Upconversion properties, multiphonon relaxation, and local environment of rare-earth ions in fluorophosphate glasses

Abstract

The upconversion properties of ${\mathrm{Er}}^{3+}$ ions were studied for the fluorophosphate glasses (45-x)${\mathrm{AlF}}_3$⋅x${\mathrm{AlPO}}_4$⋅${5\mathrm{E}\mathrm{r}\mathrm{F}}_3$⋅${30\mathrm{C}\mathrm{a}\mathrm{F}}_2$⋅${20\mathrm{B}\mathrm{a}\mathrm{F}}_2$, with use of the infrared radiation from a (Ga,Al)As laser diode (λ=802 nm) as an excitation source. Green upconversion fluorescence due to the ${}_{}{}^4{}_{}{}^{}$ ${\mathit{S}}_{3/2}$ ${\to }^4$ ${\mathit{I}}_{15/2}$ transition could be observed for the fluoride glass, while the fluorescence intensity for fluorophosphate glasses decreased drastically with increasing ${\mathrm{AlPO}}_4$ content. It was found from the phonon sideband spectra of ${\mathrm{Eu}}^{3+}$ that the phonon with ħω=1060 ${\mathrm{cm}}^{\mathrm{-}1}$, corresponding to P-${\mathrm{O}}^{\mathrm{-}}$ stretching vibrations, became coupled to the multiphonon relaxation of rare-earth ions, and its electron-phonon coupling strength increased greatly with increasing phosphate content. The temperature dependence of upconversion intensity was large for the fluoride system, while it was small for the fluorophosphate system. These tendencies could be well explained by considering the multiphonon decay rate of the ${\mathrm{Er}}^{3+}$ intermediate level and its temperature dependence, which are functions of the phonon energy of the host and the energy gap to the next-lower level of ${\mathrm{Er}}^{3+}$. Combined with the results of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ Mössbauer spectroscopy, it was concluded that the upconversion properties in these glasses were largely influenced by the local structure and the phonon mode coupled to the rare-earth ions.