Investigation of the mechanisms of upconversion luminescence in Ho 3+ doped CaF 2 crystals and ceramics upon excitation of 5 I 7 level

The mechanisms of upconversion luminescence of CaF2:Ho crystals and ceramics from 5F3, 5S2(5F4), 5F5 and 5I6 levels upon excitation of 5I7 level of Ho3+ ions were investigated. Different mechanisms are responsible for the populating and depletion of the energy levels of Ho3+ ion in CaF2:Ho crystals and ceramics upon excitation of 5I7 level. The upconversion luminescence from 5F3, 5F5, and 5I6 levels in CaF2:Ho crystals and ceramics is explained by energy transfer upconversion processes. Our results also confirmed that both excited-state absorption and energy transfer upconversion are responsible for the populating of 5S2(5F4) level.