Blue upconversion emission of Cu²⁺ ions sensitized by Yb³⁺-trimers in CaF₂

Under 978 nm near-infrared (NIR) excitation, blue upconversion (UC) emissions from CaF₂:Cu²⁺,Yb³⁺ were first observed at ∼420 nm. It was ascribed to the 3d⁸4s¹ → 3d⁹ transition of Cu²⁺ ions. From transient measurements, the UC process was found to be dominated by the energy transfer process in which three excited Yb³⁺ ions simultaneously transfer their energy to one Cu²⁺ ion. The influence of Cu²⁺ concentration and temperature on the UC emission as well as the Jahn–Teller effect was also investigated.