Multisite optical spectra and energy levels of trivalent thulium ions in yttrium scandium gallium garnet

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Abstract
Intrinsic structural disorder in scandium‐substituted garnets, attributed to mixed occupancy of certain sites in the crystal lattice by different cations, has direct consequences for the optical spectra of rare‐earth activator ions dispersed over multiple sites. In trivalent thulium‐doped yttrium scandium gallium garnet (Tm3+:YSGG), site‐selective laser excitation spectra reveal the presence of Tm3+ ions in regular D2 sites, disturbed regular sites, and in octahedral C3i sites. Absorption spectra obtained at 4 K between 0.26 and 1.85 μm are broader than those observed in more‐ordered crystal hosts and include structure attributed to Tm3+ ions in sites of other than D2 symmetry. A crystal‐field splitting calculation was carried out in which a parametrized Hamiltonian (including Coulombic, spin‐orbit, and crystal‐field terms for Tm3+ ions in D2 symmetry) was diagonalized for all manifolds of the Tm3+ (4f12) configuration. The rms deviation between 52 experimental and calculated Stark levels of Tm3+ in regular D2 sites was 5 cm−1.