Site-selective excitation and polarized absorption spectra of Nd3+ in Sr5(PO4)3F and Ca5(PO4)3F

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Abstract
Polarized absorption and fluorescence spectra were analyzed to establish individual energy (Stark) levels of Nd3+ ions in host crystals of Sr5(PO4)3F (SFAP) and Ca5(PO4)3F (FAP). Site-selective excitation and fluorescence facilitated differentiation between Nd3+ ions in emitting sites associated with 1.06 μm stimulated emission, and nonemitting Nd3+ ions in other sites. Measurements were made on samples containing different concentrations of Nd3+ at 4 K and higher temperatures. Substitution of Nd3+ for Sr2+ or Ca2+ was accompanied by passive charge compensation during crystal growth. Crystal-field splitting calculations were performed according to site for Stark levels of Nd3+ ions identified spectroscopically. We obtained a final set of crystal-field parameters Bnm for Nd3+ ions in fluorescing sites with a rms. deviation of 7 cm−1 (52 levels in Nd:SFAP) and 8 cm−1 (59 levels in Nd:FAP). For one of the nonemitting sites in Nd:FAP we obtained a final set of Bnm parameters which gave a rms deviation of 6 cm−1 between 46 experimental and calculated levels.