Luminescence Control Of Rare Earth Nanocrystals In The External Field

Rare earth ions have special 4f electron structure and can be doped into a variety of upconversion luminescent materials,which absorb two or more photons of low energy and emit photons of high energy.The luminescence of rare earth ions features good fluorescence characteristics,pure luminescent chroma,stable physical and chemical properties,and high conversion efficiency.When being doped in paramagnetic host,the luminescence of rare earth ions can be effectively influenced by applying an external magnetic field.This thesis is dedicated to studies on control of the upconversion luminescence of rare-earth-ion doped nanocrystals with external magnetic or laser fields.GdF₃:Er³⁺nanoparticles are first made using hydrothermal method and their luminescent behavior under 797 nm laser excitation is then experimentally investigated under the influence of an external magnetic field(MF).By changing the magnitude of the MF between zero and 1.3 T,the intensity of the visible luminescence can be reversibly controlled within a dynamic range of nearly two orders of magnitude.Further experimental study indicates that the MF induced suppression of luminescence is mainly ascribed to the reduced absorption of excitation laser.The prepared nanoparticles of high MF sensitivity can be used for locally and optically sensing of intermediate MFs in nanoscale volumes and MF controlled bio-imaging etc.The upconversion luminescence behavior of 1%Er³⁺-doped Na YF₄ glass ceramics under broadband 800 nm femtosecond laser pulse excitation is investigated by using the combination of streak camera and spectrometer.The combination can provide high resolution information for the luminescence process in both temporal and spectral dimensions.Experimental results indicate that the resonance-mediated two photon absorption plays a dominant role in exciting the upconversion luminescence for this low dopant concentration sample under femtosecond laser pulse irradiation.Other excitation mechanisms,like excited state absorption and energy transfer upconversion,which used to play major roles in conventional experiments with cw and pulsed lasers,only make rather minor contributions.The laser power dependence of the upconversion luminescence efficiency is also explored by tailoring the laser pulse bandwidth.Experimental results show that there exists an optimal pulse bandwidth for maximal laser power excitation efficiency for the sample under study.And predictions from a theoretical model well agree with the experimental observations.