Influence of Crystal Field Parameters on Near-Infrared to Visible Photon Upconversion in Ti²⁺ and Ni²⁺ Doped Halide Lattices

Abstract

The near-infrared to visible photon upconversion properties of Ti²⁺ and Ni²⁺ doped chloride and bromide lattices are summarized and discussed. They are used to illustrate the young and emerging field of transition metal based upconversion systems. It is demonstrated that transition metal upconversion properties can be tuned by chemical and structural variation. The change from NaCl to MgCl₂ as a host for Ti²⁺ induces a crossover in the intermediate metastable excited state of Ti²⁺ from ³T_2g to ¹T_2g, leading to an order of magnitude enhancement of the 15 K near-infrared to visible upconversion efficiency. The change from CsCdCl₃ to MgBr₂ as host lattices for Ni²⁺ dopant ions induces a crossover in the upper emitting excited state of Ni²⁺ from ¹T_2g to ¹A_1g, manifesting itself in a change from broad-band to sharp-line visible upconversion luminescence. These results are rationalized on the basis of varying covalency and crystal field strength.