Improved Infrared Emissions of Er³⁺-Tm³⁺ Co-Doped Al₂O₃ Thin Films: The Role of Cross Relaxation Among Rare Earth Ions

We report the infrared emissions of Er³⁺-Tm³⁺ co-doped amorphous Al₂O₃ thin films pumped at 791 nm by a Ti:sapphire laser. The as-deposited films were annealed to improve the photoluminescence performance. Three cross relaxation channels among Er³⁺-Tm³⁺ and Tm³⁺-Tm³⁺ ions incorporated in the films were investigated as annealing temperature increases especially from 800 to 850 °C. In order to understand the Stark effect and cross relaxations, the photoluminescence spectra were deconvoluted by Gaussian fittings. Our results indicate that the luminescence intensity of 1.62 μm in comparison to 1.5 μm can be enhanced by the cross relaxation process [Er³⁺(⁴I_13/2)+Tm³⁺(³H₆) -> Er³⁺(⁴I_15/2)+Tm³⁺1(³F₄], and the longer-wavelength side of Er³⁺ emission can be improved by the CR process [Er³⁺(⁴I_15/2)+Tm³⁺(³H₄) -> Er³⁺ (⁴I_13/2)+Tm³⁺(³F₄)] at expense of the Tm³⁺ 1.47 μm emission which is also maybe quenched by the CR effect between themselves. These results suggest one possible approach to achieve broadband infrared emissions at the wavelength region of 1.45-1.65 μm from the Er³⁺-Tm³⁺ co-doped systems.