Nonradiative relaxation and inhomogeneous splitting of aggregated optical centers in the Nd 3+-doped CaF 2 and SrF 2 crystals (FLN and decay study)

Abstract

Site-selective fluorescence and laser excitation spectroscopy for different types of optical centers in SrF 2 : Nd 3+ (0.3 – 1.0%) and SrF 2 : La 3+ (1.0%) : Nd 3+ (0.2%) were performed at 4.2 K. The crystal-field levels of the 4I 9/2, 4F 3/2 , and 4G 5/2 multiplets for different Nd 3+ optical centers were determined in SrF 2 : Nd 3+ (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 4F 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 4I 9/2(1)→ 4G 5/2(1) crystal-field transition (1.5 cm −1) similar to that in CaF 2 (2.4 cm −1) was found for the Nd–Nd pair in SrF 2. The variation of the splitting from CaF 2 to SrF 2 nicely demonstrates the R −5 dependence of coherent quadrupole–quadrupole interaction.