Intense blue upconversion fluorescence of Tm³⁺/Yb³⁺-codoped bismuth tellurite glasses

In this paper, Tm³⁺/Yb³⁺-codoped bismuth tellurite glasses with high refractive index have been synthesized. The refractive index of Tm³⁺ / Yb³⁺-codoped bismuth tellurite glasses were measured by Metricon 2010 prism coupler at two wavelengths, and the values n₁ = 2.0365 at 632.8 nm and n₂ = 1.9795 at 1550 nm were obtained. The absorption spectra, fluorescence spectra and infrared transmittance spectra of this glass were measured and analyzed. Based on Judd-Ofelt theory, and intensity parameters Ω_t ( t = 2, 4 and 6) were obtained to be 3.90 × 10^-20, 2.03 × 10^-20 and 9.03 × 10^-21 cm², respectively. Then the radiative transition probabilities, radiative lifetimes and fluorescence branching ratio were calculated. Intense blue three-photon upconversion fluorescence and near-infrared two-photon upconversion fluorescence were investigated under the excitation of 980 nm diode laser at room temperature. Wide infrared transmission window, high refractive index and strong blue three-photon upconversion emission of Tm³⁺ indicate that Tm³⁺ / Yb³⁺-codoped LKBBT glasses are promising upconversion optical and laser materials.