Upconversion studies in Er3+ doped TeO2–M2O (M=Li, Na and K) binary glasses

The luminescence properties of Er3+ doped alkali tellurite [ TeO2–M2O (M=Li, Na and K)] glasses are investigated. Infrared to visible upconversion emissions are observed at 410, 525, 550 and 658 nm using 797 nm excitation. These bands are assigned to the 2H9/2  →4I15/2, 2H11/2  →4I15/2, 4S3/2  →4I15/2, 4F9/2  →4I15/2 transitions of Er3+ respectively. Detailed study reveals that the 2H9/2  →4I15/2 transition at 410 nm involves a three-step process while the other transitions involve two-steps. Excitation with 532 nm radiation gives additional bands at 380, 404, 475 and 843 nm wavelengths due to the 4G11/2  →4I15/2, 2P3/2  →4I13/2, 2P3/2  →4I11/2, and 4S3/2  →4I13/2 transitions, respectively, along with the bands observed on NIR excitation. The fluorescence yield is found to be largest for the TeO2–Na2O glass. The lifetime of the 4S3/2 level has been measured for all the three cases and used to explain the upconversion mechanisms. The fluorescence intensity ratio corresponding to the two thermally coupled levels (2H11/2, 4S3/2) has been used to estimate the temperature of the glass. It is observed that the temperature sensing capacity of TeO2–Li2O glass is better than the other two glasses.