Up-conversion processes of 5d transition metal ions in crystals

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.