Excited-state absorption mechanisms in red-laser-pumped uv and blue upconversions inTm3+-doped fluoroaluminate glass

The mechanism of red-pumped frequency-upconversion fluorescence observed in the uv and blue ranges of frequencies for a ${\mathrm{Tm}}^{3+}$-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-μm emissions, which originate from ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$, and for the 0.48-μm emission from ${}_{}{}^1{}_{}{}^{}$ ${\mathit{G}}_4$, 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 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{F}}_4$← ${}_{}{}^3{}_{}{}^{}$ ${\mathit{H}}_6$ transition, while those for upconversion emissions shifted to the higher-energy side. Moreover, the excitation process to the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{G}}_4$ level was found to be different from that to the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$ 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 ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$ level is ESA from ${}_{}{}^3{}_{}{}^{}$ ${\mathit{F}}_4$, while that to the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{G}}_4$ level is from the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{F}}_4$ level after nonradiative decay from the upper levels. The results were discussed in terms of the slow multiphonon decay rate from the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$ to the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{G}}_4$ level in this glass.