Upconversion luminescence from aluminoborate glasses doped with Tb(3+), Eu(3+) and Dy(3+) under the excitation of 2.6-μm femtosecond laser pulses.

We investigated the upconversion luminescence of three aluminoborate glasses doped with Tb(3+), Eu(3+), and Dy(3+) under the excitation of 2.6-μm femtosecond (fs) laser pulses. Efficient upconversion luminescence appearing in the visible light spectral region was observed in all three glasses and the emission spectra are quite similar to those obtained under single photon excitation. From the dependence of the luminescence intensity on the excitation intensity in the low excitation intensity regime, it was revealed that a four-photon process is involved in the generation of the upconversion luminescence in the Tb(3+)- and Eu(3+)-doped glasses while a mixed two- and three-photon process is involved in the Dy(3+)-doped glass. In the high excitation intensity regime, a reduction of the slope to about 1.0 was observed for all glasses. A physical mechanism based on the super saturation of the intermediate states of the rare-earth ions was employed to interpret the upconversion luminescence under the excitation of long-wavelength fs laser pulses. Significantly broadened luminescence spectra were observed in thick glasses under high excitation intensities and it can be attributed to the self-focusing of the laser beam in the thick glasses.