Influence of Al/Er ratio on the optical properties and structures of Er³⁺/ Al³⁺ co-doped silica glasses

In this study, Er³⁺/Al³⁺ co-doped silica glasses with various Al/Er ratios ranging from 0 to 200 and a constant Er₂O₃ amount were prepared using the sol-gel method combined with high-temperature vacuum sintering. The absorption, emission spectra, and fluorescence lifetime of Er³⁺ ions were recorded and the absorption and emission cross sections were calculated. Using Raman, ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR), ²⁷Al triple-quantum (TQ)-MAS-NMR, and electron paramagnetic resonance (EPR) spectroscopies, the physical and optical properties were correlated with the glass structural modifications due to the Al/Er ratio change. With the increase in the Al/Er ratio, the content of Al³⁺ ions around the Er³⁺ ions gradually increased. Meanwhile, the AlO₅ and AlO₆ polyhedrons increased at the expense of the AlO₄ and SiO₄ tetrahedrons, which resulted in an improved symmetry and better ionic Er–O bonds. These structural changes led to the site-to-site variations in the Er³⁺ local environment and the changes in the ligand field strength of the Er–O bonds. This, in turn, resulted in the inhomogeneous broadening of the absorption and emission spectra, with a blue shift of both absorption and emission peaks and a decrease in both absorption and emission at 1.53 μm. With the increase in Al/Er ratio, the full width at half maximum (FWHM) of the emission increased from 27.2 to 54.3 nm. The FWHM × emission cross section (⁠$σemi$⁠) parameter for evaluating broadband amplification behavior was 30.1 × 10⁻²⁰ cm² · nm. This work reveals the close relationship between the glass structure and the spectroscopic properties of Er³⁺ ions and provides an important reference for the design of broadband amplifier materials in the field of optical communication.