Preparation of Upconversion Fluorescent Glasses by the Sol-Gel Method and Their Properties

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Near infrared to visible upconversion fluorescence has been observed in Er^<3+> doped polycrystalline Nb_2O_5 and Al_2O_3-GeO_2 glasses prepared by the sol-gel method. The results of upconversion fluorescence and electron spin resonance (ESR) for Er^<3+> ions have been interpreted in terms of clustering of Er^<3+> ions in these materials. The details are described below.Heat-treatment of a dried gel 100Nb_2O_5-1Er_2O_3 at 500ﾟC gave a glass, whereas the dried gel crystallized into the TT-Nb_2O_5 phase by the heat-treatment at 550ﾟC.The crystal showed weak green upconversion fluorescence at 544 nm under 805 nm excitation, while the glass did not. In both the crystal and glass, ESR spectra measured at 4.2 K showed signals due to Er^<3+> at about 80 mT,indicating that Er^<3+> ions may be well dispersed in these materials without clustering.In glasses xAl_2O_3-(99-x)GeO_2-1Er_2O_3(x=0-5), upconversion fluorescence due to Er^<3+> ions was observed at 525,550, and 660nm, which is assigned to the transition to the ground state ^4I_<15/2> from excited states ^2H_<11/2>, ^4S_<3/2>, and ^4F_<9/2>, respectively. The relative intensity of the 660-nm fluorescence increases with decreasing the value of x. This is because clustering of Er^<3+> ions occurs and then the populations of the ^4F_<9/2> level increases by the energy transferring cross relaxation between close-lying Er^<3+> ions in a cluster. An ESR signal at 80 mT due to Er^<3+> decreases in intensity with decreasing the value of x, implying that it is also associated with clustering. The ESR signal may weaken or disappear due to the paramagnetic dipole-dipole interaction or the antiferromagnetic interaction between clustered Er^<3+> ions.