Er/sup 3+/ to Tm/sup 3+/ energy transfer in YLiF/sub 4/ (YLF)

The Er/sup 3+/ to Tm/sup 3+/ energy transfers are analyzed in doubly doped YLiF/sub 4/:Er/sup +/, Tm/sup 3+/ single crystals. A good description of the excited state dynamics is achieved with models based on rate equations and using lifetime data. In particular, it is shown that the two lowest excited states /sup 4/I/sub 13/2/ of Er/sup 3+/ and /sup 3/F/sub 4/ of Tm/sup 3+/ are in thermal equilibrium at room temperature because of efficient Er from or to Tm energy transfers between these two levels.>