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《Chinese Journal of Luminescence》 1993-01
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Shi Ruohua Yu Jiaqj Wang Zhongka(Changchun Institute of Physics, Academia Sinica, Changchun 130021)  
The phenomena of upconversion have attracted considerable interest of scientists in recent years and have been extensively studied in systems containing rare-earth ions. The upconversion pumping of rare-earth laser systems has been demonstrated.We have observed upconverted blue fluorescence in LiYF4:Nd3+ single crystal at room temperature and 77K under pulsed-pumping by a Rhodamine 580 dye laser. The pumping wavelength is 578.4nm. There are four regions in which emission is observed.They are near 357.8nm, 383.3nm, 414.9nm and 448.6nm at room temperature (near 357.6nm, 383.2 nm, 414.7nm and 456nm at 77K) which correspond to transitions from 4D3/2 + 4D5/2 manifolds to 4I9/24I11/24I13/2. and 4I15/2,respectively. That the peaks in the emission spectra at 77K do not appear in the corresponding emission spectra at room temperature and vice versa is dur to the change of ion distribution on energy levels by changing the temperature.The blue fluorescence intensity versus the square of pump intensity has been measured at room temperature. The linear relation shows that this upconversion process involves two photons.Decay cuive measurements of the excited 4D3/2 energy level fluorescence at room temperature and at 77K show that after excitation the fluorescence increases first,reaches maximum,and then decay.This shows that energy transfer is involved in upconversion process at room temperature and at 77K.At room temperature, most of the absorption peaks in the absorption spectrum appear in the excitation spectrum. This indicates that two--step excitation is involved in upconversion process.
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