Green Upconversion Luminescence in Er³⁺/Yb³⁺ Codoped CaWO₄ Polycrystals

CaWO₄ polycrystals with fixed Yb³⁺ and various Er³⁺ concentrations are synthesized via the high temperature solid state method. The crystal structure of the polycrystals is characterized by means of x−ray diffraction. The upconversion properties of the polycrystals under the 980 nm excitation are investigated. Intense emission bands centered at 530 nm and 552 nm correspond to the transitions ²H_11/2→⁴I_15/2 and ⁴S_3/>2→⁴I_15/2 of Er³⁺, respectively. The dependence of intensity of the green emission on the pump power and possible upconversion mechanism are discussed. Quantitative analysis of dependence of upconversion emission intensity on the pump power of a laser diode indicates that two-photon processes are responsible for both 530 nm and 552 nm green upconversion emissions.