Ultraviolet upconversion emission of Pb2+ ions sensitized by Yb3+-trimers in CaF2

Under 978 nm near infrared (NIR) excitation, ultraviolet (UV) upconversion (UC) emissions from Pb2+ ions in CaF2:Pb2+, Yb3+ were first observed at room temperature. The UC emission centered at similar to 383 nm was ascribed to the P-3(0) -> (1)A(1g) (S-1(0)) transition of Pb2+ ions. From transient measurements, the UC process was found to be dominated by the energy transfer process: three excited Yb3+ ions simultaneously transfer their energy to one Pb2+ ion. With the increase in Pb2+ concentration, the cooperative luminescence from 3-Yb3+ clusters decreased gradually until it was extinguished. The cooperative sensitization to one Pb2+ ion comes from three excited Yb3+ ions, which is a four-ion process and exhibits a third power dependence on the NIR pumping power.