Broadband green upconversion luminescence of Ni2+ in KZnF3

The upconversion properties of divalent nickel doped into KZnF3 are reported. By exciting into the 3T1g (3F) or 1Eg excited state of Ni2+ with monochromatic light, a broad green luminescence around 20,000 cm−1 is observed that originates in the 1T2g 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 1T2g→3A2g luminescence is located in the red spectral range and is quenched well below room temperature.