| Trivalent erbium ions (Er3+) doped materials have attracted considerable interests because of the ability to convert low frequency radiation to high frequency via upconversion (UC) processes. Er3+-doped materials have found significant potential in a wide range of applications, such as solid-state lasers,3D display, LED, bio-labelling, and so on. Therefore, it has attracted considerable interest in the research on the enhancement, tuning and mechanism of upconversion (UC) fluorescence in lanthanide ions. In this thesis, ultrafast manipulation on the photoluminescence of Er3+ions was realized with the excitation of Er+doped nanomaterials by applying femtosecond laser. Meanwhile, the UC fluorescence mechanisms were also investigated. The results are as follows:1. UC fluorescence properties and mechanisms of Er3+:NaYF4 doped nano-crystalline glass were investigated with the excitation of fs laser at 800 nm and 1490 nm, respectively. Based on the analysis of the polarization and power dependence of the fluorescence intensities, the UC mechanism of Er3+with the excitation of fs laser was proposed. With the excitation of fs laser, simultaneous two-photo absorption is mainly involved in the UC process of Er3+, which can be proved by the polarization dependence of the fluorescence intensity. With the excitation at 800 nm, the green emission mainly at 550 nm from Er3+is originated from the simultaneous absorption of two photons at 800 nm, I15/2â†'2H9/2â†'2H11/2/4S3/2. However, the red emission at 660 nm is originated from energy cross relaxation (CR) between Er3+ ions,4I11/2+4I13/2â†'4Fg/2+4I15/2-With the excitation at 1490 nm, CR between Er3+ ions was occurred after two-photon absorption from ground states and subsequently phonon-assisted relaxation. It leads to the emission at 600 nm, which reveals a polarization dependence of two photon fluorescence.2. UC fluorescence based on co-excitation of two-wavelength lasers was investigated; achieving UC emission enhancement and tuning of Er3+ doped nano-crystalline glass with the co-excitation at 800 nm & 1490 nm. And UC mechanism excited by two fs laser was investigated based on pump-probe method. Theoretical model based on Schrodinger Equation was founded, which well explained the experimental results and indicated the main mechanism of UC emission. Excited by lasers at 800 and 1490 nm simultaneously, emission at 550 nm was obviously enhanced. By adjusting the intensity of the two excitation lasers separately, red-green tuning can be acquired. Meanwhile, on the basis of the analysis results on delay time and wavelength dependence of emission intensities, the mechanism of green emission enhancement was proposed, which is originated from simultaneous (1+1) absorption of photons at 800 and 1490 nm. It is well in accordance with the simulation results of the theoretical model based on time-dependent Schrodinger Equation.3. UC fluorescence mechanism of Er+:NaYF4 doped nano-crystalline glass were investigated with the excitation of cw laser at 800 nm. The systematic research was performed on fluorescence properties of UC with cw excitation at 800 nm. And the possible UC transitions of red and green emission in Er3+were also investigated by applying multi-color fs laser generated from OPA and the pump detection methods, leading to the determination of the main UC transition of red and green emission. With the cw excitation at 800 nm, the emission at 550 nm was mainly from excited state absorption (ESA) with a transition of4I2â†'>4I9/2â†'-4I13/2â†'2H11/2/4S3/2, while the emission at 660 nm was mainly from CR process with a transition of 4I11/2+4I13/2â†'4F9/2+4I15/2-These findings provide important evidences for deeper understanding the UC transition of Er3+ and improving the UC properties of lanthanide doped materials. |
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