Spectroscopy of thulium ions in solid-solution sesquioxide laser ceramics: Inhomogeneous spectral line broadening, crystal-field engineering and C3i sites

Abstract

We present a detailed spectroscopic study of Tm3+ ions in stoichiometric and “mixed” (solid-solution) cubic (C-type, sp. gr. Ia3‾) sesquioxides R2O3 (RY, 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 R3+ in the isostructrural R2O3 series is presented. The crystal-field splitting of Tm3+ multiplets is determined for C2 sites and justified using a barycenter plot, and the first evidence of C3i Tm3+ species is presented. A remarkable inhomogeneous spectral broadening for compositionally “mixed” sesquioxides (in particular with Sc3+) 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 3F4 → 3H6 Tm3+ transition are determined. Guidelines for material engineering of Tm3+-doped gain media for mode-locked 2-μm lasers are also provided.