The mechanism of Tm to Ho energy transfer in LiYF4

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Abstract
The energy transfer properties of the Tm, Ho:LiYF4 are examined, and special attention is given to the Tm(3F4) to Ho(5I7) transfer dynamics and equilibration. The measured transfer times are compared with the values calculated on the basis of the static Forster-Dexter and the migration-assisted Burshtein energy transfer models (which require only spectral information and rare-earth concentrations as input data). The detection of anomalously fast measured transfer rates at high Tm doping levels is interpreted within the context of percolation theory, since migration pathways from the excited Tm ion to a Ho ion must necessarily exist above a particular threshold concentration. Lastly, it is suggested that the use of low Tm concentrations below the percolation threshold may serve to minimize the Auger upconversion losses, and also will shift the excited-state equilibrium to the Ho population; this type of scenario requires that the Tm ions be pumped directly into the 3F4 state with 1.68 mu m InGaAs diode sources.