Judith Grimm, Oliver S. Wenger and Hans U. Güdel
Broadband green upconversion luminescence of Ni²⁺ in KZnF₃
J. Lumin. 102-103, 380-385 (2003)                 

Abstract: The upconversion properties of divalent nickel doped into KZnF₃ are reported. By exciting into the ³T_1g (³F) or ¹E_g excited state of Ni²⁺ with monochromatic light, a broad green luminescence around 20,000 cm^–1 is observed that originates in the ¹T_2g higher excited state. The upconversion is shown to be a single ion process, consisting of a sequential two-photon absorption. The green luminescence is observed at all temperatures between 15 K and room temperature. This is strikingly different from what is observed in Ni²⁺ doped chloride and bromide materials in which the ¹T_2g³A_2g luminescence is located in the red spectral range and is quenched well below room temperature.