Spectroscopy of Thulium ions in solid-solution sesquioxide laser ceramics: Inhomogeneous spectral line broadening, crystal-field engineering and C$_{3i}$ sites

Overview & Synopsis

We present a detailed spectroscopic study of Tm 3+ ions in stoichiometric and "mixed" (solid-solution) cubic (C-type, sp. gr. Ia3 ̅ ) sesquioxides R2O3 (R = Y, Lu, Sc or their mixture), with the goal of developing broadband-emitting gain media for ultrafast lasers at ~2 μm. Evidence of a linear variation (increase) of the crystal-field strength when decreasing the ionic radius of the host-forming cation R 3+ in the isostructrural R2O3 series is presented. The crystal-field splitting of Tm 3+ multiplets is determined for C2 sites and justified using a barycenter plot, and the first evidence of C3i Tm 3+ species is presented. A remarkable inhomogeneous spectral broadening for compositionally "mixed" sesquioxides (in particular such with Sc 3+ ) with respect to the parent compounds is revealed at low temperatures. It is proven that binary and ternary "mixed" R2O3 compounds form substitutional solid-solutions with a mixing of the host-forming cations at the atomic level. The stimulated-emission and gain cross-sections for the 3 F4 → 3 H6 Tm 3+ transition are determined. Guidelines for material engineering of Tm 3+ -doped gain media for mode-locked 2-μm lasers are also provided.