Blue Upconversion Luminescence in Tm³⁺/Yb³⁺ Codoped CaWO₄ Polycrystals

We investigate the upconversion emission of CaWO₄:Tm³⁺/Yb³⁺ polycrystals prepared by the high-temperature solid-state method. The crystal structure of the polycrystals is characterized by means of x-ray diffraction. Under the excitation of a 980 nm continuous wave diode laser, the samples show intense blue upconversion emissions centered at 473 nm, corresponding to the ¹G₄→³H₆ transition of Tm³⁺. The dependence of the upconversion emission intensity on the pump power of a laser diode is measured, and the results indicate that the two-photon and three-photon processes contribute simultaneously to the blue upconversion emissions. The possible multi-photon upconversion process and upconversion mechanisms are discussed.