Spectroscopic and upconversion properties of Er³⁺/Yb ³⁺-codoped KLTN single crystal

A high optical quality erbium and ytterbium codoped potassium lithium tantalate niobate single crystal is grown by top-seed solution growth method, and the crystal structure is confirmed to be 4 mm point group and P4/mbm space group by powder X-ray diffraction analysis. The ultraviolet-visible-near infrared absorption spectrum, upconversion fluorescence spectrum excited by 975 nm continuous wave laser, power dependence, and fluorescence decay curves of Er³⁺ ions in the crystal are measured at room temperature. The Judd-Ofelt (JO) intensity parameters are calculated using the JO theory and absorption spectrum. The relationship of JO parameters Ω₄ < Ω₆ of Er³⁺ in the crystal is different from the general trend of Ω₂ > Ω₄ > Ω₆. The absorption cross sections and emission cross sections of H11/22, S3/24, and F9/24 levels are calculated. The upconversion mechanism of Er³⁺ in the crystal is discussed based on pump power dependence analyses and decay curves. The emission intensities exhibit quadratic behaviors against pump energy, which are verified by the deduction of transition rate equation model.