Some of our recent spectral and energy transfer results on
Tm3+-based 2-?m solid state laser systems are presented. Based on
experimental data, with an improved algorithm and parameterized crystal
field calculations, complete energy level diagrams for Tm3+ in YAG and
GGG are obtained. The sets of crystal field parameters could be used to
estimate a series of important spectral data for laser emission. A discussion
of the spectral characteristics and the nature of Tm3+ multisite
structure is also presented. The static spectral modifications in Cr3+
codoped with Tm3+ YAG or GGG crystals, due to mutual crystal field
perturbations of the two dopants, are connected with Cr3+-Tm3+ pairs.
The high-resolution data offer direct evidence that the Cr31-Tm3+ energy
transfer is governed for near neighbor Cr3+(a)-Tm3+(c) pairs by a shortrange,
most probably superexchange, interaction. The contribution of
multipolar interactions should be considered only for more distant pairs,
while at large distances, dipole-dipole interaction is dominant. The data
suggest that the sensitization with Cr3+ determines not only an efficient
way of transfer of excitation from the pumping sources to active ions but
could produce changes of the spectal and temporal behavior of the latter.
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