Selective laser excitation of charge compensated sites in CaF2:Er3+

Abstract

A technique of selective laser excitation has been developed and used to study the optical spectrum of the charge compensated sites in CaF₂:Er³⁺. The technique allows the unambiguous interpretation and classification of the spectral lines observed from crystals with multiple sites. The absorption lines of specific sites have been excited with a narrow band dye laser. Nonresonant energy transfer between sites has not been observed and fluorescence from a single site can be obtained if no overlap occurs between absorption lines of different sites. The fluorescence transitions of CaF₂:Er³ have been classified into two sites having an Er³⁺ ion and fluoride interstitial compensation and 16 sites having more than one Er³⁺ ion. The single‐pair sites correspond to those previously identified with C_4v and C_3v site symmetry. The identification of the remaining sites with clusters containing more than one Er³⁺ ion is confirmed by the observation of ion–ion interactions, including up‐conversion, cross relaxation, and pair transfer phenomena. No evidence was found for ion–ion interactions involving the identified single‐pair sites. We have also succeeded in distinguishing the 16 multiple pair sites into 11 sites with similar crystal field splittings, four sites exhibiting efficient energy transfer between spectroscopically differentiable Er³⁺ ions with the site, and one site having spectral features similar to those of the single‐pair site of C_3v symmetry. The low dopant concentrations at which sites of multiple Er³⁺ ions are formed will affect theoretical interpretations of previous experiments.