Site-selective spectra and energy levels of trivalent erbium in calcium fluorophosphate

Abstract

Site-selective excitation and fluorescence spectra, and conventional polarized transverse and axial absorption spectra of trivalent erbium-doped single crystals of calcium fluorophosphate (Ca 5 (PO 4 ) 3 F), a fluorapatie mineral known as FAP, have been analyzed over the wavelength range 1.60 μm to 0.34 μm at temperatures between 4 K and room temperature. There was no spectroscopic evidence to indicate a phase transition from the hexagonal phase at room temperature to the monoclinic phase around 133 K as reported earlier. Most spectra are interpreted in terms of Er 3+ ions replacing Ca 2+ ions in Ca(I) sites and Ca(II) sites. A majority of the Er 3+ ions prefer the Ca(II) sites, where charge compensation is possible, by replacing F - with O 2- during the growth process. Measured room temperature fluorescence lifetimes are reported for Er:FAP, YbEr:FAB, and Yb:FAP. Absorption cross-sections are reported for Er:FAP. The absorption cross-sections for Er 3+ ( 4 I 15/2 → 4 I 13/2 ) are larger than the emission cross-sections for the lasing transition in Er:phosphate glass. Overlap of the two spectra makes Er:FAP a passive Q -switch for the Er:phosphate glass laser operating at 1.53 μm. A crystal-field splitting calculation for Er 3+ ions in both Ca 2+ sites is described. The initial crystal-field parameters were obtained from a lattice-sum calculation. By varying the initial set of crystal-field splitting parameters, B nm (cm -1 ), by only a small amount, we obtained good agreement between calculated and observed Stark levels.