Nonradiative relaxation and inhomogeneous splitting of aggregated optical centers in the Nd3+-doped CaF2 and SrF2 crystals (FLN and decay study)

Site-selective fluorescence and laser excitation spectroscopy for different types of optical centers in SrF2 : Nd3 + (0.3 - 1.0%) and SrF2 : La3 + (1.0%) : Nd3 + (0.2%) were performed at 4.2 K. The crystal-field levels of the I-4(9/2), F-4(3/2), and (4)G(5/2) multiplets for different Nd3 + optical centers were determined in SrF2 : Nd3 + (0.3%). An attempt was made to identify some of the new optical centers on the basis of their fluorescence lifetimes and crystal-field splitting. Some models of these centers have been discussed. Two-six times reduction of the F-4(3/2) multiplet lifetime was observed in the pair M-center in comparison with the single optical centers due to energy cross-relaxation in the pair. A fine splitting of the levels at the I-4(9/2)(1)-->(4)G(5/2)(1) crystal-field transition (1.5 cm(-1)) similar to that in CaF2 (2.4 cm(-1)) was found for the Nd-Nd pair in SrF2. The variation of the splitting from CaF2 to SrF2 nicely demonstrates the R-5 dependence of coherent quadrupole-quadrupole interaction. (C) 1999 Elsevier Science B.V. All rights reserved.