Blue Upconversion Luminescence in Tm^(3+)/Yb^(3+) Codoped CaWO_(4) Polycrystals

We investigate the upconversion emission of CaWO4:Tm^(3+)/Yb^(3+)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 ^(1)G4→^(3)H_(6) transition of Tm^(3+).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.