Optical, vibrational, and photoluminescence properties of holmium-doped boro-bismuth-germanate glasses.

Holmium (Ho)-doped boro-bismuth-germanate glasses having the chemical composition (30-x)BO + 20GeO + 20BiO + 20NaO + 10YO + xHoO, where x = 0.1, 0.5, 1.0, and 2.0 mol% were prepared by melt-quenching technique. The prepared glasses were examined for thermal, optical, vibrational, and photoluminescent properties. The prepared glasses were found to be thermally very stable. The optical bandgap and Urbach energies of 0.1 mol% HoO-doped boro-bismuth-germanate glass were calculated to be 3.3 eV and 377 MeV, respectively, using the absorption spectrum. The Judd-Ofelt analysis was performed on the 0.1 mol% HoO-doped glass and compared the obtained parameters with literature. The branching ratio (β) and emission cross-section (σ) of the green band were determined to be 0.7 and 0.24 × 10 cm, respectively. Under 450 nm excitation, a strong green emission around 550 nm was observed and assigned to the (S + F) → I (Ho) transition. Upon an increase of HoO content from 0.1 to 2.0 mol%, the intensities of all observed emission bands as well as decay time of the (S + F) → I transition have been decreased gradually. The reasons behind the decrease in emission intensity and decay time were discussed. The strong green emission suggests that these glasses may be a better option for display devices and green emission applications.