet band edge emission and defect emission in ZnO microtubes induced by two-photon absorption process. Further works indicate that the 800 nm femtosecond pulses are possible to excite the band edge emission from the ZnO bulk crystal, micro-tubes, and nanowires. The near quadratic dependence of the ultraviolet emission intensity on the excitation flux reveals the contribution of the two-photon absorption in the excitation process of the ultraviolet emission. We propose a model of two-photon Rabi flopping assisted off-resonant two-photon absorption process to explain the observed phenomena, and attributed the ultraviolet emission to be co-excited by the two-photon absorption and three-photon absorption processes.2. The multi-photon absorption induced ultraviolet stimulated emission was also observed in these samples by pumping of the femtosecond lasers. The intrinsic mechanisms of these emission behaviors were also discussed. The optical gain spectrum from ZnO bulk crystals were attributed to the electron hole plasma, the first observed lasing action from ZnO nanowires induced by multi-photon absorption were attributed to the exciton-exciton scattering process, and the case for ZnO microtubes is attributed to both. Possible resonant Fabry-Perot cavities in ZnO nanowires and microtubes were also discussed.3. By using the optical Kerr gating of picosecond temporal resolution, the transient dynamic behavior of the emission from ZnO samples were measured and used to investigate the mechanisms. In ZnO bulk crystals, the excitation power dependent time resolved photoluminescence spectra indicate that the optical gain should be attributed to electron hole plasma. Moreover, a peak at the red side of the electron hole plasma emission was found to exist at～20 ps delay which might be attributed to the biexciton contribution. Transient behavior results confirmed that the lasing mechanism in ZnO nanowires near the threshold should be attributed to the exciton-exciton scattering. We also investigated the transient behavior of the resonant enhanced multi-photon photoluminescence and second harmonic generation.The conclusions of this work give a positive answer to the questions whether multi-photon process could be as effective as linear process in wide band gap semiconductor confessedly. This work is of importance in investigating the colorful nonlinear optical phenomena in wide band gap semiconductors and could be taken as a significant contribution to the multi-photon induced photoluminescence emission and lasing in ZnO material.