Spectroscopic evaluation of Ho3+-incorporated alkali and alkaline zinc fluorophosphate glasses: Potential for visible green emission applications

Abstract

Alkali and alkaline zinc fluorophosphate (ZFP) glasses doped with Ho³⁺ ions were synthesized and characterized for potential visible green emission applications. The glasses were prepared using the melt-quenching technique with varying concentrations of Ho₂O₃ (0.5, 1.0, 1.5, and 2.0 mol%). The amorphous nature of the glasses was confirmed using X-ray diffraction (XRD) analysis. The presence of functional groups and structural units was determined by Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. The optical absorption spectra exhibited ten peaks attributed to transitions from the ground state (⁵I₈) to various excited states of Ho³⁺. Judd-Ofelt (JO) intensity parameters (Ω₂, Ω₄, and Ω₆) were evaluated, and the trend Ω₂>Ω₄>Ω₆ was observed for all glasses, indicating low symmetry and high covalency around Ho³⁺ ions. The emission spectra obtained under 450 nm excitation, revealed three main peaks corresponding to the ⁵F₄→⁵I₈, ⁵F₅→⁵I₈, and ⁵F₄→⁵I₇ transitions. The ⁵F₅→⁵I₈ transition exhibited the highest branching ratio, stimulated emission cross section, and bandwidth gain among the observed transitions. The ZFPHo_0.5 glass, containing 0.5 mol% Ho₂O₃, demonstrated the highest branching ratio (96.30 %), stimulated emission cross-section (3.28 × 10⁻²² cm²), and bandwidth gain (9.68 × 10⁻²⁸ cm³) for the ⁵F₅→⁵I₈ transition, suggesting its suitability for visible laser applications. The decay profiles of the glasses followed a double exponential decay, and the lifetime decreased with increasing Ho³⁺ concentration, owing to concentration quenching. The CIE chromaticity coordinates of the glasses fell within the yellowish-green region, with high colour purity (86.1–87.3 %) and correlated colour temperatures in the range of direct sunlight (4620–4773 K). The results indicate that Ho³⁺-doped alkali and alkaline zinc fluorophosphate glasses, particularly ZFPHo_0.5, are promising candidates for visible-green emission applications.