Enhanced Upconversion Luminescence and Physical Characteristics of Ho³⁺-Doped Tungsten Tellurite Glasses for Photonic Applications
Holmium (Ho³⁺)-doped tungsten tellurite glasses with the composition (70−x)TeO₂–10Na₂O–20WO₃–xHo₂O₃ (x = 0.5–1.5 mol%) were successfully synthesized using the conventional melt-quenching technique to investigate their suitability for visible upconversion and photonic applications. X-ray diffraction analysis confirmed the amorphous nature of all prepared samples, while differential scanning calorimetry demonstrated good thermal stability with a glass transition temperature of approximately 310 °C and a crystallization temperature near 500 °C. Various physical parameters, including density, molar volume, lanthanide ion concentration, polaron radius, interionic distance, field strength, and oxygen packing density, were evaluated to understand the structural modifications induced by Ho³⁺ incorporation. The density and lanthanide ion concentration increased systematically with increasing Ho³⁺ content, whereas the molar volume, interionic distance, and polaron radius decreased, indicating progressive densification and strengthening of the glass network. Optical absorption spectra exhibited characteristic Ho³⁺ transitions in the visible and near-infrared regions, confirming efficient incorporation of rare-earth ions into the tellurite glass matrix. Under 980 nm laser excitation, intense upconversion emissions centered at approximately 547 nm (green), 660 nm (red), and 760 nm (near-infrared) were observed, corresponding to the ⁵F₄/⁵S₂ → ⁵I₈, ⁵F₅ → ⁵I₈, and ⁵S₂ → ⁵I₇ transitions of Ho³⁺ ions, respectively. The emission intensity increased with excitation power, demonstrating efficient excited-state absorption and cross-relaxation processes responsible for the observed upconversion mechanism. The combination of favorable thermal stability, enhanced physical characteristics, and strong visible upconversion emission demonstrates that Ho³⁺-activated tungsten tellurite glasses are promising candidates for solid-state lasers, optical amplifiers, color display devices, and other advanced photonic applications.