Optical transitions and frequency upconversion emission of Er3+ ions in Ga2S3–GeS2–La2S3 glasses

Spectroscopic properties of ${\mathrm{Er}}^{\mathrm{3+}}$ in ${\mathrm{Ga}}_{\mathrm{2}}{\mathrm{S}}_{\mathrm{3}}{\mathrm{–GeS}}_{\mathrm{2}}{\mathrm{–La}}_{\mathrm{2}}{\mathrm{S}}_{\mathrm{3}}$ glasses are investigated on the basis of the spontaneous emission probabilities calculated by the use of the Judd–Ofelt theory and lifetime data. Frequency upconversion spectra are also measured with excitation at 800 and 980 nm. The spontaneous emission probabilities are much larger than those of the other glass systems such as oxides and fluorides. This is mainly attributed to the large refractive indices of the sulfide glasses. The quantum efficiencies of ${}^4{I}_{\text{11/2}}$ and ${}^4{F}_{\text{9/2}}$ levels are comparable or larger than the typical values of ${\mathrm{Er}}^{\mathrm{3+}}$ in a fluoride glass. This is a consequence of the large spontaneous emission probabilities and low phonon energies of the sulfide glasses. However, the quantum efficiency of the ${}^4{S}_{\text{3/2}}$ level is less than one-half of the value in the fluoride glass. The nonradiative transition probability of the ${}^4{S}_{\text{3/2}}$ level is much larger than the value which is evaluated from the multiphonon relaxation theory. This suggests the presence of the other nonradiative relaxation process in addition to the multiphonon relaxation. This process is considered to be the energy transfer from the excited ${\mathrm{Er}}^{\mathrm{3+}}$ to the band-gap excitation of the matrix glass. In upconversion spectra, the 497 nm emission from the ${}^4{F}_{\text{7/2}}$ level is observed in addition to the red (665 nm) and the green (533 and 552 nm) emission bands.