Optical transitions and upconversion properties of Ho3+ doped ZnO−TeO2 glass

Optical absorption, luminescence spectra and upconversion of both red to green and infrared to green luminescence have been studied for a Ho3+ doped zinc tellurite glass of composition 19ZnO−80TeO2−1Ho2O3. From the absorption data, Judd–Ofelt parameters for the Ho3+ ion are calculated along with the radiative transition rates, branching ratios and radiative lifetimes of the major Ho3+ emitting levels. The upconversion luminescence for both 646 and 754 nm excitation was attributed to a two photon excited state absorption upconversion mechanism that occurs via the I75 level. For an excitation power density of 610 W/cm2 an upconversion efficiency of 1.22% was determined for the case of 646 nm excitation, which is considerably higher than what has been reported in Ho3+ doped fluoride single crystals. Under 754 nm excitation, an upconversion efficiency of 0.27% was observed for a excitation power density of 610 W/cm2, which is reasonably low compared to other Ho3+ doped materials.