Influence of Er3+ ion concentration on spectroscopic properties and luminescence behavior in Er3+ doped Strontium telluroborate glasses

Er3+ doped Strontium telluroborate glasses were prepared following melt quenching technique by varying the concentration of Er3+ ion with respect to Borate content as 0.01, 0.05, 0.1, 0.5, 1.0 and 2.0 mol%. The presence of different vibrational groups was identified through FTIR and Raman spectral measurements. The variation in JO parameters with the increase in Er3+ ions concentration was discussed with respect to the covalency of the prepared glasses. The direct and indirect band gap values were found to decrease with the increase in Er3+ ion content and are discussed in terms of metallization factor and non-bridging oxygens. The green luminescence corresponding to the thermally coupled 2H11/2 and 4S3/2 states under 379 nm and 980 nm excitations was discussed. The luminescence intensity quenches beyond 0.5 mol% of Er3+ ion concentration in both the excitations due to the energy transfer process which takes place between Er3+ and Er3+ ions. The different possible mechanisms (ESA, ETU) responsible for the upconversion luminescence were also discussed with partial energy level diagram. The radiative parameters such as A, β and τ were determined for the 4S3/2 excited level which in turn helps one to calculate the stimulated emission cross-section (σPE). Among the studied glasses, STB0.5E glass exhibits higher A,σpE and βR values and these in turn specify its suitability for laser applications.