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

Holmium (Ho³⁺)-doped boro-bismuth-germanate glasses having the chemical composition (30-x)B₂O₃ + 20GeO₂ + 20Bi₂O₃ + 20Na₂O + 10Y₂O₃ + xHo₂O₃, 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% Ho₂O₃-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% Ho₂O₃-doped glass and compared the obtained parameters with literature. The branching ratio (β) and emission cross-section (σ_em) of the green band were determined to be 0.7 and 0.24 × 10^-20 cm², respectively. Under 450 nm excitation, a strong green emission around 550 nm was observed and assigned to the (⁵S₂ + ⁵F₄) → ⁵I₈ (Ho³⁺) transition. Upon an increase of Ho₂O₃ content from 0.1 to 2.0 mol%, the intensities of all observed emission bands as well as decay time of the (⁵S₂ + ⁵F₄) → ⁵I₈ 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.