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By excitation with 800 nm femtosecond laser, we observed the green up-conversion fluorescence at 526 nm (2H11/2-4I15/2) and 548 nm (4S3/2-4I15/2) for Er3+ ions doped in 76TeO2·20Nb2O5·4ZnO glass. From IH,S ~ Pn relationship (IH,S; fluorescence intensity of 2H11/2-4I15/2 and 4S3/2-4I15/2, P: laser power density, n: the number of photons absorbed per visible photon emitted), the n values of 1.4 and 1.3 were obtained at room temperature for IH and IS, respectively, and approached "2" at lower temperatures and at lower excitation power densities. This indicates that two photons were involved in the excitation of the up-conversion fluorescence. Considering the up-conversion intensity at various repetition frequencies on fs laser excitation and the emission decay behaviors as well as the results of Judd-Ofelt analysis, we could explain this excitation mechanism in terms of energy transfer (ET: 4I11/2 + 4I11/2 → 4F7/2 + 4I15/2, and 4I9/2 + 4I9/2 → 4S3/2 + 4I13/2). We shall also discuss on a back transfer process of the latter ET for the reduction of the n value at room temperature.

Excitation-emission properties of Er3+ ions doped in nonlinear optical TeO2-Nb2O5-ZnO glass by 800 nm femtosecond laser excitation