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.