| Rare earth (RE) ions become the research interests because of their unique electronic structure of the 4f and abundant energy levels. Upconversion fluorescence materials can be obtained by doping some RE ions into certain host materials. Upconversion fluorescence materials may be used in fields of upconversion laser, laser anti-counterfeiting, three-dimensional display, optical temperature sensor, dye-sensitised solar cells, biomarkers and medical diagnostics, etc. RE ions (Yb3+, Tm3+, Ho3+) doped YAG single crystal optical fibers are successfully grown by laser heated pedestal growth (LHPG) method. The RE ions doped YAG single crystal optical fibers are respectively marked sample 1# 5.0% Yb3+/1.0% Tm3+:YAG, sample 2# 5.0% Yb3+/0.5% Ho3+: YAG, sample 3# 5.0% Yb3+/1.0% Tm3+/0.2% Ho3+:YAG, sample 4# 5.0% Yb3+/1.0% Tm3+/0.25% Ho3+:YAG and sample 5# 5.0% Yb3+/1.0% Tm3+/0.5% Ho3+:YAG. The diameters of prepared YAG single crystal fibers are about 300um～1100μm and lengths are about 2mm～45mm. YAG single crystals have many advantages, such as high melting point, good properties of physical and chemical, the characteristic of the waveguide, good beam direction, low loss and so on. So the YAG single crystal is a very interesting host material. Specific research results are as follows:(1) The absorption spectrums of sample 1# and 2# were investigated. They have a large absorption near 980nm, which is mainly due to the energy level transition of Yb3+:2F7/2â†' 2F5/2.So Yb3+ ions are often used as sensitizer to absorb the excited energy and transfer to other ions. This is the reason why we use the 974 run semiconductor laser as the excitation light.(2) Under the excitation of 974nm the upconversion spectrums of sample 1# and 2# were investigated at the room temperature. We found that sample 1# has stronger blue upconversion emission and weaker red upconversion emission in the visible light region. The fluorescence spectrums are respectively at the near of 486nm(blue),650nm and 680nm(red), which are respectively from the transition of Tm3+:1G4â†'3H6(486nm),1G4â†'3F4(650nm) and 3F2,3â†'3H6(680nm). Sample 2# has stronger red (667nm) upconversion emission and weaker green(550nm) upconversion emission, which are respectively from the transition of Ho3+:5F5â†'5I8 (667nm) and 5F4,5S2â†'5I8(550nm).(3) The dependences of upconversion emission intensities on pump powers of sample 1#and 2# were investigated. The results show that their slopes are close to 1. So the results indicated that the correlation energy levels of Tm3+ and Ho3+ have reached saturation. We infer that it may be result from the thin material or may be related to host. But because of the limitation of experimental conditions, lower laser power will lead to the noise of spectrometer.(4) Under the excitation of 974nm the upconversion fluorescence of Yb3+/Tm3+/Ho3+:YAG single crystal fibers were investigated. By measuring the upconvesion spectrum of sample 3#,4# and 5#, we found that blue upconconvesion emission significantly reduced with the increase of Ho3+ions concentration. However no matter how to change the Ho3+ doping concentration we can’t get good performance white light emissions because the intensity of the green light is too low. The luminescence mechanism was analyzed in order to well understand the three doped YAG single crystal fibers luminescence phenomenon.(5) The upconversion fluorescence temperature dependence of sample 1# was investigated. The results show that the transition of 3F2,3F2,3â†'3H6 and 3H4â†'3H6 belong to a pair of thermally coupled energy levels by fitting the expression of the temperature and the fluorescence intensity ratio. The fitting energy levels gap between the two levesl is â–³E= 1607 cm-1, which is very close to the actual energy levels gap between the two levels â–³Ef= 1842 cm-1. It’s absolute sensitivity achieve to 2313.09/T2. So it is a good potential temperature sensing material. |
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