EXCITED-STATE ABSORPTION MECHANISMS IN RED-LASER-PUMPED UV AND BLUE UPCONVERSIONS IN TM-3+-DOPED FLUOROALUMINATE GLASS

The mechanism of red-pumped frequency-upconversion fluorescence observed in the uv and blue ranges of frequencies for a TM3+-doped fluoroaluminate glass was investigated by single-wavelength pumping of a tunable 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-amino-styryl)-4H-pyran dye laser. It was found that the dependences of fluorescence intensity on the incident excitation power were quadratic both for the 0.36- and 0.45-mum emissions, which originate from 1D2, and for the 0.48-mum emission from 1G4, indicating two-step excitation mechanisms. The excitation profile for the normal emission varied in the same way as that of the ground-state absorption spectra of the F-3(4) <-- H-3(6) transition, while those for upconversion emissions shifted to the higher-energy side. Moreover, the excitation process to the 1G4 level was found to be different from that to the 1D2 level. It was concluded that the mechanisms leading to these upconversion emissions are excited-state absorption (ESA) for both levels and that the process to the 1D2 level is ESA from F-3(4), while that to the 1G4 level is from the F-3(4) level after nonradiative decay from the upper levels. The results were discussed in terms of the slow multiphonon decay rate from the 1D2 to the 1G4 level in this glass.