ELECTRONIC ENERGY-LEVEL STRUCTURE OF TRIVALENT HOLMIUM IN YTTRIUM-ALUMINUM-GARNET

The optical-absorption spectrum of Ho3+:Y3Al 5O12 (Ho:YAG) has been analyzed between 220 and 2160 nm under variable temperature conditions between 4 and 60 K. Measurements between 220 and 470 nm were obtained from a Ho:YAG crystal that contained 15 at. % holmium. A crystal containing 0.5 at. % holmium was used to obtain the spectrum between 470 and 2160 nm. Within the temperature and total spectral range, more than 1500 transitions due to Ho3+(4f10) were observed and attributed to transitions between individual crystal-field (Stark) levels of the ground-state multiplet 5I8, and 49 different excited multiplet manifolds, 4f10(2S+1LJ), of trivalent holmium. From an analysis of these transitions it was possible to establish 419 of the 486 Stark levels predicted below 45 000 cm-1. This large number of experimentally characterized energy levels provided a suitable basis for a detailed analysis of the 4f10(Ho3+) electronic state structure in Ho:YAG. The analysis was carried out using a model Hamiltonian that assumes D2 site symmetry for the Ho3+ ions and incorporates all of the interactions known to have a significant influence on the 4fN electronic state structure of trivalent lanthanide ions in crystalline hosts. The radial-dependent parts of the interaction terms in the model Hamiltonian are represented in parametric form, and the resulting parameters are used as variables in fitting calculated-to-experimental energy-level data. The parametric fits of energy-level data yield a rms deviation of ∼11 cm-1 between the calculated and observed locations of the 419 experimentally assigned levels (between 0 and 45 000 cm-1), and both the isotropic and anisotropic (crystal-field) interaction parameters of the model Hamiltonian are well characterized by these data fits. Among the 50 4f10[SL]J multiplet manifolds represented in the energy-level analysis, only three show major discrepancies between calculated and observed crystal-field splitting energies. The three problematic multiplet regions are: (5G6, 5F1), centered at 22 258 cm-1, 5G5, centered at ∼24 040 cm-1; and (5G3, 3L 9), centered close to 28 950 cm-1. These same multiplet regions have posed problems in energy-level analyses of Ho3+(4f 10) in other systems. The possible influence of correlation crystal-field (CCF) interactions on these multiplet manifolds is explored in the present study, but CCF effects appear to be small. Overall, the model Hamiltonian derived gives an excellent representation of 4f10(Ho 3+) energy-level structure in Ho:YAG, and the eigenvectors of this Hamiltonian should provide a satisfactory basis for future calculations and modeling studies of Ho:YAG optical and magneto-optical properties. © 1995 American Institute of Physics.