Hans U. Güdel and Markus Pollnau
Near-infrared to visible photon upconversion processes in lanthanide doped chloride, bromide and iodide lattices
J. Alloys Comp. 303-304, 307-315 (2000)                 

Abstract: The near-infrared to visible upconversion behavior of some selected Er³⁺, Tm³⁺, Dy³⁺ and Ho³⁺ doped chloride, bromide and iodide host lattices is summarized and discussed. The step from the well studied oxide and fluoride materials to the heavier halides can lead to dramatic changes in the upconversion behavior. This is mainly due to the lower vibrational energies which reduce the efficiency of multiphonon relaxation processes. As a consequence, new upconversion, cross-relaxation and luminescence processes become competitive. The relevance of these new materials lies in their potential for upconversion laser and phosphor applications.