Daniel R. Gamelin, Markus Wermuth and Hans U. Güdel
Up-conversion processes of 5d transition metal ions in crystals
J. Lumin. 83-84, 405-410 (1999)                 

Abstract: The phenomenon of photon up-conversion, widespread in lanthanide compounds, is extremely rare in transition metal compounds. We have observed near-IR (NIR) to visible (VIS) up-conversion in Ti²⁺, Ni²⁺, Mo³⁺, Re⁴⁺ and Os⁴⁺ doped chloride and bromide host lattices. In the system Cs₂ZrCl₆ : Re⁴⁺ the NIR to VIS up-conversion occurs nonradiatively by energy transfer. It has a high efficiency up to room temperature. In Cs₂ZrBr₆ : Os⁴⁺ there is efficient up-conversion below 80 K by an excited-state absorption mechanism. For one specific excitation energy the typical signature of an excitation avalanche is observed. The steady-state populations for CW excitation and the excited state dynamics after pulsed excitation can be reproduced by simple rate equation models for both the Re⁴⁺ and Os⁴⁺ systems.